FUNDAMENTALS
OF THE
NERVOUS SYSTEM
AND
NERVOUS TISSUE
Nervous System
• Has three overlapping
functions:
• 1.It uses its millions of
sensory receptors to
monitor changes occurring
both inside and outside the
body
– The gathered information is
called sensory input
• 2.It processes and interprets
sensory input and decides
what should be done at each
moment—a process called
integration
• 3.It causes a response, called
motor output, by activating
effector organs
NERVOUS SYSTEM’S
FUNCTIONS
Organization of the Nervous System
• Divided into two principal parts:
– Central nervous system (CNS): consist of the brain
and spinal cord, which occupy the dorsal body cavity
• Integrating and command center of the nervous system
• Interprets sensory input and dictates motor responses based
on past experience, reflexes, and current conditions
– Peripheral nervous system (PNS): the part of the
nervous system outside the CNS
• Consists mainly of the nerves (bundles of axons) that
extend from the brain and spinal cord
– Spinal nerves carry impulses to and from the spinal cord
– Cranial nerves carry impulses to and from the brain
• Serve as the communication lines that link all parts of the
body to the CNS
ORGANIZATION
OF
THE NERVOUS SYSTEM
•
The peripheral nervous system
(PNS) has two functional
subdivisions:
– 1.The sensory, or afferent,
division of the peripheral nervous
system consist of nerve fibers that
convey impulses to the central
nervous system from sensory
receptors located throughout the
body
• Somatic afferent fibers:
sensory fibers conveying
impulses from the skin,
skeletal muscles, and joints
• Visceral afferent fibers:
sensory fibers that transmit
impulses from the visceral
organs (organs within the
ventral body cavity)
NERVOUS SYSTEM
ORGANIZATION
ORGANIZATION
OF
THE NERVOUS SYSTEM
•
•
The peripheral nervous system (PNS) has two functional subdivisions:
2.Motor, or efferent, division of the peripheral nervous system carries
impulses from the central nervous system to effector organs, which
are muscles and glands:
– These impulses activate muscles to contract and glands to secrete; that is,
they effect (bring about) a motor response
– Two main parts:
• The somatic nervous system (SNS) consists of somatic nerve fibers (axons) that
conduct impulses from the CNS to skeletal muscles, and allow conscious control of
motor activities
– Referred to a the voluntary nervous system because it allows us to consciously control
our skeletal muscles
• The autonomic nervous system (ANS) consists of visceral motor nerve fibers that
regulate the activity of smooth muscle, cardiac muscle, and glands
– Referred to as the involuntary nervous system
– Has two functional subdivisions: which typically work in opposition to each other—what one
subdivision stimulates, the other inhibits
» Sympathetic
» Parasympathetic
NERVOUS SYSTEM
ORGANIZATION
NERVOUS SYSTEM
ORGANIZATION
Histology of Nervous Tissue
• Nervous tissue is made up of just two
principal types of cells:
• 1.Supposting cells:
– Smaller cells that surround and wrap the more
delicate neurons
• 2.Neurons:
– The excitable nerve cells that transmit
electrical signals
Neuroglia
• Smaller cells that are associated with
neurons: provide a supportive and protective
network for neurons
– Also called glial cells
• Like neurons, most glial cells have branching processes
(extensions) and a central cell body
• They outnumber neurons in the CNS by about 10 to 1
– Make up about ½ the mass of the brain
– Six types:
• Four in the CNS (central nervous system)
• Two in the PNS (peripheral nervous system)
Neuroglia (Glial) in the CNS
•
•
•
•
•
Astrocytes (a) are glial cells of
the CNS that regulate the
chemical environment around
neurons and exchange between
neurons and capillaries
Most abundant and versatile
glial cells
Radiating processes cling to the
neurons and nearby capillaries
anchoring and bracing them to
their nutrient supply (blood
capillaries)
Mop up leaked potassium ions
and recapture (recycle)
neurotransmitters
Held together by gap junctions
that enable them to signal one
another (and perhaps neurons)
via intracellular calcium pulses
CELLS OF THE NERVOUS
SYSTEM
Neuroglia (Glial) in the CNS
• Microglia (b) are glial cells of
the CNS that monitor health
and perform defense functions
for neurons:
– Protective role is important
because cells of the immune
system are denied access to
the CNS (under normal
circumstances)
• Long thorny processes that
touch nearby neurons
• Transform into a special
type of macrophage that
phagocytizes
microorganisms or neuronal
debris
CELLS OF THE NERVOUS
SYSTEM
Neuroglia (Glial) in the CNS
• Ependymal cells (c) are glial
cells of the CNS that line the
central cavities of the brain
and spinal cord and help
circulate cerebrospinal fluid:
– Permeable barrier between
the cerebrospinal fluid that fills
the central cavities (of the
brain and spinal cord) and the
tissue fluid bathing the cells of
the CNS
• Many are ciliated
– Helps to circulate the
cerebrospinal fluid that
cushions the brain and spinal
cord
CEREBROSPINAL FLUID
CELLS OF THE NERVOUS
SYSTEM
Neuroglia (Glial) in the CNS
• Oligodendrocytes
(d) are glial cells of
the CNS that wrap
around neuron
fibers, forming
myelin sheaths—
insulating covering
• Also branched (fewer
processes)
Neuroglia in the PNS (Peripheral Nervous System)
• Satellite cells (e) are
glial cells of the PNS
whose function is
largely unknown
– They are found
surrounding neuron
cell bodies within
ganglia
– Name comes from the
resemblance to the
moons (satellites)
around a planet
Neuroglia in the PNS (Peripheral Nervous System)
• Schwann cells (e), or
neurolemmocytes, are
glial cells of the PNS that
surround nerve fibers,
forming the myelin sheath
• Function similar to
oligodendrocytes of CNS
• Vital to regeneration of
peripheral nerve fibers
CELLS OF THE NERVOUS
SYSTEM
NEURON
NEURONS (NERVE CELLS)
• Structural units of the nervous system
• Specialized cells that:
– Conduct messages in the form of electrical impulses
throughout the body
– Have extreme longevity:
• Function optimally for a lifetime (over 100 years)
– Largely amitotic:
• No ability to divide
– Exceptions:
» Olfactory epithelium: smell
» Hippocampus: region of brain involved in memory
– Have an exceptionally high metabolic rate requiring oxygen
and glucose
• Cannot survive for more than a few minutes without oxygen
NEURONS (NERVE CELLS)
CELL BODY
• The neuron cell body, also
called the perikaryon or
soma, is the major
biosynthetic center containing
the usual organelles except for
centrioles
• Most are located in the CNS,
where they are protected by
the bones of the skull and
vertebral column
• Clusters of cell bodies in the
CNS are called nuclei,
whereas those that lie along
the nerves in the PNS are
called ganglia
NEURONS (NERVE CELLS)
• Processes:
– Armlike extensions from the cell body of all
neurons
– CNS (brain and spinal cord): contain both
neuron cell bodies and their processes
• Bundles of neurons in the CNS are called tracts
– PNS (peripheral): consist mainly of neuron
processes
• Bundles of neurons in the PNS are called
nerves
NEURONS (NERVE CELLS)
DENDRITE
• Dendrites are cell processes
that are the receptive regions
of the cell
• Provide an enormous surface
area for receiving signals from
other neurons or the
environment
• Bristle with thorny appendages
having bulbous or spiky ends
called dendritic spines
• Convey incoming messages
toward the cell body
• These electrical signals are
NOT nerve impulses (action
potentials) but are shortdistance signals celled graded
potentials
NEURONS (NERVE CELLS)
AXON
•
•
•
Initial region of the axon arises from a
cone-shaped area of the cell body
called the axon hillock and then
narrows
Each neuron has a single axon that
generates and conducts nerve
impulses away from the cell body to
the axon terminals
Short or long
–
–
•
•
Example: axons of the motor neurons
controlling the skeletal muscles of your
toes extend from the lumbar region of
your spine to your foot (3-4 feet)
Long axons are called nerve fibers
Conducting component of the
neuron
Generates nerve impulses and
transmit them, typically away from
the cell body
NEURONS (NERVE CELLS)
TERMINAL BRANCHES
• Each neuron has only
one axon, but axons
may have occasional
branches, called axon
collaterals
– Extend from the axon at
more or less right angles
• Axon usually branches
profusely at its end
(terminus)
– 10,000 or more terminal
branches per neuron is not
unusual
NEURONS (NERVE CELLS)
TERMINAL BRANCHES
• When the impulse
reaches the axonal
terminals, it causes
neurotransmitters,
signaling chemicals
stored in vesicles there,
to be released into the
extracellular space
– Either excite or inhibit
neurons (or effector cells)
• Axon plasma
membrane (axolemma)
NEURONS (NERVE CELLS)
MYELIN SHEATH
• The myelin sheath is a whitish, fatty,
segmented covering that protects, insulates, and
increases conduction velocity of axons
– Myelinated fibers (axons bearing a myelin sheath)
conduct nerve impulses rapidly
– Unmyelinated fibers (axons without a myelin sheath)
conduct impulses slower
• Associated only with axons
• Dendrites are always unmyelinated
NEURONS (NERVE CELLS)
MYELIN SHEATH
• Formed in the PNS by
Schwann cells
– Wrapped around the
axon (jelly roll fashion)
• Initially loose but as the
wrapping gets tight the cell
cytoplasm is gradually
squeezed from between
the membrane layers
– Many concentric layers of
Schwann cell plasma
membrane enclose the
axon (like gauze wrapped
around an injured finger)
• This tight coiling is the
myelin sheath
NEURONS (NERVE CELLS)
MYELIN SHEATH
• Plasma membranes of myelinating cells
contain much less protein than the
plasma membranes of most body cells
– Channel and carrier proteins are absent, a
characteristic that makes myelin sheaths
exceptionally good electrical insulators
NEURONS (NERVE CELLS)
NEURILEMMA
• The nucleus and most of the cytoplasm of the
Schwann cell and up as a bulge just external to the
myelin sheath
– This portion of the Schwann cell which includes the
exposed part of its plasma membrane, is called the
neurilemma
• Adjacent Schwann cells along an axon do not touch one
another, so there are gaps in the sheath
– These gaps, called nodes of Ranvier (neurofibril nodes),
occur at regular intervals (1 mm) along the myelinated axon
