Nonmyelinated nerve fibres

Myelinated Nerve Fiber

Vertebrates have two main types of nerve fibre, the larger fast-conducting axons, 1 to 25 fxm in diameter, being myelinated, and the small slowly conducting ones (under 1 fxm) being non-myelinated. Most of the fibres of the autonomic system are non-myelinated, as are peripheral sensory fibres subserving sensations like pain and temperature where a rapid response is not required. Almost all invertebrates are equipped exclusively with non-myelinated fibres, but where rapid conduction is called...

Factors affecting the threshold for excitation

As seen, for example, in Fig. 2.7, excitation of a nerve fibre involves the rapid depolarization of the membrane to a critical level normally about 15 mV less negative than the resting potential. The critical level for excitation is the membrane potential at which the net rate of entry of Na+ ions becomes exactly equal to the net rate of exit of K+ ions plus a small contribution from an entry of Cl- ions. Greater depolarization than this tips the balance in favour of Na+, and the regenerative...

Extracellular recording of the nervous impulse

Diagram Monophasic Recording

There are many experimental situations where it is impracticable to use intra-cellular electrodes, so that the passage of impulses can only be studied with the aid of external electrodes. It is therefore necessary to consider how the picture obtained with such electrodes is related to the potential changes at membrane level. Since during the impulse the potential across the active membrane is reversed, making the outside negative with respect to the inside, the active region of the nerve...