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Nervous Tissue and Neuron Function
Fundamentals Of The Nervous System And
Nervous Tissue
Learn and Understand
1. Like muscle cells, neurons use membrane polarity upset
(AP) as a signal therefore keeping their membranes
constantly ready (RMP).
2. Neuroglia help create and maintain the environmental
conditions necessary for optimal neuron functioning.
3. In order to carry their message, some neurons have axons
greater than 1 m in length.
4. Increasing the frequency of action potentials, not its
strength, is how the NS controls the intensity of its
message.
5. Graded potentials may sum to threshold depolarization
causing AP in the neuron. The source of graded potentials
is the up to 10,000 synapses with other neurons.
Functions of the Nervous System
Master controlling and communicating system of body
1. Sensory: Receiving internal and external sensory input.
2. Integration: Process and evaluate, coordinate and
control response
3. Motor: Generate response signals
A. Controlling muscles and glands
B. Maintaining homeostasis
Rapid and specific - usually causes almost immediate
responses
Establishing and maintaining mental activity,
consciousness, thinking, behavior, memory, emotion
Figure 11.1 The nervous system’s functions.
Sensory input
Integration
Motor output
Anatomic Divisions of the Nervous
System100 Billion Neurons
100 Million Neurons
CNS: Integration and
control center.
Interprets sensory
input and dictates
motor output
PNS:
Consists mainly of
nerves that extend
from brain and
spinal cord.
Cranial nerves to
and from brain.
Spinal nerves to and
from spinal cord.
Plexus – network of
sensory input, motor
output and
integration outside
of the CNS
Figure 11.2 Levels of organization in the nervous system.
Central nervous system (CNS)
Brain and spinal cord
Integrative and control centers
Peripheral nervous system (PNS)
Cranial nerves and spinal nerves
Communication lines between the CNS
and the rest of the body
Sensory (afferent) division
Somatic and visceral sensory
nerve fibers
Conducts impulses from
receptors to the CNS
Motor (efferent) division
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Somatic sensory fiber SkinSomatic nervous
system
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glandsVisceral sensory fiber
Motor fiber of somatic nervous system
StomachSkeletal
muscle
Sympathetic division
Mobilizes body systems
during activity
Parasympathetic
division
Conserves energy
Promotes house-
keeping functions
during rest
Sympathetic motor fiber of ANS Heart
Parasympathetic motor fiber of ANS Bladder
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
Histology of Nervous Tissue
• Highly cellular; little extracellular space
• Two principal cell types
– Neurons (nerve cells)—excitable cells that
transmit electrical signals
– Neuroglia – small cells that surround and wrap
delicate neurons• CNS:
– Astrocytes
– Microglial cells
– Ependymal cells
– Oligodendrocytes
• Satellite cells (PNS)
• Schwann cells (PNS)
Neurons
• Structural units of nervous system
• Large, highly specialized cells that conduct
impulses
• Extreme longevity (100 years or more)
• Amitotic—with few exceptions
• High metabolic rate—requires continuous
supply of oxygen and glucose
• All have cell body and one or more
processes
Dendrites(receptiveregions)
Cell body(biosynthetic centerand receptive region)
Nucleus
Nucleolus
Axon hillock
Chromatophilicsubstance (roughendoplasmicreticulum)
Axon(impulse-generatingand -conductingregion)
Impulsedirection
Schwann cell
Myelin sheath gap(node of Ranvier)
Terminal branches
Axonterminals(secretoryregion)
Soma = Biosynthetic center of neuron
Synthesizes proteins, membranes, and other chemicalsRough ER (chromatophilic substance or Nissl bodies)
Most active and best developed in body
Most neuron cell bodies in CNSNuclei are clusters of neuron cell bodies in CNS
DendritesConvey incoming messages toward cell body as graded potentials
Structure of a Motor Neuron
The Axon: Structure
• One axon per cell arising from axon hillock– Cone-shaped area of cell body
• In some, axon short or absent, in others most of length of cell
• Long axons called nerve fibers
• Occasional branches (axon collaterals)
• Branches profusely at end (terminus)– Can be 10,000 terminal branches
• Distal endings called axon terminals or terminal boutons, axon bulbs, presynaptic terminals
The Axon: Functional Characteristics
• Generates and conducts AP
• Transmits AP along axolemma to axon terminal Neurotransmitters released into extracellular space
• Synapsed with many other neurons at same time
• Lacks rough ER and Golgi apparatus– Relies on cell body to renew proteins and membranes
• Quickly decay if cut or damaged
Schwanncell plasmamembrane
Schwann cellcytoplasm
AxonSchwann cellnucleus
Myelinsheath
Schwann cell cytoplasm
Myelination of a nerve fiber (axon)
1
2
3
Figure 11.5a Nerve fiber
myelination by Schwann
cells in the PNS.