• It is at this nodes that axon collaterals can emerge from the
axon
NEURON
SCHWANN AXON
BRAIN
• Regions of the brain and spinal cord
containing dense collections of
myelinated fibers are referred to as
white matter and are primarily fiber tracts
– Gray matter contains mostly nerve cell
bodies and unmyelinated fibers
Classification of Neurons
Structural
• There are three structural classes of neurons: classified
according to the number of processes extending from their cell
body
– Multipolar neurons:
• Have three or more processes (many dendrites and single axon)
• 99% of neurons belong to this class
• Major neuron type in the CNS
– Bipolar neurons have a single axon and dendrite
• Found only in some of the special sense organs, where they act as receptor
cells
– Retina of the eye and olfactory mucosa
– Unipolar neurons have a single process extending from the cell
body and divides T-like into proximal and distal branches
• Mainly in ganglia in the PNS, where they function as sensory neurons
Classification of Neurons
Functional
•
There are three functional classes of neurons: classified according to the direction in which
the nerve impulse travels relative to the central nervous system
– Sensory, or afferent, neurons conduct impulses toward the CNS from receptors
• Except for the bipolar neurons found in some special sense organs, virtually all
sensory neurons are unipolar, and their cell bodies are located in sensory ganglia
outside the CNS
• Some can be very long:
– Example, fibers carrying sensory impulses from the skin of your toes travel for
more than a meter before reaching their cell bodies in a ganglion close to the
spinal cord
– Motor, or efferent, neurons conduct impulses from the CNS to effectors (muscles and
glands) of the body periphery
• Multipolar, except for some neurons of the autonomic nervous system
• Cell bodies are located in the CNS
– Interneurons, or association, neurons conduct impulses between sensory and motor
neurons
• Most are confined within CNS
• In CNS integration pathways
• 99% of the neurons of the body
• Almost are multipolar, but there is considerable diversity in both size and fiberbranching patterns
NEUROPHYSIOLOGY
• Neurons are highly responsive to
stimuli:
– When adequately stimulated, an electrical
impulse is generated and conducted along the
length of its axon
• This response, called the action potential
(nerve impulse), is always the same, regardless
of the source or type of stimulus
Basic Principles of Electricity
•
•
•
Opposite charges attract each other, energy must be used (work must
be done) to separate them
Coming together of opposite charges liberates energy that can be
used to do work
Voltage is a measure of the amount of difference in electrical charge
between two points, called the potential difference
– Measured in volts or millivolts ( 1 mV = 0.001 V )
– Greater the difference in charge between two points, the higher the voltage
•
The flow of electrical charge from point to point is called current, and
is dependent on voltage and resistance (hindrance to current flow)
– Used to do work
– The amount of charge that moves between the two points depends on two
factors: voltage and resistance
•
Resistance: hindrance to charge flow
– Provided by substances through which the current must pass
– Substances with high electrical resistance are called insulators
– Substances with low electrical resistances are called conductors
Basic Principles of Electricity
• Ohm’s Law: relationship between voltage,
current, and resistance
– Current (I) = Voltage (V) / Resistance (R)
• Current (I) is directly proportional to voltage (V)
– Greater the voltage (potential difference), the greater the
current
• Current (I) is inversely related to resistance (R)
– Greater then resistance, the smaller the current
Basic Principles of Electricity
• In the body, electrical currents are due to the
movement of ions (rather than free electrons)
across cellular membranes
– There are NO free electrons in a living system
– There is a slight difference in the numbers of
positive and negative ions on the two sides of
cellular plasma membranes (there is a charge
separation), so there is a potential across those
membranes
• The resistance to current flow is provided by the plasma
membrane
The Role of Membrane Ion Channels
• The plasma membrane has many ion channels made
of proteins
– Some of these channels are passive, or leakage, channels
which are always open
– Some are active, or gated that change shape or open/close in
response to the proper signal
• Chemically gated, or ligand-gated, channels open when the
appropriate chemical (in this case, neurotransmitter) bind
• Voltage-gated channels open and close in response to changes in
the membrane potential
• Mechanically gated channels open in response to physical
deformation of the receptor (sensory receptors for touch and
pressure)
– Each type of channel is selective as to the type of ion (or ions) it
allows to pass
• Example: a potassium ion channel allows only potassium ions to
pass
GATED CHANNELS
The Role of Membrane Ion Channels
• When gated channels are open, ions diffuse quickly
across the membrane following their electrochemical
gradients, creating electrical currents and voltage
changes across the membrane according to the
rearranged Ohm’s law equation:
– Voltage (V) = current (I) X resistance (R)
• Ions move along chemical gradients when they diffuse
passively from an area of their higher concentration
to an area of lower concentration
• Ions move along electrical gradients when they move
toward an area of opposite charge
• It is ion flows along electrochemical gradients that
underlie all electrical phenomena in neurons
The Resting Membrane Potential
•
•
The potential difference between
two points is measured with a
voltmeter
When one microelectrode of a
voltmeter is inserted into the neuron
and the other rests on the neuron’s
outside surface, a voltage across the
membrane of approximately -70 mV is
recorded
–
•
This potential difference in a resting
neuron (Vr) is called the resting
membrane potential, and the
membrane is said to be polarized
–
•
Minus (-) sign indicates that the
cytoplasmic side (inside) of the
membrane is negatively charged
relative to the outside
Values vary from -40 mV to -90 mV
The resting potential exists only
across the membrane; that is, the
bulk solutions inside and outside
the cell are electrically neutral
MEASURING MEMBRANE
POTENTIAL
The Resting Membrane Potential
• The neuron cell membrane is polarized,
being more negatively charged inside than
outside
• The degree of this difference in electrical charge is the
resting membrane potential
• The resting membrane potential is generated by
differences in ionic makeup of intracellular
and extracellular fluids, and differential
membrane permeability of the plasma
membrane to those ions
The Resting Membrane Potential
•
Cell cytosol (inside cell) contains a
lower concentration of Na+ and a
higher concentration of K+ than the
extracellular (outside) fluid
– In the extracellular fluid, the
positive charges of sodium and
other cations (+ ion) are balanced
chiefly by chloride ions (Cl-)
– Negatively charged (anionic)
proteins (A-) help to balance the
positive charges of intracellular
cations (primarily K+ ions)
• Although there are many
other solutes (glucose,
urea, and other ions) in
both fluids, potassium
plays the most important
role in generating the
membrane potential
The Resting Membrane Potential
• At rest the membrane is:
– Impermeable to the large
anionic (- ions) cytoplasmic
proteins
– Slightly permeable to
sodium
– Approximately 75 times more
permeable to potassium
than to sodium
– Freely permeable to chloride
ions
• These resting permeabilities
reflect the properties of the
passive (leakage) ion
channels in the membrane
The Resting Membrane Potential
• Potassium ions diffuse out
of the cell along their
concentration gradient much
more easily and quickly than
sodium ions can enter the cell
along theirs
– K+ loss from the cell continues
until the force of its
concentration gradient is
balanced exactly by the pull
created by the negativity of the
cell interior
– At this point K+ diffusion
across the membrane into
and out of the cell is
equalized and the resting
membrane potential is
established
RESTING MEMBRANE
POTENTIAL
Membrane Potentials That Act as Signals
• Neurons use changes in membrane potential
as communication signals
– These can be brought on by changes in membrane
permeability to any ion, or alteration of ion
concentrations on the two sides of the membrane
• Changes in membrane potential relative to
resting membrane potential can either be
depolarizations, in which the interior of the
cell becomes less negative, or
hyperpolarizations, in which the interior of the
cell becomes more negatively charged
DEPOLARIZATION
• Is a reduction in membrane
potential
• The inside of the membrane
becomes less negative
(moves closer to zero) than the
resting potential
• Example: a change in resting
potential from -70 mV to -65
mV is a depolarization
• Also includes events in which
the membrane potential
reverses and moves above
zero to become positive
• Increases the probability of
producing nerve impulses
HYPERPOLARIZATION
• Occurs when the
membrane potential
increases, becoming
more negative than the
resting potential
• Example: a change from 70 mV to -75 mV is
hyperpolarization
• Reduces the probability
of producing nerve
impulses
MEMBRANE POLARIZATION
Membrane Potentials That Act as Signals
• Neurons use changes in their
membrane potential as communication
signals for receiving, integrating, and
sending information
• A change in membrane potential can be
produced by:
– 1. Anything that changes membrane
permeability to any ion
– 2. Anything that alters ion concentrations
on the two sides of the membrane
GRADED POTENTIALS
• Short-lived, local changes in membrane
potential that can be either depolarizations
or hyperpolarizations
– These changes cause current flows that
decrease in magnitude with distance
• Called “graded” because their magnitude
varies directly with stimulus strength
– Stronger the stimulus, the more the
voltage changes and the farther the
current flows
GRADED POTENTIALS
• Triggered by some change (a stimulus) in
the neuron’s environment that causes
gated ion channels to open
– Named according to the stimulus:
