into the axon. This exchange of potassium and sodium is called depolarization. As
these electrolytes change positions, an electrical charge is set up and the impulses will
travel down the axon until it reaches the terminal bulbs. When the impulse reaches the
terminal bulbs, it will cause a release of neurotransmitters stored there into the synaptic
cleft. Once in the synaptic cleft, the neurotransmitters will diffuse across the synapse to
the dendrite of the postsynaptic neuron causing it to depolarize (see Figure 5-4).
b. Once the postsynaptic neuron has depolarized, the neurotransmitters must be
removed from the synaptic cleft to prevent further depolarization. This is accomplished
by two means. The neurotransmitter is either reabsorbed into the terminal bulb or an
enzyme destroys it. This process ends the impulse.
c. Before the neuron can depolarize again, the electrolyte sodium and
potassium must resume their original positions. The sodium pump theory states that
before the neuron can depolarize again the sodium is pumped out and the potassium is
pumped back in (repolarized).
A neurotransmitter is a chemical substance that aids in the transmission of an
impulse across the synapse. An impulse will cause the release of a neurotransmitter,
which is synthesized and stored in terminal bulbs of the axon. The neurotransmitter will
diffuse across the synaptic cleft and Initiate an impulse in the postsynaptic nerve. The
neurotransmitter reacts with a receptor-site on the postsynaptic nerve initiating an
impulse. The neurotransmitter must be removed from the synaptic cleft to stop the
a. Acetylcholine. Acetylcholine (Ach) is destroyed by acetylcholinesterase
(AchE) in the synaptic cleft.
Norepinephrine. Norepinephrine (NE) is removed from the synaptic cleft by:
Reabsorption (reuptake) into the terminal knob.
Destroyed by catechol-o-methyl transferase (COMT).
Destroyed by monoamine oxidase (MAO).
Dilution by diffusion out of the junctional cleft.
5-9. THE ALL OR NONE LAW
This law states that if a stimulus is strong enough to cause a nerve impulse, it will
cause the entire fiber to depolarize and not just part of it.