Essentials of Human Physiology for Pharmacy

chapter four: Electrical signals 25depolarization, the membrane is said to undergo repolarization, returning toits resting potential.The mechanism by which the signal is transmitted along the cell membraneis referred to as local current flow or the movement of positively chargedions. In the area of a stimulus causing a depolarization, the inside of the cellbecomes positive (less negative) relative to the outside of the cell. Becauseopposite charges attract, the (+) charges in this area are attracted to and movetoward the negative charges on the adjacent areas of the internal surface ofthe cell membrane. As a result, these adjacent areas become depolarized dueto the presence of these (+) charges. This process continues and the electricalsignal travels along the cell membrane away from the initial site of thestimulus; however, these graded or local potentials travel only short distances.The cell membrane is not well insulated and the current (positivecharges) tends to drift away from the internal surface of the cell membrane.Consequently, as the signal travels along the membrane, the number of (+)charges causing the depolarization of the next region of membrane continuallydecreases and the magnitude of the depolarization therefore decreases.The further away from the initial site of stimulation, the smaller the magnitudeof the signal is until it eventually dies out.4.3 Action potentialsAction potentials are long-distance electrical signals (see Figure 4.2). Thesesignals travel along the entire neuronal membrane. Unlike graded potentialsin which the magnitude of the signal dissipates, the magnitude of the actionpotential is maintained throughout the length of the axon. Furthermore, incontrast to graded potentials whose magnitude is stimulus dependent, actionpotentials are always the same size. If a stimulus is strong enough to depolarizethe membrane to a critical level referred to as threshold, then themembrane continues to depolarize on its own, independent of the stimulus.Typically, threshold is approximately 20 mV less negative than the restingmembrane potential. Once threshold is reached, the continued depolarizationtakes place automatically. This is due to the diffusion of ions accordingto their concentration and electrical gradients and not due to the originalstimulus itself.Given that action potentials are always of a similar magnitude, how canstimuli of varied strengths be distinguished? A suprathreshold stimulus, onethat is larger than necessary to depolarize the membrane simply to threshold,does not produce a larger action potential, but it does increase the frequencyat which action potentials are generated. In other words, a stronger stimuluswill trigger a greater number of action potentials per second.The generation of an action potential involves changes in permeabilityto Na + ions and K + ions through voltage-gated ion channels. However, thesepermeability changes take place at slightly different times (see Figure 4.2).Voltage-gated ion channels open and close in response to changes in membranepotential. Initially, a stimulus will cause the membrane to depolarize