Neurones provide a rapid means of communication over long distances, by passing impulses (action potentials) very rapidly along their axonal membranes. They can be separated into several groups with different physiological functions.
Neurones provide a rapid means of communication over long distances within the body by passing action potentials - electrical impulses - along their axonal membranes.
Myelinated axons have larger diameters, lower thresholds and higher conduction velocities than unmyelinated axons; the speed of conduction of the impulse is directly related to the degree of myelination.
Nerve trunks contain many axons with different degrees of myelination. They are classified according to diameter or conduction velocity into two systems: A-alpha, A-beta, A-delta, and C; or Groups I, II, III and IV.
Key Words: Action potential, 'Spike', voltage gated sodium channels, voltage gated potassium channels, saltatory conduction, myelin, the relation between conduction velocity and axon diameter
The Compound Action Potential is recorded from a nerve trunk containing many axons
Image source: michaeldmann.net
The thresholds of large axons are lower than the thresholds of small diameter axons. The diagram shows the effects of increasing stimulus strength, as a result of which more and more small axons become active. Smaller axons conduct more slowly and this is the basis of identifying A-beta (large myelinated), A-gamma and A-delta fibres (finely myelinated), and C (unmyelinated) axons.
For mammalian myelinated axons the conduction velocity (m/sec) is approximated 6x the diameter of the axon.
Compound Action Potential and Conduction Velocity
Nerves contain many axons of different sizes - some heavily myelinated, others finely myelinated, and others unmyelinated. These have different thresholds to electrical stimulation and different rates of conduction along the axon.
Large diameter axons are heavily myelinated, have low thrsholds to electrical stimulation, and are rapidly conducting.
At the opposite end of the spectrum, unmyelinated axons have very high thresholds and conduct very slowly.
In the diagram opposite, recordings were made using electrodes applied to the nerve, and the nerve was stimulated electrically some distance away form the receording site. The vertical axis shows the receorded compound action potential, and the time scale shows the timing of the arrival of the action potentias in each group of nerve fibres.
The top trace in the diagram opposite shows the A-alpha only peak over a longer time course.
The stimulus necessary to initiate an action potential in small axons is larger than for larger diameter axons.
As the stimulus is increased, smaller axons begin to generate their action potential, and these potentials are more slowly. Examples are the A-beta, A-gamma and A-delta potentials; it takes more time for the action potentials of smaller axons to arrive at the recording site.
All of the A waves are due to action potentials in myelinated axons: A alpha axons have a lot lf myelin, whereas A-delta axons are finely myelinated.
Finally at very high stimulus strengths the unmyelinated C-fibres are activated, and it can be seen that the conduct very slowly.
Conduction velocities of different axons
The Conduction Velocity is the speed at which the nerve impulse is conducted in an axon, and is measured as metres/second.
Axons conduct action potentials at different velocities, and the fastest use Saltatory Conduction, in which the impulse jumps from one node of Ranvier to the next.
Large myelinated axons conduct rapidly (100+ m/sec). The high speed is due largely to the myelin: the AP jumps from one node of Ranvier to the next because currents follow pathways of least resistance, and myelin (rolled fatty cell membrane) has a high resistance
A fibres conduct at high speed: A-alpha conducts more rapidly than A-beta and A-delta (because of the lower degree of myelination of the slower conducting axons).
C fibres are unmyelinated and conduct very slowly (1 m/s).
For myelinated axons the conduction velocity (m/sec) is approximately 6 times the diameter in microns.
Classification of Nerve Fibres
Nerve trunks contain many axons with different degrees of myelination, and therefore different conduction velocities.
Two systems of classification of nerve fibres have evolved according to (a) diameter or (b) conduction velocity, by scientists studying the (a) sensory or (b) motor systems. These are two different ways of describing axons of the same size.
Erlanger and Gasser classified motor axons into two groups of A fibres and C (unmyelinated) axons.
<=1 micrometres (unmyelinated)
Sensory Axons were classified using a different system of Fibre Types: