Action Potential Overview
Formation of an action potential
The formation of an action potential can be divided into five steps. (1) A stimulus from a
sensory cell or another neuron causes the target cell to depolarize toward the threshold
potential. (2) If the threshold of excitation is reached, all Na+ channels open and the
membrane depolarizes. (3) At the peak action potential, K+ channels open and K+
begins to leave the cell. At the same time, Na+ channels close. (4) The membrane
becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized
membrane is in a refractory period and cannot fire. (5) The K+ channels close and the
Na+/K+ transporter restores the resting potential.
A neuron can receive input from other neurons via a chemical called a neurotransmitter.
If this input is strong enough, the neuron will pass the signal to downstream neurons.
Transmission of a signal within a neuron (in one direction only, from dendrite to axon
terminal) is carried out by the opening and closing of voltage-gated ion channels, which
cause a brief reversal of the resting membrane potential to create an action potential
(Figure 1). As an action potential travels down the axon, the polarity changes across the
membrane. Once the signal reaches the axon terminal, it stimulates other neurons.
Depolarization and the Action Potential
When neurotransmitter molecules bind to receptors located on a neuron’s dendrites,
voltage-gated ion channels open. At excitatory synapses, positive ions flood the interior
of the neuron and depolarize the membrane, decreasing the difference in voltage
between the inside and outside of the neuron. A stimulus from a sensory cell or another
neuron depolarizes the target neuron to its threshold potential (-55 mV), and Na
channels in the axon hillock open, starting an action potential. Once the sodium
channels open, the neuron completely depolarizes to a membrane potential of about
+40 mV. The action potential travels down the neuron as Na+ channels open.
Hyperpolarization and Return to Resting Potential
Action potentials are considered an "all-or nothing" event. Once the threshold potential
is reached, the neuron completely depolarizes. As soon as depolarization is complete,
the cell "resets" its membrane voltage b