Answer
Excitability is the capacity of certain cells to generate action potentials at their membranes in response to depolarization, that is, the reverse of ion concentrations across the plasma membrane-- caused by certain stimuli. Most living cells are excitable in that can receive and react to stimuli, but neurons and muscle cells have special capacities to receive stimuli, generate action potentials and transmit messages through neurotransmitters.This capacity enables neurons to coordinate the body's physiological processes, and enables muscle cells to contract and relax and thus effect the vital and complex processes of voluntary movement of body parts, heart functions, movement of food along the digestive tract, and movement of urine along the urethra.
Work Step by Step
An electrical potential is the voltage difference caused by the differences in electrically charged particles across a barrier--in the case of cells, across a plasma membrane. Actually, the differences in concentrations of sodium and potassium ions(Na+ and K+) are mainly responsible for the voltage difference, but Calcium cations(Ca++), chloride anions, and organic acids also contribute to this phenomenon. The resting potential in human excitable cells is about -76 millivolts. If a stimulus causes the mechanisms of the selectively permeable membrane--ion channels, protein pumps and voltage-gated ion channels-- to let in large numbers of Na+ ions, then depolarization occurs and the transmembrane potential changes from about -75 millivolts to about +50 millivolts. The action potential may last variable periods-- 1-300 milliseconds-- dependent on ions, and type of tissue (nerves, voluntary muscle, or cardiac muscle fibers. It is reversed when the sodium (Na+) channels close, and the K+ channels open ; this allows the outflow of K+ ions establishing equal concentrations of Na+ and K+ ions in the cytosol and in the Iinterstitial fluid (ISF).. The resting potential is re-established as the transport mechanisms of the plasma membrane once again pump more sodium ions out of, and more K+ ions into the cytosol to restore the resting potential of the cell.