Segmented sheath
around most long or
large-diameter axonsMyelinated fibers
Function of myelinProtects and electrically
insulates axon
Increases speed of nerve
impulse transmission
Nonmyelinated fibers
conduct impulses more
slowly
Figure 11.5b Nerve fiber myelination by Schwann cells in the PNS.
Myelin sheath
Outer collar of perinuclear cytoplasm (of Schwann cell)
Cross-sectional view of a myelinated axon (electron
micrograph 24,000x)
Axon
Functional Classifications:
SensoryTransmit impulses from sensory receptors toward CNS
Cell bodies in ganglia in PNS – ganglion is a grouping of NCBs outside of the CNS
MotorCarry impulses from CNS to effectors
Most cell bodies in CNS (except some autonomic neurons)
Interneuron (association neuron)Lie between motor and sensory neurons
Shuttle signals through CNS pathways; most are entirely within CNS
99% of body's neurons
Functional Classification of Neurons
• Sensory– Transmit impulses from sensory receptors toward CNS
– Almost all are Unipolar
– Cell bodies in ganglia in PNS – ganglion is a grouping of NCBs outside of the CNS
• Motor– Carry impulses from CNS to effectors
– Multipolar
– Most cell bodies in CNS (except some autonomic neurons)
• Interneurons (association neurons)– Lie between motor and sensory neurons
– Shuttle signals through CNS pathways; most are entirely within CNS
– 99% of body's neurons
The Resting Membrane Potential
• Potential difference across membrane of resting
cell
– Approximately –70 mV in neurons
• Actual voltage difference varies from -40 mV to -90 mV
– Membrane termed polarized
• Generated by:
– Differences in ionic makeup of ICF and ECF • ECF has higher concentration of Na+ than ICF
– Balanced chiefly by chloride ions (Cl-)
• ICF has higher concentration of K+ than ECF– Balanced by negatively charged proteins
• K+ plays most important role in membrane potential
– Differential permeability of the plasma membrane
Measuring Membrane Potential in Neurons
Figure 11.6 Operation of gated channels.
Open and close to change which ions move across membrane and when.
One stimulated by messenger; one stimulated by electrical charge
Chemically gated ion channels Voltage-gated ion channels
Open in response to binding of the
appropriate neurotransmitter
Open in response to changes
in membrane potential
Receptor
Closed
Neurotransmitter chemical attached to receptor
Open Closed Open
Chemicalbinds
Membranevoltagechanges
Each Na+ channel has two voltage-sensitive gates
• Activation gates
• Closed at rest; open with depolarization allowing Na+ to enter cell
• Inactivation gates
• Open at rest; block channel once it is open to prevent more Na+ from entering cell
Differences in Plasma Membrane
Permeability
• Impermeable to large anionic proteins
• Slightly permeable to Na+ (through leakage channels)
– Sodium diffuses into cell down concentration gradient
• 25 times more permeable to K+ than sodium (more leakage channels)
– Potassium diffuses out of cell down concentration gradient
• Quite permeable to Cl–
Membrane Potential Changes
Used as Communication Signals
• Membrane potential changes when
– Concentrations of ions across membrane change
– Membrane permeability to ions changes
• Changes produce two types signals
– Graded potentials
• Incoming signals operating over short distances
• Mostly arrive at axodendritic and axosomatic synapses
• Collectively control the post-synaptic neuron
– Action potentials
• Long-distance signals of axons
Action Potentials (AP)
• Principle way neurons send signals
• Principal means of long-distance neural
communication
• Occur only in muscle cells and axons of
neurons
• Brief reversal of membrane potential with
a change in voltage of ~100 mV
• Do not decay over distance as graded
potentials do
Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of membrane that is
depolarized by local currents.
