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Chapter 12
Nervous Tissue
Lecture Outline
Principles of Human Anatomy and Physiology,
11e
1
INTRODUCTION
 The nervous system, along with
the endocrine system, helps to
keep controlled conditions
within limits that maintain health
and helps to maintain
homeostasis.
 The nervous system is
responsible for all our
behaviors, memories, and
movements.
 The branch of medical science
that deals with the normal
functioning and disorders of the
nervous system is called
neurology.
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Chapter 12
Nervous Tissue
 Controls and integrates all body activities
within limits that maintain life
 Three basic functions

sensing changes with sensory receptors



fullness of stomach or sun on your face
interpreting and remembering those changes
reacting to those changes with effectors


muscular contractions
glandular secretions
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Major Structures of the Nervous System
 Brain, cranial nerves, spinal cord, spinal nerves,
ganglia, enteric plexuses and sensory receptors
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Structures of the Nervous System - Overview
 Twelve pairs of cranial nerves emerge from the base of
the brain through foramina of the skull.

A nerve is a bundle of hundreds or thousands of axons, each
of which courses along a defined path and serves a specific
region of the body.
 The spinal cord connects to the brain through the foramen
magnum of the skull and is encircled by the bones of the
vertebral column.

Thirty-one pairs of spinal nerves emerge from the spinal
cord, each serving a specific region of the body.
 Ganglia, located outside the brain and spinal cord, are
small masses of nervous tissue, containing primarily cell
bodies of neurons.
 Enteric plexuses help regulate the digestive system.
 Sensory receptors are either parts of neurons or
specialized cells that monitor changes in the internal or
external environment.
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12 pair of cranial nerves
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Cranial Nerves
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Cranial Nerve Names
On
Olfactory CN I
Old
Optical CN II
Olympus’
Occulomotor CN III
Towering
Trochlear CN IV
Top
Trigeminal CN V
A
Abducens CN VI
Fat
Facial CN VII
Vivacous
Vestibulocochlear CN VIII
Goat
Glossopharyngeal CN IX
Victomized
Vagus CN X
A
Accessory CN XI
Hat
Hypoglossal CN XII
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Functions of the Nervous Systems
 The sensory function of the nervous system is to
sense changes in the internal and external
environment through sensory receptors.

Sensory (afferent) neurons serve this function.
 The integrative function is to analyze the sensory
information, store some aspects, and make decisions
regarding appropriate behaviors.

Association or interneurons serve this function.
 The motor function is to respond to stimuli by
initiating action.

Motor(efferent) neurons serve this function.
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Nervous System Divisions
 Central nervous system (CNS)

consists of the brain and spinal cord
 Peripheral nervous system (PNS)


consists of cranial and spinal nerves that contain
both sensory and motor fibers
connects CNS to muscles, glands & all sensory
receptors
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Subdivisions of the PNS
 Somatic (voluntary) nervous system (SNS)
 neurons from cutaneous and special sensory
receptors to the CNS
 motor neurons to skeletal muscle tissue
 Autonomic (involuntary) nervous systems
 sensory neurons from visceral organs to CNS
 motor neurons to smooth & cardiac muscle and
glands


sympathetic division (speeds up heart rate)
parasympathetic division (slow down heart rate)
 Enteric nervous system (ENS)
 involuntary sensory & motor neurons control GI
tract
 neurons function independently of ANS & CNS
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Organization of the Nervous System
 CNS is brain and spinal cord
 PNS is everything else
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Enteric NS
 The enteric nervous system (ENS) consists of
neurons in enteric plexuses that extend the
length of the GI tract.


