The Synapse

Axons end close to, or in some cases at the point of contact with, another cell. Once action potentials reach the end of an axon, they directly or indirectly stimulate (or inhibit) the other cell. In specialized cases, action potentials can directly pass from one cell to another. In most cases, however, the action potentials stop at the axon ending, where they stimulate the release of a chemical neurotransmitter that affects the next cell.

A synapse is the functional connection between a neuron and a second cell. In the CNS, this other cell is also a neuron. In the PNS, the other cell may be either a neuron or an effector cell within a muscle or gland. Although the physiology of neuron-neuron synapses and neuron-muscle synapses is similar, the latter synapses are often called myoneural, or neuromuscular, junctions.

Neuron-neuron synapses usually involve a connection between the axon of one neuron and the dendrites, cell body, or axon of a second neuron. These are called, respectively, axodendritic, axosomatic, and axoaxonic synapses. In almost all synapses, transmission is in one direction only—from the axon of the first (or presynaptic) neuron to the second (or postsy-naptic) neuron. Most commonly, the synapse occurs between the axon of the presynaptic neuron and the dendrites or cell body of the postsynaptic neuron.

In the early part of the twentieth century, most physiologists believed that synaptic transmission was electrical—that is, that action potentials were conducted directly from one cell to the next. This was a logical assumption given that nerve endings appeared to touch the postsynaptic cells and that the delay in synap-tic conduction was extremely short (about 0.5 msec). Improved histological techniques, however, revealed tiny gaps in the synapses, and experiments demonstrated that the actions of autonomic nerves could be duplicated by certain chemicals. This led to the hypothesis that synaptic transmission might be chemical— that the presynaptic nerve endings might release chemicals called neurotransmitters that stimulated action potentials in the postsynaptic cells.

In 1921, a physiologist named Otto Loewi published the results of experiments suggesting that synaptic transmission was indeed chemical, at least at the junction between a branch of the vagus nerve (see chapter 9) and the heart. He had isolated the heart of a frog and, while stimulating the branch of the vagus that innervates the heart, perfused the heart with an isotonic salt solution. Stimulation of this nerve slowed the heart rate, as expected. More importantly, application of this salt solution to the heart of a second frog caused the second heart also to slow its rate of beat.

Loewi concluded that the nerve endings of the vagus must have released a chemical—which he called Vagusstoff—that inhibited the heart rate. This chemical was subsequently identified as acetylcholine, or ACh. In the decades following Loewi's discovery, many other examples of chemical synapses were discovered, and the theory of electrical synaptic transmission fell into disrepute. More recent evidence, ironically, has shown that electrical synapses do exist in the nervous system (though they are the exception), within smooth muscles, and between cardiac cells in the heart.

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