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...

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...