Many neurons of the enteric plexuses function
independently of the ANS and CNS.
Sensory neurons of the ENS monitor chemical
changes within the GI tract and stretching of
its walls, whereas enteric motor neurons
govern contraction of GI tract organs, and
activity of the GI tract endocrine cells.
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Neuronal Structure & Function
14
Neurons
 Functional unit of nervous system
 Have capacity to produce action potentials
electrical excitability
 Cell body
 single nucleus with prominent nucleolus
 Nissl bodies (chromatophilic substance)
 rough ER & free ribosomes for protein
synthesis
 neurofilaments give cell shape and support
 microtubules move material inside cell
 lipofuscin pigment clumps (harmless aging)
 Cell processes = dendrites & axons

15
Parts of a Neuron
Neuroglial cells
Nucleus with
Nucleolus
Axons or
Dendrites
Cell body
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Rat Neuron
17
18
Cell membrane
 The dendrites are the receiving or input
portions of a neuron.
 The axon conducts nerve impulses from the
neuron to the dendrites or cell body of
another neuron or to an effector organ of the
body (muscle or gland).
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Dendrites
 Conducts impulses
towards the cell body
 Typically short, highly
branched &
unmyelinated
 Surfaces specialized
for contact with other
neurons
 Contains neurofibrils
& Nissl bodies
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Axons
 Conduct impulses away





from cell body
Long, thin cylindrical
process of cell
Arises at axon hillock
Impulses arise from initial
segment (trigger zone)
Side branches (collaterals)
end in fine processes called
axon terminals
Swollen tips called synaptic
end bulbs contain vesicles
filled with neurotransmitters
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Axonal Transport
 Cell body is location for most protein synthesis
 neurotransmitters & repair proteins
 Axonal transport system moves substances
 slow axonal flow



movement in one direction only -- away from cell body
movement at 1-5 mm per day
fast axonal flow




moves organelles & materials along surface of
microtubules
at 200-400 mm per day
transports in either direction
for use or for recycling in cell body
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Axonal Transport & Disease
 Fast axonal transport route by which
toxins or pathogens reach neuron cell
bodies


tetanus (Clostridium tetani bacteria)
disrupts motor neurons causing painful
muscle spasms
 Bacteria enter the body through a
laceration or puncture injury

more serious if wound is in head or neck
because of shorter transit time
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Diversity in Neurons
 Both structural and functional features are used to
classify the various neurons in the body.
 On the basis of the number of processes extending
from the cell body (structure), neurons are classified
as multipolar, biopolar, and unipolar (Figure 12.4).
 Most neurons in the body are interneurons and are
often named for the histologist who first described
them or for an aspect of their shape or appearance.
Examples are Purkinje cells (Figure 12.5a) or
Renshaw cells (Figure 12.5b).
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Structural Classification of Neurons
 Based on number of processes found on cell body
 multipolar = several dendrites & one axon
 most common cell type
 bipolar neurons = one main dendrite & one axon
 found in retina, inner ear & olfactory
 unipolar neurons = one process only(develops from a
bipolar)
 are always sensory neurons
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Functional Classification of Neurons
 Sensory (afferent) neurons

transport sensory information from skin,
muscles, joints, sense organs & viscera
to CNS
 Motor (efferent) neurons

send motor nerve impulses to muscles &
glands
 Interneurons (association) neurons


connect sensory to motor neurons
90% of neurons in the body
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Association or Interneurons
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Neuroglial Cells
 Half of the volume of the CNS
 Smaller cells than neurons
 50X more numerous
 Cells can divide

