Neurons synthesize and store specific chemicals called neurotransmitters which are released at the terminal following the arrival of an electrical impulse. For example, acetylcholine1 neurons synthesize acetylcholine by a series of enzymatic reactions that takes place in the neuron terminal. Acetylcholine is stored in the terminal in small sacs, or vesicles. When an electrical impulse originating in the cell body travels down the axon to the terminal, it triggers the release of acetylcholine from the vesicles into the synaptic space (also called the synapse2) (Figure 5).
When acetylcholine is released from the axon terminals, it binds to specific proteins called acetylcholine receptors3 on neighboring neurons or on other types of cells, like muscle cells. When acetylcholine binds to its receptor, it causes a change in the protein structure, opening a channel through which Na+ ions4 move (with the concentration gradient) inside the cell. The influx of Na+ generates a membrane current that triggers a new electrical impulse (Figure 5).
Figure 5 Watch what happens at an acetylcholine synapse; the axon terminal releases acetylcholine, which binds to acetylcholine receptors to open ion channels. Then a current forms across the membrane to trigger an electrical impulse. From: Gross de Nunez & Schwartz-Bloom, 1997; for full citation, see Resources
The electrical impulse causes the cell to do some work. In the case of a muscle cell, it causes muscle contraction; this occurs in smooth muscle, like the intestines and the bronchioles of the lung, and in skeletal muscle. In sweat, salivary and tear glands, acetylcholine causes secretion. In the heart, acetylcholine slows conduction of electrical impulses and thus decreases the heart rate. In the brain, acetylcholine affects the firing rate of neurons and participates in memory and learning, motor control, and wakefulness. So depending on the location of the acetylcholine receptors, acetylcholine has many actions throughout the body.
1 a neurotransmitter stored in vesicles of nerve terminals; it is found in neurons within the central nervous system, the somatic nervous system, the parasympathetic nervous system and the sympathetic nervous system.
2 the connection between two neurons or a neuron and a muscle cell; neurotransmitters are released from the terminal into the synaptic space and bind to receptors on the neighboring cell.
3 a protein to which hormones, neurotransmitters and drugs bind. They are usually located on cell membranes and elicit a function once bound.
4 an atom, radical, or molecule that has gained or lost one or more electrons. Therefore it acquires a net negative or positive charge.