• Sensory neuron stimulated by some form of
energy (heat, light, etc), the graded potential is
called a generator potential
• Stimulus is a neurotransmitter, the graded potential
is called a postsynaptic potential
GRADED POTENTIALS
• Assume that a small area of
a neuron’s plasma
membrane has been
depolarized by a stimulus (a)
– Current will flow on both
sides of the membrane
between the depolarized
(active) membrane area and
the adjacent polarized
(resting) areas
– (b) Positive ions migrate
toward more negative areas
(the direction of cation[+
ion] movement is the
direction of current flow),
and negative ions
simultaneously move
toward more positive areas
GRADED POTENTIALS
• Inside the cell:
– Positive ions (mostly K+) move away from the depolarized area and
accumulate on the neighboring membrane areas, where they displace
negative ions
• Outside the cell:
– Positive ions on the outer membrane are moving toward the region of
reversed membrane polarity (depolarized region), which is momentarily
less positive
• As these positive ions move, their “places” on the membrane
become occupied by negative ions (such as: Cl- and HCO3-) (ionic
musical chairs)
• Thus, at regions abutting the depolarized region, the inside becomes
less negative and the outside becomes less positive
– That is, neighboring membrane is, in turn, depolarized
GRADED POTENTIALS
•
Impression given is that the circuit is
completed by ions passing into and
out of the cell through the membrane
–
•
*****NOT THE CASE*****
Only the inward current across the
membrane is caused by the flow of
ions through gated channels
–
The outward current, the so-called
capacitance (the property of an electric
nonconductor that permits the storage
of energy) current, reflects changes in
the charge distribution as the ions
migrate along the two membrane
face
•
•
Reflects the fact that the fatty
membrane interior is a poor
conductor of current
It is a capacitor that temporarily stores
the charge, forcing the ions of
opposite charge to accumulate
opposite one another on either side
of the membrane
GRADED POTENTIAL
GRADED POTENTIALS
•
The flow of current to adjacent
membrane areas changes the
membrane potential in that area
– The plasma membrane is
permeable, and most of the
charge is quickly lost through the
membrane
• Current is decremental: dies out
within a few millimeters of its
origin
•
•
Because the current dissipates
quickly and dies out with
increasing distance from the site
of initial depolarization, graded
potential can act as signals only
over very short distances
HOWEVER, they are essential in
initiating action potentials, the
long-distance signals
MEMBRANE POTENTIAL
CHANGES
ACTION POTENTIALS
•
•
•
Principal way neurons
communicate is by generating and
propagating action potentials (APs)
– Cells with excitable
membranes—neurons and
muscle cells—can generate
action potentials
GRAPH:shows a brief reversal of
membrane potential with a total
amplitude (change in voltage) of about
100 mV (from -70 mV to +30 mV)
– A depolarization phase is
followed by a repolarization
phase and often a short period
of hyperpolarization
• Total of approximately a few
milliseconds
DO NOT DECREASE in strength and
distance
ACTION POTENTIAL PHASES
ACTION POTENTIALS
• Events of action potential generation and
transmission are identical in skeletal muscle cells
and neurons
• Also celled a nerve impulse in a neuron
– Only generated in axons
• Stimulus changes the permeability of the neuron’s
membrane by opening specific voltage-gated channels
on the axon
– These channels open and close in response to changes in the
membrane potential and are activated by local currents
(graded potentials) that spread toward the axon along the
dendritic and cell body membranes
– In many neurons, the transition from local graded potential to
action potential takes place at the axon hillock
Generation of an Action Potential
•
Involves three consecutive but overlapping
changes in membrane permeability resulting
from the opening and closing of active ion
gates
– All induced by depolarization of the axonal
membrane
• These permeability changes are a
transient increase in Na+
permeability, followed by restoration
of Na+ impermeability, and then a
short-lived increase in K+
permeability
• GRAPH:
– The first two permeability
changes occur during the
depolarization phase of action
potential generation, indicated by
the upward-rising part of the AP
curve or spike
– The third permeability change is
responsible for both the
repolarization (the downward part
of the AP spike) and
hyperpolarization phases shown
in then graph
ACTION POTENTIAL PHASES
Generation of an Action Potential
•
(1) Resting state: Voltage-gated
channels closed
–
–
Virtually all the voltage-gated Na+ and
K+ channels are close, but, small
amounts of K+ leave the cell via
leakage channels and even smaller
amounts of Na+ diffuse in
Each Na+ channel has two voltagesensitive gates:
•
•
•
•
–
Activation gate: Is closed at rest and
responds to depolarization by opening
rapidly
Inactivation gate: Open at rest and
responds to depolarization by closing
slowly
Thus, depolarization opens and closes
sodium channels
Both gates must be open in order for
Na+ to enter, but the closing of either
gate effectively closes the channel
Active potassium channel has a
single voltage-sensitive gate
•
•
Closed in the resting state
Opens slowly in response to
depolarization
ACTION POTENTIAL PHASES
Generation of an Action Potential
• (2) Depolarizing phase: Increase in Na+
permeability and reversal of membrane potential
– As the axonal membrane is depolarized by local
currents, the sodium channel activation gates open
quickly and Na+ rushes into the cell
• This influx of positive charge depolarizes that local “patch” of
membrane further, opening more activation gates so that the
cell interior becomes progressively less negative
• When depolarization at the stimulation site reaches a certain
critical level called threshold (between -55 and -50 mV),
depolarization becomes self-generating, urged on by positive
feedback
– That is, after being initiated by the stimulus, depolarization
is driven by the ionic currents created by Na+ influx
Generation of an Action Potential
•
(2) Depolarizing phase: Increase
in Na+ permeability and reversal of
membrane potential
– As more Na+ enters, the
membrane depolarizes further
and opens still more activation
gates until all Na+ channels
are open
– As a result, the membrane
potential becomes less and
less negative and then
overshoots to about +30 mV
as Na+ rushes in along its
electrochemical gradient
• This rapid depolarization
and polarity reversal
produce the sharply
upward spike of the
action potential in the
graph
ACTION POTENTIAL PHASES
Generation of an Action Potential
• Propagation, or transmission, of an action
potential occurs as the local currents of an
area undergoing depolarization cause
depolarization of the forward adjacent
area
• Repolarization, which restores resting
membrane potential, follows
depolarization along the membrane
Generation of an Action Potential
• (3) Repolarization phase:
Decrease in Na+ permeability
– As the membrane potential
passes 0 mV and becomes
increasingly positive, the
positive intracellular charge
resists further Na+ entry
– The slow inactivation gates
of the Na+ channels begin to
close after a few milliseconds
of depolarization
• Membrane permeability to
Na+ declines to resting
levels, and the net influx of
Na+ stops completely
– AP spike stops rising and
reverses direction
ACTION POTENTIAL PHASES
Generation of an Action Potential
• (3) Repolarization phase: Increase in K+
permeability
– As Na+ entry declines, the slow voltage-sensitive
K+ gates open and K+ rushes out of the cell,
following its electrochemical gradient
– Internal negativity of the resting neuron is
restored, an event called repolarization
– Both the abrupt decline in Na+ permeability and the
increased permeability to K+ contribute to
repolarization
ACTION POTENTIAL PHASES
Generation of an Action Potential
•
(4) Hyperpolarization: K+ permeability
continues
–
–
Because potassium gates are
sluggish gates that are slow to
respond to the depolarization signal,
the period of increased K+
permeability typically lasts longer
than needed to restore the resting state
As a result of the excessive K+
efflux, an after-hyperpolarization,
also called the undershoot, is seen on
the AP curve as a slight dip
following the spike (and before the
potassium gates close)
•
Note: Both the activation and
inactivation gates of the Na+ channels
are closed during afterhyperpolarization
–
Hence, the neuron is insensitive to a
stimulus and depolarization at this
time
ACTION POTENTIAL PHASES
Generation of an Action Potential
• Although repolarization restores resting
electrical conditions, it does not restore
resting ionic conditions
– Because an axonal membrane has
thousands of sodium-potassium ( Na+-K+ )
pumps, the ions are redistributed following
repolarization
Propagation of an Action Potential
•
Propagation, or transmission, of
an action potential occurs as the
local currents of an area
undergoing depolarization
cause depolarization of the
forward adjacent area (away
from the origin of the nerve
impulse), which opens voltagegated channels and triggers an
action potential there
– In the body, action potentials are
initiated at the end of the axon
and conducted away from that
point toward the axon’s terminals
– Once initiated, an action
potential is self-propagating
and continues along the axon at
constant velocity
PROPAGATION OF ACTION
POTENTIAL
PROPAGATION OF ACTION
POTENTIAL
PROPAGATION OF ACTION
POTENTIAL
Threshold and the ALL-or-NONE Phenomenon
• A critical minimum, or threshold,
depolarization is defined by the amount of influx
of Na+ that at least equals the amount of efflux of
K+
– Typically reached when the membrane has been
depolarized by 15-20 mV from the resting value
– If one more Na+ enters, further depolarization occurs,
opening more Na+ channels and allowing more
sodium ions entry
• Action potentials are an all-or-none
phenomena: they either happen completely, in
the case of a threshold stimulus, or not at all, in
the event of a subthreshold stimulus
Coding for Stimulus Intensity
•
•
•
•
•
How can the CNS determine whether a
particular stimulus is intense or weak?
