Nervous Tissue and Function
 Function of the Nervous System
 Organization (Structural and Functional)
 Supporting Cells of the Nervous System
 Anatomy of a Neuron
 Classification of Neurons by Function
 Graded and Action Potentials
 Myleination and MS
 Reflexes
 Synapses
 EPSPs and IPSPs
 Neurotransmitters
Functional Classification of the Peripheral Nervous System
Organization of the Nervous System
and reflexes
inhibitory
stimulatory
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
Nervous Tissue and Function
 Function of the Nervous System
 Organization (Structural and Functional)
 Supporting Cells of the Nervous System
 Anatomy of a Neuron
 Classification of Neurons
 Neuron Function Graded and Action Potentials
 Myleination and MS
 Reflexes
 Synapses
 EPSPs and IPSPs
 Neurotransmitters
Nervous Tissue: Support Cells (Neuroglia)
 Astrocytes
• Abundant, star-shaped cells
• Brace neurons
• Form barrier
between capillaries
and neurons
• Control the chemical
environment of
the brain
• Help transfer nutrients between capillaries and neurons
• Mop up and recapture potassium ions and
neurotransmitters
"Star cells connect and protect"
Nervous Tissue: Support Cells
 Microglia
• Spider-like phagocytes
• Dispose of debris by
phagocytosis
• Monitor neuron health
"Tiny garbage spiders"
Ependymal Cells (literally, “wrapping garment” 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
Nervous Tissue: Support Cells
 Oligodendrocytes
• Produce myelin sheath around nerve fibers
in the central nervous system
"Oligos insulate"
Nervous Tissue: Support Cells
 Schwann cells
• Form myelin sheath in the peripheral nervous
system
• Satellite cells
• Surround neuron cell bodies in the PNS
Satellite
cells
Cell body of neuron
Schwann cells
(forming myelin sheath)
Nerve fiber
Nervous Tissue and Function
 Function of the Nervous System
 Organization (Structural and Functional)
 Supporting Cells of the Nervous System
 Anatomy of a Neuron
 Classification of Neurons
 Neuron Function Graded and Action Potentials
 Myleination and MS
 Reflexes
 Synapses
 EPSPs and IPSPs
 Neurotransmitters
Dendrites
(receptive regions)
Cell body
(biosynthetic center
and receptive region)
Nucleolus
Axon
(impulse generating
and conducting region)
Nucleus
Nissl bodies
Impulse
direction
Node of Ranvier
(Rough ER)
Axon hillock
(b)
Schwann cell
Neurilemma (one interTerminal
node)
branches
Axon
terminals
(secretory
region)
Figure 11.4b
Neuron Cell Body Names and Locations
Clusters of cell bodies Bundles of nerve fibers
(neuronal processes)
CNS
Nuclei
Tracts
 White matter -dense
myelinated fibers
 Gray matter- unmyelinated
fibers and cell bodies
PNS
Ganglia
Nerves
(bundles of axons)
Axons and Connection(s) to Other Neurons
Synapse
Nerve Fiber Coverings
 In the PNS, Schwann cells
produce myelin sheaths in
jelly-roll like fashion
 Nodes of Ranvier – gaps in
myelin sheath along the axon
 In the CNS,
oligodendrocytes produce
myelin sheaths and have no
neurilemma (so cannot
regenerate if damaged).
 In multiple sclerosis (MS) the
myelin sheaths are destroyed
(autoimmunity) leading to
poorly functioning muscles
Nervous Tissue and Function
 Function of the Nervous System
 Organization (Structural and Functional)
 Supporting Cells of the Nervous System
 Anatomy of a Neuron
 Classification of Neurons by Function
 Graded and Action Potentials
 Myleination and MS
 Reflexes
 Synapses
 EPSPs and IPSPs
 Neurotransmitters
Functional Classification of Neurons
 Sensory (afferent) neurons
•
Carry impulses from the sensory
receptors
•
Cell bodies outside the CNS
o
Cutaneous sense organs
o
Proprioceptors – detect stretch
or tension
 Motor (efferent) neurons
•
Carry impulses from the central
nervous system
•
Cell bodies inside the CNS
 Interneurons (association neurons)
•
Found in neural pathways in the
central nervous system
•
Connect sensory and motor
neurons
•
Cell bodies inside the CNS
From skin receptors,
muscles and tendons
Nervous Tissue and Function
 Function of the Nervous System
 Organization (Structural and Functional)
 Supporting Cells of the Nervous System
 Anatomy of a Neuron
 Classification of Neurons by Function
 Graded and Action Potentials
 Myleination and MS
 Reflexes
 Synapses
 EPSPs and IPSPs
 Neuraltransmitters
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 charges
 Potential difference: the voltage or difference in charge
measured between two points
 Current (I): the flow of electrical charge (ions) between
two points
 Resistance (R): hindrance to charge flow (provided by the
plasma membrane)
 Insulator: substance with high electrical resistance
 Conductor: substance with low electrical resistance
 Intracellular fluid (ICF) - cytoplasm of neuron
 Extracellular fluid (ECF) - fluid outside a neuron cell
Role of Membrane Ion Channels (Protein “Gates”)

