absolute refractory period Brief time period (0.5 microsecond) following membrane depolarization during which the membrane is incapable of depolarizing again.
absorption Energy that stimulates a particular tissue to perform its normal function.
accommodation Adaptation by the sensory receptors to various stimuli over an extended period of time.
ACTH Adrenocorticotropic hormone. This hormone stimulates the release of glucocorticoids (cortisol) from the adrenal glands.
acidic reaction The accumulation of negative ions under the positive pole that produces hydrochloric acid.
acoustic microstreaming The unidirectional movement of fluids along the boundaries of cell membranes resulting from the mechanical pressure wave in an ultrasonic field.
action potential A recorded change in electrical potential between the inside and outside of a nerve cell, resulting in muscular contraction.
active electrode(1) The electrode that is used to drive ions into the tissues. (2) An electrode attached directly to the skin over a muscle that picks up the electrical activity produced by a muscle contraction.
acupressure The technique of using finger pressure over acupuncture points to decrease pain.
acute injury An injury in which active inflammation is present that includes the classic symptoms of tenderness, swelling, redness, and so on.
afferent (1) Conduction of a nerve impulse toward an organ. (2) Axons from neurons carrying a signal toward the spinal cord (a sensory fiber).
air space plate A capacitor type electrode in which the plates are separated from the skin by the space in a glass case. Used with shortwave diathermy.
alkaline reaction The accumulation of positive ions under the negative electrode that produces sodium hydroxide.
all-or-none response The depolarization of nerve or muscle membrane is the same once a depolarizing intensity threshold is reached; further increases in intensity do not increase the response.
alternating current Current that periodically changes its polarity or direction of flow.
ampere Unit of measure that indicates the rate at which electrical current is flowing.
amplitude (1) The variation in pressure found along the path of the wave in units of pressure (N/m2). (2) The intensity of current flow as indicated by the height of the waveform from baseline. (3) The size of the potential. In a sensory nerve assessment, this represents the summed action potential traveling across one point of that nerve. In a motor nerve assessment, this represents the summed action potentials traveling across the collective muscle fibers under the pickup electrode. Motor amplitudes are typically much larger than sensory action potentials.
analgesia Loss of sensibility to pain.
annulus fibrosus The interlacing cross-fibers of fibroelastic tissue that are attached to adjacent vertebral bodies that contain the nucleus pulposus.
anode The positively charged electrode.