CHAPTER 12 NERVOUS TISSUE
2
body control systems are responsible for maintaining homeostasis:
1. Nervous--acts quickly with nerve impulses
as messengers
2. Endocrine--acts more slowly with hormones
as messengers
Basic
functions of the nervous system:
1. Sensory function--senses changes within
the body or in the environment
2. Integrative function--analyzes sensory
information, may store it, makes decisions on appropriate response
3. Motor function--responds to stimuli by directing
muscle contractions or glandular secretions
Neurology--branch
of science/medicine
DIVISIONS OF THE NERVOUS SYSTEM
I.
CENTRAL NERVOUS SYSTEM (CNS)--brain & spinal cord
II.
PERIPHERAL NERVOUS SYSTEM (PNS)--all other nervous
tissue--connects the CNS to the sensory receptors, muscles and glands of the
rest of the body (includes cranial and spinal nerves)
A. SOMATIC NERVOUS SYSTEM
1. Sensory (afferent) neurons--input
to the CNS from somatic receptors in head, body, limbs, and special senses
2. Motor (efferent) neurons--originate
in the CNS and conduct voluntary impulses from the CNS to skeletal
muscle only
B. AUTONOMIC NERVOUS SYSTEM
1. Sensory (afferent) neurons that
carry impulses from autonomic receptors in internal organs into the CNS
2. Motor (efferent) neurons that carry
involuntary impulses to smooth muscle, cardiac muscle, glands and
adipose tissue
a. Sympathetic division-impulses
that promote energy use--"fight-or-flight" response is an extreme
example
b. Parasympathetic division--impulses than promote processes that restore and conserve energy—digestion is an example
C. ENTERIC NERVOUS SYSTEM---“brain
of the gut”---involuntary and previously considered part of the autonomic
nervous system. It consists of neurons (100 million) in enteric plexuses (nerve
networks) along the gastrointestinal (GI) tract. These nerves communicate with
the CNS but can also function independently .
1. Sensory neurons monitor chemical
changes and degree of stretch in the digestie tract
organs
2. Motor neurons control:
a. Contraction of smooth muscle in
the digestive tract
b. Exocrine secretions by
digestive tract organs
c. Secretion by GI tract endocrine
cells
CELLS OF THE NERVOUS SYSTEM
The
nervous system contains 2 types of cells:
1. Neuroglia (glial cells)--special
cells that support, protect and nourish the true nerve cells (neurons). These
cells are smaller than neurons and greatly outnumber them (5-50X). They fill
about 1/2 the space in the central nervous system. Neuroglia
(unlike neurons) reacily divide. If an injury causes
loss of neurons, neuroglia fill the space. Brain tumors often arise from neuroglia. Types—see Figure 12-6P. 410
a. Astrocytes---star-shaped
with many processes
1) Support of neurons
2) Help form blood-brian
barrier
3) Secretion
of chemicals that appear to regulate brain development in the embryo
4) Help provide an ideal chemical
environment for neurons
5) Influence
formation of neural synpases
b. Oligodendrocytes---most
numerous
1) Support of neurons
2) Myelination
of CNS fibers
c. Microglia---very
small
1) Phagocytosis
within the CNS--microbes & debris
d. Ependyma---cuboidal to columnar, many are ciliated
1) Line brain ventricles
2) Form CSF and assist in circulation of
it
e. Schwann cells
1) Myelination
of PNS fibers
2) Assist in axon regeneration
f. Satellite cells---support neurons in
ganglia (clusters of neuron cell bodies located outside the CNS)
2. Neurons--true nerve cells. They are responsible for thinking, sensing, remembering, controlling muscle activity and regulating glandular secretion. Like muscle cells, they have the property of electrical excitability—that is the ability to generate action potentials (nerve impulses) in response to stimuli. This is due to the presence of special ion channels. Parts of a neuron:
a. Cell body---main central part of
the cell. Contains the nucleus surrounded by cytoplasm with typical organelles.
