Class notes PHYSIOLOGY 2130

Nerve Cells Nerve cells and muscle cells are considered "excitable" because they can use the resting membrane potential to generate an electrochemical impulse called an action potential. The action potential is considered to be the "language" of the nervous system because this is the way nerve cells communicate with one-another. Action potentials are also necessary for muscle contractions. The Action Potential The action potential is a rapid reversal of the resting membrane. During this event, while recording from inside the cell, the membrane potential rapidly changes from rest (-70 mV) to roughly +35 mV. This sudden change to a more positive value is called depolarization. After this phase the membrane potential rapidly returns to -70 mV, which is called repolarization. The membrane potential then briefly becomes more negative, reaching roughly -90 mV. This is called hyperpolarization. After this very negative phase, the membrane potential returns to the resting levels of -70 mV. The movement of ions across the membrane (principally Na+ and K+) causes these rapid changes in membrane potential. These ions are allowed to move across the membrane through special channels. Voltage-Gated Channels In the neuron, these channels are generally found on the axon and are essential for the generation of the action potential. They are sensitive to changes in the membrane potential and open when the inside of the cell becomes more positive (i.e., the membrane changes from -70 mV to -60 mV). This change is called a depolarization. Voltage-Gated Sodium Channels This channel is specific for sodium and will allow no other molecule through. These channels have gates that open only when there is a depolarization of the membrane. ..............................CONTINUED................................