Peripheral Nervous System Flashcards
The peripheral nervous system involves
the peripheral nerves and the ganglia.
Normal peripheral nerve types
Structural+ functional componenets of each
Where do cell bodies (soma) of each lie?
The dorsal root ganglion
Dorsal root ganglion is surrounded by a connective tissue capsule.
Inside the capsule is a mixture of neuronal cell bodies and axons (cut in various planes).
Single sensory neurone
There is a dark nucleus inside a pale nucleus.
The small nuclei are satellite cells which support the neurones.
These sensory neurones are pseudo-unipolar (no dendrites)
Sensory neurone cell bodies (somatic and autonomic) lie in the ganglia associated with spinal dorsal roots or some cranial nerves.
Post ganglionic neuronal soma lie in the autonomic ganglia, either arranged in:
▪ Rows paravertebral (sympathetic).
▪ Closer to internal organs (parasympathetic).
There are THREE layers of connective tissue in nerves:
- Epineurium – Connective tissue surrounding nerve.
- Perineurium – Connective tissue surrounding fascicles.
- Endoneurium – Connective tissue surrounding axons.
Functional component of PNS
3 neurones
▪ Somatic motor neurones have their soma in the CNS.
▪ Autonomic motor neurones consist of a preganglionic
and postganglionic motor neurone.
o There is an autonomic ganglion in between the
CNS and the effector.
▪ Sensory neurones have a ganglion in the middle.
Types of neurones in PNS
There are 4 types of neurone:
1. Somatic Sensory – Information from non-internal
organs.
2. Somatic Motor – Controls striated muscle.
3. Autonomic Sensory – Sensory from internal organs.
4. Autonomic Motor – Controls internal organs.
Somatic sensory and Somatic motor are found in ALL
spinal nerves whereas autonomic is found in MOST.
Sensory and motor neurons in PNS
There is a Dorsal (sensory) Horn, Ventral (motor) Horn
and Spinal Nerve (both sensory and motor).
Dermatome
Dermatome = the skin that a spinal nerve innervates.
o The pattern of dermatomes in the thorax are a
series of horizontal stripes because they are
arranged along the ribs.
The dermatomes can be thought of in terms of spinal OR peripheral nerves.
o Spinal nerves = STRIPEY pattern.
o Peripheral nerves = PATCHY pattern
Clinical relevance = if someone has sensory loss, you can plot out the area where sensation is lost.
o If the area is a stripe, it is a spinal nerve injury.
o If the area is a patch, it is a peripheral nerve injury
Myotome
Myotome = the muscle area that a spinal nerve innervates.
Ramus
Dorsal?
Ventral?
Purpose?
As the spinal nerves leave the spinal cord, some merge
together and later divide (into rami)
after they bunch up into a spinal nerve, the ramus are the branches
The dorsal rami innervate muscle and skin of the back.
The ventral rami innervate muscles and skin of the rest of the body.
Ventral rami of spinal nerves C5-T1 innervate the upper limb through the brachial plexus.
Ventral rami of spinal nerves L2-S2 innervate the lower limb through the lumbosacral plexus.
Rami communicantes provide interconnections between some spinal nerves and ganglia of the
sympathetic chain
Plexus
Spinal nerves are combined to form peripheral nerves
There is a plexus which goes to each of the four limbs
o I.E. Brachial Plexus goes to the upper limb.
o C5-T1 nerves innervate the brachial plexus.
NOTE: Before the plexus is SPINAL NERVES but after the plexus is the PERIPHERAL NERVES.
In a plexus, the axons of different spinal nerves are recombined to form a peripheral nerve therefore
they may contain axons from several spinal nerves.
Effects of injury+ disease on peripheral nerve function
Compression injury healing
(picture 1, lecture 10)
If injury is close to soma?
If the nerve is cut?
If damage is severe?
- With the compression injury (forceps), the axoplasm is completely separated but the
Endoneurium remains intact (important as a scaffold to build off). - Once the compression has occurred, the part distal to the soma of the nerve degenerates.
- The macrophage influx clears the debris.
- The Schwann cells undergo mitosis and proliferate.
- Axonal sprouts grow from the proximal stump and there is competition between them to
synapse with the target organ first. - Once an axonal sprout synapses, the other axonal sprouts regress and the successful axonal
sprout grows and is myelinated. - End-result is an axon which looks similar to the original but the internode distance (between
nodes of ranvier) is less than original (so conduction speed decreases).
▪ If the injury is close to the soma, the shock may kill the whole cell.
▪ If a compression injury occurs, the endometrial sheath is left intact, providing a scaffold to re-grow
off but if the nerve is cut, the axonal sprouts have no guidance cues and so you ay have reinnervation of the wrong target.
o Clean cut = Surgeon can re-join in microsurgery.
o Mess cut = Nerve graft from the back of the calf.
▪ The damage could be so severe that the axonal sprouts have no guidance at all and form a
tangled bundle called a neuroma → very painful and need to be surgically removed.
Peripheral neuropathies Causes? What is affected? Mechanism? Types?
Progressive degeneration of nerves
Usually start distally and progress towards the CNS.
▪ Cause may be metabolic, infectious or hereditary.
o Common cause is badly controlled diabetes – Metabolic.
o Alcoholism – Metabolic.
o Leprosy – Infectious.
▪ Sensory and/or motor axons are affected.
▪ Mechanism – either due to direct problem with the axon or due to a secondary effect of damage to
the myelin sheath leading to damage of the axon.
- Segmental Demyelination.
a. Schwann cells sporadically die and fall away.
b. This does not kill the axon or stop conduction of
an AP but it will slow down conduction as there is
continuous conduction rather than saltatory. - Axonal Degeneration.
a. After degeneration of the axon, the myelin also
degenerates and you get a conduction block.
Conduction block can occur in segmental demyelination
but it requires adjacent Schwann cells to die and fall away, making conduction fade away.
Main diagnostic techniques for peripheral nerve disorders
▪ Conduction Velocity:
o Measure conduction velocity and seeing if it slows or if the shape of the wave changes when you stimulate the nerve using Nerve Conduction Study
▪ Nerve Biopsy:
o Biopsy a piece of the nerve, a pathologist will be able to see the different features according to the various causes of neuropathy.
Somatosensory Evoked Potential (SEP)- evaluation of site of lesion by assessment of amplitude+ latency of responses
Electromyography
List the factors that affect conduction velocity of peripheral axons
▪ Degree of Myelination:
o More insulation means quicker impulses due to nodes of ranvier being present to allow saltatory conduction.
▪ Diameter of axon:
o Thicker axons allow a faster conduction speed as there are more receptors and thus free ions to transmit a current.
