24B15: Exam Report

  1. Outline the general classification of nerve fibres including details on their function, size and conduction speed (30% of marks).

  2. Describe the mechanisms of action potential generation and propagation along a myelinated peripheral nerve fibre (70% of marks).

47% of candidates passed this question.

The first component of this question required a list of the different nerve fibre types including the breakdown of A, B and C fibres along with their primary function, for example motor / sensory / parasympathetic / sympathetic, presence of myelin, size and velocity of conduction.

The second component of this question required the mechanism by which an action potential arises and is propagated in a myelinated nerve.

This required a detailed overview of electrolyte movement across the neuronal membrane during depolarisation, timing and explanation of the refractory period, the basis for unidirectional movement and the mechanism of saltatory conduction which occurs in a myelinated nerve.

K1vi / 24B15: Outline the general classification of nerve fibres including details on their function, size and conduction speed

Classification of nerve fibres

Fibre

Function

Diameter (μm)

Conduction velocity (m/s)

Aα

Skeletal motor, joint position

10-20

60-120

Aβ

Touch, pressure

5-10

40-70

Aγ

Muscle spindle motor

3-6

15-30

Aδ

Pain, temperature touch

2-5

10-30

B

Preganglionic autonomic

 1-3

3-15

C

Pain

0.5-1

0.5-2

Nerve Action potential

Graph of membrane potential phases over time.

Phases

1. Depolarization. Inward Na+ current

The action potential is all or none because, once the threshold is attained, there is a ‘positive feedback’ between membrane depolarization and the opening of more and more Na+ channels

  • Tens of microseconds

2. Repolarization

Opening of voltage gated K+ channels – outward K+ current > over the next millisecond or so 

3. Hyperpolarization (afterpotential)

Due to the Na+ and K+ channels not having returned to their previous states.

The K+ channels recover more slowly.

Refractory period

Absolute

  • During the first millisecond of the nerve action potential, it is impossible to evoke another in the absolute refractory period because the Na+ channels are inactivated 

Relative

  • Period of 10−15 ms. responsive and open only to a larger-than-normal stimulus.

Propagation in a myelinated peripheral neuron

  • Known as Saltatory Conduction
  • Myelin formed by Schwann cells and glial cells acts as an insulator and greatly increases the resistance of the membrane.
  • Between the myelin blocks are myelin-free sites called nodes of Ranvier
  • Characterized by a high concentration of voltage-dependent Na+ channels> where action potentials are generated and jump along from node to node in a ‘saltatory’ manner.
  • It is unidirectional since each node is refractory for a time after stimulation 

Author: Nazma Navilehal Rajasab