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.
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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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.