gallbladder: The pear-shaped reservoir for bile after it is secreted by the liver.
gametes: Reproductive cells (spermatozoa or oocytes) that contain half the normal chromosome complement.
gametogenesis: The formation of gametes.
gamma aminobutyric acid (GABA): A neurotransmitter of the central nervous system whose effects are generally inhibitory.
gamma motor neurons: Motor neurons that adjust the sensitivities of muscle spindles (intrafusal fibers).
ganglion/ganglia: A collection of neuron cell bodies outside the central nervous system.
gangliosides: Glycolipids that are important components of cell membranes in the central nervous system.
gap junctions: Connections between cells that permit electrical coupling.
gaster: The stomach; the body, or belly, of a skeletal muscle.
gastrectomy: The partial or total surgical removal of the stomach.
gastric: Pertaining to the stomach.
gastric glands: The tubular glands of the stomach whose cells produce acid, enzymes, intrinsic factor, and hormones.
gastric inhibitory peptide (GIP): A duodenal hormone released when the arriving chyme contains large quantities of carbohydrates; triggers the secretion of insulin and a slowdown in gastric activity.
gastrin: A hormone produced by enteroendocrine cells of the stomach, after exposure to mechanical stimuli or stimulation of the vagus nerve, and of the duodenum, after exposure to chyme that contains undigested proteins.
gastritis: An inflammation of the stomach.
gastroenteric reflex: An increase in peristalsis along the small intestine; triggered by the arrival of food in the stomach.
gastroileal reflex: Peristaltic movements that shift materials from the ileum to the colon; triggered by the arrival of food in the stomach.
gastrointestinal (GI) tract: See digestive tract.
gastroscope: A fiber-optic instrument that permits visual inspection of the stomach lining.
gastrulation: The movement of cells of the inner cell mass that creates the three primary germ layers of the embryo.
gene: A portion of a DNA strand that functions as a hereditary unit and is located at a particular site on a specific chromosome.
genetic engineering: Research and experiments involving the manipulation of the genetic makeup of an organism.
genetics: The study of mechanisms of heredity.
geniculate: Like a little knee; the medial geniculates and the lateral geniculates are nuclei in the walls of the thalamus of the brain.
genitalia: The reproductive organs.
genotype: An individual’s genetic complement, which determines the individual’s phenotype.
germinal centers: Pale regions in the interior of lymphoid tissues or nodules, where cell divisions are under way.
gestation: The period of intrauterine development.
gingivae: The gums.
gingivitis: An inflammation of the gums.
gland: Cells that produce exocrine or endocrine secretions.
glans: The expanded tip of the penis that surrounds the external urethral meatus; continuous with the corpus spongiosum.
glaucoma: An eye disorder characterized by rising intraocular pressures due to inadequate drainage of aqueous humor at the canal of Schlemm.
glenoid cavity: A rounded depression that forms the articular surface of the scapula at the shoulder joint.
glial cells: See neuroglia.
globular proteins: Proteins whose tertiary structure makes them rounded and compact.
glomerular capsule: The expanded initial portion of the nephron that surrounds the glomerulus.
glomerular filtration rate: The rate of filtrate formation at the glomerulus.
glomerulonephritis: An inflammation of the glomeruli of the kidneys.
glomerulus: A ball or knot; in the kidneys, a knot of capillaries that projects into the enlarged, proximal end of a nephron; the site of filtration, the first step in the production of urine.
glossopharyngeal nerve: Cranial nerve IX.
glottis: The passageway from the pharynx to the larynx.
glucagon: A hormone secreted by the alpha cells of the pancreatic islets; elevates blood glucose concentrations.
glucocorticoids: Hormones secreted by the zona fasciculata of the adrenal cortex to modify glucose metabolism; cortisol, cortisone, and corticosterone are important examples.
glucogenic amino acids: Amino acids that can be broken down, converted to pyruvic acid, and used in gluconeogenesis.
gluconeogenesis: The synthesis of glucose from protein or lipid precursors.
glucose: A six-carbon sugar, C6H12O6; the preferred energy source for most cells and normally the only energy source for neurons.
glycerides: Lipids composed of glycerol bound to fatty acids.
glycogen: A polysaccharide that is an important energy reserve; a polymer consisting of a long chain of glucose molecules.
glycogenesis: The synthesis of glycogen from glucose molecules.
glycogenolysis: Glycogen breakdown and the liberation of glucose molecules.
glycolipids: Compounds created by the combination of carbohydrate and lipid components.
glycolysis: The anaerobic cytoplasmic breakdown of glucose into lactic acid by way of pyruvic acid, with a net gain of two ATP molecules.
glycoprotein: A compound containing a relatively small carbohydrate group attached to a large protein.
glycosuria: The presence of glucose in urine.
goblet cell: A goblet-shaped, mucus-producing, unicellular gland in certain epithelia of the digestive and respiratory tracts.
goiter: An enlargement of the thyroid gland.
Golgi apparatus: A cellular organelle consisting of a series of membranous plates that give rise to lysosomes and secretory vesicles.
Golgi tendon organ: A receptor sensitive to tension in a tendon.
gomphosis: A fibrous synarthrosis that binds a tooth to the bone of the jaw; see periodontal ligament.
gonadotropin-releasing hormone (GnRH): A hypothalamic releasing hormone that causes the secretion of follicle-stimulating hormone and luteinizing hormone by the anterior pituitary gland.
gonadotropins: Follicle-stimulating hormone and luteinizing hormone, hormones that stimulate gamete development and sex hormone secretion.
gonads: Reproductive organs that produce gametes and hormones.
gout: A condition resulting from elevated uric acid concentrations in blood and in peripheral tissues.
granulocytes: White blood cells containing granules that are visible with the light microscope; includes eosinophils, basophils, and neutrophils; also called granular leukocytes.
gray matter: Areas in the central nervous system that are dominated by neuron cell bodies, neuroglia, and unmyelinated axons.
gray ramus: A bundle of postganglionic sympathetic nerve fibers that are distributed to effectors in the body wall, skin, and limbs by way of a spinal nerve.
greater omentum: A large fold of the dorsal mesentery of the stomach; hangs anterior to the intestines.
greater vestibular glands: Mucous glands in the vaginal walls that secrete into the vestibule; the equivalent of the bulbourethral glands of males.
greenstick fracture: A fracture in which a bone cracks and bends, most commonly involving the long bones of young children.
groin: The inguinal region.
gross anatomy: The study of the structural features of the body without the aid of a microscope.
growth hormone (GH): An anterior pituitary hormone that stimulates tissue growth and anabolism when nutrients are abundant and restricts tissue glucose dependence when nutrients are in short supply.
growth hormone–inhibiting hormone (GH-IH): A hypothalamic regulatory hormone that inhibits growth hormone secretion by the anterior pituitary; also called somatostatin.
guanine: A purine; one of the nitrogenous bases in the nucleic acids RNA and DNA.
gustation: Taste.
gynecologist: A physician specializing in the female reproductive system.
gyrus: A prominent fold or ridge of neural cortex on the surfaces of the cerebral hemispheres.