rapid mitosis in tumor formation (gliomas)
 4 cell types in CNS

astrocytes, oligodendrocytes, microglia &
ependymal
 2 cell types in PNS

schwann and satellite cells
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Astrocytes
 Star-shaped cells
 Form blood-brain
barrier by
covering blood
capillaries
 Metabolize
neurotransmitters
 Regulate K+
balance
 Provide structural
support
29
Microglia
 Small cells found
near blood vessels
 Phagocytic role -clear away dead cells
 Derived from cells
that also gave rise to
macrophages &
monocytes
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Ependymal cells
 Form epithelial
membrane lining
cerebral cavities &
central canal
 Produce
cerebrospinal fluid
(CSF)
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Satellite Cells
 Flat cells
surrounding
neuronal cell
bodies in peripheral
ganglia
 Support neurons in
the PNS ganglia
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Oligodendrocytes
 Most common glial
cell type
 Each forms myelin
sheath around more
than one axons in
CNS
 Analogous to
Schwann cells of
PNS
33
Myelination
 A multilayered lipid and protein covering
called the myelin sheath and produced by
Schwann cells and oligodendrocytes
surrounds the axons of most neurons (Figure
12.8a).
 The sheath electrically insulates the axon and
increases the speed of nerve impulse
conduction.
34
Schwann
Cell
 Cells encircling PNS axons
 Each cell produces part of the myelin sheath
surrounding an axon in the PNS
35
Axon Coverings in PNS
 All axons surrounded by a lipid &
protein covering (myelin sheath)
produced by Schwann cells
 Neurilemma is cytoplasm & nucleus
of Schwann cell
 gaps called nodes of Ranvier
 Myelinated fibers appear white
 jelly-roll like wrappings made of
lipoprotein = myelin
 acts as electrical insulator
 speeds conduction of nerve
impulses
 Unmyelinated fibers
 slow, small diameter fibers
 only surrounded by neurilemma
but no myelin sheath wrapping
36
Myelination
in PNS
 Schwann cells myelinate (wrap around) axons in the PNS
during fetal development
 Schwann cell cytoplasm & nucleus forms outermost layer
of neurolemma with inner portion being the myelin sheath
 Tube guides growing axons that are repairing themselves
37
Myelination in the CNS
 Oligodendrocytes myelinate axons in the CNS
 Broad, flat cell processes wrap about CNS axons,
but the cell bodies do not surround the axons
 No neurilemma is formed
 Little regrowth after injury is possible due to the
lack of a distinct tube or neurilemma
38
Gray and
White
Matter
 White matter = myelinated processes (white in color)
 Gray matter = nerve cell bodies, dendrites, axon
terminals, bundles of unmyelinated axons and neuroglia
(gray color)


In the spinal cord = gray matter forms an H-shaped inner
core surrounded by white matter
In the brain = a thin outer shell of gray matter covers the
surface & is found in clusters called nuclei inside the CNS
 A nucleus is a mass of nerve cell bodies and dendrites
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inside the CNS.
Electrical Signals in Neurons
 Neurons are electrically excitable due to
the voltage difference across their
membrane
 Communicate with 2 types of electric
signals


action potentials that can travel long
distances
graded potentials that are local
membrane changes only
 In living cells, a flow of ions occurs
through ion channels in the cell
membrane
40
Two Types of Ion Channels
 Leakage (nongated) channels are always open



nerve cells have more K+ than Na+ leakage
channels
as a result, membrane permeability to K+ is higher
explains resting membrane potential of -70mV in
nerve tissue
 Gated channels open and close in response to a
stimulus

results in neuron excitability
41
Ion Channels
 Gated ion channels respond to voltage
changes, ligands (chemicals), and
mechanical pressure.



Voltage-gated channels respond to a direct
change in the membrane potential (Figure
12.10a).
Ligand-gated channels respond to a specific
chemical stimulus (Figure 12.10b).
Mechanically gated ion channels respond to
mechanical vibration or pressure.
42
Gated Ion Channels
43
Resting Membrane Potential
 Negative ions along inside of cell membrane & positive ions
along outside


potential energy difference at rest is -70 mV
cell is “polarized”
 Resting potential exists because
 concentration of ions different inside & outside
 extracellular fluid rich in Na+ and Cl
 cytosol full of K+, organic phosphate & amino acids
 membrane permeability differs for Na+ and K+
 50-100 greater permeability for K+
 inward flow of Na+ can’t keep up with outward flow of K+
 Na+/K+ pump removes Na+ as fast as it leaks in
44
45
Graded
Potentials
 Small deviations from resting
potential of -70mV


