Functions of the Nervous System
1. Sensory input
•
Information gathered by sensory receptors
about internal and external changes
2. Integration
•
Interpretation of sensory input
3. Motor output
•
Activation of effector organs (muscles and
glands) produces a response
Sensory input
Integration
Motor output
Figure 11.1
Divisions of the Nervous System
• Central nervous system (CNS)
• Brain and spinal cord
• Integration and command center
• Peripheral nervous system (PNS)
• Paired spinal and cranial nerves carry
messages to and from the CNS
Peripheral Nervous System (PNS)
•
Two functional divisions
1. Sensory (afferent) division
•
Somatic afferent fibers—convey impulses
from skin, skeletal muscles, and joints
•
Visceral afferent fibers—convey impulses
from visceral organs
2. Motor (efferent) division
•
Transmits impulses from the CNS to
effector organs
Motor Division of PNS
1. Somatic (voluntary) nervous system
•
Conscious control of skeletal muscles
Motor Division of PNS
2. Autonomic (involuntary) nervous system
(ANS)
•
Visceral motor nerve fibers
•
Regulates smooth muscle, cardiac muscle,
and glands
•
Two functional subdivisions
•
Sympathetic
•
Parasympathetic
Peripheral nervous system (PNS)
Central nervous system (CNS)
Cranial nerves and spinal nerves
Communication lines between the
CNS and the rest of the body
Brain and spinal cord
Integrative and control centers
Sensory (afferent) division
Somatic and visceral sensory
nerve fibers
Conducts impulses from
receptors to the CNS
Somatic sensory
fiber
Motor (efferent) division
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Somatic nervous
system
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Skin
Visceral sensory fiber
Stomach
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Mobilizes body
systems during activity
Sympathetic motor fiber of ANS
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
Parasympathetic motor fiber of ANS
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Parasympathetic
division
Conserves energy
Promotes housekeeping functions
during rest
Heart
Bladder
Figure 11.2
Histology of Nervous Tissue
• Two principal cell types
1. Neurons—excitable cells that transmit
electrical signals
Histology of Nervous Tissue
2. Neuroglia (glial cells)—supporting cells:
•
Astrocytes (CNS)
•
Microglia (CNS)
•
Ependymal cells (CNS)
•
Oligodendrocytes (CNS)
•
Satellite cells (PNS)
•
Schwann cells (PNS)
Astrocytes
• Most abundant, versatile, and highly branched
glial cells
• Cling to neurons, synaptic endings, and
capillaries
• Support and brace neurons
Astrocytes
• Help determine capillary permeability
• Guide migration of young neurons
• Control the chemical environment
• Participate in information processing in the
brain
Capillary
Neuron
Astrocyte
(a) Astrocytes are the most abundant
CNS neuroglia.
Figure 11.3a
Microglia
• Small, ovoid cells with thorny processes
• Migrate toward injured neurons
• Phagocytize microorganisms and neuronal
debris
Neuron
Microglial
cell
(b) Microglial cells are defensive cells in
the CNS.
Figure 11.3b
Ependymal Cells
• Range in shape from squamous to columnar
• May be ciliated
• Line the central cavities of the brain and spinal
column
• Separate the CNS interstitial fluid from the
cerebrospinal fluid in the cavities
Fluid-filled cavity
Ependymal
cells
Brain or
spinal cord
tissue
(c) Ependymal cells line cerebrospinal
fluid-filled cavities.
Figure 11.3c
Oligodendrocytes
• Branched cells
• Processes wrap CNS nerve fibers, forming
insulating myelin sheaths
Myelin sheath
Process of
oligodendrocyte
Nerve
fibers
(d) Oligodendrocytes have processes that form
myelin sheaths around CNS nerve fibers.
Figure 11.3d
Satellite Cells and Schwann Cells
• Satellite cells
• Surround neuron cell bodies in the PNS
• Schwann cells (neurolemmocytes)
• Surround peripheral nerve fibers and form
myelin sheaths
• Vital to regeneration of damaged peripheral
nerve fibers
Satellite
cells
Cell body of neuron
Schwann cells
(forming myelin sheath)
Nerve fiber
(e) Satellite cells and Schwann cells (which
form myelin) surround neurons in the PNS.
