The Peripheral Nervous System

Question 1

What are dorsal root ganglia and what are their features?

Answer 1

Groups of sensory neurone cell bodies
Associated with spinal dorsal roots
Have no dendrites
Pseudounipolar cells

Question 2

Where do postganglionic cell bodies lie and how are they arranged?

Answer 2

Lie in Autonomic ganglia

Either arranged in rows paravertebrally (sympathetic NS) or closer to internal organs (parasympathetic NS)

Question 3

How are bundles of axons arranged?

Answer 3

Bundles of individual axons = Fascicles

Bundles of fascicles = Nerves

Question 4

How are axons packaged to prevent damage by movement?

Answer 4

Endoneurium - loose connective tissue wrapping individual axons and their associated Schwann cells
Perineurium - dense connective tissue wrapping fascicles (groups of axons)
Epineurium - loose connective tissue surrounding the whole nerve

Question 5

Features of unmyelinated axons?

Answer 5

Usually small diameter
Clothed in cytoplasm of Schwann cells (neurolemma)
Slow conduction speed by continuous conduction (1m/s)

Question 6

Features of myelinated axons?

Answer 6

Large diameter
Clothed in succession of tightly wrapping Schwann cells (wrapping forces loss of cytoplasm so sheath of cell membranes is formed)
Fast conduction speed by saltatory conduction (120m/s)

Question 7

Process of saltatory conduction?

Answer 7

Depolarisation of axon only occurs at nodes of Ranvier
No Na+ channels in membrane of insulating Schwann cells so depolarisation cannot occur
So depolarisation “jumps” between adjacent nodes of Ranvier

Question 8

Features of sensory neurones in PNS?

Answer 8

Autonomic and somatic neurones are the same
Receptors on skin/viscera trigger action potential when stimulated
Impulses travel via dorsal root ganglion to CNS

Question 9

Features of motor neurones in PNS? There are differences.

Answer 9

Somatic - soma in CNS, impulses travel along ventral root through spinal nerve to appropriate skeletal muscle to trigger contraction
Autonomic - Preganglionic neurone in CNS, impulse travels along axon where is synapses at autonomic ganglion with postganglionic neurone, impulse then carried to appropriate target

Question 10

Stages of ANS sensory and motor transmission?

Answer 10

Sensory - Periphery of body to dorsal root ganglion to dorsal horn of grey matter
Motor - From ventral horn of grey matter to preganglionic neurone to autonomic ganglion to postganglionic neurone

Question 11

Stages of SNS sensory and motor transmission?

Answer 11

Sensory - Periphery of body to dorsal root ganglion to dorsal horn of grey matter
Motor - Ventral horn of grey matter via the ventral root

Question 12

Spinal nerve formed where and from what?

Answer 12

Intervertebral foramen

From junction of dorsal and ventral roots

Question 13

What do spinal nerves divide into and what do these innervate?

Answer 13

Dorsal and ventral rami
Dorsal rami innervate muscle and skin of the back
Ventral rami innervate muscles and skin of the rest of body

Question 14

What is a dermatome?

Answer 14

An area of skin innervated by a particular spinal nerve

Question 15

What is a myotome?

Answer 15

An area of muscle innervated by a particular spinal nerve

Question 16

Where are spinal nerves recombined to form peripheral nerves?

Answer 16

At a point called a plexus

Question 17

Which spinal nerves innervate the upper limbs and how?

Answer 17

C5-T1

Ventral rami innervate upper limbs through the BRACHIAL plexus

Question 18

Which spinal nerves innervate the lower limbs and how?

Answer 18

L2-S2

Ventral rami innervate the lower limbs through the LUMBOSACRAL plexus

Question 19

Peripheral nerve changes after compression injury?

Answer 19

Compression injury breaks continuity of axoplasm so no action potentials can be propagated.
Distal part of nerve degenerates but proximal stump remains intact, macrophages phagocytose axonal/myelin debris.
Growth of axonal sprouts from proximal stump guided by scaffold of proliferating Schwann cells.
First axonal sprout makes contact with target organ, growth/myelination occurs.
Nodes of Ranvier in regenerated axon will be closer together than initially (slower transmission)

Question 20

What is and what are the 2 types of peripheral neuropathy?

Answer 20

Progressive degeneration of peripheral nerves
Segmental demyelination - Schwann cells die so slower continuous conduction replaces saltatory conduction in regions without Schwann cells
Axonal degeneration - Leads to complete conduction block

Question 21

What can be used to diagnose peripheral nerve disorders?

Answer 21

Conduction velocity - can determine is a peripheral neuropathy is present and what type
Nerve biopsy - can be used to study pathogenesis of the disease