Nerve impulses propagation speeds

When large areas of the membrane are depolarized in this manner, the electrochemical disturbance propagates in wave-like form down the membrane, generating a nerve impulse. Myelin sheets, formed by Schwann cells, wrap around nerve fibers and provide an electrical insulator that surrounds most of the nerve and greatly speeds up the propagation of the wave (signal) by allowing ions to flow in and out of the membrane... 

The idea that signals are transmitted along the nerve channels as an electric current had arisen as early as the middle of the nineteenth century. Yet even the first measurements performed by H. Helmholtz showed that the transmission speed is about lOm/s (i.e., much slower than electric current flow in conductors). It is known today that the propagation of nerve impulses along the axons of nerve cells (which in humans are as long as 1.5m) is associated with an excitation of the axon s outer membrane. 

What we believe to be particularly important in the result [Eq. (52)] is that the impulse speed depends strongly on the sodium current activation rate. Thus by measuring the impulse speed we obtain information not only about passive electric characteristics of the nerve fiber but also about the dynamics of the molecular structures responsible for the fiber s activity. A more comprehensive comparison of the above theory with experiment, in particular with the computer-aided treatment of the H-H model carried out in Reference (24), is given elsewhere, in which theory modifications that are more adequate to the H-H model are also analyzed. It should be noted, besides, that qualitatively similar results were obtained by Rinzel and Keller who studied impulse propagation in a FitzHugh-Nagumo model (which takes into account the inertial nature of the variable in the same manner as it does potassium conductance). 

We have already noted above that in analyzing excitable systems one has, more often than not, to deal with a parabolic equation with a nonlinear source. In this section we will concern ourselves with an excitable medium of a different type, where the signals are transmitted in the neuron network not by the local currents but by the nervous impulses traveling along the axons. The propagation speed of the activity wave will, if this transmission mode is possible at all, depend not only on the signal transmission speed but also on the other characteristics of nerve cells such as cell body capacitance, conductance, etc. 

The transmission of a nerve impulse through a synaptic junction differs in several respects from its conduction along a nerve fibre. A synaptic delay in the speed propagation of the impulse is characteristic of junctional transmission. The transmission is possible only in one direction, for example from the nerve to the muscle, but not in the opposite direction. The phenomena of summation of subthreshold impulses, of facilitation of the transmission of a series of impulses, are the specific features of synaptic transmission. 

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