Figure 11.3e
Neurons (Nerve Cells)
• Special characteristics:
• Long-lived ( 100 years or more)
• Amitotic—with few exceptions
• High metabolic rate—depends on continuous
supply of oxygen and glucose
• Plasma membrane functions in:
• Electrical signaling
• Cell-to-cell interactions during development
Cell Body (Perikaryon or Soma)
• Biosynthetic center of a neuron
• Spherical nucleus with nucleolus
• Well-developed Golgi apparatus
• Rough ER called Nissl bodies (chromatophilic
substance)
Cell Body (Perikaryon or Soma)
• Network of neurofibrils (neurofilaments)
• Axon hillock—cone-shaped area from which
axon arises
• Clusters of cell bodies are called nuclei in the
CNS, ganglia in the PNS
Dendrites
(receptive regions)
Cell body
(biosynthetic center
and receptive region)
Nucleolus
Axon
(impulse generating
and conducting region)
Nucleus
Nissl bodies
Axon hillock
(b)
Impulse
direction
Node of Ranvier
Schwann cell
Neurilemma (one interTerminal
node)
branches
Axon
terminals
(secretory
region)
Figure 11.4b
Processes
• Dendrites and axons
• Bundles of processes are called
• Tracts in the CNS
• Nerves in the PNS
Dendrites
• Short, tapering, and diffusely branched
• Receptive (input) region of a neuron
• Convey electrical signals toward the cell body
as graded potentials
The Axon
• One axon per cell arising from the axon hillock
• Long axons (nerve fibers)
• Occasional branches (axon collaterals)
The Axon
• Numerous terminal branches (telodendria)
• Knoblike axon terminals (synaptic knobs or
boutons)
• Secretory region of neuron
• Release neurotransmitters to excite or inhibit
other cells
Axons: Function
• Conducting region of a neuron
• Generates and transmits nerve impulses
(action potentials) away from the cell body
Axons: Function
• Molecules and organelles are moved along
axons by motor molecules in two directions:
• Anterograde—toward axonal terminal
• Examples: mitochondria, membrane
components, enzymes
• Retrograde—toward the cell body
• Examples: organelles to be degraded, signal
molecules, viruses, and bacterial toxins
Dendrites
(receptive regions)
Cell body
(biosynthetic center
and receptive region)
Nucleolus
Axon
(impulse generating
and conducting region)
Nucleus
Nissl bodies
Axon hillock
(b)
Impulse
direction
Node of Ranvier
Schwann cell
Neurilemma (one interTerminal
node)
branches
Axon
terminals
(secretory
region)
Figure 11.4b
Myelin Sheath
• Segmented protein-lipoid sheath around most
long or large-diameter axons
• It functions to:
• Protect and electrically insulate the axon
• Increase speed of nerve impulse transmission
Myelin Sheaths in the PNS
• Schwann cells wraps many times around the
axon
• Myelin sheath—concentric layers of Schwann
cell membrane
• Neurilemma—peripheral bulge of Schwann
cell cytoplasm
Myelin Sheaths in the PNS
• Nodes of Ranvier
• Myelin sheath gaps between adjacent
Schwann cells
• Sites where axon collaterals can emerge
Schwann cell
plasma membrane
Schwann cell
cytoplasm
Axon
1
A Schwann cell
envelopes an axon.
Schwann cell
nucleus
2
The Schwann cell then
rotates around the axon,
wrapping its plasma
membrane loosely around
it in successive layers.
Neurilemma
Myelin sheath
(a) Myelination of a nerve
fiber (axon)
3
The Schwann cell
cytoplasm is forced from
between the membranes.
The tight membrane
wrappings surrounding
the axon form the myelin
sheath.
Figure 11.5a
Unmyelinated Axons
• Thin nerve fibers are unmyelinated
• One Schwann cell may incompletely enclose
15 or more unmyelinated axons
Myelin Sheaths in the CNS
• Formed by processes of oligodendrocytes, not
the whole cells
• Nodes of Ranvier are present
• No neurilemma
• Thinnest fibers are unmyelinated
Myelin sheath
Process of
oligodendrocyte
Nerve
fibers
(d) Oligodendrocytes have processes that form
myelin sheaths around CNS nerve fibers.