hyperpolarization =
membrane has become
more negative
depolarization = membrane
has become more positive
 The signals are graded,
meaning they vary in
amplitude (size), depending
on the strength of the
stimulus and localized.
 Graded potentials occur
most often in the dendrites
and cell body of a neuron.
46
How do Graded Potentials Arise?
 Source of stimuli


mechanical stimulation of membranes with mechanical
gated ion channels (pressure)
chemical stimulation of membranes with ligand gated
ion channels (neurotransmitter)
 Graded/postsynaptic/receptor or generator potential


ions flow through ion channels and change membrane
potential locally
amount of change varies with strength of stimuli
 Flow of current (ions) is local change only
47
Generation of an Action Potential
 An action potential (AP) or impulse is a sequence of
rapidly occurring events that decrease and eventually
reverse the membrane potential (depolarization) and
then restore it to the resting state (repolarization).

During an action potential, voltage-gated Na+ and K+
channels open in sequence (Figure 12.13).
 According to the all-or-none principle, if a
stimulus reaches threshold, the action potential is
always the same.

A stronger stimulus will not cause a larger impulse.
48
Action Potential
 Series of rapidly occurring events that change and then
restore the membrane potential of a cell to its resting state
 Ion channels open, Na+ rushes in (depolarization), K+
rushes out (repolarization)
 All-or-none principal = with stimulation, either happens one
specific way or not at all (lasts 1/1000 of a second)
 Travels (spreads) over surface of cell without dying out
49
Depolarizing Phase
of Action Potential
 Chemical or mechanical stimulus
caused a graded potential to reach
at least (-55mV or threshold)
 Voltage-gated Na+ channels open
& Na+ rushes into cell




in resting membrane, inactivation gate of sodium channel is
open & activation gate is closed (Na+ can not get in)
when threshold (-55mV) is reached, both open & Na+ enters
inactivation gate closes again in few ten-thousandths of
second
only a total of 20,000 Na+ actually enter the cell, but they
change the membrane potential considerably(up to +30mV)
 Positive feedback process
50
Repolarizing Phase
of Action Potential
 When threshold potential of





-55mV is reached, voltage-gated
K+ channels open
K+ channel opening is much
slower than Na+ channel
opening which caused depolarization
When K+ channels finally do open, the Na+ channels have
already closed (Na+ inflow stops)
K+ outflow returns membrane potential to -70mV
If enough K+ leaves the cell, it will reach a -90mV membrane
potential and enter the after-hyperpolarizing phase
K+ channels close and the membrane potential returns to the
resting potential of -70mV
51
Refractory Period of
Action Potential
 Period of time during which
neuron can not generate
another action potential
 Absolute refractory period



even very strong stimulus will
not begin another AP
inactivated Na+ channels must return to the resting state
before they can be reopened
large fibers have absolute refractory period of 0.4 msec
and up to 1000 impulses per second are possible
 Relative refractory period
 a suprathreshold stimulus will be able to start an AP
 K+ channels are still open, but Na+ channels have closed
52
The Action
Potential:
Summarized
 Resting membrane
potential is -70mV
 Depolarization is
the change from 70mV to +30 mV
 Repolarization is
the reversal from
+30 mV back to -70
mV)
53
Local Anesthetics
 Local anesthetics and certain neurotoxins


