Nerve Physiology


Nerve Impulses
     Neurons have two major functions:
  Irritability is the ability to respond to a stimulus and change it into an impulse.
  Conductivity is the ability to transmit the impulse to other neurons, muscles and glands.


Polarization of the Plasma Membrane Animation showing creation of resting membrane potential
     All cells have polarized plasma membranes. This polarization results from the fact that there is an uneven distribution of ions, or charged particles, on either side of the membrane. For example, there is a higher concentration of sodium ions (Na+) outside the cell in the extracellular fluid than inside the cell, and there is a larger concentration of potassium ions (K+) inside the cell than outside the cell.
     The unequal distribution of ions also results in the plasma membrane being electrically polarized. Typically, the outside of the cell has more positive charges than the inside of the cell.
     The electrical polarity across the plasma membrane is like a battery that contains energy in the form of volts. The charges involved are so small that this energy is actually expressed as thousandths of a volt, or millivolts. The energy contained in this polarity is released when ions pass through channels that open in the plasma membrane. It is the movement of charged particles through the channels that causes the impulse and it is the stimulus that causes these channels to open.


     The neuron responds to a stimulus by opening Na+ channels in the plasma membrane. The Na+ ions rush into the cell down their concentration gradients. This inrush of positive charges causes the interior of the cell to go from being more negative than the outside to more positive. Although this actually represents a reversal in polarity this event is called a depolarization because it changes the polarity the cell has when it is not being stimulated, or is resting. Action Potential Animation
     The depolarization results in one of two types of reversal in polarity, or potentials:
  Graded potential
  A graded potential simply represents a change in polarity that will vary in degree depending upon the strength of the stimulus. In other words, the stronger the stimulus, the more positive the inside of the cell becomes, as more Na+ rush in.
  Graded potentials occur on the dendrites and cell bodies of neurons.
  Action potential
  An action potential occurs when the change in polarity reaches a certain level called the threshold. When the threshold is reached, the depolarization becomes self-propagating and a nerve impulse travels along the axon of the neuron. Propagation of the Action Potential
  Action potentials occur only on the axons of neurons and either occur at full strength when the threshold is reached or donít occur at all. This is an example of what is called the all-or-none response.


     The depolarization, whether a graded potential or an action potential, ends when the plasma membrane returns to its original resting potential with the inside of the cell being more negative than the outside again. This is called repolarization.
     The repolarization occurs because after the depolarization the Na+ channels close and Na+ stops rushing into the cell and K+ channels open permitting the K+ to rush out of the cell down its concentration gradient.
     Until repolarization occurs the axon cannot conduct another nerve impulse.


Chemical Synapse Norton books animation
      The nerve impulse travels along the axon, as a wave of depolarization, and reaches the axon terminal where there is usually a chemical synapse. When the plasma membrane at the axon terminal is depolarized the following events occur:
  1. Vesicles containing a chemical signal molecule called a neurotransmitter fuses with the plasma membrane.
  2. Neurotransmitter is released into the synaptic cleft.
  3. The neurotransmitter binds with receptors on the plasma membrane of the next cell.
     The binding of the neurotransmitter on the plasma membrane of the cell receiving the signal causes a change in the permeability of the membrane to ions that often results in depolarization (although hyperpolarization is also possible).