A nerve signal soon reaches the end of an axon and can go no farther. In most cases, it triggers the release of a neuro-transmitter that stimulates a new wave of electrical activity in the next cell across the synapse. The most thoroughly studied type of synapse is the neuromuscular junction described in chapter 11, but here we consider synapses between two neurons. The first neuron in the signal path is the presynaptic neuron, which releases the neurotransmitter. The second is the postsynaptic neuron, which responds to it.
The presynaptic neuron may synapse with a den-drite, the soma, or the axon of a postsynaptic neuron and form an axodendritic, axosomatic, or axoaxonic synapse, respectively. A neuron can have an enormous number of synapses (fig. 12.16). For example, a spinal motor neuron is covered with about 10,000 synaptic knobs from other neurons—8,000 ending on its dendrites and another 2,000 on the soma. In part of the brain called the cerebellum, one neuron can have as many as 100,000 synapses.
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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.