The big picture
Resting state1 2 Depolarization
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+30
0
–55
–70
Action
potential2
3
411
0 1 2 3 4
Threshold
Time (ms)
Repolarization
Hyperpolarization
3
4
The AP is caused by permeability changes in the
plasma membrane:
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–70
–55
+30
0
Time (ms)
Actionpotential
Na+
permeability
K+ permeability
Re
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me
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bilit
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0 1 2 3 4
411
2
3
Outside cell
Inside cell
Activationgate
Inactivationgate
Closed Opened Inactivated
The events
The key players
Voltage-gated Na+ channels
Closed Opened
Outside cell
Inside cell
Voltage-gated K+ channels
Sodiumchannel
Potassiumchannel
Activationgates
Inactivationgate
Resting state
Depolarization
Repolarization
Hyperpolarization
1
4
3
2
At threshold (–55 to –50 mV) positive feedback causes opening of all Na+
channels → a reversal of membrane polarity to +30mV
Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of
membrane that is depolarized by local currents. (1 of 3)
Resting state. No
ions move through
voltage-gated
channels.
Depolarization
is caused by Na+
flowing into the cell.
Repolarization is
caused by K+ flowing
out of the cell.
Hyperpolarization is
caused by K+ continuing to
leave the cell.Action
potential
Threshold
Time (ms)
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nti
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(mV
)
+30
0
–70
0 1 2 3 4
–55
1 2
1
2
3
4
3
4
1
Each K+ channel has one voltage-sensitive gate
• Closed at rest; Opens slowly with depolarization
Repolarization and hyperpolarization:
• Slow voltage-gated K+ channels open
• K+ exits the cell and internal negativity is restored
Role of the Sodium-Potassium Pump
• Repolarization resets electrical conditions,
not ionic conditions
• After repolarization Na+/K+ pumps
(thousands of them in an axon) restore
ionic conditions
Me
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nti
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V)
+30
–70
Voltageat 0 ms
Recordingelectrode
Time = 0 ms. Action potential hasnot yet reached the recording electrode.
Resting potential
Peak of action potential
Hyperpolarization
Figure 11.12a Propagation of an action potential (AP).
Na+ influx causes local
currents
Local currents cause
depolarization of adjacent
membrane areas in
direction away from AP
origin (toward axon's
terminals)
Figure 11.12b Propagation of an action potential (AP).
Me
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ran
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nti
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+30
–70
Voltageat 2 ms
Resting potential
Peak of action potential
Hyperpolarization
Time = 2 ms. Action potentialpeak reaches the recording electrode.
Since Na+ channels closer to
AP origin are inactivated no new
AP is generated there
Once initiated an AP is self-
propagating
Figure 11.12c Propagation of an action potential (AP).
Me
mb
ran
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ote
nti
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V)
+30
–70
Voltageat 4 ms
Resting potential
Peak of action potential
Hyperpolarization
Time = 4 ms. Action potentialpeak has passed the recordingelectrode. Membrane at therecording electrode is stillhyperpolarized.
AP to propagates AWAY from the AP
origin
Absolute and Relative Refractory Periods
• a period when a neuron is unable to respond to a new
stimulus or is less responsive to stimulus
• Absolute refractory period
– Time from opening of Na+ channels until resetting of the
channels
– Ensures that each AP is an all-or-none event
– Enforces one-way transmission of nerve impulses
• Relative refractory period
– Follows absolute refractory period
• Most Na+ channels have returned to their resting state
• Some K+ channels still open
• Repolarization is occurring
– Threshold for AP generation is elevated
• Inside of membrane more negative than resting state
Figure 11.15 Action potential propagation in nonmyelinated and myelinated axons.
Stimulus Size of voltage
In bare plasma membranes, voltage decays. Without voltage-gated channels, as on a dendrite,voltage decays because current leaks across themembrane.
Stimulus Voltage-gatedion channel
In nonmyelinated axons, conduction is slow(continuous conduction). Voltage-gated Na+ and K+
channels regenerate the action potential at each pointalong the axon, so voltage does not decay. Conductionis slow because it takes time for ions and for gates ofchannel proteins to move, and this must occur beforevoltage can be regenerated.
Stimulus Myelinsheath
Myelinsheath gap
Myelinsheath
In myelinated axons, conduction is fast (saltatoryconduction). Myelin keeps current in axons(voltage doesn’t decay much). APs are generated onlyin the myelin sheath gaps and appear to jump rapidlyfrom gap to gap.
1 mm
Saltatory conduction is about
30 times faster
Group C fibers
Group A & B
fibers
‘receptive zone’ graded potentials
Nerve Fiber Classification
• Group A fibers
– Large diameter, myelinated somatic sensory and
motor fibers of skin, skeletal muscles, joints
– Transmit at 150 m/s
• Group B fibers
– Intermediate diameter, lightly myelinated fibers
– Transmit at 15 m/s
• Group C fibers
– Smallest diameter, unmyelinated ANS fibers
– Transmit at 1 m/s
Coding for Stimulus Intensity
• All action potentials are alike and are
independent of stimulus intensity
– How does CNS tell difference between a weak
stimulus and a strong one?
