Neuroscience 8 - The peripheral nervous system

Question 1

What are the 4 types of neuron in the PNS? And what are their functions?

Answer 1

1. Somatic sensory - carries information from skin/muscle/joints.
2. Somatic motor - controls striated muscle.
3. Autonomic sensory - brings sensory information from the internal organs.
4. Autonomic motor - controls glands or smooth muscle.

Question 2

What is a myotome and a dermatome?

Answer 2

1. Myotome - the muscle that a spinal nerve innervates.

2. Dermatome - the skin that a spinal nerve innervates.

Question 3

Where do somatic neurones originate from?

Answer 3

The primary motor cortex in the brain.

Question 4

Is SNS autonomic or voluntary?

Answer 4

Can be both voluntary or autonomic (reflexive).

Question 5

Where do autonomic neurones originate from?

Answer 5

From the visceral nuclei in the hypothalamus and leave via the autonomic nuclei in the brain stem or spinal cord.

Question 6

What are the 2 divisions of the ANS?

Answer 6

1. Sympathetic - formed by neurones leaving from spinal nerves T1 - L2 (fight or flight).
2. Parasympathetic - formed by neurones leaving from cranial and sacral nerves (resting and digesting).

Question 7

What are pre and post ganglionic neurones?

Answer 7

1. Pre-ganglionic neurones originate from the brain stem or spinal cord.
2. Post-ganglionic neurones originate from autonomic ganglia to target organ/tissue.

Question 8

How many pairs of nerves does the PNS consist of? And what are they split into?

Answer 8

There are 43 pairs of nerves in the PNS. Split into 12 cranial nerves and 31 spinal nerves.

Question 9

What are the 5 differences between ANS and SNS?

Answer 9

1. SNS has voluntary effectors and ANS has involuntary effectors.
2. SNS has single motor neurone from spinal cord to target whereas ANS has 2 neuones (ganglionic synapse).
3. SNS is always stimulatory whereas ANS is both stimulatory and inhibitory.
4. SNS uses ACh but ANS uses ACh + NE.
5. SNS does not fire at rest but ANS has a baseline firing.

Question 10

What are ganglia?

Answer 10

Cell bodies, synapses and dendrites in the PNS.

Question 11

What is the dorsal and ventral root?

Answer 11

1. The dorsal root is posteriorly located and brings afferent signals from periphery to CNS (sensory).
2. The ventral root is anteriorly located and brings efferent signals from CNS to periphery (motor).

Question 12

What are rami?

Answer 12

They are lateral branches of the spinal cord that carry both sensory and motor fibres. Not the same as ventral and dorsal root ganglia.

Question 13

What are the groups of neurones in the PNS?

Answer 13

1. A group - Have large diameters and are myelinated (high conduction velocity) - alpha, beta, delta and gamma types.
2. B group - Have smaller diameters but myelinated (low conduction velocity) - autonomic information.
3. C group - small diameters and not myelinated (low conduction velocity) - dull, aching burning pain and temperature sensation.

Question 14

What are nerve plexuses?

Answer 14

They are networks of intersecting spinal nerves. They include the cervical, brachial, lumbar, sacral, celiac and coccygeal plexuses.

Question 15

What is the clinical relevance of dermatomes being innervated by spinal and peripheral nerves?

Answer 15

Spinal nerves innervate in a stripey pattern whereas peripheral nerves innervate in a patchy pattern.

Question 16

What are peripheral neuropathies?

Answer 16

1. It is progressive damage affecting PNS nerves.
2. May impair:
   - Sensation (sensory nerves), tingling numbness.
   - Movement (motor nerves), weakness of hand + feet.
   - Involuntary functions (autonomic nerves), change heart rate and blood pressure.

Question 17

What are causes or peripheral neuropathies? (5)

Answer 17

Has many causes.

1. Metabolic.
2. Toxic.
3. Inflammatory.
4. Traumatic.
5. Genetic.

Question 18

What is mononeuropathy and polyneuropathy?

Answer 18

1. Mononeuropathy is single nerve affected.

2. Polyneuropathy is when multiple nerves are affected.

Question 19

What is neuropraxia?

Answer 19

1. Selective demyelination of axon sheath causing a reversible conduction block.
2. Endoneurium and axon still intact.

Question 20

What is axonotmesis?

Answer 20

1. Demyelination and axon loss.
2. Epineurium and perineurium still intact.
3. Damage occurs slightly below and proximal to the site of injury.

Question 21

What is neurotmesis?

Answer 21

1. Most severe type of nerve injury.
2. Complete nerve division and disruption.
3. Damage to epineurium - no nerve growth.

Question 22

What is electromyography?

Answer 22

It is a technique for evaluating and recording electrical activity produced by muscles. Involves placing a needle electrode in the muscle and another on the skin.

Question 23

What are nerve conduction velocity tests?

Answer 23

1. They measure the conduction velocity of nerves in the body. A nerve is stimulated and an electrode is placed downstream that measures the time taken for signal to reach - hence calculate the speed.
2. Sower than normal could indicate damage to the nerve.