DEFINITIONS - LETTER A

ABSORPTION - here, we use the term absorption in the pharmacological sense, meaning that a substance has appeared in the bloodstream; the substance may have been absorbed into the blood from the digestive tract, from the lungs, through the skin, or from other tissues (for example, following injection) (see reabsorption)

ACTION POTENTIAL - this is an electrical signal that travels along the membrane of an excitable cell such as a nerve, muscle or glandular cell.  An action potential is a signal that tells a cell to do something.  For example, an action potential makes a muscle cell contract.  Action potentials are very brief positive fluctuations in the electrical potential of a cell membrane (see membrane potentials).

ACTIVE TRANSPORT - generally, any mechanism requiring the use of ATP energy to transport a solute particle across a membrane; for example, the Na⁺/K⁺-ATP-dependent pump is an example of an active transport mechanism

ADEQUATE STIMULUS - a stimulus which is appropriate for a specific type of sensory receptor... for example, light is the adequate (appropriate) stimulus for the cells of the retina.  However, mechanical pressure on the eyeball (not the adequate or appropriate stimulus) can stimulate the cells of the retina. Pressing gently on the lateral aspect of the eyeball, against the eyelid rather than directly against the eye, causes a dark circle to appear in the middle of the visual field.  Here, the brain interprets a mechanical stimulus as an image which does not exist!

AFFERENT - entering the spinal cord; a sensory nerve carrying information from an organ and into the spinal cord is an afferent nerve. (see efferent)

ALLOSTERIC - effects of ligands which result in changes to the shape of molecules to which the ligands bind; for example, the binding of a hormone might change the shape of a receptor... this would be an allosteric effect

ALKALOSIS - a condition in which plasma pH falls significantly below 7.4

AMBULATORY - able to walk

AMMENHOREA - an absence of menses

ANASTAMOSES - locations where one tissue fuses with another, usually where they would not do so; for example, where 2 blood vessels fuse in a location where they normally would not; an anastomosis may be natural or artificially created in a surgical procedure, for example

ANTIGENIC EPITOPES - 3-dimensional outcroppings on molecules which act as a stimulus (antigen) against which cells can raise an immune response (produce antibodies) 

AQUEOUS SOLUTION - a mixture dissolved in water; biological solutions are generally aqueous

ATROPHY/ATROPHIC - tissue wasting; eg. muscles atrophy or "waste away" from disuse

AUTOCRINE - one cell type signals for a response by cells of the same type (eg. a platelet signalling for ADH release by platelets)

AUTONOMIC NERVOUS SYSTEM - the involuntary nervous system with its sympathetic and parasympathetic branches

AUTOSOMAL CHROMOSOMES - all chromosomes other than the sex (X,Y) chromosomes.  Occur as 23 pair (n=46) in humans.

AXON TERMINAL - the end of the nerve that sends messages to other cells