Conduction of Nerve Impulse

the transmission of a signal in the form of a wave of excitation within a single neuron or from one cell to another.

Conduction of nerve impulses along nerve conductors occurs by means of electrotonic and action potentials, which move along the fiber in both directions without transferring to neighboring fibers. Transmission of intercellular signals is achieved through the synapses, usually by means of mediators that induce postsynaptic potentials. Nerve conductors may be regarded as cables with a relatively low axial resistance (axoplasm resistance, r_i) and a higher membrane resistance (r_m). The nerve impulse moves along the nerve conductor by means of a local current passing between the resting and active parts of the nerve. As the distance from the excitation’s point of origin increases, there occurs in the conductor a gradual reduction of the impulse; the distance (constant of length) becomes 2.7 times greater than the impulse. When the conductor’s structure is homogeneous, the reduction is exponential. Since r_m and r_i are in inverse ratio to the diameter of the conductor, reduction of the nerve impulse occurs earlier in the thin fibers than in the thick ones. The imperfection of the cable properties of the nerve conductor is compensated for by their excitability.

The main prerequisite for excitation is the presence of a resting potential in the nerve. If the local current passing through the resting section causes a depolarization of the membrane reaching a critical threshold, this process leads to a spreading action potential (AP). The ratio of the level of threshold depolarization to the amplitude of the AP, usually no less than 1:5, ensures high reliability of conduction: the sections of the conductor with the capacity to generate AP may be so far apart that in order to overcome their distance from one another the nerve impulse decreases its amplitude to a little over one-fifth of its original amplitude. This attenuated signal will again be intensified to a standard level (AP amplitude) and will then be able to continue along the nerve.

The rate at which a nerve impulse is conducted depends on the velocity with which the membrane capacity on the section in front of the impulse discharges to the level of the AP’s generation threshold. This threshold, in turn, is determined by the geometric characteristics of the nerves, by changes in the nerves’ diameter, and by the presence of branching ganglia. Specifically, thin fibers have a higher r_i and a greater surface capacity; hence the conduction rate of the nerve impulse along them is lower. At the same time, thickness in nerve fibers prevents the existence of many parallel channels of communication.

The conflict between the physical properties of nerve conductors and requirements for compactness in the nervous system was resolved in the course of vertebrate evolution by the appearance of myelinated fibers. The rate of nerve-impulse conduction in the myelinated fibers of warm-blooded animals (despite their small diameter of 4–20 microns) reaches 100-120 m/sec. Generation of an AP occurs only in limited sections of their surface, called the nodes of Ranvier, whereas nerve-impulse conduction along the internodal sections is achieved electrotonically. Some medicinal substances, such as anesthetics, decelerate nerve-impulse conduction to the point of total block. Such substances are used in medical practice for the relief of pain.