Prevent opening of voltage-gated Na+
channels
Nerve impulses cannot pass the
anesthetized region
Examples:
 Novocaine and lidocaine
54
Propagation of Action Potential
 An action potential spreads (propagates) over
the surface of the axon membrane


as Na+ flows into the cell during
depolarization, the voltage of adjacent areas
is effected and their voltage-gated Na+
channels open
self-propagating along the membrane
 The traveling action potential is called a nerve
impulse
55
Continuous versus Saltatory Conduction
 Continuous conduction (unmyelinated fibers)

step-by-step depolarization of each portion of the
length of the axolemma
 Saltatory conduction


depolarization only at nodes of Ranvier where
there is a high density of voltage-gated ion
channels
current carried by ions flows through extracellular
fluid from node to node
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Saltatory Conduction
 Nerve impulse conduction in which the impulse jumps
from node to node
57
Speed of Impulse Propagation
 The propagation speed of a nerve impulse is not
related to stimulus strength.

larger, myelinated fibers conduct impulses faster
due to size & saltatory conduction
 Fiber types

A fibers largest (5-20 microns & 130 m/sec)


B fibers medium (2-3 microns & 15 m/sec)


myelinated somatic sensory & motor to skeletal muscle
myelinated visceral sensory & autonomic preganglionic
C fibers smallest (.5-1.5 microns & 2 m/sec)

unmyelinated sensory & autonomic motor
58
Encoding of Stimulus Intensity
 How do we differentiate a light touch from
a firmer touch?

frequency of impulses


firm pressure generates impulses at a higher
frequency
number of sensory neurons activated

firm pressure stimulates more neurons than
does a light touch
59
Action Potentials in Nerve and Muscle
 Entire muscle cell membrane versus only the
axon of the neuron is involved
 Resting membrane potential


nerve is -70mV
skeletal & cardiac muscle is closer to -90mV
 Duration


nerve impulse is 1/2 to 2 msec
muscle action potential lasts 1-5 msec for skeletal
& 10-300msec for cardiac & smooth
 Fastest nerve conduction velocity is 18 times
faster than velocity over skeletal muscle fiber
60
SIGNAL TRANSMISSION AT
SYNAPSES
 A synapse is the functional junction between
one neuron and another or between a neuron
and an effector such as a muscle or gland.
61
Signal Transmission at
Synapses
 2 Types of synapses

electrical



ionic current spreads to next cell through gap junctions
faster, two-way transmission & capable of synchronizing
groups of neurons
chemical

one-way information transfer from a presynaptic neuron
to a postsynaptic neuron
 axodendritic -- from axon to dendrite
 axosomatic -- from axon to cell body
 axoaxonic -- from axon to axon
62
63
Chemical Synapses
 Action potential reaches end bulb




and voltage-gated Ca+ 2 channels
open
Ca+2 flows inward triggering
release of neurotransmitter
Neurotransmitter crosses synaptic
cleft & binding to ligand-gated
receptors
 the more neurotransmitter
released the greater the change
in potential of the postsynaptic
cell
Synaptic delay is 0.5 msec
One-way information transfer
64
Excitatory & Inhibitory Potentials
 The effect of a neurotransmitter can be either
excitatory or inhibitory

a depolarizing postsynaptic potential is called an
EPSP



it results from the opening of ligand-gated Na+ channels
the postsynaptic cell is more likely to reach threshold
an inhibitory postsynaptic potential is called an IPSP



it results from the opening of ligand-gated Cl- or K+
channels
it causes the postsynaptic cell to become more negative
or hyperpolarized
the postsynaptic cell is less likely to reach threshold 65
Removal of Neurotransmitter
 Diffusion

move down concentration
gradient
 Enzymatic degradation

acetylcholinesterase
 Uptake by neurons or glia
cells


neurotransmitter
transporters
Prozac = serotonin
reuptake
inhibitor
66
Three Possible Responses
 Small EPSP occurs

potential reaches -56 mV only
 An impulse is generated


threshold was reached
membrane potential of at least -55 mV
 IPSP occurs


membrane hyperpolarized
potential drops below -70 mV
67
Comparison of Graded & Action
Potentials
 Origin


GPs arise on dendrites and cell bodies
APs arise only at trigger zone on axon hillock
 Types of Channels


AP is produced by voltage-gated ion channels
GP is produced by ligand or mechanicallygated channels
 Conduction