Figure 11.3d
White Matter and Gray Matter
• White matter
• Dense collections of myelinated fibers
• Gray matter
• Mostly neuron cell bodies and unmyelinated
fibers
Structural Classification of Neurons
• Three types:
1. Multipolar—1 axon and several dendrites
• Most abundant
• Motor neurons and interneurons
2. Bipolar—1 axon and 1 dendrite
• Rare, e.g., retinal neurons
Structural Classification of Neurons
3. Unipolar (pseudounipolar)—single, short
process that has two branches:
•
Peripheral process—more distal branch,
often associated with a sensory receptor
•
Central process—branch entering the
CNS
Table 11.1 (1 of 3)
Table 11.1 (2 of 3)
Functional Classification of Neurons
• Three types:
1. Sensory (afferent)
• Transmit impulses from sensory receptors
toward the CNS
2. Motor (efferent)
• Carry impulses from the CNS to effectors
Functional Classification of Neurons
3. Interneurons (association neurons)
•
Shuttle signals through CNS pathways;
most are entirely within the CNS
Table 11.1 (3 of 3)
Neuron Function
• Neurons are highly irritable
• Respond to adequate stimulus by generating
an action potential (nerve impulse)
• Impulse is always the same regardless of
stimulus
Principles of Electricity
• Opposite charges attract each other
• Energy is required to separate opposite
charges across a membrane
• Energy is liberated when the charges move
toward one another
• If opposite charges are separated, the system
has potential energy
Definitions
• Voltage (V): measure of potential energy
generated by separated charge
• Potential difference: voltage measured
between two points
• Current (I): the flow of electrical charge (ions)
between two points
Definitions
• Resistance (R): hindrance to charge flow
(provided by the plasma membrane)
• Insulator: substance with high electrical
resistance
• Conductor: substance with low electrical
resistance
Role of Membrane Ion Channels
•
Proteins serve as membrane ion channels
• Two main types of ion channels
1. Leakage (nongated) channels—always open
Role of Membrane Ion Channels
2. Gated channels (three types):
•
Chemically gated (ligand-gated)
channels—open with binding of a
specific neurotransmitter
•
Voltage-gated channels—open and
close in response to changes in
membrane potential
•
Mechanically gated channels—open and
close in response to physical
deformation of receptors
Receptor
Neurotransmitter chemical
attached to receptor
Na+
Na+
Na+
Chemical
binds
K+
Closed
Na+
Membrane
voltage
changes
K+
Open
(a) Chemically (ligand) gated ion channels open when the
appropriate neurotransmitter binds to the receptor,
allowing (in this case) simultaneous movement of
Na+ and K+.
Closed
Open
(b) Voltage-gated ion channels open and close in response
to changes in membrane voltage.
Figure 11.6
Gated Channels
• When gated channels are open:
• Ions diffuse quickly across the membrane
along their electrochemical gradients
• Along chemical concentration gradients from
higher concentration to lower concentration
• Along electrical gradients toward opposite
electrical charge
• Ion flow creates an electrical current and
voltage changes across the membrane
Resting Membrane Potential (Vr)
• Potential difference across the membrane of a
resting cell
• Approximately –70 mV in neurons
(cytoplasmic side of membrane is negatively
charged relative to outside)
• Generated by:
• Differences in ionic makeup of ICF and ECF
• Differential permeability of the plasma
membrane
Voltmeter
Plasma
membrane
Ground electrode
outside cell
Microelectrode
inside cell
Axon
Neuron
Figure 11.7
Resting Membrane Potential
• Differences in ionic makeup
• ICF has lower concentration of Na+ and Cl–
than ECF
• ICF has higher concentration of K+ and
negatively charged proteins (A–) than ECF
Resting Membrane Potential
• Differential permeability of membrane
• Impermeable to A–
• Slightly permeable to Na+ (through leakage
channels)
• 75 times more permeable to K+ (more leakage
channels)
• Freely permeable to Cl–
Resting Membrane Potential
• Negative interior of the cell is due to much
greater diffusion of K+ out of the cell than Na+
diffusion into the cell
• Sodium-potassium pump stabilizes the resting
membrane potential by maintaining the
concentration gradients for Na+ and K+
The concentrations of Na+ and K+ on each side of the membrane are different.
Outside cell
The Na+ concentration
is higher outside the
cell.
K+
(5 mM )
Na+
(140 mM )
The K+ concentration
is higher inside the
cell.
K+
(140 mM )
Na+
(15 mM )
Inside cell
The permeabilities of Na+ and K+ across the
membrane are different.
Suppose a cell has only K+ channels...
K+ loss through abundant leakage
channels establishes a negative
membrane potential.
