Overview of the Peripheral Nervous System

Question 1

Is the distinction between the central and peripheral nervous systems arbitrary?

Answer 1

No, the nerve cells have different immune and antigenic properties

Question 2

Where are nerves often damaged?

Answer 2

When they enter/leave the spinal (vertebral) canal, as this ordinarily works to protect the spinal cord

Question 3

How might damage to vertebrae or IV discs lead to damage of sensory nerves?

Answer 3

Dorsal root ganglia may lie in the intervertebral foramen and therefore damage can crush these spinal nerves

Question 4

How do peripheral nerves obtain a blood supply?

Answer 4

Have small arteries within them to supply oxygen to the nerves themselves, and these arteries are branches of the spinal cord arteries

Question 5

What is a dermatome?

Answer 5

The area of skin innervated by a spinal nerve

Question 6

Describe lower motor neurones

Answer 6

Connect directly with skeletal muscle and have cell body in ventral horn (axons project from ventral root and synapse on muscle) and have a single axon projecting from the cell body which has a number of dendrites

Question 7

Describe upper motor neurones

Answer 7

Emerge from the brainstem or brain and synapse with lower motor neurones

Question 8

Describe sensory neurones

Answer 8

Have two sets of dendrite-like processes (one in periphery, other in spinal cord), cell body is T junction off of the axon and lies in the DRG, they end in the skin or muscle as sensory receptors which have a specialised connective tissue capsule

Question 9

Briefly describe myelination of a peripheral nerve

Answer 9

During growth of peripheral nerve the Schwann cell wraps itself many times around the axon to gradually squeeze out the cytoplasm until multiple layers of cell membranes are left which is then known as the myelin sheath

Question 10

What is endoneurium?

Answer 10

A thin protective membrane surrounding individual sensory and motor fibres

Question 11

What is perineurium?

Answer 11

Groups of functionally-related nerve fibres are collected in fascicles and surrounded by layer of CT

Question 12

What is epineurium?

Answer 12

Deep fascia that surrounds a whole peripheral nerve which consists of several fascicles bundled together with blood vessels

Question 13

Where are sensory receptors predominantly in the skin

Answer 13

Epidermal-dermal junction

Question 14

How are action potentials initiated in sensory nerve fibres?

Answer 14

Receptors have non-voltage-gated sodium channels which are opened by mechanical bending, when this occurs –> sodium influx via mechanosensitive channels –> graded depolarisation of a few mV –> generator potential formed –> generates action potentials in the axon where sodium-channels start to occur (first node of Ranvier)

Question 15

What is a generator potential?

Answer 15

The graded depolarisation formed in sensory afferents as a result of stimulation (mechanical bending)

Question 16

What does a greater generator potential lead to?

Answer 16

The greater the generator potential, the greater the frequency of action potentials produced

Question 17

What type of receptor are free sensory nerve endings?

Answer 17

Chemoreceptors

Question 18

Describe free sensory nerve endings

Answer 18

Form very fine nerve plexus which will then respond to chemical stimuli such as changes in pH, or chemicals such as peptides in ECF

Question 19

What is ‘noxious’ stimuli?

Answer 19

Actually, or potentially damaging tissue event

Question 20

What is the nerve fibre for free nerve endings?

Answer 20

C or Aδ

Question 21

Define conduction velocity

Answer 21

Speed at which an action potential travels along a nerve fibre

Question 22

How do you calculate conduction velocity?

Answer 22

Myelinated fibres: 6 x diameter

Unmyelinated fibres = diameter

Question 23

Describe and give examples of nerves with Aα nerve fibres

Answer 23

Largest diameter and velocity: alpha motorneurones (motor to skeletal muscle), 1a afferents (sensory from muscle spindle) and Ib afferents (sensory from Golgi tendon organ)

Question 24

Describe and give examples of nerves with Aβ nerve fibres

Answer 24

2nd fastest velocity; general sensory afferents (sensory from skin, viscera and secondary endings)

Question 25

Describe and give examples of nerves with Aγ nerve fibres

Answer 25

Medium velocity; gamma motoneurones (motor to muscle spindles)

Question 26

Describe and give examples of nerves with Aδ nerve fibres

Answer 26

Small diameter myelinated fibres; nociceptor and thermoreceptors (carries ‘fast’ pain from skin, muscle, joints and thermoreceptors)

Question 27

Describe and give examples of nerves with C nerve fibres

Answer 27

Unmyelinated fibres; nociceptor or thermoreceptor (carries ‘slow’ pain from skin, muscle, viscera and thermoreceptors)

Question 28

Describe nerve regeneration

Answer 28

Nerve fibres of PNS regenerate at about 1.5mm/day, however some never regenerate’ the further distal the nerve injury is, the more likely there is to be successful regeneration

Question 29

Explain how action potentials are generated in motor neurones

Answer 29

Sensory neurones synapse on the dendrites and cause a small depolarisation (EPSP) and summation (spatial or temporal) leads to the development of an action potential in the motor neurone

Question 30

Describe temporal summation

Answer 30

when one input fires two EPSPs close together in time, this may trigger an AP

Question 31

Describe spatial summation

Answer 31

when two different synaptic inputs are active at the same time so the two EPSPs add together and may trigger an AP

Question 32

Explain the biochemistry of summation on motor neurones

Answer 32

Summed EPSPs are due to an inward ionic current (sodium influx) caused by sensory axons releasing glutamate anion into the synapse and causing the AMPA receptor to open, allowing sodium influx into the cell. Current flows down and depolarises the whole cell, and the action potential begins at the axon hillock.