Stimulus intensity is coded in the frequency
of action potentials
Graph:
– Red (↑) arrow: point of stimulus
application
– Red (↓) arrow: point of cessation
– Arrow length: indicates strength of
stimulus
– Action potential shown by Blue
vertical lines (more line=greater
frequency)
Strong stimuli (red arrows) cause nerve
impulses to be generated more often in a
given time interval than do weak stimuli
(above the threshold)
Thus, stimulus intensity is coded for by
the number of impulses generated per
second—that is, by the frequency of
impulse transmission—rather than by
increases in the strength (amplitude) of
the individual action potential
Relationship between Stimulus Strength and
Action Potential Frequency
Refractory Periods
•
The refractory (stubborn,
obstinate) period of an axon is
related to the period of time
required so that a neuron can
generate another action potential
– When a patch of neuron
membrane is generating an action
potential and its sodium channels
are open, the neuron cannot
respond to another stimulus, no
matter how strong
– This period from opening of the
activation gates of Na+ channels
to the closing of the inactivation
gates is called the absolute
refractory period
• Ensures that each action
potential is a separate, all-ornone event and enforces oneway transmission of the action
potential
Refractory Periods
• Relative Refractory
Period:
– Interval following the
absolute refractory period
– Repolarization is occurring
– Threshold stimulus will not
trigger an action potential
but an exceptionally strong
stimulus can reopen the
Na+ gates and allow
another impulse to be
generated
Refractory Periods
Conduction Velocities
• Influence of Axon Diameter and the Myelin
Sheath on Conduction Velocity
– Nerve fibers that transmit impulses most
rapidly (100 m/s) are found in neural
pathways where speed is essential (postural
reflexes)
– Axons that conduct impulses more slowly
typically serve internal organs such as:
gastrointestinal, glands, blood vessels
Conduction Velocities
• Rate of impulse depends largely on two
factors:
• 1. Axon diameter:
– Larger diameters conduct impulses faster
– Smaller diameters conduct impulses slower
Conduction Velocities
• 2. Degree of myelination:
– Unmyelinated axons conduct impulses relatively slowly
• Continuous conduction:
– Action potentials are generated at sites immediately adjacent to each other and
conduction is relatively slow
– Myelinated axons have a high conduction velocity
• Myelin acts as an insulator to prevent leakage of charge
• Saltatory conduction:
– Current can pass only at the nodes of Ranvier, where the myelin sheath is
interrupted and the axon is bare
– Essentially all the voltage-gated Na+ channels are concentrated at the nodes
– Thus, when an action potential is generated in a myelinated fiber, the local
depolarizing current does not dissipate through the adjacent (nonexcitable)
membrane regions but instead is maintained and moves to the next node, a
distance of approximately 1 mm, where it triggers another action potential
– Action potentials are triggered only at the nodes
– Electrical signal jumps from node to node
SALTATORY CONDUCTION
HOMEOSTATIC IMBALANCE
• Multiple sclerosis (MS):
– Myelin sheaths in the CNS are gradually destroyed,
reduced to nonfunctional hardened lesions
(scleroses)
– Autoimmune disease
• Immune system attacks protein in the myelin
– Causing short-circuiting of impulses
– Axons not damaged
• Visual disturbances (including blindness)
• Muscle control problems (weakness, clumsiness, and
ultimately paralysis)
• Speech disturbances
• Urinary incontinence
HOMEOSTATIC IMBALANCE
• A number of chemical and physical factors
impair impulse propagation
– Mechanisms of action differ:
• Alcohol, sedatives, and injected anesthetics all block nerve
impulses by reducing membrane permeability to ions, mainly
Na+
• As we have seen, no Na+ entry—NO action potential
– Cold and continuous pressure interrupt blood
circulation (and hence the delivery of oxygen and
nutrients) to neurons, impairing their ability to conduct
impulses
• Examples:
– Fingers get numb when you hold an ice cube
– Foot goes to sleep when you sit on it
The Synapse
•
•
•
A junction that mediates information
transfer between neurons or
between a neuron and an effector
cell
The neuron conducting impulses
toward the synapse is the
presynaptic neuron and the neuron
transmitting the electrical signal away
from the synapse is the
postsynaptic neuron
Types:
–
–
–
Axodendritic synapses: between the
axonal endings of one neuron and the
dendrites of other neurons
Axosomatic synapses: between
axonal endings of one neuron and cell
bodies of other neurons
Less understood:
•
•
•
Axoaxonic synapses: between axons
Dendrodendritic synapses: between
dendrites
Dendrosomatic synapses: between
dendrites and cell bodies
TYPES OF SYNAPSES
The Synapse
• Neurons have anywhere from 1,000 to
10,000 axonal terminals making synapses
and are stimulated by an equal number of
other neurons
• There are two varieties of synapses:
– Electrical
– Chemical
Electrical Synapses
•
•
•
•
•
•
Electrical synapses have neurons
that are electrically coupled via
protein channels and allow direct
exchange of ions from cell to cell
Specialized to allow flow of ions
between neurons
Less common variety
Correspond to the gap junctions found
between certain body cells
Contain protein channels that connect
the cytoplasm of adjacent neurons and
allow ions to flow directly from one
neuron to the next (electrically
coupled)
Appear to be important in CNS:
– Mental attention
– Conscious perception
– Between glial cells playing a role
in ion and water homeostasis
RAT SYNAPSE
Chemical Synapses
•
Specialized for release and reception of chemical neurotransmitters
• Neurons conducting impulses toward the synapse are
presynaptic cells, and neurons carrying impulses away from
the synapse are postsynaptic cells
•
•
Typical chemical synapse is made up of three parts:
– Axonal terminal of the presynaptic neuron
• Contains many tiny, membrane-bounded sacs (synaptic vesicles)
– Contain thousands of neurotransmitter molecules
– Neurotransmitter receptor region on the membrane of a dendrite or
cell body of the postsynaptic neuron
– Synaptic cleft:
• A fluid-filled space (30-50 nm) space between the presynaptic and
postsynaptic neurons
Transmission of signals across these synapses is a chemical event
that depends on the release, diffusion, and receptor binding of
neurotransmitter molecule and results in unidirectional
communication between neurons
Information Transfer Across
Chemical Synapses
• When a nerve impulse reaches the
axonal terminal, it sets into motion a
chain of events that triggers
neurotransmitter release
• The neurotransmitter crosses the
synaptic cleft and, on binding to
receptors on the postsynaptic
membrane, causes changes in the
postsynaptic membrane permeability
Information Transfer Across
Chemical Synapses
• 1.Calcium channels
open in the presynaptic
axonal terminal:
– When the nerve impulse
(depolarization wave)
reaches the axonal
terminal, membrane
depolarization opens not
only Na+ channels but
voltage-gated Ca2+
channels
– During the brief time the
Ca2+ gates are open, Ca2+
(influx) floods into the
terminal from the
extracellular fluid
CHEMICAL SYNAPSE
Information Transfer Across
Chemical Synapses
•
2.Neurotransmitter is released
– The surge of Ca2+ into the axonal
terminal acts as an intracellular
messenger, directing docked
synaptic vesicles to fuse with
the axonal membrane and
empty their contents by
exocytosis into the synaptic
cleft
– The Ca2+ is then quickly removed
from the terminal, either taken up
into the mitochondria or ejected
from the neuron by an active Ca2+
pump
– The precise Ca2+ sensor that
initiates neurotransmitter
exocytosis is still a question, but
a Ca2+ binding protein called
synaptotagmin found in the
synaptic vesicles seems a likely
candidate
CHEMICAL SYNAPSE
Information Transfer Across
Chemical Synapses
• 3.Neurotransmitter
binds to
postsynaptic
receptors
– The neurotransmitter
diffuses across the
synaptic cleft and
binds reversibly to
specific protein
receptors clustered
on the postsynaptic
membrane
CHEMICAL SYNAPSE
Information Transfer Across
Chemical Synapses
• 4.Ion channels open in the
postsynaptic membrane
– As the receptor proteins
bind neurotransmitter
molecules, the threedimensional shape of the
proteins changes
– This causes ion channels to
open, and the resulting
current flows produce local
changes in the membrane
potential (voltage changes)
– Depending on the receptor
protein to which the
neurotransmitter binds and the
type of channel the receptor
controls, the postsynaptic
neuron may be either
excited or inhibited
CHEMICAL SYNAPSE
Information Transfer Across
Chemical Synapses
• For each nerve impulse reaching the
presynaptic terminal, many vesicles (perhaps
300) are emptied into the synaptic cleft
• The higher the impulse frequency (that is, the
more intense the stimulus), the greater the
number of synaptic vesicles that fuse and
spill their contents, and the greater the effect
on the postsynaptic cell
Information Transfer Across
Chemical Synapses
• 5.Neurotransmitter is
quickly destroyed by
enzymes present at
the synapse or taken
back into the
presynaptic terminal
– Depletion of
neurotransmitter
closes the ion
channels and
terminates the
synaptic response
CHEMICAL SYNAPSE
Termination of Neurotransmitter Effects
• As long as the neurotransmitter is bound to a
postsynaptic receptor, it continues to affect
membrane permeability and to block reception
of additional “messages” from presynaptic
neurons
– Neurotransmitter effects last a few milliseconds
before being terminated in one of three ways:
• Degradation by enzymes from the postsynaptic cell or within
the synaptic cleft
– acetylcholine
• Reuptake by astrocytes or the presynaptic terminal, where
the neurotransmitter is stored or destroyed by enzymes
– norepinephrine
• Diffusion away from the synapse
Synaptic Delay
• Although some neurons can transmit impulses at
150 m/s (300 mph), neural transmission
across a chemical synapse is comparatively
slow and reflects the time required for
neurotransmitter release, diffusion across the
synapse, and binding to receptors
– This synaptic delay, which lasts 0.3-5.0 ms, is the rate
limiting (slowest) step of neural transmission
• Explains why transmission along short neural pathways
involving only two or three occurs rapidly, but transmission
along multisynaptic pathways typical of higher mental
functioning occurs much more slowly
• However, in practical terms these differences are not
noticeable
Postsynaptic Potential and
Synaptic Integration
• Many receptors present on postsynaptic
membranes at chemical synapses are
specialized to open ion channels, thereby
converting chemical signals to electrical
signals
– Neurotransmitters mediate local changes in
membrane potential that are graded according to
the amount of neurotransmitter released and the
time it remains in the area
• Action potentials on the postsynaptic cell that may be
excitatory or inhibitory
Excitatory Synapses and EPSPs
•
•
•
EPSPs: excitatory postsynaptic
potentials (a)
At excitatory synapses, neurotransmitter
binding causes depolarization of the
postsynaptic membrane
Only a single type of channel opens on
postsynaptic membranes (those of
dendrites and neuronal cell bodies)