Also present only in neurons:
1) Lipofuchsin--pigment
that is probably an end-product of lysosomal
activity, increases with age
2) Nissl bodies
(chromatophilic substance)—prominent clusters of
rough ER with ribosomes to synthesize proteins needed for growth of neurons and
regeneration of damaged parts
3) Cytoskeleton that provides support
& shape
a) Neurofibrils—cell
shape & support (bundles of intermediate filaments)
b)
Microtubules—assist in movement within the neuron
Remaining
parts are fibers or processes that are extensions of the cell membrane and
contain a small amount of cytoplasm:
b. Dendrites--short, tapering
highly branched processes that conduct impulses TOWARD the cell body
c. Axons--long thin processes that
may be myelinated. Axons conduct impulses AWAY FROM
the cell body. They join the cell body at a cone-shaped elevation called an
axon hillock. Cytoplasm of the neuron extends down the axon and is called axoplasm; cell membrane surrounds it and is called axolemma. After the axon has emerged from the cell it may
give off side branches called axon collaterals. The axon and collaterals end by
dividing into fine branches called axon terminals. The ends of most axon
terminals swell into bulb-shaped synaptic end bulbs, which contain synaptic
vesicles that store neurotransmitters (acetylcholine or some other). Release of
neurotransmitter can influence the activity of other neurons, muscles or
glands. Occasionally axon terminals have a string of swellings called
varicosities instead of a single synaptic end bulb.
Most
of the synthesis of the neuron occurs in the cell body. Axonal transport
systems move materials needed in the axon or axon terminals to the area and
return worn-out parts to the cell body.
Axons
vary in length from 1 mm to 3 feet or more. Nerves in the PNS contain hundreds
to thousands of axons bundled together and surrounded by CT coats.
STRUCTURAL CLASSIFICATION OF NEURONS:
(Based on number of processes)
1. Multipolar
neurons--several dendrites and 1 axon. Most neurons of the
CNS.
2. Bipolar neurons--1 dendrite, 1 axon.
Retina, inner ear, olfactory (smell) area
3. Unipolar neurons—have only one process connecting to the cell body. It is formed when the axon and single dendrite fuse during development. This single process divides a short distance from the cell body and both parts are characteristic of an axon except that one part (the one that extends toward the periphery) has dendrites at its tip. All of these are sensory neurons.
FUNCTIONAL CLASSIFICATION OF NEURONS:
(Based on direction in which they conduct
impulses)
1. Afferent (sensory) neurons--transmit
impulses from receptors in skin, sense organs, muscles, joints and viscera into
the CNS
2. Efferent (motor) neurons--carry motor
impulses from the CNS to effectors (muscles and glands)
3. Interneurons--carry
nerve impulses from 1 neuron to another neuron. 90 % of our neurons are interneurons and there are thousands of different types.
MYELINATION
Axons
of neurons are often surrounded by a white, segmented, phospholipid
and protein sheath called the myelin sheath. This insulates the axon and
increases the speed of nerve impulse conduction.
In
the PNS, glial cells called Schwann cells produce the
myelin sheath by wrapping around peripheral axons during fetal development and
the first year of life. The end result is that the inner layers of the Schwann
cell membrane (up to 100 layers) form the myelin sheath. The outer layer
contains the Schwann cell nucleus and cytoplasm and is called the neurolemma. In axons of the PNS the neurolemma
aids in regeneration of injured axons. It forms a tube that guides and
stimulates the axon. At intervals the myelin sheath has gaps called Nodes of Ranvier.
In
the CNS oligodendrocytes myelinate
many axons but in such a way that no neurolemma is
formed. The oligodendrocyte sends
processes that wrap around the axon but the cell body and nucleus do not wrap.
CNS axons do not regrow following injury, mainly due
to 2 factors:
·
No neurolemma
·
Inhibition of regrowth
(instead of encouragement as in PNS)
Nerves
of a newborn are not fully myelinated and this is one
reason why an infant's responses are slow and uncoordinated.
Multiple
sclerosis and Tay-Sachs disease involve destruction
of myelin sheath.
TERMS RELATED TO NERVOUS TISSUE
NERVE
FIBER---general term for any process of a neuron (axon or dendrite) but used
most often to refer to axons
NERVE-a
group of axons bundled together and running along the same path in the PNS
TRACT--group
of axons bundled together and running along the same path in the CNS. May be
entirely within the brain, connecting different brain areas, or may run up or
down the spinal cord, connecting specific brain regions with the periphery
GANGLION--group
of nerve cell bodies in the PNS
NUCLEUS--group
of nerve cell bodies in the CNS
WHITE
MATTER--aggregations of myelinated processes from
many neurons (whitish color comes from myelin). Wherever it is located, white
matter is concerned with transmitting impulses.
