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Absolute refractory period: The period after a nerve’s depolarization during which a subsequent depolarization cannot occur, used for recharging the electrical potential.

Absolute zero: Theoretically, the lowest possible temperature, equal to –273°C or –460°F. At this point, all atomic and molecular motion ceases.

Absorption: The process of a medium collecting thermal energy and changing it to kinetic energy.

Accommodation: The decrease in a nerve’s action potential frequency over time when exposed to an unchanging depolarization stimulus.

Acetylcholine: Neurotransmitter responsible for transmitting motor nerve impulses.

Acoustical interface: A surface where two materials of different densities meet.

Acoustical spectrum: Energy transmitted through mechanical waves.

Acoustical streaming: The unidirectional flow of fluids within the tissues caused by the application of therapeutic ultrasound.

Actin: A contractile muscle protein.

Actinomycosis: A disease state of actin caused by a fungus.

Action potential: The change in the electrical potential of a nerve or muscle fiber when stimulated.

Activities of daily living: Fundamental skills required for a certain lifestyle, including mobility, self-care, and grooming.

Acupuncture points: Points on the skin theorized to control systemic functions. These points lie along 12 main channels, eight secondary channels, and a network of subchannels.

Acute: Of recent onset. The period after an injury when the local inflammatory response is still active.

ADA: See Americans With Disabilities Act.

A-delta fibers: A type of nerve that transmits painful information that is often interpreted by the brain as burning or stinging pain.

Adenosine triphosphate (ATP): An important source of energy for intracellular metabolism.

Adriamycin: An antibiotic medication.

Aerobic: Requiring the presence of oxygen.

Afferent: Carrying impulses toward a central structure, for example, the brain.

Alarm stage: The first stage in the general adaptation syndrome in which the body readies its defensive systems.

Albinism: A condition in which the individual lacks pigmentation of the skin, hair, and eyes. The skin is prone to sunburns and the eyes are particularly sensitive to light (photophobia).

Allograft: A replacement or augmentation of a biological structure with a synthetic one.

Alpha-motoneurons: Efferent motor neurons that innervate muscle fibers.

ALS: See Anterior lateral system.

Alternating current: The uninterrupted flow of electrons marked by a change in the direction and magnitude of the movement.

Americans With Disabilities Act: Legislation passed in 1990 (Public Law 101-336) that protects the rights of disabled individuals by creating standards to ensure access and prohibit discrimination in transportation, accommodation, public services, and so on.

Amino acids: Building blocks of protein.

Amperage: The rate of flow of an electrical current. One ...

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