Response to Changes in Environments

• Acetylcholine: A type of neurotransmitter that is used for communication between neurones.
• Actin: A type of protein filament found in myofibrils. It forms thin filaments consisting of two long twisted chains.
• Actinomyosin bridge: The cross-bridge formed when a myosin head attaches to the myosin binding site on an actin filament.
• Action potential: The temporary change in electrical potential across the membrane of an axon in response to the transmission of a nerve impulse.
• Adenylate cyclase: An enzyme that catalyses the conversion of ATP to cAMP.
• Adrenaline: A hormone that is secreted by the adrenal glands under stressful conditions. It increases blood glucose concentration by activating enzymes involved in glycogenolysis.
• Afferent arteriole: The blood vessel that stems from the renal artery and supplies blood to the nephron. It has a larger diameter than the efferent arteriole and divides into a complex system of capillaries, the glomerulus.
• Anisotropic (A) bands: The darker bands in a myofibril, which consist of overlapping actin and myosin filaments.
• Antidiuretic hormone (ADH): A hormone made by the hypothalamus and secreted by the posterior pituitary gland in response to a fall in blood water potential. It increases the permeability to water of the distal convoluted tubule and the collecting duct, allowing more water to be reabsorbed into the blood.
• Ascending limb: The limb of the loop of Henle that rises into the cortex. It is wider in diameter than the ascending limb and its walls are impermeable to water. Sodium ions are moved out of the ascending limb by active transport.
• Atrioventricular node (AVN): A group of cells located between the atria that slow down the wave of excitation and pass it between the ventricles, along the bundle of His.
• Atrioventricular septum: A layer of non-conductive tissue between the right atrium and left ventricle of the mammalian heart.
• Autonomic nervous system: A branch of the motor nervous system that carries nerve impulses to muscles and glands. It controls involuntary activities and has two divisions: the sympathetic nervous system and the parasympathetic nervous system.
• Auxins: A class of plant hormones that control cell elongation.
• Axon: A long fibre that conducts nerve impulses away from the cell body.
• Bundle of His: A collection of Purkyne fibres which run from the AVN down to the apex of the ventricles.
• Cell body: The region of the neurone that contains the organelles, notably the nucleus and the rough endoplasmic reticulum.
• Chemoreceptor: A type of receptor found in the walls of the carotid arteries that detects changes in blood pH and transmits nerve impulses to the medulla oblongata. For example, if blood pH decreases, chemoreceptors increase the frequency of nerve impulses to the medulla oblongata.
• Cholinergic synapse: An excitatory or inhibitory synapse formed between neurones or neurones and other effector organs. It uses the neurotransmitter, acetylcholine.
• Collecting duct: The final region of the nephron that collects urine from the distal convoluted tubules and empties it into the renal pelvis. Its permeability to water is altered by ADH.
• Cone cells: A type of light receptor cell that transduces light energy into a generator potential. Cone cells are concentrated in the fovea, detect light of high intensity, and lead to colour images. One cone cell forms a synapse with a single bipolar cell, giving high visual acuity.
• Control mechanism: A self-regulating system consisting of five features: optimum point, receptor, coordinator, effector, and feedback mechanism.
• Coordinator: Coordinates information from the receptors and sends instructions to the effectors.
• Cyclic AMP (cAMP): A ‘second messenger’ involved in the action of adrenaline that activates protein kinase.
• Dendrites: Short, branched extensions of the cell body that receive nerve impulses from other neurones.
• Dendrons: Extensions of the cell body which branch into smaller fibres, dendrites.
• Depolarisation: A sudden, temporary change in the membrane potential of a neurone in response to the transmission of a nerve impulse. The inside of the axon is less negative than the outside.
• Descending limb: The limb of the loop of Henle that dips down into the medulla. It is smaller in diameter than the ascending limb. The walls of the descending limb are permeable to water, so the filtrate loses water as it moves down.
• Diabetes: A disorder of metabolism in which blood glucose concentration is not regulated properly. There are two forms: Type I and Type II diabetes.
• Distal convoluted tubule: The twisted region of the nephron between the loop of Henle and the collecting duct. It controls blood pH by reabsorbing ions and alters the concentration of water and salts reabsorbed. Its permeability to water is altered by ADH.
• Effector: An organ, tissue, or cell that produces a response to a stimulus.
• Efferent arteriole: The blood vessel that carries blood away from the glomerulus and sub-divides to form a network of capillaries. Its diameter is smaller than the afferent arteriole, creating a build up of hydrostatic pressure in the glomerulus.
• Excitatory synapse: A synapse that produces new action potentials when neurotransmitters bind with receptor proteins on the postsynaptic neurone.
• Fast-twitch muscle fibres: A type of muscle fibre that contracts more rapidly, with more power, over a shorter period. They are adapted for anaerobic respiration and intense activity.
• Feedback mechanism: The mechanism by which the change to a system, brought about by the effector, is detected by the receptor.
• Fovea: The point on the retina, opposite the pupil, that receives the highest intensity of light. It contains the greatest concentration of cone cells but no rod cells.
• Generator potential: Depolarisation of the membrane of a sensory receptor cell that occurs in response to a stimulus.
• Glomerular filtrate: The fluid produced by ultrafiltration of the blood into the renal capsule. It contains water, glucose, mineral ions and urea.
• Glomerulus: A bundle of capillaries located in the renal capsule which are adapted for the filtration of blood. They later merge to form the efferent arteriole.
• Glucagon: A hormone that is produced by α cells of the islets of Langerhans. It increases blood glucose concentration by activating enzymes involved in gluconeogenesis and the conversion of glycogen to glucose.