K+ leakage channels
K+
K+
K+
K+
K+
K+
Na+
K
K+
Na+
K+
K+
Na+
K+
K+
Na+
Na+-K+ ATPases (pumps)
maintain the concentration
gradients of Na+ and K+
across the membrane.
Cell interior
–90 mV
Now, let’s add some Na+ channels to our cell...
Na+ entry through leakage channels reduces
the negative membrane potential slightly.
Cell interior
–70 mV
Na+-K+ pump
Finally, let’s add a pump to compensate
for leaking ions.
Na+-K+ ATPases (pumps) maintain the
concentration gradients, resulting in the
resting membrane potential.
Cell interior
–70 mV
Figure 11.8
Membrane Potentials That Act as Signals
• Membrane potential changes when:
• Concentrations of ions across the membrane
change
• Permeability of membrane to ions changes
• Changes in membrane potential are signals
used to receive, integrate and send
information
Membrane Potentials That Act as Signals
• Two types of signals
• Graded potentials
• Incoming short-distance signals
• Action potentials
• Long-distance signals of axons
Changes in Membrane Potential
• Depolarization
• A reduction in membrane potential (toward
zero)
• Inside of the membrane becomes less
negative than the resting potential
• Increases the probability of producing a nerve
impulse
Depolarizing stimulus
Inside
positive
Inside
negative
Depolarization
Resting
potential
Time (ms)
(a) Depolarization: The membrane potential
moves toward 0 mV, the inside becoming
less negative (more positive).
Figure 11.9a
Changes in Membrane Potential
• Hyperpolarization
• An increase in membrane potential (away from
zero)
• Inside of the membrane becomes more
negative than the resting potential
• Reduces the probability of producing a nerve
impulse
Hyperpolarizing stimulus
Resting
potential
Hyperpolarization
Time (ms)
(b) Hyperpolarization: The membrane
potential increases, the inside becoming
more negative.
Figure 11.9b
Graded Potentials
• Short-lived, localized changes in membrane
potential
• Depolarizations or hyperpolarizations
• Graded potential spreads as local currents
change the membrane potential of adjacent
regions
Stimulus
Depolarized region
Plasma
membrane
(a) Depolarization: A small patch of the
membrane (red area) has become depolarized.
Figure 11.10a
(b) Spread of depolarization: The local currents
(black arrows) that are created depolarize
adjacent membrane areas and allow the wave of
depolarization to spread.
Figure 11.10b
Graded Potentials
• Occur when a stimulus causes gated ion
channels to open
• E.g., receptor potentials, generator potentials,
postsynaptic potentials
• Magnitude varies directly (graded) with
stimulus strength
• Decrease in magnitude with distance as ions
flow and diffuse through leakage channels
• Short-distance signals
Membrane potential (mV)
Active area
(site of initial
depolarization)
–70
Resting potential
Distance (a few mm)
(c) Decay of membrane potential with distance: Because current
is lost through the “leaky” plasma membrane, the voltage declines
with distance from the stimulus (the voltage is decremental ).
Consequently, graded potentials are short-distance signals.
Figure 11.10c
Action Potential (AP)
• Brief reversal of membrane potential with a
total amplitude of ~100 mV
• Occurs in muscle cells and axons of neurons
• Does not decrease in magnitude over
distance
• Principal means of long-distance neural
communication
The big picture
1 Resting state
3 Repolarization
Membrane potential (mV)
2 Depolarization
3
4 Hyperpolarization
2
Action
potential
Threshold
1
4
1
Time (ms)
Figure 11.11 (1 of 5)
Generation of an Action Potential
• Resting state
• Only leakage channels for Na+ and K+ are
open
• All gated Na+ and K+ channels are closed
Properties of Gated Channels
• Properties of gated channels
• Each Na+ channel has two voltage-sensitive
gates
• Activation gates
• Closed at rest; open with depolarization
• Inactivation gates
• Open at rest; block channel once it is
open
Properties of Gated Channels
• Each K+ channel has one voltage-sensitive
gate
• Closed at rest
• Opens slowly with depolarization
Depolarizing Phase
• Depolarizing local currents open voltagegated Na+ channels
• Na+ influx causes more depolarization
• At threshold (–55 to –50 mV) positive
feedback leads to opening of all Na+
channels, and a reversal of membrane
polarity to +30mV (spike of action potential)
Repolarizing Phase
• Repolarizing phase
• Na+ channel slow inactivation gates close
• Membrane permeability to Na+ declines to
resting levels
• Slow voltage-sensitive K+ gates open
• K+ exits the cell and internal negativity is
restored
Hyperpolarization
• Hyperpolarization
• Some K+ channels remain open, allowing
excessive K+ efflux
• This causes after-hyperpolarization of the
membrane (undershoot)
3
2
Action
potential
Na+ permeability
K+ permeability
1
4
1
Relative membrane permeability
Membrane potential (mV)
The AP is caused by permeability changes in
the plasma membrane
Time (ms)
Figure 11.11 (2 of 5)
Role of the Sodium-Potassium Pump
• Repolarization
• Restores the resting electrical conditions of the
neuron
• Does not restore the resting ionic conditions
• Ionic redistribution back to resting conditions
is restored by the thousands of sodiumpotassium pumps
Propagation of an Action Potential
• Na+ influx causes a patch of the axonal
membrane to depolarize
• Local currents occur
• Na+ channels toward the point of origin are
inactivated and not affected by the local
currents
Propagation of an Action Potential
• Local currents affect adjacent areas in the
forward direction
• Depolarization opens voltage-gated channels
and triggers an AP
• Repolarization wave follows the depolarization
wave
• (Fig. 11.12 shows the propagation process in
unmyelinated axons.)
