Download The peripheral nerves

The peripheral
nerves
Dr. Faten
introduction
the nervous system contains many neurons + glial
cells (neuroglia).
 Glial cells are very numerous; there are 10-50
times as many glial cells as neurons.
 The Schwann cells that invest axons in peripheral
nerves are classified as glia.
 In the CNS, there are three main types of neuroglia.
1-Microglia= (resemble tissue macrophages )
2- Oligodendrogliocytes =are involved in myelin
formation inside CNS.
3- Astrocytes=(form the blood-brain barrier & produce
substances that are trophic to neurons) .

summary

Neurons
Cell bodies
Dendrites
Axons
NS
+
glial cells
supporting cells
Schwann cells
(PNS)
+
Oligodendrogliocytes
(CNS)
(PNS)
peripheral nerves
conduct Impulses
myelination
regeneration of PN
myelination of CNS
Glial cells
Cell Type
Functions
CENTRAL
NERVOUS
SYSTEM
Astrocytes
Maintain blood-brain barrier; provide structural support; regulate ion, nutrient, and dissolved gas
concentrations; absorb and recycle neurotransmitters; assist in tissue repair after injury
Oligodendrocytes
Myelinate CNS axons; provide structural framework
Microglia
Remove cell debris, wastes, and pathogens by phagocytosis
Ependymal cells
Line ventricles (brain) and central canal (spinal cavity); assist in production, circulation, and
monitoring of cerebrospinal fluid
PERIPHERAL
NERVOUS
SYSTEM
Satellite cells
Surround neuron cell bodies in ganglia
Schwann cells
Cover all axons in PNS; responsible for myelination of some peripheral axons; participate in
repair process after injury
Definition of peripheral nerve

Axons of neurons with associated blood
vessels and connective tissues are called
peripheral nerves or simply nerves that
carry sensory information into CNS and
motor information from CNS.

An axon is a long cytoplasmic process
capable of propagating an action potential
classification:
Anatomical classification:
1. Cranial nerves: connected to the brain
2. Spinal nerves: attached to the spinal cord
Functional Classification of nerves
1.Sensory = carry only sensory information
2.Motor = carry only motor information ( somatic,
autonomic nerves).
3. Mixed = carry all.
*Cranial nerves: some sensory; motor or mixed.
* Spinal nerves are mixed nerves
Axon Diameter, mylination and conduction velocity
=morphological and functional classification

Axons are classified into three groups according to the relationships among
diameter, myelination, and propagation speed:
1. Type A fiber: largest axon (from 4 to 20 µm), myelinated and its speed
is 140 meters per second.
2. Type B fiber: Smaller (2-4 µm), myelinated axon, and its speed is 18
meters per second.
3. Type C fiber: Very small (less than 2 µm in diameter), unmyelinated and
its speed is 1 meter per second.

We can understand the relative importance of myelin by noting that in going
from Type C to Type A fibers, there is a tenfold increase in diameter, but
the propagation speed increases by 140 times! For that the myelin
increases the conduction speed.
The myelin






The myelin, a protein-lipid complex that is wrapped
around the axon .
Outside the CNS, the myelin is produced by Schwann
cells, glia-like cells found along the axon.
Myelin forms when a Schwann cell wraps its membrane
around an axon up to 100 times.
The myelin sheath envelopes the axon except at its
ending and at the nodes of Ranvier
The myelin has the insulating function.
The loss of myelin is associated with delayed or
blocked conduction in the demyelinated diseases .
In the CNS , most neurons are myelinated,
but the cells that form the myelin are
oligodendrogliocytes rather than Schwann
cells.
 oligodendrogliocytes send off multiple
processes that form myelin on many
neighboring axons.
 In
multiple sclerosis, a crippling
autoimmune disease, there is patchy
destruction of myelin in the CNS.