– This channel allows Na+ and K+ to
diffuse simultaneously through the
membrane in opposite directions
– Although this two-way cation flow may
appear to be self-defeating when
depolarization is the goal, remember
that the electrochemical gradient for
sodium is much steeper than that for
potassium
• Hence, Na+ influx is greater
than K+ efflux, and net
depolarization occurs
Excitatory Synapses and EPSPs
• If enough neurotransmitter binds,
depolarization of the postsynaptic membrane
can successfully reach 0 mV, which is well
above an axon’s threshold (about -50 mV) for
“firing off” an action potential
– However, postsynaptic membranes do not
generate action potentials; only axons (with their
voltage-gated channels) have this capability
• The dramatic polarity reversal seen in axons never
occurs in membranes containing only chemically gated
channels because the opposite movements of K+ and Na+
prevent accumulation of excessive positive charge inside the
cell
Excitatory Synapses and EPSPs
• Hence , instead of action potentials, local graded depolarization
events (EPSPs) occur at excitatory postsynaptic membranes (a)
• The only function of EPSPs is to help trigger an action potential
distally at the axon hillock of the postsynaptic neuron
• If currents reaching the hillock are strong enough to depolarize the
axon to threshold, axonal voltage-gated channels open and an
action potential is generated
Inhibitory Synapses and IPSPs
•
•
IPSPs: inhibitory postsynaptic
potentials (b)
Binding of neurotransmitters at
inhibitory synapses reduces a
postsynaptic neuron’s ability to
generate an action potential
–
Most inhibotory neurotransmitters
induce hyperpolarization of the
postsynaptic membrane by making the
membrane more permeable to K+
and/or Cl- (sodium ion permeability is
not affected)
•
•
If K+ channels are opened, K+ moves
out of the cell
If Cl- channels are opened, Cl- moves
into the cell
–
–
In either case, the charge on the inner
face of the membrane becomes more
negative
As the membrane potential increases
and is driven farther from the axon’s
threshold, the postsynaptic neuron
becomes less likely to fire (b)
POSTSYNAPTIC POTENTIALS
Integration and Modification of
Synaptic Events
Summation by the Postsynaptic Neuron
•
A single EPSP cannot induce an action potential in the postsynaptic neuron
– But if thousands of excitatory axonal terminals are firing on the same
postsynaptic membrane, or if a smaller number of terminals are delivering
impulses rapidly, the probability of reaching threshold depolarization increases
greatly
• Thus, EPSPs can add together (summate) to influence the activity of a postsynaptic
neuron
•
•
E1 and E2 are excitatory
I1 is inhibitory
Summation by the Postsynaptic Neuron
Subthreshold (a)
• No summation
• Synapse E1 is stimulated and then stimulated again
shortly thereafter
• The two EPSPs do not overlap in time, so no
summation occurs; threshold is not reached in the
axon of the postsynaptic neuron
Summation by the Postsynaptic Neuron
Temporal Summation (b)
• Occurs when one or more presynaptic neurons transmit impulses in
rapid-fire order and bursts of neurotransmitter are released in
quick succession (occurs in response to several successive
releases of neurotransmitter)
• Synapse E1 is stimulated a second time before the initial EPSP
has died away
• Axon’s threshold is reached, causing an action potential to be
generated
Summation by the Postsynaptic Neuron
Spatial Summation (c)
•
•
•
Occurs when the postsynaptic neuron is stimulated at the same time by
a large number of terminals from the same or, more commonly,
different neurons (postsynaptic cell is stimulated at the same time by
multiple terminals)
Huge numbers of its receptors bind neurotransmitter and simultaneously
initiate EPSPs, which summate and dramatically enhance depolarization
Synapses E1 and E2 are stimulated simultaneously (spatial
summation), resulting in a threshold depolarization) (c)
Summation by the Postsynaptic Neuron
Inhibitory Summation
•
•
•
•
•
•
•
Both temporally and spatially inhibited to a greater degree
Not only do EPSPs summate and IPSPs summate, but also EPSPs summate with IPSPs
If the stimulatory effects of EPSPs dominate the membrane potential enough to reach threshold,
the neuron will fire
If summation yields only subthreshold depolarization or hyperpolarization, the neuron fails to
generate an action potential
Synapse I1 is stimulated, resulting in a short-lived IPSP (hyperpolarization)
When E1 and I1 are simulataneously stimulated, the changes in potential cancel each other
Axon hillock membranes function as neural integrators, and their potential at any time
reflects the sum of all incoming neural information
Neural integration of EPSPs and IPSPs at the
Axonal Membrane of the Postsynaptic Cell
Synaptic Potentiation
• Repeated or continuous use of a synapse
enhances the presynaptic neuron’s ability to
excite the postsynaptic neuron, producing
larger than expected postsynaptic potentials
– Synaptic potentiation results when a presynaptic cell
is stimulated repeatedly or continuously, resulting in
an enhanced release of neurotransmitter
• Increases the efficiency of neurotransmission along a
particular pathway:
– Example: hippocampus of the brain (special role in
memory and learning)
Presynaptic Inhibition and Neuromodulation
• Presynaptic inhibition results when
another neuron inhibits the release of
excitatory neurotransmitter from a
presynaptic cell
– Neuromodulation occurs when a
neurotransmitter acts via slow changes in
target cell metabolism, or when chemicals
other than neurotransmitter modify neuronal
activity (hormones)
CHEMICAL SYNAPSE
Neurotransmitters and Their Receptors
• Neurotransmitters are one of the ways
neurons communicate, and they have
several chemical classes:
– More than 50 neurotransmitters have been
identified
– Classified chemically and functionally
– TABLE 11.3
Classification of Neurotransmitters
Chemical Structure
•
Acetylcholine (ACh)
– First neurotransmitter identified
– Released at neuromuscular junctions
– Synthesized and enclosed in synaptic vesicles in axonal teerminals in a reaction
catalyzed by the enzyme choline acetyltransferase
• Acetic acid is bound to coenzyme A (CoA) to form acetyl-CoA, which then
combines with choline
– Coenzyme A is released
–
choline
–
acetyltransferase
–
Acetyl-CoA + choline → ACh + CoA
– Once released by the presynaptic terminal, ACh binds to the postsynaptic
receptors briefly
• Then it is released and degraded to acetic acid and choline by the enzyme
acetylcholinesterase (AChE), located in the synaptic cleft and on postsynaptic
membranes
• The released choline is recaptured by the presynaptic terminals and reused to
synthesize more ACh
– ACh is released by all neurons that stimulate skeletal muscles and by
some neurons of the autonomic nervous system
– ACh-releasing neurons are also prevalent in the CNS
Novel Messengers
•
Adenosine Triphosphate (ATP):
–
–
•
Stored in synaptic vesicles
Major neurotransmitter in both the CNC and PNS
Nitric oxide (NO):
–
Short-lived toxic gas
•
–
–
–
–
–
•
Passes swiftly into cells binding briefly to metal-containing enzymes, and then disappears
Stored in vesicles and released by exocytosis
Instead of attaching to surface receptors, it zooms through the plasma membrane of nearby
cells to bind with a peculiar intracellular receptor—iron in guanylyl cyclase, the enzyme that
makes the second messenger cyclic GMP (guanosine monophosphate)
Excessive release in the brain is responsible for much of the damage in stroke patients
Relaxes intestinal smooth muscles
Relaxes vascular smooth muscles of the arterioles of the penis causing the erectile tissues to
fill with blood in an erection
Carbon monoxide: gas messenger
–
–
Stimulates synthesis of cyclic GMP (guanosine monophosphate)
Found in brain regions like NO
•
Different pathways but similar mode of action
Classification of Neurotransmitters
Chemical Structure
• Biogenic Amines:
– Catecholamines:
• Dopamine
• Norepinephrine (NE)
• Epinephrine
– Indolamines:
• Serotonin
• Histamine
Classification of Neurotransmitters
Chemical Structure
• Catecholamines:
– Dopamine and NE are
synthesized from the amino
acid tyrosine in a common
pathway consisting of several
steps
• Neurons contain only the
enzymes needed to
produce their own
neurotransmitter(s)
– Thus, the sequence stops at
dopamine in dopaminereleasing neurons but
continues on to NE in NEreleasing neurons
» The same pathway is
used by the
epinephrine-releasing
cells of the brain and
adrenal medulla
NEUROTRANSMITTERS
PATHWAYS
Catechol
BIOGENIC AMINES
• Biogenic:A substance produced by a Life
Process with a Amine Group
CATECHOLAMINES
• Prepares body for physical activity
(increase heart rate, blood pressure, and
blood glucose level)
– DOPAMINE
– EPINEPHRINE (ADRENALINE)
– NOREPINEPHRINE(NORADENALINE)
CATECHOLAMINES
• Named because they share
the Catechol Group
• Benzene with two Hydroxyl
side groups
• Produce from amino acids
phenylalanine and tyrosine
• Water soluble, therefore,
circulate in the blood stream
• Produced by the Adrenal
Medulla and Postganglionic
fibers of the Sympathetic
Nervous System (PNS)
Dopamine
DOPAMINE
• Found in both Plants and Animals
• Plants: may help protect fruits and
vegetables against bacterial and fugal
growth
• Animals: functions as a neurohormone
DOPAMINE
• Is a neurohormone:
– Both a
neurotransmitter and a
hormone
• Hormone is released by
the Hypothalamus which
inhibits the release of
Prolactin from the
Anterior Lobe of the
Pituitary
– In blood circulation but
cannot cross Blood-Brain
barrier
DOPAMINE
• Neurotransmitter mainly produced in the Brain
Stem Neurons
• Involved in:
– Neuromodulation of CNS
• Transmitting nerve impulses throughout the Brain
– Coordination of body movements
– Emotional ability to experience pleasure and pain
• Pleasure from food and sex (High Levels)
• Pain processing in the CNS (Low Levels)
– Fibromyalgia
– Restless Leg Syndrome
– Parkinson’s disease
DOPAMINE
• Linked to a personality trait and happiness
– High Levels:
• One personality trait in humans is how sensitive
and responsive we are to incentives and rewards
• When our Dopamine system is activated, we are
more positive, excited and eager to go after goals
or rewards, such as food, sex, money, education,
or professional achievement
– Low Levels:
• More susceptible to depression/low self
esteem/suicide
DOPAMINE
• Many chronic diseases result
from the overproduction or
underproduction of Dopamine
DOPAMINE
• Low:
• Parkinson’s Disease: inability of Dopamine to move into the Frontal
Lobe of the brain results in the inability to control fine motor
movement
• Schizophrenic: Flow of Dopamine throughout the CNS is not
allowed to circulate as usual
– Mental disorder that makes it difficult to tell the difference
between real and unreal experiences
– Difficult to think logically
– Lack of organization in the PreFrontal Cortex: lack of
organization of thoughts and preception
– Difficult have normal emotions responses
– Difficult to behave normally in a social situation
DOPAMINE
• HOWEVER: could
be HIGH in the
Striatum region of
the Brain:
Overactivity
DOPAMINE
• High:
• Epilepsy
• Speech Disorders
– Stuttering
• Drug Addiction propensity
DOPAMINE
• When an organism is exposed to stress
(psychological and/or environmental),
Dopamine and Serotonin levels will rise
to help cope with additional stress.
