The Peripheral Nervous System

Question 1

Sensory ( afferent ) division

Answer 1

CNS+PNS
PNS divides to sensory (afferent division): sensory neuron
getting info from skin and muscles to brain
and motor (efferent division)
Motor division divides to autonomic nervous system and somatic nervous system:voluntary control or automatic- skeletal muscles of the body
- we have 2 neurons, upper motor neuron ( CNS)
-lower motor neuron (PNS)
- synapse either in the brainstem or spinal cord.
Lower motor neuron will continue its way to the skeletal muscle.
Autonomic nervous system divides to sympathetic and parasympathetic.

Question 2

Autonomic nervous system

Answer 2

divided into 2 components sympathetic and parasympathetic.
3 neurons
-1 neuron from hypothalamic nuclei to brain stem nuclei ( parasympathetic) or spinal cord (sympathetic), or sacral region ( parasympathetic sacral nerve)
- 2nd: preganglionic Neuron from brain stem nuclei/ or spinal cord to autonomic ganglia
-3rd: postganglionic Neuron from autonomic ganglia to Visceral Effectors

Question 3

What is the equivalent of astrocytes in the PNS?

Answer 3

Satellite cells

Question 4

Describe the packaging of nerve axons into nerves

Answer 4

Axons are packaged into fascicles and the fascicles are packaged into nerves.

Question 5

What are the three layers of connective tissue found in nerves?

Answer 5

Endoneurium - wraps around individual axons
Perineurium - wraps around fascicles. Gives main tensile strength to nerve
Epineurium - wraps around nerves.

Question 6

what is the difference between myelinated and unmyelinated axons?

Answer 6

unmyelinated axons also have Schwann cells wrapped around them, the difference is that they only have one layer of membrane around them and one Schwann cell can accommodate many axons

Question 7

Describe the arrangement of the somatic motor neuron, autonomic motor neuron and sensory neuron

Answer 7

-Somatic motor neuron: have their cell bodies in the CNS.

CNS—–>SMN—>skeletal muscle
- Autonomic motor neuron: preganglionic neuron and post ganglionic motor neuron. There is an autonomic ganglionic in between the CNS and the effector.
CNS—> preganglionic—–> autonomic ganglion—-> post-ganglionic e.g. blood vessels, glands, viscera
Sensory neurons: have a ganglion in the middle.
CNS

Question 8

Describe the arrangement of autonomic motor neurons.

Answer 8

Autonomic motor neurons have a preganglionic neuron and a postganglionic neuron with an autonomic ganglion in the middle. The location of the autonomic ganglion varies depending on whether it is sympathetic or parasympathetic.

Question 9

What are the 4 types of neuron

Answer 9

• Somatic sensory: carries information in FROM the skin/muscles/joints
• somatic motor: controls striated muscle
• Autonomic sensory: brings sensory information from the internal organs
• Autonomic Motor: travels to glands to make them secrete or to smooth muscle in the wall of the gut etc

Question 10

where are the somatic sensory and motor found and where are the autonomic nerves found

Answer 10

Somatic sensory and somatic motor: found in ALL spinal nerves
Autonomic: found in MOST spinal nerves.

Question 11

where does the cell body of the somatic sensory axon and autonomic sensory come from and where is this found?

Answer 11

In the dorsal root ganglion.
this is found in the spinal nerve.
axon comes through the spinal nerve and it has a cell body in the Dorsal Root Ganglion.

Question 12

How is the autonomic motor structured?

Answer 12

2 cells between the CNS and the effector instead of just one.
-preganglionic autonomic motor neuron sends its axon through the ventral root and into the spinal nerve.
-it will move towards the effector but at some stage in the journey it will move into an autonomic ganglion.
- in the autonomic ganglion, it will synapse with a postganlionic motor neuron and the axon of the post ganglionic motor neuron ill continue towards the target organ.
therefore the location of the autonomic ganglion is very variable

Question 13

Which roots do motor and sensory nerves go out of?

Answer 13

Motor = ventral
Sensory = dorsal

Question 14

Where do autonomic sensory neurons have their cell bodies?

Answer 14

In the dorsal root ganglion

Question 15

What is a myotome and dermatome?

Answer 15

Muscle= The muscle that a spinal nerve innervates
Dermatome= the skin that a spinal nerve innervates

Question 16

What spinal nerves make up the brachial plexus?

Answer 16

C5-T1

Question 17

How is the brachial plexus different to thoracic nerves

Answer 17

some of the spinal nerves merge together then diverge later.

• this means you get mixing up of the individual spinal nerves so by the time you have 3 peripheral nerves emerging from the bottom of the plexus, each of these nerves will have axons which have originated from more than one spinal nerve.
• this is different from thoracic nerves which only have axons from a particular segment.

Question 18

What are the nerves called that are leaving the brachial plexus and before the plexus?