GPs are localized (not propagated)
APs conduct over the surface of the axon
68
Comparison of Graded & Action
Potentials
 Amplitude


amplitude of the AP is constant (all-or-none)
graded potentials vary depending upon stimulus
 Duration

The duration of the GP is as long as the
stimulus lasts
 Refractory period

The AP has a refractory period due to the
nature of the voltage-gated channels, and the
GP has none.
69
Summation
 If several presynaptic end bulbs release their
neurotransmitter at about the same time, the
combined effect may generate a nerve
impulse due to summation
 Summation may be spatial or temporal.
70
Spatial Summation
 Summation of effects of
neurotransmitters
released from several
end bulbs onto one
neuron
71
Temporal Summation
 Summation of effect of
neurotransmitters released
from 2 or more firings of the
same end bulb in rapid
succession onto a second
neuron
72
Summation
 The postsynaptic neuron is an integrator, receiving
and integrating signals, then responding.
 If the excitatory effect is greater than the inhibitory
effect but less that the threshold level of stimulation,
the result is a subthreshold EPSP, making it easier to
generate a nerve impulse.
 If the excitatory effect is greater than the inhibitory
effect and reaches or surpasses the threshold level of
stimulation, the result is a threshold or suprathreshold
EPSP and a nerve impulse.
 If the inhibitory effect is greater than the excitatory
effect, the membrane hyperpolarizes (IPSP) with
73
failure to produce a nerve impulse.
Neurotransmitters
 Both excitatory and inhibitory
neurotransmitters are present in the CNS and
PNS; the same neurotransmitter may be
excitatory in some locations and inhibitory in
others.
 Important neurotransmitters include
acetylcholine, glutamate, aspartate, gamma
aminobutyric acid, glycine, norepinephrine,
epinephrine, and dopamine.
74
Neurotransmitter Effects
 Neurotransmitter effects can be modified




synthesis can be stimulated or inhibited
release can be blocked or enhanced
removal can be stimulated or blocked
receptor site can be blocked or activated
 Agonist

anything that enhances a transmitters effects
 Antagonist

anything that blocks the action of a neurotranmitter
75
Small-Molecule Neurotransmitters
 Acetylcholine (ACh)



released by many PNS neurons & some CNS
excitatory on NMJ but inhibitory at others
inactivated by acetylcholinesterase
 Amino Acids


glutamate released by nearly all excitatory neurons
in the brain ---- inactivated by glutamate specific
transporters
GABA is inhibitory neurotransmitter for 1/3 of all
brain synapses (Valium is a GABA agonist -enhancing its inhibitory effect)
76
Small-Molecule Neurotransmitters
 Biogenic Amines

modified amino acids (tyrosine)




norepinephrine -- regulates mood,
dreaming, awakening from deep sleep
dopamine -- regulating skeletal muscle
tone
serotonin -- control of mood, temperature
regulation, & induction of sleep
removed from synapse & recycled or
destroyed by enzymes (monoamine
oxidase or catechol-0-methyltransferase)
77
Small-Molecule Neurotransmitters
 ATP and other purines (ADP, AMP & adenosine)


excitatory in both CNS & PNS
released with other neurotransmitters (ACh & NE)
 Gases (nitric oxide or NO)


formed from amino acid arginine by an enzyme
formed on demand and acts immediately



diffuses out of cell that produced it to affect neighboring
cells
may play a role in memory & learning
first recognized as vasodilator that helps lower
blood pressure
78
Neuropeptides
 3-40 amino acids linked by peptide bonds
 Substance P -- enhances our perception of
pain
 Pain relief

enkephalins -- pain-relieving effect by blocking
the release of substance P (bind to morphine
receptors in the central nervous system and have
opioid properties)