Two main types of ion channels:
1.
Leakage (nongated) channels —
always open
2.
Gated channels (three types):



Receptor
Neurotransmitter chemical
attached to receptor
Na+
Na+
Chemical
binds
Chemically gated (ligandgated) channels—open with
binding of a specific
neurotransmitter
Closed
Voltage-gated channels —
open and close in response to
changes in membrane
potential
Na+
K+
K+
Membrane
voltage
changes
Open
Na+
Mechanically gated
channels —open and close
in response to physical
deformation of receptors
Closed
Open
Potential difference across the membrane of a resting cell
(= Resting Potential)
Voltmeter
Ground electrode
outside cell
Plasma
membrane
Microelectrode
inside cell
Intracellular fluid (ICF)
Extracellular fluid (ECF)
Electrochemical gradient
or potential difference is
established by the powered
pumping of more positive
ions in ECF than ICF.
Axon
Neuron
Figure 11.7
Resting Membrane Potential
 Differences in ionic makeup
+
K+
Na+
Na+ Na+ Na+
+ Na Cl–
Na
+
+
ECF Na+ Na Na+ Na+ NaNa
Cl– Na+
+
• ICF has lower concentration of Na+ and Cl– than
ECF
• ICF has higher concentration of
charged proteins (A–) than ECF
K+
ICF
A-
and negatively
 Differential permeability of membrane
+
K+ K K+
K+
K+
A-
• Impermeable to A–
Cl–
Na+
ECF
K+ ACl–
ICF
• Slightly permeable to Na+ (through leakage channels)
A-
K+
Cl–
• 75 times more permeable to K+ (more leakage channels)
• Freely permeable to Cl–
 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+
ECF
ICF
++ + + + + +
+ + + + + +
+
+
+
K+
To Recap….
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
Graded potentials
potentials
o Incoming short-distance signals
o Short-lived, localized changes in membrane potential
o Depolarizations or hyperpolarizations
o Graded potential spreads as local currents change the
membrane potential of adjacent regions
o Act to enhance or limit chances of an action potential but does
not send an electrical “message”
• Action potentials
o Long-distance signals of axons
Graded Potential: Depolarization
Depolarizing stimulus
Inside
positive
Inside
negative
Depolarization
Resting
potential