Answer 18

Before the plexus= SPINAL NERVES

After the plexus= PERIPHERAL NERVES

Question 19

Describe the differences in the dermatomes of spinal nerves and peripheral nerves.

Answer 19

Spinal nerves = stripey

Peripheral nerves = patchy

Question 20

what is the clinical relevance of spinal nerve and peripheral nerve appearance?

Answer 20

If someone has sensory loss and you don’t know where the lesion is, plot out the area on the skin where the sensation is lost. If the pattern is a STRIPE you know the injury is to the spinal nerve. If a PATCH of skin has lost sensation, then it is most likely a peripheral nerve.

Question 21

Describe the process of regeneration following peripheral nerve injury.

Answer 21

With compression injuries, the axoplasm is separated but the endoneurium is still in tact. The axon distal to the compression (further away from the cell body) degenerated - macrophages invade and phagocytose the cell debris. Schwann cells divides and increase in number. Axons sprouts then grow down the endoneurium. There is competition between the axonal sprouts to reach the target organ. When the first axon sprout reaches the target cell, the other axon sprouts regress. The axon then widens and a myelin sheath forms. The only difference is that the internodal distance is reduced so the conduction speed is reduced.

Question 22

What could happen if the damage to the neuron is so severe that there are no guidance cues at all for the axonal sprouts?

Answer 22

A neuroma could form which is very painful and needs to be surgically removed.

Question 23

what does the quality of regeneration depend on?

Answer 23

-distance between the point of injury and the cell body:
if the injury is close to the cell body, then the shock to the cell ma be so severe that the whole cell dies.
-also depends on the type of damage:
compression injury leave the endoneruial sheath intact, so you have a good conduit for the growth of axonal sprouts.

Question 24

what is peripheral neuropathies?

Answer 24

-Progressive degeneration of nerves
-usually start distally and progress towards the CNS.
Mechanism:
-due to a direct problem with the axon or it will be due to a secondary effect of damage to the myelin sheath leading to damage to the axon.
May impair sensation, movement, gland or organ function

Sensory nerves (sensation): cause tingling, pain, numbness 
Motor nerves (movement): cause weakness to hands and feet
Autonomic nerves (involuntary functions): cause changes in heart rate or blood pressure

Question 25

What are the two types of peripheral neuropathy? Describe them.

Answer 25

Segmental Demyelination - loss of a Schwann cell hear and there. This will not kill the cell but it will slow down the action potential because the action potential will propagate via continuous conduction rather than saltatory conduction.
Axonal Degeneration - once you already have degeneration of an axon, the myelin also degenerates and you get a conduction block- this can also happen with segmental demyelination later on in the course of disease if several adjacent schwann cells die making conduction fade away.

Question 26

What are two techniques of diagnosing peripheral neuropathy?

Answer 26

Nerve Conduction Study (NCS): can be diagnosed by measuring nerve conduction velocity, amplitude of responses and seeing if it is slow and seeing the shape of the wave that you get when you stimulate the nerve
Nerve biopsy: take a little bit of damaged nerve from the patient- a pathologist will be able to see the different features according to the various causes of neropathy.

Question 27

How many plexuses do we have?

Answer 27

4

There is a plexus which goes to EACH of the four limbs.

Question 28

What is the clinical significance of skin and muscles?

Answer 28

Clinical significance: can determine the site of spinal damage by simple pin prick exam.
Clinical significance: Testing for root level muscle weakness

Question 29

what are the PNS nerve injury classification?

Answer 29

Neuropraxia:

• reversible conduction block
• selective demyelination of the axon sheath
• endoneurium and axon still intact - e.g., nerve compression

Axonotmesis:

• demyelination and axon loss
• epineurium and perineurium remain intact
• still some continuity within the nerve
• degeneration occurs below and slightly proximal to the site of injury

Neurotmesis:

• most severe form of nerve injury
• associated with complete nerve division and disruption
• commonly seen after toxic or ischaemic injuries
• Damage to the epineurium (around the entire nerve) - no nerve growth

Question 30

what are the ways to diagnose PNS nerve injury?

Answer 30

Electromyography EMG:
Needle EMG: Diagnostic technique that distinguishes between myopathies (intrinsic to muscle) and muscle weakness due to nerve pathology (neurogenic). Looks at the waveforms of Motor Unit Action potentials.

Somatosensory Evoked Potential (SEP):

Evaluation of site of lesion (PNS or CNS) in somatosensory pathways by assessment of amplitude and latency of responses. Example, MS

Very small amplitude of NERVE responses (mV) therefore constant averaging over MANY responses is necessary to negate noise, compared to surface EMG (mV) looking at MUSCLE.

Question 31

Group the tests together as to what they are looking for?

Answer 31

EMG, NCS in assessment of peripheral nerves and SEPs in assessment of peripheral and central pathways.