Voltage
at 0 ms
Recording
electrode
(a) Time = 0 ms. Action
potential has not yet
reached the recording
electrode.
Resting potential
Peak of action potential
Hyperpolarization
Figure 11.12a
Voltage
at 2 ms
(b) Time = 2 ms. Action
potential peak is at the
recording electrode.
Figure 11.12b
Voltage
at 4 ms
(c) Time = 4 ms. Action
potential peak is past
the recording electrode.
Membrane at the
recording electrode is
still hyperpolarized.
Figure 11.12c
Threshold
• At threshold:
• Membrane is depolarized by 15 to 20 mV
• Na+ permeability increases
• Na influx exceeds K+ efflux
• The positive feedback cycle begins
Threshold
• Subthreshold stimulus—weak local
depolarization that does not reach threshold
• Threshold stimulus—strong enough to push
the membrane potential toward and beyond
threshold
• AP is an all-or-none phenomenon—action
potentials either happen completely, or not at
all
Coding for Stimulus Intensity
• All action potentials are alike and are
independent of stimulus intensity
• How does the CNS tell the difference between
a weak stimulus and a strong one?
• Strong stimuli can generate action potentials
more often than weaker stimuli
• The CNS determines stimulus intensity by the
frequency of impulses
Action
potentials
Threshold
Stimulus
Time (ms)
Figure 11.13
Absolute Refractory Period
• Time from the opening of the Na+ channels
until the resetting of the channels
• Ensures that each AP is an all-or-none event
• Enforces one-way transmission of nerve
impulses
Absolute refractory
period
Relative refractory
period
Depolarization
(Na+ enters)
Repolarization
(K+ leaves)
After-hyperpolarization
Stimulus
Time (ms)
Figure 11.14
Relative Refractory Period
• Follows the absolute refractory period
• Most Na+ channels have returned to their
resting state
• Some K+ channels are still open
• Repolarization is occurring
• Threshold for AP generation is elevated
• Exceptionally strong stimulus may generate
an AP
Conduction Velocity
• Conduction velocities of neurons vary widely
• Effect of axon diameter
• Larger diameter fibers have less resistance to
local current flow and have faster impulse
conduction
• Effect of myelination
• Continuous conduction in unmyelinated axons
is slower than saltatory conduction in
myelinated axons
Conduction Velocity
• Effects of myelination
• Myelin sheaths insulate and prevent leakage
of charge
• Saltatory conduction in myelinated axons is
about 30 times faster
• Voltage-gated Na+ channels are located at
the nodes
• APs appear to jump rapidly from node to
node
Stimulus
Size of voltage
(a) In a bare plasma membrane (without voltage-gated
channels), as on a dendrite, voltage decays because
current leaks across the membrane.
Voltage-gated
Stimulus
ion channel
(b) In an unmyelinated axon, voltage-gated Na+ and K+
channels regenerate the action potential at each point
along the axon, so voltage does not decay. Conduction
is slow because movements of ions and of the gates
of channel proteins take time and must occur before
voltage regeneration occurs.
Stimulus
Myelin
sheath
(c) In a myelinated axon, myelin keeps current in axons
(voltage doesn’t decay much). APs are generated only
in the nodes of Ranvier and appear to jump rapidly
from node to node.