– Coordinating the Frontal Lobe of the Brain
with the Limbic System
– Reducing stress hormones cortisol and
adrenaline
DOPAMINE
• People who have a naturally high level of
Dopamine have the propensity to become
addicted to certain drugs. High levels of
Dopamine unregulated by the Brain protein
called Dopamine Transporter cause the behavior
of the “HIGH”. This Brain Protein normally
cleans up Dopamine that has already excited
Brain Cells. Because Dopamine controls
movement, comprehension, and some social
behavior, these three behaviors manifest
hyperactivity: CONCLUSION is that addictive
novelty-seeking behaviors are the result of high
levels of Dopamine.
DOPAMINE
• Cocaine: increases production of Dopamine by
preventing re-uptake at synapse (necessary for
homeostasis)
– Thus, more Dopamine remains to stimulate neurons
– Stimulant of the CNS
– Obtained from the leaf of the coca plant
– No longer in coke: extracted
• Amphetamines: increase production of Dopamine by
stimulating the release of more Dopamine
– Stimulant to increase wakefulness with decreased
fatigue and appetite
– Performance enhancer
DOPAMINE
• Essential Amino acids required for the production of Dopamine can
be found in the following Foods:
– Tea (black/green)
– Apples, bananas, watermelon, blueberries
– Red wine
– Beets, beans
– Meat
– Dairy
– Eggs
– Fish
– Soy products
• If you are low on amino acids required by the body to manufacture
Dopamine, you can experience depression and fuzzy thinking
DOPAMINE
• Since Dopamine cannot cross the bloodbrain barrier, Dopamine given as a drug
does not not directly affect the CNS.
• To increase the amount of dopamine in the
brains of patients with Parkinson’s
disease, the precursor of Dopamine is
often given because it crosses the bloodbrain barrier relatively easily
BLOOD-BRAIN BARRIER
• An anatomic-physiologic
feature of the Brian thought to
consist of walls of capillaries in
the CNS and surrounding
astrocytic glial membranes
• The barrier separates the CNS
from the blood
• Prevents or slows the passage
of some drugs and other
chemical compounds,
radioactive ions, and diseasecausing organisms such as
viruses from the blood into the
CNS
Norepinephrine
Norepinephrine
• Also called Noradrenaline
• Neurotransmitter involved in
sleep and wakefulness,
attention, and feeding
behavior.
• Neuromodualtor in the CNS
• Stress hormone released by
the Adrenal Glands into the
blood that regulates the
Sympathetic Nervous System
(SNS) and the visceral organs
Norepinephrine
• Neurotransmitter released from the sympathetic neurons:
– Increases rate of contraction of the Heart
– Increases release of glucose from energy sources
– Increases blood flow to skeletal muscles
– Increase Brains oxygen supply
– Can suppress neuroinflammation
• Brain: the noradrenergic neurons in the Brain form a
neurotransmitter system, that, when activated, exerts effects on
large areas of the Brain (cerebral cortex, limbic system, and spinal
cord)
– Effects are alertness and arousal, and influences on the reward
system
• Stress hormone (Adrenal Medulla), affects parts of the Brain, such
as the amygdala, where attention responses are controlled
Norepinephrine
•
•
•
•
•
Cannot cross the Blood-Brain barrier
Role in attention and focusing
Low levels implicated in depression
High levels in Schizophrenia
Help in Hypotension
Norepinephrine
• Nutritional Sources:
– Same as Dopamine
Epinephrine
Epinephrine
•
•
•
•
•
Also called Adrenaline
An Adrenal stress hormone
– Increases Heart rate
Actions vary by Tissue Type:
– Causes smooth muscle
relaxation in the bronchioles
but causes constriction of the
smooth muscle that lines most
arterioles
• Constricts blood vessels
• Dilates air passages
A neurotransmitter present at
lower levels in the Brain
“Flight-or-Fight” hormone
response of the Sympathetic
Nervous System
DIFFERENCE BETWEEN
EPINEPHRINE and NOREPINEPHRINE
Methyl group attached to nitrogen
DIFFERENCE BETWEEN
EPINEPHRINE and NOREPINEPHRINE
• Both released by the Adrenal Medulla
• Output of Adrenal Medulla:
– 80% Epinephrine
– 20% Norepinephrine
• Sympathetic postganglionic fibers:
– 80% Norepinephrine
– 20% Epinephrine
• Both derived from the amino acid Tyrosine
• Both essential in stress responses
• Both reduce digestive activity
• Vary in affinities for adrenergic receptors
INDOLEAMINE
• Any of various indole derivatives, such as,
serotonin, containing a primary, secondary,
or tertiary amine group
Serotonin
Serotonin
• CNS neurotransmitter derived from the amino acid Tryptophan:
precursor to Serotonin
• Produced in Midbrain (Pons)
• Involved in regulating mood, sleep, appetite, sexuality, and memory
• Low levels in diet: violent behavior
• Sleep/Wake Cycle
• A number of antipsychotic drugs used in treatment of depression
and anxiety are thought to act specifically on sertonergic neurons
• Controls emotions: plays a role in emotional disorders such as
depression, suicide, impulsive behavior, and aggression
• Feel-good neurotransmitter
• Low levels: autistic children,arthritis, eating disorders
(bulimia), suicide
Serotonin
• Helps to relay messages from one area of the brain to
another
• Because of the widespread distribution of the brain cells,
it is believed to influence a variety of psychological and
other body functions
• Most brain cells are influenced either directly or indirectly
– This includes brain cells related to mood,
depression, emotions, aggression, anxiety,
sexual desire and function, appetite, sleep,
memory and learning, perception, temperature
regulation, and social behavior
• Regulates these processes through pathways
that innervate (connect to) different brain
regions
Serotonin
• Also, affects the functioning of our
cardiovascular system, muscles, and
various elements in the endocrine system
• May play a role in milk production in the
breast
• Defect in the serotonin network may be
one underlying cause of SIDS (sudden
infant death syndrome)
Serotonin
• Imbalance in serotonin levels may
influence mood in a way that leads to
depression
– Possible problems include low brain cell
production, a lack of receptor sites able to
receive the serotonin that is made, inability
of serotonin to reach receptor site, or a
shortage in tryptophan
• Could lead to depression, as well as,
obsessive-compulsive disorder, anxiety, panic,
and even anger
Serotonin
• The function of Serotonin depends on the
region of the Brain into which it is released
– Also depends on the type of Serotonin
receptor present in that region
• Example:
– Serotonin neurons in the Frontal Cortex regulate
cognition, memory and perception
– Serotonin in the Hippocampus regulate memory and
mood
– Serotonin in other limbic regions such as the Amygdala
regulates mood
SEROTONIN
SEROTONIN
•
Low levels are often attributed to:
–
Alcohol abuse
–
Anger
–
Anxiety
–
Aspartame (artifical sweeteners)
–
Caffeine
–
Chronic pain
–
Cigarette smoking
–
Depression
–
Diabetes
–
Diet pills
–
Eating disorders
–
Ecstasy
–
Fibromyalgia
–
Genetic predisposition
–
Hormone (thyroid, adrenal, estrogen imbalances)
–
Hypoglycemia (low blood sugar)
–
Inflammation
–
Infection
–
Insomnia
–
Lack of exercise
–
Migraines
–
Obesity
–
Poor diet
–
Panic attacks
–
Problems converting Tryptophan to Serotonin
–
Problems with digestion
–
Stress
SEROTONIN
• Symptoms of low levels:
–
–
–
–
Negative thoughts
Low self esteem
Obsessive thoughts and behaviors
Irritable bowel syndrome
• Abdominal pain
• Constipation
• Diarrhea
• Shortage of Tryptophan is believed to be a
major culprit leading to Depression
SEROTONIN
• Foods high in Tryptophan and B Vitamins
(Serotonin precursor) allow our Brain to
naturally produce Sertonin
– Foods high in Serotonin (Turkey) can make
you tired
• Body will produce high levels of Serotonin and thus
reduce possibility of depression
SEROTONIN
• Foods/Behavior that increases Serotonin
– Baked potato with skin
– Beef
– Complex carbohydrates
– Exercise
– Fresh fruit/vegetables
– Manage stress/negative thoughts
– Oates
– Poultry
– Pumpkin
– Quality sleep (6-8 hours)
– Salmon
– Sunflower seeds
– Tuna
– Water helps in absorption
SEROTONIN
•
Protein-rich foods, contain high levels of Tryptophan. However, levels of
both Tryptophan and Serotonin drop after eating a meal packed with
protein. WHY?
