M1.4 — 1 PNS neurophysiology

Question 1

What makes up the CNS? and PNS?

Answer 1

CNS = brain and spinal cord
PNS = 12 cranial nerves and remaining nerves of the body (includes peripheral nerves, neuromuscular junction, and spinal nerves)

Question 2

What are the divisions of the PNS

Answer 2

somatic nervous system
autonomic nervous system

Question 3

Differentiate Somatic and Autonomic nerves - how many neurons in the neuron system? where do they begin and end?

Answer 3

somatic = single neuron system to skeletal muscle
- motor neuron lies inside the brainstem or spinal cord
- sensory neuron lies outside CNS in DRG

autonomic - two neuron system to smooth muscle, cardiac muscle and glands
- preganglionic nerve bodies located in the CNS connect with ganglia outside the CNS
- postganglionic efferent fibers go from ganglia to end organ

Question 4

What neurotransmitters are involved in sympathetic and parasympathetic nerves?

Answer 4

sympathetic motor neuron sends acetylcholine, sensory neuron norepinephrine and epinephrine

parasympathetic both pre and postganglionic send acetylcholine

Question 5

What level of the spinal cord are sympathetic cell bodies?

Answer 5

preganglionic cell bodies between T1-L2

Question 6

What level of the spinal cord are parasympathetic cell bodies?

Answer 6

preganglionic cell bodies in brain stem and sacral portions and exit via cranial nerves III, VII, IX, X, S2 and S3

Question 7

what are the glia of the PNS

Answer 7

Schwann cells - form the myelin sheath around axons in PNS (similar to oligodendrocytes in CNS)

Satellite cells - surround neuron cell bodies in ganglia and provide support (similar to astro types in CNS)

Question 8

what are the connective tissues the surround peripheral nerves?

Answer 8

endoneurium - cushion between individual nerve fibers

perineurium - surrounds group of nerve fibers

epineurium - surrounds entire nerve trunk

Question 9

what type of information is sent through the dorsal root ganglia

Answer 9

sensory information from the body

Question 10

How does an action potential travel down an axon

Answer 10

stimulus received = depolarization

sodium ion permeability

potassium ion permeability = repolarize

Question 11

what mineral influences the excitability of nerves? what occurs when there is deficiency?

Answer 11

low calcium increases excitability
high calcium decreases excitability

tetany in observed in cases of calcium deficiency

Question 12

What is Wallerian degeneration?

Answer 12

when the nerve axons becomes damaged, schwann cells proliferate, axis cylinder dies, myelin disintegrates - can cause profound effects on end organ or complete atrophy of muscle fibers

Question 13

how might a patient describe the feeling of peripheral nerve damage?

Answer 13

• gradual onset of numbness, prickling, or tingling in feet or hands, which can spread upwards into legs and arms
• sharp, jabbing, throbbing, freezing, or burning pain
• extreme sensitivity to touch

Question 14

what are other symptoms of peripheral nerve damage?

Answer 14

• reduced or loss of function associated structures to damaged nerves
• lack of coordination and falling
• muscle weakness, atrophy or paralysis if motor nerves are affected
• chronic severe disproportionate pain: complex regional pain syndrome

Question 15

What is the first degree of the Sunderland Scale for degree of nerve degeneration?

Answer 15

• a reversible local conduction block at the site of the injury
• no degeneration

Question 16

What is the second degree of the Sunderland Scale for degree of nerve degeneration?

Answer 16

• pressure destroys point on axon and all distal to injury
• supporting structures remain intact

Question 17

What is the third degree of the Sunderland Scale for degree of nerve degeneration?

Answer 17

• axon is completely severed and most of supporting structures
• endoneurium severed, fascicles intact

Question 18

What is the fourth degree of the Sunderland Scale for degree of nerve degeneration?

Answer 18

• axon, endoneurium and fascicles severed
• nerve graft required

Question 19

What is the fifth degree of the Sunderland Scale for degree of nerve degeneration?

Answer 19

entire nerve trunk severed including epineurium

Question 20

how are neurons regenerated in the PNS?

Answer 20

• axons still attached to cell bodies are viable and capable of regrowth
• extensive connective tissue allows for proximal ion and attachment for regrowth possibility
• debris from denervation can be removed
• proliferation neurilemma has channels for the axon to grow into

Question 21

what is the rate of neuron regeneration in the PNS? what does it depend on?

Answer 21

thought to be 1-2mm/day

• axoplasmic flow
• axon finding proper path to end organ
• overproliferation of connective tissue (scar formation at severed end)
• infection