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
Time (ms)
(a) Depolarization: The membrane potential
moves toward 0 mV, the inside becoming
less negative (more positive). Increases the
probability of producing a nerve impulse.
Figure 11.9a
Graded Potential: Hyperpolarization
Hyperpolarizing stimulus
Resting
potential
Hyperpolarization
Time (ms)
(b) Hyperpolarization: The membrane
potential increases, the inside becoming
more negative. Decreases the probability of
producing a nerve impulse.
Figure 11.9b
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
Membrane Potentials That Act as Signals
 Two types of signals
• Graded potentials
o Incoming short-distance signals
o Short-lived, localized changes in membrane potential
o Depolarizations or hyperpolarizations
o Graded potential spreads as local currents change the membrane
potential of adjacent regions
• Action potentials
o Long-distance signals of axons
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
Anatomy of an Action Potential
•Na+ channel slow inactivation gates close
• Only leakage channels for
Na+ and K+ are open
Depolarizing local
currents open voltagegated Na+ channels
•Membrane permeability to Na+ declines
to resting levels
• All gated Na+ and K+
channels are closed
Na+ influx causes more
depolarization
•K+ exits the cell and internal negativity
is restored
3 Repolarization
2 Depolarization
33
22
Action
potential
Na+
permeability
Action
potential
K+ permeability
Threshold
1
1
44
Time (ms)
1
Relative membrane permeability
Membrane potential (mV)
1 Resting state
•Slow voltage-sensitive K+ gates open
•Some K+ channels remain
open, allowing excessive K+
efflux
•This causes afterhyperpolarization of the
membrane (undershoot)
4 Hyperpolarization
Channel gating (online
animation)
Figure 11.11 (1 of 5)
Voltage Change at Point in Neuron as Action Potential Passes By
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 Change at Point in Neuron as Action Potential Passes By
Voltage
at 2 ms
(b) Time = 2 ms. Action
potential peak is at the
recording electrode.
Figure 11.12b
Voltage Change at Point in Neuron as Action Potential Passes By
Voltage
at 4 ms
Action
potential
online
Figure 11.12c
(c) Time = 4 ms. Action potential peak is past
the recording electrode. Membrane at the
recording electrode is still hyperpolarized.
Threshold Stimulus
 Subthreshold stimulus—weak local depolarization that does not reach
threshold
 Threshold stimulus—strong enough to push the membrane potential
toward and beyond threshold (Membrane is depolarized by 15 to 20 mV)
 AP is an all-or-none phenomenon—action potentials either happen
completely, or not at all
 All action potentials are alike and are independent of stimulus intensity
Action
potentials
Threshold
Stimulus
Time (ms)
Figure 11.13
Refractory Periods
Absolute refractory
period
Depolarization
(Na+ enters)
Relative refractory
period
ARP
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
RRP
•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
Repolarization(K+ leaves)
After-hyperpolarization
Stimulus
Time (ms)
Figure 11.14
AP Velocity a Function of Axon Diameter and Myelination
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, about 30 times faster than a bare axon.
Continuous
conduction
Saltatory
conduction
Node of Ranvier
1 mm
Myelin sheath
Figure 11.15
Multiple Sclerosis (MS)
 Nature
• An autoimmune disease that mainly affects young adults
• Symptoms: visual disturbances, weakness, loss of muscular control,
speech disturbances, and urinary incontinence
• Myelin sheaths in the CNS become nonfunctional scleroses
• Shunting and short-circuiting of nerve impulses occurs
• Impulse conduction slows and eventually ceases
 Treatment
• Some immune system–modifying drugs, including interferons and
Copazone:
o Hold symptoms at bay
o Reduce complications
o Reduce disability
Nervous Tissue and Function
 Function of the Nervous System
 Organization (Structural and Functional)
 Supporting Cells of the Nervous System
 Anatomy of a Neuron
 Classification of Neurons by Function
 Graded and Action Potentials
 Myleination and MS
 Reflexes
 Synapses
 EPSPs and IPSPs
 Neurotransmitters
The Reflex Arc
Types of Reflexes and Regulation
 Autonomic reflexes
• Smooth muscle regulation
• Heart and blood pressure regulation
• Regulation of glands
• Digestive system regulation
 Somatic reflexes
• Activation of skeletal muscles
Nervous Tissue and Function
 Function of the Nervous System
 Organization (Structural and Functional)
 Supporting Cells of the Nervous System
 Anatomy of a Neuron
 Classification of Neurons by Function
 Graded and Action Potentials
 Myleination and MS
 Reflexes
 Synapses
 EPSPs and IPSPs
 Neurotransmitters
Two Kinds of Synapses
 Chemical Synapses
• Specialized for the release and reception of neurotransmitters
• Typically composed of two parts
o Axon terminal of the presynaptic neuron, which contains synaptic vesicles
o Receptor region on the postsynaptic neuron
 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:
o Embryonic nervous tissue
o Some brain regions
How Neurons Communicate at Synapses
Events at the Synapse (online animation)
Narrated synapse (online)
 Irritability – ability to respond to stimuli
 Conductivity – ability to transmit an
impulse`
SodiumPotassium pump (online animation)
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.
Events at the Synapse (online animation)
Narrated synapse (online)
6 Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse.
SodiumPotassium pump (online animation)
Figure 11.17
Nervous Tissue and Function
 Function of the Nervous System
 Organization (Structural and Functional)
 Supporting Cells of the Nervous System
 Anatomy of a Neuron
 Classification of Neurons by Function
 Graded and Action Potentials
 Myleination and MS
 Reflexes
 Synapses
 EPSPs and IPSPs
 Neurotransmitters
Excitatory Synapses and EPSPs
Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na + and
K+ in opposite directions

