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11-7. 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 11-2) is a "connection" between two neurons.

(1) First neuron. An axon terminates in tiny branches. At the end of each branch is found a terminal bulb. Synaptic vesicles (bundles of neurotransmitter) are located within each terminal bulb. That portion of the terminal bulb which faces the synaptic cleft is thickened and is called the presynaptic membrane. This is the membrane through which neurotransmitters pass to enter the synaptic cleft.

(2) Synaptic cleft. The synaptic cleft is the space between the terminal bulb of the first neuron and the dendrite or cell body of the second neuron.

(3) Second neuron. The terminal bulb 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 11-3) is a "connection" between the terminal of a motor neuron and a muscle fiber. The neuromuscular junction has an organization identical to a synapse. However, the bulb is larger. The postsynaptic membrane is also larger and has foldings to increase its surface area.

(1) Motor neuron. The axon of a motor neuron ends as it reaches a striated muscle fiber (of a skeletal muscle). At this point, it has a terminal bulb. Within this bulb are synaptic vesicles (bundles of neurotransmitter). The presynaptic membrane lines the surface of the terminal bulb and lies close to the muscle fiber.

(2) Synaptic cleft. The synaptic cleft is a space between the terminal bulb of the motor neuron and the membrane of the muscle fiber.

(3) Muscle fiber. The terminal bulb of the motor neuron protrudes into the surface of the muscle fiber. The membrane lining the synaptic space has foldings and is called the postsynaptic membrane. Beneath the postsynaptic membrane is a chemical which inactivates the used neurotransmitter.

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