• Strong stimuli cause action potentials to occur
more frequently
– # Of impulses per second or frequency of APs
• CNS determines stimulus intensity by the
frequency of impulses
– Higher frequency means stronger stimulus
Figure 11.13 Relationship between stimulus strength and action potential frequency.
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+30
–70
Actionpotentials
Sti
mu
lus
vo
lta
ge Threshold
Stimulus
Time (ms)
0
Synapses
Synapse Classification
• Axodendritic—between axon terminals of
one neuron and dendrites of others
• Axosomatic—between axon terminals of
one neuron and soma of others
• Less common types:
– Axoaxonal (axon to axon)
– Dendrodendritic (dendrite to dendrite)
– Somatodendritic (dendrite to soma)
Important Terminology
• Presynaptic neuron
– Neuron conducting impulses toward synapse
– Sends the information
• Postsynaptic neuron (in PNS may be a
neuron, muscle cell, or gland cell)
– Neuron transmitting electrical signal away
from synapse
– Receives the information
• Most function as both
Figure 11.16 Synapses.
Axodendritic
synapses
Dendrites
Cell body
Axoaxonal
synapses
Axon
Axosomatic
synapses
Axon
Axosomatic
synapses
Cell body (soma)
of postsynaptic
neuron
Varieties of Synapses: Electrical Synapses
• Less common than chemical synapses
– Neurons electrically coupled (joined by gap
junctions that connect cytoplasm of adjacent
neurons)
• Communication very rapid
• May be unidirectional or bidirectional
• Synchronize activity
– More abundant in:
• Embryonic nervous tissue
• Cardiac muscle
• Nerve impulse remains electrical
Varieties of Synapses: Chemical Synapses
• Specialized for release and reception of chemical neurotransmitters
• Typically composed of two parts – Axon terminal of presynaptic neuron
• Contains synaptic vesicles filled with neurotransmitter
– Neurotransmitter receptor region on postsynaptic neuron's membrane
• Usually on dendrite or cell body
• Two parts separated by synaptic cleft– Fluid-filled space
• Electrical impulse changed to chemical across synapse, then back into electrical
Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynapticneuron
Action potentialarrives at axonterminal.
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Ca2+ entrycauses synapticvesicles to releaseneurotransmitterby exocytosis
Neurotransmitter diffusesacross the synaptic cleft andbinds to specific receptors onthe postsynaptic membrane.
Mitochondrion
Axon terminal
Synapticcleft
Synapticvesicles
Postsynapticneuron
Postsynapticneuron
Presynapticneuron
1
2
3
4
Enzymaticdegradation
Diffusion awayfrom synapse
Reuptake
Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Ion movement
Graded potential
Binding of neurotransmitter opension channels, resulting in gradedpotentials.
5
Synaptic Delay
• Time needed for neurotransmitter to be
released, diffuse across synapse, and bind
to receptors
0.3–5.0 ms
• Synaptic delay is rate-limiting step of
neural transmission
Neurotransmitters
• Language of nervous system
• 50 or more neurotransmitters have been
identified
• Most neurons make two or more
neurotransmitters
– Neurons can exert several influences
• Usually released at different stimulation
frequencies
• Classified by chemical structure and by
function
Classification of Neurotransmitters:
Function
• Effects - excitatory versus inhibitory
– Neurotransmitter effects can be excitatory
(depolarizing) and/or inhibitory
(hyperpolarizing)
– Effect determined by receptor to which it binds
• Acetylcholine and NE bind to at least two receptor
types with opposite effects
– ACh excitatory at neuromuscular junctions in skeletal
muscle
– ACh inhibitory in cardiac muscle
Figure 11.20 Direct neurotransmitter receptor mechanism: Channel-linked receptors.
Ion flow blocked
Closed ion
channel
Ligand
Ions flow
Open ion
channel
Direct action
Neurotransmitter binds to and opens ion channels
Promotes rapid responses by altering membrane potential
Examples: ACh and amino acids
Graded Potentials
• Short-lived, localized changes in membrane
potential
– Magnitude varies with stimulus strength
– Stronger stimulus → more voltage changes; farther
current flows
• Either depolarization or hyperpolarization
• Triggered by stimulus that opens gated ion
channels
• Current flows but dissipates quickly and decays
– Graded potentials are signals only over short
distances
Figure 11.9a Depolarization and hyperpolarization of the membrane.
