Nervous impulse, propagation

Figure 9. Diagram of local currents in the excited region accompanying nervous impulse propagation.

Figure 10. Relationship between nervous impulse propagation speed and rate of sodium current activation. The solid curve was calculated via Eqs. (49)-(50), the dotted curve via Eq. (52). The experimental value is marked with an asterisk.

Local anaesthetics are drugs that reversibly interrupt impulse propagation in peripheral nerves thus leading to autonomic nervous system blockade, analgesia, anaesthesia and motor blockade in a desired area of the organism. 

The more detailed analysis given in Reference (9) leads to the same formula. Below it will be shown that the above stimulus propagation behaviour bears a profound analogy with propagation of a nervous impulse in the sense that in both cases the impulse speed essentially depends on the characteristic activation time which in this model is the film Recomposition time and, in the H-H model, the sodium channel activation time. The properties of the Lillie model have been described in more detail in Reference (11) where a long list of references can also be found. 

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. 

No stimulant drugs are known in this class which consists mainly of the local anaesthetics, much used in dental and spinal anaesthesia. These preferentially block the smaller fibres with the result that sensory nerves are affected more than motor nerves. What local anaesthetics do is to elevate the threshold for excitation and thus they block propagation of the nervous impulse without depolarizing the fibre. 

The a2 receptor, when stimulated, inhibits adenylate cyclase (in a reaction that needs GTP) and the level of cAMP falls. The ai-receptor, when stimulated, propagates the nervous impulse by liberating calcium ions, and does not affect adenylate cyclase. 

Choline functions in fat metaboHsm and transmethylation reactions. Acetylcholine functions as a neurotransmitter in certain portions of the nervous system. Acetylcholine is released by a stimulated nerve cell into the synapse and binds to the receptor site on the next nerve cell, causing propagation of the nerve impulse. 

It is now well ascertained that dendrites are capable of propagating action potentials not only in distal to proximal direction, but also in the reverse direction by back-propagation after initiation at the cell body (Ludwig and Pittman, 2003). The so-called law of dynamic polarization enunciated by Cajal (see Berlucchi, 1999) was aimed at stating the unidirectional propagation of excitations within the nervous system, and assumed that nerve impulses are conducted from the dendrite or soma to axon terminals. This dogma is now being reconsidered, not only in view of the evidence of dendrodendritic synapses, but also in view of the existence of electrical synapses in which the flow of information can be bidirectional. 

Schwann s ceDs cells that produce myelin in the peripheral nervous system. These cells are located between the axon and axon terminal of neurons and create the myelin sheath which aides in insulating axons and in increasing impulse speeds as they are propagated through the neuron (allowing for salutatory conduction). 

In contrast to the endocrine system that achieves long-term control via chemical (hormonal) mechanisms, the nervous system relies on more rapid mechanisms of chemical and electrical transmission to propagate signals and commands. The rapid conduction of impulses is essential in allowing the nervous system to mediate shortterm and near immediate communication and control between various body systems. 

A striking example of the importance of ion channels is their role in the propagation of impulses by neurons, the fundamental units of the nervous system. Here we give a thermodynamic description of the process. 

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