GRAY
MATTER--nerve cell bodies, dendrites, unmyelinated
axons. Function is concerned with integration and generation of impulses.
In
the spinal cord: white matter surrounds an inner core of gray matter
In
the brain: a thin outer layer of gray matter covers the surface and the white
matter makes up the rest except for masses of gray matter (nuclei) deep within
the brain
NEUROPHYSIOLOGY
The
first step in understanding generation of a nerve impulse is understanding
the resting membrane potential (RMP). In a resting neuron there is a buildup of
negative charges just inside the cell membrane and a buildup of positive
charges on the outside. This is true for most cells of the body, but is
especially important in the function of the excitable cells (nerve and muscle
cells). This separation of charges is a form of potential energy, measured in millivolts (1/1000 of a volt). The RMP of a neuron ranges
from -40 to -90 millivolts---the greater the
difference in charge the greater the voltage. A membrane in this condition is
said to be polarized. With proper stimulation, the RMP can change suddenly,
producing a response called an action potential.
In
living cells, a flow of ions is produces tiny amounts of electrical current.
Since the phospholipid portion of the plasma membrane
is impermeable to ions, they flow across the membrane by way of openings called
ion channels. Most of these channels can open and close. Ion channels are of 2
basic types:
1. Leakage channels---open and close at
random but some are usually open---numerous K+ leakage channels are the reason
the membrane is normally somewhat permeable to K+. There are also a few Na+
leakage channels, but not as many.
2. Gated channels---open and close in
response to specific stimuli. They give nerve and muscle cell membranes the
property of excitability.
a. Voltage-gated channels---these open
in response to a direct change in the membrane potential. These channels
participate in the generation and conduction of action potentials.
b. Ligand-gated
channels---open and close in response to a specific chemical. The presence of
chemical ligands such as neurotransmitters, hormones
and ions regulate these channels.
c. Mechanically-gated channels open and
close in response to mechanical pressure, stretching, or vibration. Found in
hearing receptors, stretch receptors, and touch receptors.
d. Light-gated ion channels---found in
the eye
RESTING MEMBRANE POTENTIAL
MEMBRANE PERMEABILITY:
Moderately permeable to K+
Moderately
permeable to Cl-
Resting neuron Membrane is polarized
Almost impermeable to Na+
Impermeable
to organic anions
RESULT:
K+ next to the membrane leaks out due to
the concentration gradient, so the area at the inside of the membrane takes on
a negative charge
Only a very small amount of Na+ leaks in
but Na+ gathers at the outside edge of the membrane, giving it
To
maintain these normal conditions in the cell, an active transport mechanism
called the Na+/K+ pump continually pushes Na+ out and K+ back in. This is vital
to maintain the RMP as the difference in charge across the membrane produces a
resting membrane potential of – 40 to – 90 millivolts
( - 70 millivolts is
average).
NERVE ACTION POTENTIAL (NERVE IMPULSE)
This
is a series of events that changes the charge inside the neuron from negative
to positive (the depolarizing phase) and then returns it to the original state
(repolarizing phase). For this to occur, a stimulus
of the proper type and of sufficient strength must be applied to the neuron,
usually to the dendrite.
Steps:
1.
Proper type of stimulus of sufficient (threshold) strength arrives. The
dendrite is the most common part of the neuron to receive and begin the
response to this stimulus.
2.
The response to the stimulus causes the charge inside to go from – 70 to
approximately – 55.
3. At
this point, large numbers of voltage-gated Na+ channels open and large numbers
of Na+ ions rush in.
4.
Entry of these positive ions changes the charge inside the cell from negative
to 0 to positive. Membrane potential goes from negative to positive ( an
average of + 30 millivolts).
This is depolarization.
5.
After remaining open only a small fraction of a second, Na+ channels close, so
no more Na+ ions enter.
6. At
the same time the Na+ channels close, K+ channels open.
7. K+
ions rush out, because of the concentration gradient.
8.
This returns the charge inside the cell to approximately the original value
(this is called repolarization).
9.
Na+ and K+ are in the wrong place, but the Na+/K+ pump quickly returns them to
where they belong and gets the charge just right at the same time. At the end
of repolarization, the resting membrane potential has
been fully restored. The typical time for all of these steps is 1 millisecond
(1/1000 of a second).