Functions of each type
Type A fibers carry sensory information to the
CNS concerning position, balance, and delicate
touch and pressure sensations from the surface
of the skin. The motor neurons that control
skeletal muscles also send their commands over
large, myelinated Type A axons.
Type B fibers and Type C fibers carry lessurgent information concerning temperature,
pain, itching, and general touch and pressure
sensations to the CNS and carry motor
instructions to smooth muscle, cardiac muscle,
glands, and other peripheral effectors.
NOTES
*
only around one-third of all axons carrying sensory
information are myelinated, and most sensory
information arrives over slender Type C fibers.
* Myelination improves coordination and control by
decreasing the time between reception of a sensation
and initiation of an appropriate response.
* Myelination begins relatively late in development, and
the myelination of sensory and motor fibers is not
completed until early adolescence.
Properties of nerves
1. Excitability= respond to various stimuli by electrical changes
called action potential (nerve impulse).
2. Conductivity = the nerve can conduct impulses in both
direction if stimulated at its middle.
However in the body, the nerve conducts impulses in one
direction only and this normal direction is called orthodromic
conduction, but if it occurs in opposite direction is called
antidromic direction ( example = triple response).
*myelinated fibers used saltatory or jumping conduction ( ↑
velocity) from ranvier node to next.
*non-myelinated fibers used continuous conduction (slowly
process). (Review genesis of resting membrane and AP and
propagation).
Saltatory Conduction

myelin is an effective insulator .

depolarization in myelinated axons jumps from one node of
Ranvier to the next .

This jumping of depolarization from node to node is called
saltatory conduction.

It is a rapid process, and myelinated axons conduct up to
50 times faster than the fastest unmyelinated fibers.
3. All or none =means that an increasing in stimulus intensity above threshold
produce the same action potential with its threshold.
4. the various classes of fibers in peripheral nerves differ in their sensitivity to
hypoxia and anesthetics.
* Local anesthetics depress transmission in the group C fibers before they
affect the touch fibers in the A group.
*Conversely, pressure on a nerve can cause loss of conduction in largediameter motor, touch, and pressure fibers while pain sensation remains
relatively intact.
*Patterns of this type are sometimes seen in individuals who sleep with their
arms under their heads for long periods, causing compression of the nerves
in the arms.
*Because of the association of deep sleep with alcoholic intoxication, the
syndrome is commonest on weekends and has acquired the interesting
name
Saturday
night
or
Sunday
morning
paralysis.
Compound action potential:
* Each nerve contains more than one type of
nerve fibers with different speeds of conduction.
*If an stimulus is applied at one point on a nerve
and the action potential is recorded some
distance away → several peaks will appear and
represent the arrival of action potential in the
fastest fibers then in the slower fibers.
*This action potential with multiple peaks is called
a compound action potential.
Neuronal response to nerve
transection (cutting)
1. Degeneration and is usually followed by
2. Regeneration
1. Degeneration types
A. wallerian degeneration
B. retrograde degeneration
C. transneural degeneration
degeneration
A.
wallerian
(orthograde)
degeneration=
degenerative changes occur in the distal part of
a nerve which is separated from their cell
bodies if it is transected:
*Swelling of neurofibrils and break into short
segment
*myelin hydrolysis beaked into oil droplets
*all remnants are removed by the neurilemmal
cells and tissue macrophages in the area.
*neurilemmal tube becomes empty
B. retrograde degeneration of proximal part
and cell body:
*degeneration of proximal part = like distal
part changes
* Degeneration of cell body = swelling,
eccentric nucleus, fragmentation of Golgi
apparatus and chromatolysis
C. Transneuronal degeneration:

Means degeneration of a neuron if it
becomes functionless after degeneration
of another neuron.
Regeneration
*Cell
body regeneration starts 20 days after transection
and before nerve fiber regeneration and complete in
about 80 days
*cell regains normal size, nucleus returns to center and
Golgi apparatus are restored.
*nerve fiber regeneration starts by growing of proximal
neurilemma tube till its cut ends. Then, it sprouts into
several branches , at the same time the distal part
grows towards sprouting neurofibrils by chemotaxis
secreted by schwann cells.
*The myelin sheath is regenerated by neurilemmal cells
*schwann cells also release growth factors ( neurotrophins) such
as nerve growth factor and insulin like growth factor to promote
Regeneration of a nerve after
trauma

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Tissues Macrophages migrates into trauma area
Phagocyize disintegrated myelin & axonal debris +release mitosis
stimulating chemicals
stimulate

Proliferation of shawnn cells by neurilemmal sheat that form cords to
guide regeneration of axon &
Release
Growth factors
Stimulate
Axonal growth
Sprouting across gap to reach original contact

Notes :

Rate of nerve growth = 1mm / day (as maximum)

inside CNS, nerve fibers have no neurilemmal sheath so, after
inj. no regeneration occurs.

neurilemmal sheath = schwann cells surrounded by basement
membrane, then form a tube facilitates regeneration of nerve.
But in CNS this continuous basement membrane not present ,
so no tube for regeneration.

unmyelinated nerve are also surrounded by a sheath of
schwann, but it lacks the multiple wrappings of schwann cell
membrane.