– When you eat a high-protein meal, you “flood the blood with BOTH
Tryptophan and its competing amino acids, “all fighting for entry into the
brain. That means only a small amount of Tryptophan gets through and
Serotonin levels do not rise.
•
BUT, eat a carbohydrate-rich meal, and your body triggers a release of
insulin. This causes any amino acids in the blood to be absorbed into the
body but not the brain. EXCEPT for Tryptophan! It remains in the blood at
high levels following a carbohydrate meal, which means it can freely enter
the Brain and cause Serotonin levels to rise.
– A high carbohydrate diet (starches) with B Vitamins increase Tryptophan
metabolism BUT MUST WATCH OUT FOR HIGH GLUCOSE LEVELS!
• BALANCE IS THE KEY
•
High levels of Tryptophan in the Brain directly influence increased
Serotonin production and new Brain cell production begins to rise
SEROTONIN
• Exercise can do a lot to improve your
mood
• Regular exercise can be as effective a
treatment for depression as
antidepressant medication or
psychotherapy
– Belief that it helps increase Serotonin levels
SEROTONIN
• Blood platelets take up Serotonin and store it
– When platelets bind to a clot, they release Serotonin,
where it serves as a vasoconstrictor helping to
regulate homeostasis and blood clotting
• Serves as a growth factor for some types of
cells, which may give it a role in wound healing
• Although Serotonin is manufactured in the
Brain where it performs its primary
functions, 90% of our supply is found in the
digestive tract and blood platelets
RECREATIONAL DRUGS
ECSTASY/LSD
• Both Ecstasy and LSD (lysergic acid diethylamide) are drugs
with amphetamine-like and hallucinogenic properties
• Induce Serotonin, Dopamine, and Norepinephrine release
• High release causes the Brain to become significantly depleted of
these important neurotransmitters contributing to the negative
behavioral aftereffects that users often experience:
– Brain stops doing such a good job of effectively differentiating
between yourself and the surrounding world
– Lasting confusion, depression, and selective impairment of
memory
– Symptoms can occur within minutes to hours and generally
include restlessness, hallucinations, rapid heartbeat, increased
body temperature and sweating, loss of coordination, muscle
spasms, nausea, vomiting, diarrhea, and rapid changes in blood
pressure
RECREATIONAL DRUGS
ECSTASY/LSD
• Much of the research on hallucinogenic drugs has focused on Brain
Serotonin for two reasons:
– First, it was discovered early on that many of the major
hallucinogens has a molecular structure similar to that of
Serotonin
– Second, animal studies examining brain neurotransmitters
following administration of hallucinogens invariably reported
changes in Serotonin
• All of the cell bodies containing Serotonin are found in the Brain
Stem
– From the Brain stem these neurons send their axons great
distances to influence virtually all the major areas of the CNS,
including the Cortex (especially the visual area)
BIPOLAR DISORDER
• A condition in which people go back and
forth between periods of a very good or
irritable mood and depression: “MOOD
SWINGS”
– Can be very quick
– Degree depends on levels of Dopamine,
Serotonin, and Norepinephrine
ADHD
• Attention-Deficit Hyperactivity Disorder
– Linked with the Brain’s chemical system, NOT
its structure
• Not due to Brain injury or damage
– Neurotransmitters:
• Three linked to behavioral and emotional
conditions: Dopamine, Serotonin, and
Norepinephrine use by the Brain to stimulate or
repress stimulation in Brain Cells
(HOMEOSTASIS)
– Low levels/high levels
ADHD
• Low Serotonin: linked with clinical depression
– Modern antidepressants medication increases the
level
• Low Dopamine: impluse/behavior problems
– Can not repress the urge to do or say something
• Low Norepinephrine: inattention and
distractibility
– Difficult to focus
Classification of Neurotransmitters
Chemical Structure
• Indolamines:
– Serotonin is synthesized from the amino acid
trytophan
– Histamine is synthesized from the amino acid
histidine
MELATONIN
• Produced in the Pineal Gland
• Serotonin is a precursor;
therefore, amino acid
Tryptophan is important in diet
• Stimulated by darkness
• Inhibited by light
• Duration of Melatonin
secretion each day is directly
proportional to the length of the
night
• Called the “Third Eye”
because of its sensitivity to
Light
MELATONIN
• Regulates Circadian Cycle (Biological
Clock): Circa from Latin meaning “around”
• Believed to influence our sleep habits
– Begins to rise in the afternoon
– High at night
– Decreases in early morning hours
• Winter: seasonal affective disorders (SAD)
or winter depression
• Decreases with age
MELATONIN
• Dietary supplement in treatment of:
– Headaches older people
– Jet lag
– SAD: seasonal affective disorders
– Sleep problems
– Sleep problems in the blind
HOMEOSTATIC IMBALANCE
Biogenic Amine Neurotransmitters
• Broadly distributed in the brain:
– Role in emotional behavior
– Regulate the biological clock
– Imbalances of catecholamines
• Associated with neurological problems
– Example: overproduction of dopamine occurs in
schizophrenia
• Certain drugs (LSD) bind to biogenic
amine receptors and induce hallucinations
HISTAMINE
HISTAMINE
•
•
•
Derived from the amino acid
Histidine
Neurotransmitter: mediates
arousal and attention in CNS
Immune system (allergic
response):Triggers the
inflammatory response
– responds to foreign pathogens
producing Histamine from Mast
cells and Basophils in the
Connective Tissue or Blood
causing increase permeability of
capillaries to White Blood cells
and some proteins
•
Important stimulant of HCl
secretion by the stomach
HISTAMINE
•
•
Neurotransmitter: mediates
arousal and attention in CNS
(Posterior Hypothalamus)
Neurons project throughout the
Brain to modulate sleep
– Antihistamines produce sleep
– Destruction of histamine
releasing neurons leads to the
inability to maintain vigilance
– Histamine has a stimulatory
effect on neurons
• It also has been
suggested that histamine
controls the mechanisms
by which memories and
learning are forgotten
CAFFEINE
• Is a natural component of chocolate, coffee and
tea, and is added to colas and energy drinks
• It is a common ingredient in diet pills and some
over the counter pain relievers
• Naturally occurring chemical stimulant
• Shares a number of traits with more notorious
drugs such as amphetamines, cocaine and
heroin
– Caffeine uses the same biochemical mechanisms as
these drugs to stimulate Brain function
CAFFEINE
• Cocaine:
– Coca leaf
– Coke now uses spent coca leaves
• Cocaine extracted
• Chocolate:
– Cocoa plant beans
• Heroin:
– Is an opiate drug that is synthesized from morphine, a
naturally occurring substance extracted from the seed
pod of rhe Asian opium poppy plant
CAFFEINE
• In the pure form, caffeine is a white crystalline
powder that tastes bitter
• It is medically useful to stimulate the heart and
also serves as a mild diuretic, increasing urine
production to flush fluid out of the body
• Provides a boost of energy or a feeling of
heightened alertness
– Stay awake to study for a test or a long drive
• Can be addictive
CAFFEINE
• Occurs naturally in many plants, including
coffee beans, tea leaves and cocoa beans
• In many drinks and energy drinks
• Works by fooling adenosine receptors
– Adenosine is a Purine in DNA and a
neurotransmitter in the Brain
• Adenosine slows down nerve cell activity along
neural pathways but caffeine (which binds to the
same receptors) speeds activity up
CAFFEINE
• As adenosine is created in the Brain, it
binds to adenosine receptors
– This binding causes drowsiness by slowing
down nerve cell activity
– In the Brain, this also causes blood vessels to
dilate, most likely to let more oxygen into that
organ during sleep
CAFFEINE
• To a nerve cell, caffeine looks like adenosine
– Binds to the adenosine receptor
– Caffeine is taking up all the receptors that adenosine
normally would bind with
– Instead of slowing down because of the adenosine’s
effect, the nerve cells tend to speed up
– Also, causes the Brain’s blood vessel to constrict,
because it blocks adenosine’s ability to open them up
• This effect is why some headache medicines like Anacin
contain caffeine
– Constricting blood vessels in the Brain can help stop a vascular
headache
CAFFEINE
• Like Heroin and Cocaine it slows the
reabsorption of Dopamine thus increasing
Dopamine level
CAFFEINE
• Positive benefits:
– Regular coffee drinkers were 80% less to
develop Parkinson’s
– Reduce risk for Colon cancer
– 80% drop in developing cirrhosis
– Reduces risk of gallstones
AMINO ACIDS
• Certain amino acids function as
neurotransmitters:
– Gamma-aminobutyric acid (GABA), glycine,
aspartate, and glutamate
– Amino acid neurotransmitters have so far
been found only in the CNS
PEPTIDES
• Neuropeptides, essentially strings of amino acids, include a broad
spectrum of molecules with diverse effects
• Examples:
– Neuropeptide substance P: important mediator of pain signals
– Endorphins: act as natural opiates, reducing our preception of pain
under certain stressful conditions
• Examples:
– Beta endorphin
– Dynorphin: blocks transmission of pain signals in the brain
– Enkephalins: increases dramatically in pregnant women in labor
• Release is enhanced when an athlete gets a so-called second wind and is
probably responsible for the “runner’s high”
– Gut-brain peptides: gastrointestinal tract
• Somatostatin: inhibits motility and gastric acid secretion
• Cholecystokinin: secreted in small intestine and stimulates contraction of
the gallbladder and pancreatic secretion
Novel Messengers
•
Adenosine Triphosphate (ATP):
–
–
•
Stored in synaptic vesicles
Major neurotransmitter in both the CNC and PNS
Nitric oxide (NO):
–
Short-lived toxic gas
•
–
–
–
–
–
•
Passes swiftly into cells binding briefly to metal-containing enzymes, and then disappears
Stored in vesicles and released by exocytosis
Instead of attaching to surface receptors, it zooms through the plasma membrane of nearby
cells to bind with a peculiar intracellular receptor—iron in guanylyl cyclase, the enzyme that