GRADED POTENTIALS/ACTION POTENTIALS
It is
possible for a stimulus to affect a neuron without producing an action
potential (nerve impulse). If a slight change in membrane potential results
from a stimulus, this is a graded potential. It can go 2 ways:
1. Membrane polarization becomes more
negative than it already was---hyperpolarization
2. Membrane polarization is less negative
but does not change enough to make the cell positive inside. This response is a
depolarizing graded potential. Remember, this is not ENOUGH depolarization to
produce an action potential. An action potential makes the cell positive enough
inside that a nerve impulse results.
REFRACTORY PERIOD
Immediately
following the generation of a nerve action potential, there is
ALL-OR-NONE PRINCIPAL
If a
threshold stimulus is applied to the neuron, an action potential results and
has a constant and maximum strength for conditions at that time. Conditions
such as toxic materials or fatigue could alter the strength. The threshold
stimulus can vary from one neuron to another.
We
detect differences in the intensity of stimuli even though impulses are all of
the same strength. This is due to:
1) Frequency of stimulus
2) Number of sensory neurons involved
PROPAGATION OF A NERVE IMPULSE
Nerve
impulses are transmitted from one part of the body to another (conduction). The
impulse spreads from the point where it originated along the axon to a muscle
cell, gland cell or another neuron (some type of synapse). Na+ channels open
along the length of the axon (like dominoes falling) and just behind this
spreading depolarization, repolarization follows.
Local
anesthetics prevent opening of the voltage-gated Na+ channels, so sensory
neurons cannot transmit pain impulses past the blockage.
In unmyelinated axons step-by-step depolarization of the
entire axon plasma membrane must occur. This is called continuous conduction.
It is relatively slow and requires more energy.
In myelinated axons conduction occurs somewhat differently.
The myelin sheath acts to block ionic currents across the membrane. Remember,
though, that at intervals along the axon are gaps in the myelin sheath called
nodes of Ranvier. The nodes have a very high density
of voltage-gated Na+ channels so depolarization readily occurs in these. In
between the nodes, the current flows through the extracellular
fluid from one node to the next. This is called saltatory
conduction. It results in much faster conduction (up to 50 X faster) and
uses less energy. Since depolarization and repolarization
occur only in a fraction of the length of the fiber, the Na+/K+ pump uses less
energy to restore resting conditions.
Speed
of conduction of a nerve impulse is not related to strength of impulse or
strength of stimulus. Factors that do influence speed:
1. Presence or absence of myelin
2 Diameter of fiber--the larger the
faster
3. Temperature--the warmer the faster
TYPES
OF FIBERS (AXONS)
1. A
fibers
Largest diameter
Briefest refractory period
All myelinated (saltatory conduction)
Speed: As fast as over 300 feet/second
(280 mph)
Location: Large sensory nerves and motor
nerves to skeletal muscle where split-second reactions may mean survival
2. B
fibers
Medium diameter
Somewhat longer refractory period
Myelinated
Speed: 30 - 40 feet/second (32 mph)
Location: Sensory from viscera, motor to
autonomic ganglia
3. C
fibers
Smallest diameter
Longest refractory period
Unmyelinated
Speed: 1.5 feet/second (1-4 mph)
Location: Sensory from some skin receptors and viscera, motor from autonomic ganglia to cardiac muscle, smooth muscle and glands
SYNAPSES
Axons
transmit impulses to muscles, glands or other neurons.
Neuromuscular junction--axon terminals
communicate with muscle fiber
Neuroglandular
junction--axon terminals communicate with gland cells
Synapses--axon terminals of one neuron
communicate with another neuron
Synapses are essential for homeostasis because they allow information to be integrated and filtered. Certain signals are transmitted while others are blocked. Synapses are also important because their disruption can result in disease and also because they are the site of action for many drugs (pain-killers for example).
Electrical
synapses—action potentials spread from one cell to another through
gap junctions (little protein tunnels between the two cells called connexons). No neurotransmitter is required. Relatively
rare, but in cardiac and visceral smooth muscle they provide coordinated
contractions of large numbers of fibers all at once. They are also found in the
CNS, but we do not fully understand their function there. Benefits of
electrical synapses :
1. Very fast conduction.
2. Synchronization—activation of large
numbers of neurons or muscle fibers at once
Chemical
synapses are the usual type. In many ways they are similar to
a
neuromuscular junction:
An
axon usually synapses with a dendrite, but it can also synapse with a cell body
or an axon hillock.
Here are the events when the impulse arrives at the synaptic end bulb of the presynaptic neuron:
1.
