Nerve Potentials and Impulse Propagation
Neurons in the nervous system are linked together in circuits, the simplest of which consists of a sensory neuron receiving input from a sensory receptor cell such as a stretch receptor in a muscle, and a motor neuron which would cause the muscle to contract (Wilson, 2000, 232). This is a simple reflex circuit such as one which causes the knee-jerk reaction. The passage of an electrical impulse down a neuron is caused by a flow of ions through channels in the nerve cell membrane. Changes in the electrical currents cause changes in the membrane potential of the neuron.
At rest, a neuron maintains a potential difference across its membrane known as the resting potential (232). The resting potential of a neuron is maintained by three factors: a large number of potassium channels in the membrane which allow potassium ions to flux easily across the membrane; a concentration gradient for potassium ions, with the greater concentration being inside the cell; and Na-K ATPase, which functions as a pump to maintain the concentration gradients of potassium and sodium across the neuron membrane.
Potassium can flow freely out of the cell down its concentration gradient, and the net flux causes the build up of an electrical potential, with the inside of the cell becoming negative relative to the outside (232). As a positive charge begins to build up on the outside, the flow of potassium ions will begin to slow down because the membrane potential will oppose the outward flow of positive ions. At this point, equilibrium potential is reached (about -75 millivolts, inside negative relative to outside). However, the resting potential of a neuron membrane is slightly less, -70 millivolts, because there is a slight membrane permeability to sodium ions which flow in the opposite direction, into the cell. There is a slight leakage of sodium and potassium across the membrane, and it is pumped back...