(4) Others. There are other, more specialized types of neurons found in the
body (for example, central nervous system).
5-6. NEURON "CONNECTIONS"
A neuron may "connect" either with another neuron or with a muscle fiber. A
phrase used to describe such "connections" is "continuity without contact." Neurons do
not actually touch. There is just enough space to prevent the electrical transmission
from crossing from the first neuron to the next. This space is called the synaptic cleft.
Information is transferred across the synaptic cleft by chemicals called
neurotransmitters. Neurotransmitters are manufactured and stored on only one side of
the cleft. Because of this, information flows in only one direction across the cleft.
a. The Synapse. A synapse (Figure 5-2) is a "connection" between two
Figure 5-2. A synapse.
(1) First neuron. An axon terminates in tiny branches. At the end of each
branch is found a terminal knob. Synaptic vesicles (bundles of neurotransmitters) are
located within each terminal knob. That portion of the terminal knob that faces the
synaptic cleft is thickened and is called the presynaptic membrane. This is the
membrane through that neurotransmitters pass to enter the synaptic cleft.
(2) Synaptic cleft. The synaptic cleft is the space between the terminal knob
of the first neuron and the dendrite or cell body of the second neuron.
(3) Second neuron. The terminal knob of the first neuron lies near a site on
a dendrite or the cell body of the second neuron. The membrane at this site on the
second neuron is known as the postsynaptic membrane. Within the second neuron is a
chemical that inactivates the used neurotransmitter.
b. The Neuromuscular Junction. A neuromuscular junction (Figure 5-3) is a
"connection" between the terminal of a motor neuron and a muscle fiber. The