Conduction of Nerve Impulse

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, ri) and a higher membrane resistance (rm). 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 Conduction of Nerve Impulse (constant of length) becomes 2.7 times greater than the impulse. When the conductor’s structure is homogeneous, the reduction is exponential. Since rm and ri 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 ri 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.


References in periodicals archive ?
These activities of AST-OPC1 include production of neurotrophic factors, stimulation of vascularization, and induction of remyelination of denuded axons, all of which are critical for survival, regrowth, and conduction of nerve impulses through axons at the injury site.
It plays a role in the conduction of nerve impulses, according to an (http://imr.
Choline likewise supports to maintain the assembly of cellular membranes, aids in the conduction of nerve impulses, supports in the immersion of fat and decreases chronic inflammation.
According to the company AST-OPC1 is an oligodendrocyte progenitor population derived from human embryonic stem cells, with three potentially reparative functions, including production of neurotrophic factors, stimulation of vascularization, and induction of remyelination of denuded axons, all of which are critical for survival, regrowth and conduction of nerve impulses through axons at the injury site.
MS is an autoimmune condition when antibodies produced by the body turn on itself, particularly the fatty sheath that covers all nerves, which is crucial for the conduction of nerve impulses.
Myelin is the fatty sheath that coats the brain's nerve axons, allowing for efficient conduction of nerve impulses, and iron plays a key role in myelin production.
Next to a wall of papier-mache switch plate covers, where colorful electrical wires pooled to the floor, was a sign that read: "FACT: Myelin (the white matter of brain) is an electrically insulated, fatty sheath surrounding axons (nerve fibers) allowing efficient conduction of nerve impulses in the body controlling movement.
Showing how, at the molecular level, conduction of nerve impulses can be restored even in the absence of myelin.
scleroses, or lesions) which further interrupts the conduction of nerve impulses (Rumrill, Kaleta, & Battersby, 1996).
Such studies at NYU have demonstrated that at the interruptions in the myelin sheath called the nodes of Ranvier, the nerve fiber membrane has high concentrations of protein molecules corresponding to the sodium channels essential for the conduction of nerve impulses that jump from node to node.
As the myelin becomes destroyed, the patches of myelin are replaced by scar tissue which produces lesions or plaques; thus, further interruption of the conduction of nerve impulses occur (Wolf, 1984).
Another issue dealt with in considerable detail was the mechanisms of the conduction of nerve impulses and the ways this conduction can fail, particularly in demyelinating diseases, such as multiple sclerosis (MS).