Neuro 6 – The Peripheral Nervous System
Anil Chopra
1. Describe the structural and functional components of a normal peripheral
nerve.
Peripheral Nervous System
Consists of…
Efferent divisions
And afferent
divisions
& glial cells (Schwann
cells and satellite cells)
& connective tissue and
vascular tissue.
The Peripheral Nerve
Bundles of axons divide and anastomose along the course
of the nerve. Each component of the nerve is surrounded by
its own tissue sheath.
Epineurium: surrounds whole nerve. Loose connective
tissue made from collagen. Also carries blood supply for nerve.
Perineurium: dense connective tissue that surround the fascicle which forms
many layers of fibroblast – like cells. Gives the nerve mechanical strength and
helps as a diffusion barrier to maintain ionic balances in the axon.
Endoneurium: surrounds individual axons, made of lose connective tissue.
Each spinal nerve innovates some organ or part of the skeletal muscle/viscera. The
nerve has a:
Sensory root – the dorsal root and a
Motor root – the ventral root.
These both meet at the point of dorsal root ganglion, where they divide into rami:
 Dorsal rami – innervate skin and back muscles
 Ventral rami – innervate skin, chest muscles, limbs and pelvis.
 Rami communicates – split into white ramus communicates (preganglionic
sympathetic fibres) and grey ramus communicates (postganglionic
sympathetic fibres).
SOMATIC
AUTONOMIC
2. List the factors that affect conduction velocity of peripheral axons.
Nerve fibres are classified according to their function, speed, Myelination and
diameter. Conduction velocity depends on:
(1) Axon diameter (larger diameter means less resistance which results in quicker
conduction).
(2) Myelination (if an axon is myelinated, it will conduct by saltatory conduction
where the action potential jumps between the nodes of Ranvier)
Myelinated fibres have single Schwann cells wrap around them in 100 layers of
myelin. Unmyelinated fibres have the axon lie within invaginations in the Schwann
cell.
3.
Define the terms: dermatome, myotome, ramus, plexus and explain their
significance with regard to innervation of the body.
 Dermatome – area of skin innervated by a particular spinal nerve.
 Myotome – muscle group innervated by particular spinal nerve.
 Ramus – branches into which nerves divide in the autonomic and somatic nervous
system. Can contain sensory and motor neurones.
 Plexus – where many spinal nerves combine to form the peripheral nerves, there
are 2 in the body.
 Brachial plexus – formed from C5-T1
 Lumbosacral plexus – formed from L2 – S2
If nerves are damaged then there may be some decrease in sensitivity from a certain
dermatome.
4. State the spinal levels which contribute to the nerves of the upper and lower
limb
C5 – T1 innervate upper limbs via brachial plexus
L2 – S2 innervate lower limbs via lumbosacral plexus
5. Compare and contrast the effects of injury and disease on peripheral nerve
function.
(A) Normal axon being crushed
(B) Macrophages phagocytose debris
(C) Cell bodies undergo chromatolysis and then eventual proximal axons make
contact with Schwann cells.
(D) Schwann cells grow and axon grows at around 2-5mm per day.
(E) New axon with nodes of Ranvier closer together.
Peripheral Neuropathies
- Involve degradation of peripheral nerves.
- Because of metabolic disorder, infection.
- Usually begins distally.
- Affect sensory and motor axons.
- Affects myelin or axon.
6. Outline main diagnostic techniques for peripheral nerve disorders.
- Measuring conduction velocity: this cal also help distinguish whether the
disease is demyelinating or axonal.
- A nerve biopsy can be taken to study
the pathogenesis.