acupuncture may produce loss of pain
sensation because of release of opioids-like
substances such as endorphins or dynorphins
79
Strychnine Poisoning
 In spinal cord, Renshaw cells normally release
an inhibitory neurotransmitter (glycine) onto
motor neurons preventing excessive muscle
contraction
 Strychnine binds to and blocks glycine receptors
in the spinal cord
 Massive tetanic contractions of all skeletal
muscles are produced

when the diaphragm contracts & remains
contracted, breathing can not occur
80
Neuronal Circuits
 Neuronal pools are organized into circuits
(neural networks.) These include simple
series, diverging, converging, reverberating,
and parallel after-discharge circuits (Figure
12.18 a-d).
 A neuronal network may contain thousands
or even millions of neurons.
 Neuronal circuits are involved in many
important activities



breathing
short-term memory
waking up
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Neuronal
Circuits
 Diverging -- single cell stimulates many others
 Converging -- one cell stimulated by many others
 Reverberating -- impulses from later cells repeatedly
stimulate early cells in the circuit (short-term memory)
 Parallel-after-discharge -- single cell stimulates a group of
cells that all stimulate a common postsynaptic cell (math82
problems)
Regeneration & Repair
 Plasticity maintained throughout life



sprouting of new dendrites
synthesis of new proteins
changes in synaptic contacts with other
neurons
 Limited ability for regeneration (repair)


PNS can repair damaged dendrites or axons
CNS no repairs are possible
83
Damage and Repair in the
Peripheral Nervous System
 When there is damage to an axon, usually there are
changes, called chromatolysis, which occur in the cell body
of the affected cell; this causes swelling of the cell body
and peaks between 10 and 20 days after injury.
 By the third to fifth day, degeneration of the distal portion of
the neuronal process and myelin sheath (Wallerian
degeneration) occurs; afterward, macrophages phagocytize
the remains.
 Retrograde degeneration of the proximal portion of the fiber
extends only to the first neurofibral node.
 Regeneration follows chromatolysis; synthesis of RNA and
protein accelerates, favoring rebuilding of the axon and
often taking several months.
84
Repair within the PNS
 Axons & dendrites may be
repaired if



neuron cell body remains intact
schwann cells remain active and
form a tube
scar tissue does not form too
rapidly
 Chromatolysis

24-48 hours after injury, Nissl
bodies break up into fine
granular masses
85
Repair within the PNS
 By 3-5 days,
 wallerian degeneration occurs
(breakdown of axon & myelin
sheath distal to injury)
 retrograde degeneration occurs
back one node
 Within several months,
regeneration occurs


neurolemma on each side of
injury repairs tube (schwann cell
mitosis)
axonal buds grow down the
tube to reconnect (1.5 mm per
day)
86
Neurogenesis in the CNS
 Formation of new neurons from stem cells was
not thought to occur in humans


1992 a growth factor was found that stimulates
adult mice brain cells to multiply
1998 new neurons found to form within adult
human hippocampus (area important for learning)
 There is a lack of neurogenesis in other regions
of the brain and spinal cord.
 Factors preventing neurogenesis in CNS

inhibition by neuroglial cells, absence of growth
stimulating factors, lack of neurolemmas, and
rapid formation of scar tissue
87
Multiple Sclerosis (MS)
 Autoimmune disorder causing
destruction of myelin sheaths in CNS




sheaths becomes scars or plaques
1/2 million people in the United States
appears between ages 20 and 40
females twice as often as males
 Symptoms include muscular weakness,
abnormal sensations or double vision
 Remissions & relapses result in
progressive, cumulative loss of function
88
Epilepsy
 The second most common neurological
disorder

affects 1% of population
 Characterized by short, recurrent attacks
initiated by electrical discharges in the brain



lights, noise, or smells may be sensed
skeletal muscles may contract involuntarily
loss of consciousness
 Epilepsy has many causes, including;

brain damage at birth, metabolic disturbances,
infections, toxins, vascular disturbances, head
injuries, and tumors
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end
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