{A band}{}{Length of the myosin myofilament in a sarcomere.}
{abducens nerve}{}{Cranial nerve VI; motor fibers transmit impulses to muscles that move the eyes}
{abduction}{}{Movement away from the midline.}
{absorption}{}{Taking in of substances by cells or membranes.}
{accessory nerve}{}{Cranial nerve XI; motor fibers transmit impulses to muscles that move the soft palate, pharynx, larynx, neck, and back.}
{acetylcholine}{}{Neurotransmitter substance released from motor neurons, all preganglionic neurons of the parasympathetic and sympathetic divisions, all postganglionic neurons of the parasympathetic division, some postganglionic neurons of the sympathetic division, and some central nervous system neurons.}
{acetylcholinesterase}{}{Enzyme found in the synaptic cleft that causes the breakdown of acetylcholine to acetic acid and choline, thus limiting the stimulatory effect of acetylcholine.}
{acid}{}{Molecule that is a proton donor; any substance that releases hydrogen ions (H╠).}
{acrosome}{}{Cap on the head of the spermatozoon, with hydrolytic enzymes that help the spermatozoon to penetrate the ovum.}
{actin myofilament}{}{Thin myofilament within the sarcomere; composed of two F actin molecules, tropomyosin, and troponin molecules.}
{action potential}{}{Change in membrane potential in an excitable tissue that acts as an electric signal and is propagated in an all-or-none fashion.}
{activation energy}{}{Energy that must be added to molecules to initiate a reaction.}
{active site}{}{The portion of an enzyme that binds only to its substrate.}
{active transport}{}{Carrier-mediated process that requires ATP and can move substances against a concentration gradient.}
{adduction}{}{Movement of a body part toward the midline.}
{adenine}{}{One of the four nucleotides found in DNA and RNA; abbreviated as "A."}
{adenohypophysis}{}{Portion of the hypophysis derived from the oral ectoderm; commonly called the anterior pituitary.}
{adenosine diphosphate [ADP]}{}{Adenosine, an organic base, with two phosphate groups attached to it. Adenosine diphosphate combines with a phosphate group to form adenosine triphosphate.}
{adenosine triphosphate [ATP]}{}{Adenosine, an organic base, with three phosphate groups attached to it. Energy stored in ATP is used in nearly all of the endergonic reactions in cells.}
{adipose tissue}{}{Fat-storing tissue}
{adrenal cortex}{}{Outer portion of the adrenal gland.}
{adrenal gland}{}{Also called the suprarenal gland. Located near the superior pole of each kidney, it is composed of a cortex and a medulla. The adrenal medulla is a highly modified sympathetic ganglion that secretes the hormones epinephrine and norepinephrine; the cortex secretes aldosterone and cortisol as its major secretory products.}
{adrenal medulla}{}{Inner portion of adrenal gland.}
{adventitia}{}{Outermost layer of the digestive tract, also called the serosa.}
{aerobic respiration}{}{Breakdown of glucose in the presence of oxygen to produce carbon dioxide, water, and approximately 38 ATPs; includes glycolysis, the citric acid cycle, and the electron transport chain.}
{afferent [sensory] division}{}{Nerve fibers that send impulses from the periphery to the central nervous system.}
{afferent arteriole}{}{Vessel that conveys blood to glomerulus of nephron within kidneys.}
{afterpotential}{}{A period of hyperpolarization in a cell's membrane following an action potential.}
{alimentary canal}{}{Tubular portion of digestive tract that leads from the mouth to the anus.}
{allele}{}{Any one of a series of two or more different genes that may occupy the same position or locus on a specific chromosome.}
{alpha motor neuron}{}{Neuron involved in a reflex arc that causes the withdrawal reflex.}
{alveolus [alveoli]}{}{Air sac of lung; saclike structure.}
{amine hormone}{}{A hormone formed from an amino acid.}
{amino acid}{}{Class of organic acids that comprise the building blocks for proteins.}
{amino group}{}{Organic functional group found on all amino acids; has the chemical structure of [ùNH╥].}
{amylase}{}{Enzyme that hydrolyzes starch.}
{anaerobic respiration}{}{Breakdown of glucose in the absence of oxygen to produce lactic acid and two ATPs; consists of glycolysis and the reduction of pyruvic acid to lactic acid.}
{anal canal}{}{Last portion of the digestive system, which ends in the anus. Feces travel through the anal canal before exiting the body.}
{anaphase}{}{Time during cell division when chromatids divide [or in the case of first meiosis, when the chromosome pairs divide].}
{anatomy}{}{Branch of science dealing with the form and structure of body parts.}
{anterior}{}{Front.}
{antidiuretic hormone [ADH]}{}{Hormone that the posterior pituitary lobe releases to enhance water conservation by kidneys.}
{apocrine sweat gland}{}{Sweat glands that produce organic secretions traditionally are called apocrine. These sweat glands, however, are actually merocrine glands; see also merocrine and holocrine.}
{apocrine}{}{Gland whose cells contribute cytoplasm to its secretion [e.g., mammary glands]. Sweat glands that produce organic secretions traditionally are called apocrine. These sweat glands, however, are actually merocrine glands; see also merocrine and holocrine.}
{appendicular skeleton}{}{The portion of the skeleton consisting of the upper limbs and the lower limbs and their girdles.}
{arachnoid}{}{Thin, cobweb-appearing meningeal layer surrounding the brain; the middle of the three layers.}
{archenteron}{}{Anatomical and functional portion of an embryo at the gastrula stage formed from the invagination of the blastocoel.}
{areolar tissue}{}{Loose fibrous connective tissue with a proteinaceous matrix.}
{arrector pili muscle}{}{That which raises hair; Smooth muscle attached to the hair follicle and dermis that raises the hair when it contracts.}
{arteriole}{}{Minute artery with all three tunics that transports blood to a capillary.}
{articular cartilage}{}{Hyaline cartilage covering the ends of bones within a synovial joint.}
{ascending tract}{}{Group of nerve fibers in spinal cord that transmits sensory impulses upward to brain.}
{association area}{}{Region of the cerebral cortex controlling memory, reasoning, judgment, and emotions.}
{atomic number}{}{Number of protons in each type of atom.}
{autonomic nervous system [ANS]}{}{Composed of nerve fibers that send impulses from the central nervous system to smooth muscle, cardiac muscle, and glands.}
{autorhythmic}{}{Spontaneous and periodic; for example, in smooth muscle it implies spontaneous [without nervous or hormonal stimulation] and periodic contractions.}
{autosome}{}{Any chromosome other than a sex chromosome; normally occur in pairs in somatic cells and singly in gametes.}
{axial skeleton}{}{Skull, vertebral column, and rib cage.}
{axoaxonic synapse}{}{A synapse between the axon of one neuron and the axon of another; usually plays a regulatory [excitatory or inhibitory] role in nerve impulse transmission.}
{axon}{}{Main central process of a neuron that normally conducts action potentials away from the neuron cell body.}
{axon hillock}{}{Area of origin of the axon from the nerve cell body.}