Na+ influx is greater that K+ efflux, causing a net depolarization

Excitatory postsynaptic potential (EPSP) helps trigger AP at axon hillock if EPSP is of threshold strength
and opens the voltage-gated channels
Membrane potential (mV)

Threshold
An EPSP is a local
depolarization of the
postsynaptic membrane
that brings the neuron
closer to AP threshold.
Neurotransmitter binding
opens chemically gated
ion channels, allowing
the simultaneous passage of Na+ and K+.
Stimulus
Time (ms)
Figure 11.18a
Inhibitory Synapses and Inhibitory Postsynaptic Potential (IPSPs)
Neurotransmitter binds to and opens channels for K+ or Cl–

Causes a hyperpolarization (the inner surface of membrane becomes more negative)

Reduces the postsynaptic neuron’s ability to produce an action potential
Membrane potential (mV)

Threshold
An IPSP is a local
hyperpolarization of the
postsynaptic membrane
and drives the neuron
away from AP threshold.
Neurotransmitter binding
opens K+ or Cl– channels.
Stimulus
Time (ms)
Figure 11.18b
Nervous Tissue and Function
 Function of the Nervous System
 Organization (Structural and Functional)
 Supporting Cells of the Nervous System
 Anatomy of a Neuron
 Classification of Neurons by Function
 Graded and Action Potentials
 Myleination and MS
 Reflexes
 Synapses
 EPSPs and IPSPs
 Neurotransmitters
Chemical Classes of Neurotransmitters
 Acetylcholine (Ach) (Mostly excitory in CNS, PNS if prolonged prod. tetanus (with nerve
gases), receoptors destroyed in myasthenia gravis)
•
Released at neuromuscular junctions and some ANS neurons
•
Synthesized by enzyme choline acetyltransferase
•
Degraded by the enzyme acetylcholinesterase (AChE)
 Biogenic amines
o
Catecholamines
 Dopamine (“Feeling good” CNS neurotransmitter, uptake blocked by cocaine, excitory or inhibitory), norepinephrine
(NE), and epinephrine
o
Indolamines
 Serotonin (Roles in sleep, appetite, nausea, mood; blocked by seritonin-specific reuptake inhibitors (SSRIs) like
Prozac and LSD, enhanced by ecstasy (3,4-Methylenedioxymethamphetamine)), histamine
•
Broadly distributed in the brain; play roles in emotional behaviors and the biological clock
 Amino acids include:
o
GABA—Gamma ()-aminobutyric acid (inhibitory brain NT augmented by alcohol, benzodiazepine-valium)
o
Glutamate (excitory in CNS, causes stroke when overreleased, overstimulation of neurons) , Glycine, Aspartate
 Peptides (neuropeptides) include:
o
Substance P, endorphins, somatostatin, cholecystokinin
 Purines such as ATP
 Gases and lipids
•
NO, CO, Endocannabinoids