Sensory and Motor Clocks

There are sensory and motor clocks which control the rates of transmission and reception of information within the central nervous system of various organisms.

The basic unit of sensory information in an organism is the action potential, which is excited within a nerve cell and propagated along the length of a nerve fiber by a series of electrochemical impulses.

Sensory organs transmit information to the brain using the same code, but at different frequencies. The number of impulses which are triggered per unit of time corresponds to the different frequencies associated with each of the senses. The sense which is experienced depends on which part of the brain is stimulated.

Nerve impulses oscillate at frequencies ranging from 1 or fewer cycles per second, to several hundred cycles per second, and travel at subsonic speeds ranging from 1 to 100 meters per second depending on the diameter and temperature of the fiber, the speed being less for thin fibers and greater for thick fibers.

The distance which nerve impulses travel in one-eighteenth of a second is a biological constant of the nervous system. It indicates the moment when rapidly recurring stimuli are perceived as a single sensation. This distance corresponds to the maximum time lag which occurs between two frames of a moving picture before it begins to flicker.

Time intervals between two sensory stimuli necessary to distinguish them as separate range from 2 one-hundredths of a second, to 17 one-thousandths of a second. For light, the interval is 43 one-hundredths of a second, and for touch, 27 one-hundredths of a second.

Sensory and motor activities in the human body occur at various rates in relation to one another. Eye movements occur at faster rates than gustatory responses, which occur at greater rates than monosynaptic reflexes, such as a knee-jerk.

The human ear can distinguish 132 separate stimuli within a period of a second. The eye can distinguish up to 24 separate stimuli in a second. 10 stimuli within a second will cause voluntary muscles to remain in a permanent state of contraction. Under the same conditions, involuntary muscles remain contracted for up to several seconds.