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
Action Potential Animation
depolarization results in one of two types of reversal in
polarity, or potentials:
| 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.
potentials occur on the dendrites and
cell bodies of neurons.
| 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
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