The synaptic end bulb area depolarizes.
2.
Here, depolarization, in addition to the usual effects, also opens
voltage-gated Ca+ channels, which allow Ca2+ ions to flow into the
synaptic end bulb.
3. As Ca ions flow in, the rising level of Ca
causes exocytosis of synaptic vesicles.
4.
Neurotransmitter molecules cross the synaptic cleft.
5.
Neurotransmitter molecules bind to receptors on the postsynaptic neuron.
6.
This causes ligand-gated channels to open, allowing
ions to flow into the postsynaptic neuron
7.
Transmission can only occur one way, which is important in preventing impulses
from backing up.
What
happens next in the postsynaptic neuron can vary. Neurotransmitters can have
two kinds of effects:
1. EXCITATORY---we
have studied the release of acetylcholine in skeletal muscle, The ACh acted as an excitatory neurotransmitter
there---its binding to ACh receptors caused an action
potential in the muscle cell (excitement).
Like acetylcholine in skeletal muscle, many neurotransmitters at synapses between neurons are excitatory--they bring about depolarization of the membrane of the post-synaptic neuron by opening chemically-gated channels that allow inflow of positive ions (usually Na+).
2. INHIBITORY--some
neurotransmitters have the opposite effect--these are called inhibitory
neurotransmitters. They open chemically-gated channels for Cl-
to enter or for K+ to leave. Either way, the inside of the membrane becomes
even more negative than usual (hyperpolarization),
making depolarization more difficult.
Neurotransmitters must be quickly removed from the synaptic cleft to prevent their effects from being extended. We are designed so that the effect of a single nerve impulse and its resulting burst of neurotransmitter should end quickly. If we need a more sustained effect it should be provided by a steady stream of nerve impulses and continuous release of neurotransmitter--this keeps the nervous system in control. Neurotransmitters are removed by:
1. Diffusion out of the synaptic cleft
2. Breakdown by a specific enzyme (such as acetylcholinesterase)
3. Uptake into surrounding cells, either the
neuron that released them or surrounding neuroglia
A
typical neuron in the CNS receives input from 1000-10,000 synapses. What occurs
in this neuron is summation---the sum of all these influences, both excitatory
and inhibitory. There are 3 possible results of summation:
1.
EPSP---If the excitatory effect is greater than the inhibitory effect, but
below threshold strength, the membrane may become partly depolarized (but not
enough to generate an impulse). However, subsequent impulses may generate an
impulse more easily. This is called EPSP—excitatory postsynaptic potential.
2.
Nerve impulse---If the excitatory effect is greater and is at or above
threshold strength, an impulse is generated
3.
IPSP---If the inhibitory effect is greater than the excitatory effect, the
membrane hyperpolarizes--the inside becomes even more negative than usual and
it will be harder to generate an impulse. This is called IPSP—inhibitory
postsynaptic potential.
NEUROTRANSMITTERS
There
are about 100 substances are known or suspected to act as neurotransmitters.
Acetylcholine is the best known. It is the neurotransmitter of most PNS neurons
and many neurons of the CNS also. It is excitatory at the neuromuscular
junction of skeletal muscle , but it can also be
inhibitory at some other locations.
Neurotransmitters
are placed in 2 classes:
1. Small
molecule neurotransmitters
a. Acetylcholine---excitatory at some
synapses, inhibitory at others---inactivated by acetlycholinesterase
b. Amino acids
1)Glutamate---excitatory---very
important
2) Aspartate---also
excitatory
3) GABA---not used in making proteins but
is the most common inhibitory neurotransmitter in the brain and also acts in
the spinal corde---Valium enhances its effect
4) Glycine---inhibitory
in the spinal cord
c. Biogenic amines---amino acids are
modified---may be either excitatory or inhibitory
1) Norepinephrine---mood,
awakening, dreaming
2) Epinephrine
3) Dopamine---emotions, skeletal muscle
tone, movement---if neurons that produce it are damaged. Parkinson’s disease
results
4) Serotonin---concentrated in the raphe nucleus
d. ATP and other purines---adenosine,
ATP,ADP,AMP---all are excitatory
e. Gases---nitric oxide is formed
when needed and acts immediately. It may play a role in memory and learning. It
also causes vasodilation (produced by endothelial
cells).