makes the second messenger cyclic GMP (guanosine monophosphate)
Excessive release in the brain is responsible for much of the damage in stroke patients
Relaxes intestinal smooth muscles
Relaxes vascular smooth muscles of the arterioles of the penis causing the erectile tissues to
fill with blood in an erection
Carbon monoxide: gas messenger
–
–
Stimulates synthesis of cyclic GMP (guanosine monophosphate)
Found in brain regions like NO
•
Different pathways but similar mode of action
Classification of Neurotransmitters
Function
• Functional classifications of
neurotransmitters consider whether the
effects are: excitatory or inhibitory, and
whether the effects are direct or indirect
• There are two main types of neurotransmitter
receptors:
– Channel-linked receptors mediate direct transmitter
action and result in brief, localized changes
– G protein-linked receptors mediate indirect
transmitter action resulting in slow, persistent, and
often diffuse changes
Effects:Excitatory Versus Inhibitory
• Excitatory: cause depolarization
• Inhibitory: cause hyperpolarization
– Some: are both depending on the specific
receptor types with which they interact
• Example: acetylcholine is excitatory at
neuromuscular junctions with skeletal muscle and
inhibitory in cardiac muscle
Mechanism of Action
Direct Versus Indirect
• Neurotransmitters that open ion channels are
said to act directly
– Provoke rapid responses in postsynaptic cells by
promoting changes in membrane potential
• ACh and the amino acid neurotransmitters
• Neurotransmitters that act indirectly promote
broader, longer-lasting effects by acting
through intracellular second-messenger
molecules (G protein mechanisms)
– Actions similar to that of hormones
– Biogenic amines, neuropeptides, and the dissolved
gases
Neurotransmitter Receptors
• Channel-linked: mediate fast synaptic
transmission
• G protein-linked: oversee slow synaptic
resposes
Channel-Linked Receptors
• Mediate direct transmitter
action
• Also called ionotropic
receptors
• Composed of several protein
subunits arranged in a
rosette (circular) around a
central pore
• As the ligand
(neurotransmitter) binds to
one (or more) receptor
subunits, the proteins
change shape
• This event opens the central
channel and allows ions to
pass, altering the membrane
potential of the target cell
Channel-Linked Receptors
• Channel-linked receptors are always located precisely
opposite sites of neurotransmitter release, and their ion
channels open instantly upon ligand binding and remain
open 1 ms or less while the ligand is bound
• At excitatory receptor sites (nicotine ACh channels and
receptors for amino acids [glutamate, aspartate,] and
ATP), the channel-linked receptors are cation
channels that allow small cations (Na+, K+, Ca2+) to
pass, but the greatest drive is for Na+ entry, which
contributes to membrane depolarization
• Channel-linked receptors that respond to amino acids
GABA (gamma-aminobutyric acid) and glycine, allow K+
or Cl- to pass, mediate fast inhibition
(hyperpolarization)
Channel-Linked Receptors
•
•
•
•
(a): Channel-linked receptors open in response to ligand (ACh in this case)
binding
When no ligand is bound, the channel is closed and no current passes
through it
As soon as ligand binds, the channel opens and ions flow through it
The precise ion current is determined by the structure and charge of the
channel proteins
– In ACh channels, Na+, K+, and Ca2+ pass, resulting in a depolarizing current
G Protein-Linked Receptors
• Unlike responses to neurotransmitter binding at channel-linked
receptors, which are immediate, simple, brief, and highly localized at
a single postsynaptic cell, the activity mediated by G protein-linked
receptors is indirect, slow, complex, prolonged
• Receptors that fall into this class are transmembrane protein
complexes and include muscarine ACh receptors and those that
bind the biogenic amines and neuropeptides
• When a neurotransmitter binds to a G protrein-linked receptor, the G
protein is activated
– Activated G proteins typically work by controlling the production of
second messengers, such as cyclic AMP, cyclic GMP, diacylglycerol, or
Ca2+
• Which act as go-betweens to regulate (open or close) ion channels or
activate kinase enzymes that initiate a cascade of enzymatic reactions in the
target cells
G Protein-Linked Receptors
• Some of these second messengers modify
(activate or inactivate) other proteins,
including channel proteins, by attaching
phosphate groups to them
• Others interact with nuclear proteins that
activate genes and induce synthesis of new
proteins in the target cell
• Because the effects produced tend to bring
about widespread metabolic changes, G
protein-linked receptors are commonly called
metabotropic receptors
G Protein-Linked Receptors
• (b): A G protein-linked receptor
• (1) Unbound receptor
• (2) Binding of the neurotransmitter (ligand) to the receptor results in
receptor-G protein interaction and activation of the G protein
– Once GTP replaces GDP in the G protein complex, (3) the G protein
can interact with and activate adenylate cyclase
G Protein-Linked Receptors
• (2) Once GTP replaces GDP in the G protein
complex, (3) the G protein can interact with and
activate adenylate cyclase
G Protein-Linked Receptors
• (4) Activated adenylate cyclase catalyzes the
formation of cyclic AMP (cAMP) from ATP
G Protein-Linked Receptors
• (5) cAMP, acting as an intracellular second
messenger, mediates events which activate
enzymes that bring about the postsynaptic
neuron’s response (changes in membrane
potential, protein synthesis, etc.)
NEUROTRANSMITTER
RECEPTORS
BASIC CONCEPTS OF NEURAL
INTEGRATION
•
Organization of Neurons:
Neuronal Pools
– Neuronal pools are functional
groups of neurons that integrate
incoming information from
receptors or other neuronal pools
and relay the information to
other areas
•
Simplified representation
shows the relative position of
postsynaptic neurons in the
discharge and facilitated zones
– Notice that the presynaptic fiber
makes more synapses per
neuron with neurons in the
discharge zone
•
Most neuronal pools consist of
thousands of neurons and
include inhibitory as well as
excitatory neurons
NEURONAL POOL
Types of Circuits
• Circuits: patterns of synaptic
connections in neuronal pools
– Determines the pool’s functional capabilities
• Four basic circuit patterns:
– Diverging circuits
– Converging circuits
– Reverberating (oscillating)
– Parallel after-discharge
Diverging Circuits (a,b)
• One incoming fiber
triggers responses in
ever-increasing numbers
of neurons farther and
farther along in the circuit
• Often amplifying
circuits
• Divergence can occur
along a single pathway or
along several pathways
• Common in both
sensory and motor
systems
Converging Circuits (c,d)
•
•
•
•
Opposite of divergence
Common in both sensory and
motor pathways
They are characterized by
reception of input from many
sources, and a funneling to a
given circuit, resulting in strong
stimulation or inhibition
Incoming stimuli may converge
from one area or from many
different areas, which results in
strong stimulation or inhibition
– Different types of sensory
stimuli can have the same
effect:
• Example: seeing the smiling face
of their infant, smelling the baby’s
freshly powdered skin, or hearing
the baby gurgle can all trigger a
flood of loving feelings in parents
Reverberating (Oscillating) Circuits (e)
•
•
•
•
Incoming signal travels through a
chain of neurons, each of which
makes collateral synapses with
neurons in a previous part of the
pathway
As a result of the positive feedback,
the impulses reverberate (are sent
through the circuit again and again)
Reverberating, or oscillating, circuits
are characterized by feedback by axon
collaterals to previous points in the
pathway, resulting in ongoing
stimulation of the pathway
Involved in control of rhythmic
activities
–
–
–
–
Sleep-wake cycle
Breathing
Motor activities (arm swinging when
walking)
Short-term memory
Parallel After-Discharge Circuits (f)
•
•
•
•
Incoming fiber stimulates several
neurons arranged in parallel
arrays that eventually stimulate
a common output cell
Impulses reach the output cell
at different times, creating a
burst of impulses called an after
discharge that lasts 15 ms or
more after the initial input has
ended
No positive feedback, and once all
the neurons have fired, circuit
activity ends
Parallel after-discharge circuits
may be involved in complex,
exacting types of mental
processing, such as working on
mathematical problems
CIRCUIT TYPES
Patterns of Neural Processing
• Input processing is both serial and parallel
Patterns of Neural Processing
Serial
• Serial: the input travels
along one pathway to a
specific destination
• Whole system works in a
predictable all-or-nothing
manner
– Serial processing is
exemplified by spinal
reflexes, and involves
sequential stimulation of
the neurons in a circuit
REFLEX ARC
Pattern of Neural Processing
Parallel
• Parallel: the input travels along several
different pathways to be integrated in
different CNS regions
– Parallel processing results in inputs
stimulating many pathways simultaneously,
and is vital to higher level mental functioning
– Brain derives its power from its ability to
process in parallel
DEVELOPMENTAL ASPECTS
OF
NEURONS
• The nervous system originates from a dorsal neural tube and neural
crest, which begin as a layer of neuroepithelial cells that ultimately
become the CNS
• Differentiation of neuroepithelial cells occurs largely in the second
month of development
• Growth of an axon toward its target appears to be guided by older
“pathfinding” neurons and glial cells, nerve growth factor and
cholesterol from astrocytes, and tropic chemicals from target cells
• The growth cone is a growing tip of an axon
– It takes up chemicals from the environment that are used by the cell to
evaluate the pathway taken for further growth and synapse formation
• Unsuccessful synapse formation results in cell death, and a certain
amount of apoptosis occurs before the final population of neurons is
complete