Node of Ranvier
1 mm
Myelin sheath
Figure 11.15
The Synapse
• A junction that mediates information transfer
from one neuron:
• To another neuron, or
• To an effector cell
The Synapse
• Presynaptic neuron—conducts impulses
toward the synapse
• Postsynaptic neuron—transmits impulses
away from the synapse
Types of Synapses
• Axodendritic—between the axon of one
neuron and the dendrite of another
• Axosomatic—between the axon of one neuron
and the soma of another
• Less common types:
• Axoaxonic (axon to axon)
• Dendrodendritic (dendrite to dendrite)
• Dendrosomatic (dendrite to soma)
Axodendritic
synapses
Dendrites
Axosomatic
synapses
Cell body
Axoaxonic synapses
(a)
Axon
Axon
Axosomatic
synapses
(b)
Cell body (soma) of
postsynaptic neuron
Figure 11.16
Electrical Synapses
• Less common than chemical synapses
• Neurons are electrically coupled (joined by gap
junctions)
• Communication is very rapid, and may be
unidirectional or bidirectional
• Are important in:
• Embryonic nervous tissue
• Some brain regions
Chemical Synapses
• Specialized for the release and reception of
neurotransmitters
• Typically composed of two parts
• Axon terminal of the presynaptic neuron, which
contains synaptic vesicles
• Receptor region on the postsynaptic neuron
Synaptic Cleft
• Fluid-filled space separating the presynaptic
and postsynaptic neurons
• Prevents nerve impulses from directly passing
from one neuron to the next
Synaptic Cleft
• Transmission across the synaptic cleft:
• Is a chemical event (as opposed to an
electrical one)
• Involves release, diffusion, and binding of
neurotransmitters
• Ensures unidirectional communication
between neurons
Information Transfer
• AP arrives at axon terminal of the presynaptic
neuron and opens voltage-gated Ca2+
channels
• Synaptotagmin protein binds Ca2+ and
promotes fusion of synaptic vesicles with axon
membrane
• Exocytosis of neurotransmitter occurs
Information Transfer
• Neurotransmitter diffuses and binds to
receptors (often chemically gated ion
channels) on the postsynaptic neuron
• Ion channels are opened, causing an
excitatory or inhibitory event (graded
potential)
Chemical synapses
transmit signals from
one neuron to another
using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Mitochondrion
Ca2+
Ca2+
Ca2+
3 Ca2+ entry causes
neurotransmittercontaining synaptic
vesicles to release their
contents by exocytosis.
Axon
terminal
Ca2+
Synaptic
cleft
Synaptic
vesicles
4 Neurotransmitter
diffuses across the synaptic
cleft and binds to specific
receptors on the
postsynaptic membrane.
Postsynaptic
neuron
Ion movement
Enzymatic
degradation
Graded potential
Reuptake
Diffusion away
from synapse
5 Binding of neurotransmitter
opens ion channels, resulting in
graded potentials.
6 Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse.
Figure 11.17
Chemical synapses
transmit signals from
one neuron to another
using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
Mitochondrion
Ca2+
Ca2+
Axon
terminal
Ca2+
Ca2+
Synaptic
cleft
Synaptic
vesicles
Postsynaptic
neuron
Figure 11.17, step 1
Chemical synapses
transmit signals from
one neuron to another
using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Mitochondrion
Ca2+
Ca2+
Axon
terminal
Ca2+
Ca2+
Synaptic
cleft
Synaptic
vesicles
Postsynaptic
neuron
Figure 11.17, step 2
Chemical synapses
transmit signals from
one neuron to another
using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Mitochondrion
Ca2+
Ca2+
3 Ca2+ entry causes
neurotransmittercontaining synaptic
vesicles to release their
contents by exocytosis.
Axon
terminal
Ca2+
Ca2+
Synaptic
cleft
Synaptic
vesicles
Postsynaptic
neuron
Figure 11.17, step 3
Chemical synapses
transmit signals from
one neuron to another
using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Mitochondrion
Ca2+
Ca2+
3 Ca2+ entry causes
neurotransmittercontaining synaptic
vesicles to release their
contents by exocytosis.
4 Neurotransmitter
diffuses across the synaptic
cleft and binds to specific
receptors on the
postsynaptic membrane.