2. Neuropeptides---chains of 3 - 40 amino acids. May be excitatory or inhibitory. Many are hormones that also act elsewhere in the body.
a. Enkephalins These 3 are natural painkillers and
b. Endorphins are sometimes called the opiate
c. Dynorphins peptides
d. Substance P---pain
TABLE 12.3
P. 430
In
some instances a single synaptic end bulb can release more then one
neurotransmitter.
Strychnine
poisoning causes its effects by blocking the receptors for the inhibitory
neurotransmitter glycine in the spinal cord. This
upsets the expected balance between excitatory and inhibitory effects and leads
to violent uncontrolled contractions of all skeletal muscles.
Factors
that alter conduction and effects of neurotransmitters at synapses:
1.
Alkalosis (increase in blood pH)--increased excitability of neurons can even
lead to muscle spasms and convulsions
2.
Acidosis (decrease in blood pH)--depression of neuron activity that can lead to
coma
3.
Pressure on a nerve—may block impulse conduction
4.
Hypnotics, tranquilizers, anesthetics increase the threshold for excitation and
depress activity
5.
Caffeine, benzedrine,
nicotine promote the release of excitatory neurotransmitters
6.
Botulism inhibits the release of acetylcholine and causes paralysis of skeletal
muscle
7.
"Nerve gas," insecticides inactivate acetylcholinesterase,
causing convulsion, diarrhea, vomiting as acetylcholine remains in synaptic
clefts
8.
Curare blocks acetylcholine receptors
NEURAL CIRCUITS---patterns of connections
between neurons in the CNS
1. Simple series circuit---1 presynaptic neuron synapses with 1 post synaptic neuron.
This is simplest and easiest to understand, but it is not the most common type.
(Usually things are more complicated.)
2. Diverging circuit (divergence)---a single presynaptic neuron
may synapse with several postsynaptic neurons. This is called divergence and
tends to cause stimulation of increasing numbers of neurons along the circuit,
making widespread effects likely.
3. Converging circuit (convergence)---a single postsynaptic neuron synapses with several presynaptic neurons. This permits more effective
stimulation or inhibition of the postsynaptic neuron.
4. Reverbrating
(oscillatory) circuit---once the circuit is stimulated, the impulse runs
around and around the circuit, maybe even for hours. Coordinating
muscle activities, waking & sleeping, short-term memory, breathing.
5. Parallel after-discharge circuits---single
presynaptic neuron stimulated a group of postsynaptic
neurons (divergence) but then all the postsynaptics
synapse with a common neuron.
REGENERATION & REPAIR OF NERVOUS TISSUE
The
nervous system has the characteristic of plasticity---the ability to change
based on experience. These changes can occur throughout life and include:
Growing new dendrites
Synthesis of new proteins
Changes in synapses
What
the nervous system has limited ability to do is repair
damaged neurons or produce new ones to replace them. Scientists believed
until recently that after the first year of life no new neurons at all could be
produced in mammalian brains. Recent developments have shown that, with the
stimulus of epidermal growth factor, brain cells CAN be made to divide (in the
lab, not in living brains). Even more surprising, in 1998 it was proved that
one particular area of the brain, the hippocampus, seems to produce new neurons
on its own.
However,
it is still mostly true that we do not produce new neurons or even successfully
regrow axons in the CNS, probably due to several
factors:
1. Inhibition by neuroglia,
esp. oligodendrocytes
2. Absence of signals to induce growth
3. No neurolemma
in the CNS
4. Rapid formation of scar tissue that
blocks regrowth
Injury
to the brain and spinal cord is usually permanent. However, we now believe that
even in adults the CNS contains stem cells that we might be able to stimulate
to develop into neurons. Also, transplanting neurons into the brain might work.
Unfortunately this involves use of fetal tissue, which is highly controversial.
PNS DAMAGE AND REPAIR
If an
axon in the PNS is damaged, regrowth is possible if:
1. Cell body is intact
2. Neurolemma is
present
3. Scar tissue does not block
Steps
in regrowth, if all goes well:
1. Nissl bodies
break up (chromatolysis)
2. Axon and myelin sheath distal to injury
deteriorate and are removed by macrophages (Wallerian
degeneration), but neurolemma remains
3. RNA and protein synthesis in the cell
body increase
4. Schwann cells on each side of the injury
form a regeneration tube
5. Axon regrows
down the tube to the original termination---1.5 mm per day---and reestablishes
connections
6. Schwann cells form new myelin sheath