Depolarizing stimulus
Insidepositive
Insidenegative
Depolarization
Restingpotential
Mem
bra
ne p
ote
nti
al
(volt
age,
mV
)
Depolarization: The membrane potentialmoves toward 0 mV, the inside becoming lessnegative (more positive).
Time (ms)
+50
0
–50
–70
–1000 1 2 3 4 5 6 7
Decrease in membrane potential
(toward zero and above)
Inside of membrane becomes less
negative than resting membrane
potential
Increases probability of producing a
nerve impulse
Figure 11.9b Depolarization and hyperpolarization of the membrane.
Hyperpolarizing stimulus
Mem
bra
ne p
ote
nti
al
(volt
age,
mV
)
Time (ms)
+50
0
–50
–70
–1000 1 2 3 4 5 6 7
Hyperpolarization: The membrane potential
increases, the inside becoming more negative.
Restingpotential
Hyper-polarizationAn increase in membrane
potential (away from zero)
Inside of cell more negative than
resting membrane potential)
Reduces probability of
producing a nerve impulse
Excitatory Synapses and EPSPs
• Neurotransmitter binding opens chemically
gated channels
• Allows simultaneous flow of Na+ and K+ in opposite
directions
• Na+ influx greater than K+ efflux → net
depolarization called EPSP (not AP)
• EPSP help trigger AP if EPSP is of threshold
strength
– Can spread to axon hillock, trigger opening of
voltage-gated channels, and cause AP to be
generated
Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory.
An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing Na+ and K+ to pass through simultaneously.
Threshold
Stimulus
+30
0
–55
–70
Time (ms)
10 20 30
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Excitatory postsynaptic potential (EPSP)
Inhibitory Synapses and IPSPs
• Reduces postsynaptic neuron's ability to
produce an action potential
– Makes membrane more permeable to K+ or
Cl–
• If K+ channels open, it moves out of cell
• If Cl- channels open, it moves into cell
– Therefore neurotransmitter hyperpolarizes cell
• Inner surface of membrane becomes more
negative
• AP less likely to be generated
Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory.
Threshold
Stimulus
+30
0
–55
–70
Time (ms)
10 20 30
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An IPSP is a localhyperpolarization of the postsynaptic membranethat drives the neuronaway from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.
Inhibitory postsynaptic potential (IPSP)
Synaptic Integration: Summation
• A single EPSP cannot induce an AP
• EPSPs can summate to influence
postsynaptic neuron
• IPSPs can also summate
• Temporal summation
• Spatial summation
• Most neurons receive both excitatory and
inhibitory inputs from thousands of other
neurons
– Only if EPSP's predominate and bring to
threshold → AP
Postsynaptic Potentials and Their Summation
Temporal Summation
Spatial Summation
Integration of EPSPs and IPSPs
Integration: Presynaptic Inhibition
• Excitatory neurotransmitter release by one
neuron inhibited by another neuron via an
axoaxonal synapse
• Less neurotransmitter released
• Smaller EPSPs formed
Additional Slides
• May not be shown on screen in class
Capillary
Neuron
Astrocyte
Astrocytes are the most abundant CNS neuroglia.
1. Support and brace neurons2. Play role in exchanges between capillaries and neurons3. Guide migration of young neurons4. Control chemical environment around neurons
Neuron
Microglial
cell
Microglial cells are defensive cells in the CNS.
1. Migrate toward injured neurons
2. Can transform to phagocytize microorganisms and
neuronal debris
Fluid-filled cavityCilia
Ependymal
cells
Brain or
spinal cord
tissue
Ependymal cells line cerebrospinal fluid–filled cavities.
1. Range in shape from squamous to columnar
2. May be ciliated - Cilia beat to circulate CSF
3. Line the central cavities of the brain and spinal column
4. Form permeable barrier between CSF in cavities and tissue
fluid bathing CNS cells
Myelin sheath
Process of
oligodendrocyte
Nerve
fibers
Oligodendrocytes have processes that form myelin
sheaths around CNS nerve fibers.
1.Branched cells
2.Processes wrap CNS nerve fibers, forming
insulating myelin sheaths thicker nerve fibers
Satellite
cells Cell body of neuron
Schwann cells
(forming myelin sheath)
Nerve fiber
Satellite cells and Schwann cells (which form myelin)
surround neurons in the PNS.
Satellite cells
Surround neuron cell bodies in PNS
Function similar to astrocytes of CNS
Schwann cells (neurolemmocytes)
Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve
fibers
Similar function as oligodendrocytes
Vital to regeneration of damaged peripheral nerve fibers