Axon
terminal
Ca2+
Ca2+
Synaptic
cleft
Synaptic
vesicles
Postsynaptic
neuron
Figure 11.17, step 4
Ion movement
Graded potential
5 Binding of neurotransmitter
opens ion channels, resulting in
graded potentials.
Figure 11.17, step 5
Enzymatic
degradation
Reuptake
Diffusion away
from synapse
6 Neurotransmitter effects are terminated
by reuptake through transport proteins,
enzymatic degradation, or diffusion away
from the synapse.
Figure 11.17, step 6
Chemical synapses
transmit signals from
one neuron to another
using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Mitochondrion
Ca2+
Ca2+
Ca2+
3 Ca2+ entry causes
neurotransmittercontaining synaptic
vesicles to release their
contents by exocytosis.
Axon
terminal
Ca2+
Synaptic
cleft
Synaptic
vesicles
4 Neurotransmitter
diffuses across the synaptic
cleft and binds to specific
receptors on the
postsynaptic membrane.
Postsynaptic
neuron
Ion movement
Enzymatic
degradation
Graded potential
Reuptake
Diffusion away
from synapse
5 Binding of neurotransmitter
opens ion channels, resulting in
graded potentials.
6 Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse.
Figure 11.17
Termination of Neurotransmitter Effects
• Within a few milliseconds, the
neurotransmitter effect is terminated
• Degradation by enzymes
• Reuptake by astrocytes or axon terminal
• Diffusion away from the synaptic cleft
Synaptic Delay
• Neurotransmitter must be released, diffuse
across the synapse, and bind to receptors
• Synaptic delay—time needed to do this (0.3–
5.0 ms)
• Synaptic delay is the rate-limiting step of
neural transmission
Postsynaptic Potentials
•
Graded potentials
•
Strength determined by:
•
•
Amount of neurotransmitter released
•
Time the neurotransmitter is in the area
Types of postsynaptic potentials
1. EPSP—excitatory postsynaptic potentials
2. IPSP—inhibitory postsynaptic potentials
Table 11.2 (2 of 4)
Table 11.2 (3 of 4)
Table 11.2 (4 of 4)
Neurotransmitters
• Most neurons make two or more
neurotransmitters, which are released at
different stimulation frequencies
• 50 or more neurotransmitters have been
identified
• Classified by chemical structure and by
function
Chemical Classes of Neurotransmitters
• Acetylcholine (Ach)
• Released at neuromuscular junctions and
some ANS neurons
• Synthesized by enzyme choline
acetyltransferase
• Degraded by the enzyme acetylcholinesterase
(AChE)
Chemical Classes of Neurotransmitters
• Biogenic amines include:
• Catecholamines
• Dopamine, norepinephrine (NE), and
epinephrine
• Indolamines
• Serotonin and histamine
• Broadly distributed in the brain
• Play roles in emotional behaviors and the
biological clock
Chemical Classes of Neurotransmitters
• Amino acids include:
• GABA—Gamma ()-aminobutyric acid
• Glycine
• Aspartate
• Glutamate
Chemical Classes of Neurotransmitters
• Peptides (neuropeptides) include:
• Substance P
• Mediator of pain signals
• Endorphins
• Act as natural opiates; reduce pain
perception
• Gut-brain peptides
• Somatostatin and cholecystokinin
Chemical Classes of Neurotransmitters
• Purines such as ATP:
• Act in both the CNS and PNS
• Produce fast or slow responses
• Induce Ca2+ influx in astrocytes
• Provoke pain sensation
Chemical Classes of Neurotransmitters
• Gases and lipids
• Nitric oxide (NO)
• Synthesized on demand
• Activates the intracellular receptor guanylyl
cyclase to cyclic GMP
• Involved in learning and memory
• Carbon monoxide (CO) is a regulator of cGMP
in the brain
Chemical Classes of Neurotransmitters
• Gases and lipids
• Endocannabinoids
• Lipid soluble; synthesized on demand from
membrane lipids
• Bind with G protein–coupled receptors in the
brain
• Involved in learning and memory
Functional Classification of
Neurotransmitters
• Neurotransmitter effects may be excitatory
(depolarizing) and/or inhibitory (hyperpolarizing)
• Determined by the receptor type of the postsynaptic
neuron
• GABA and glycine are usually inhibitory
• Glutamate is usually excitatory
• Acetylcholine
• Excitatory at neuromuscular junctions in skeletal
muscle
• Inhibitory in cardiac muscle