The Synapse

The gap between successive neurons is wide enough that impulses do not travel from one neuron to the next in the same way as along a single neuron. Information travels from one neuron to the next by means of a chemical neurotransmitter. Together, the gap and the "connecting" membranes of the neurons are called the synapse (Figure 12-8). The gap is called the synaptic cleft.

AXON TERMINAL

NEURON #1

NEURON #2

(RECEPTOR SITE)

(BOUTON)

AXON TERMINAL

NEURON #1

NEURON #2

(RECEPTOR SITE)

Synaptic Cleft Terminal Bouton
Figure 12-8. A synapse.

a. Many synaptic vesicles (bundles of neurotransmitters) are found in the terminal bulb (bouton) of the first neuron. Each vesicle contains a quantum, a specific amount, of neurotransmitter or a substance used to make the neurotransmitter.

b. When the impulse reaches the bouton, these vesicles are stimulated to release their neurotransmitter. The neurotransmitter then passes out of the bouton, through the presynaptic membrane, into the synaptic cleft. On the other side of the synaptic cleft is the postsynaptic membrane. This is the receptor site of the second neuron.

c. The neurotransmitter is located only in the terminal bulb of the first neuron. For this reason, impulses travel in only one direction through the synapse, from the first to the second neuron. Since this process consumes much energy, there are many well-developed mitochondria in the bouton, or terminal bulb.

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Essentials of Human Physiology

Essentials of Human Physiology

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