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 space between neurons (the synapse2) (Figure 5). Acetylcholine neurons also innervate tissues such as muscles and other organs. 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 muscles. 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 (Figure 6). The influx of Na+ generates a membrane current that triggers a new electrical impulse or some form of work. In the case of a muscle, 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 (it can also increase heart rate indirectly via the sympathetic nervous system5). 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; neurotransmitters are released from the terminal into the synaptic space and bind to receptors on the neighboring neuron.
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.
5 part of the autonomic nervous system which controls the functions of organs and tissues especially during times of stress, fear and emergencies. It consists of 2 types of neurons, pre-ganglionic and post-ganglionic. The pre-ganglionic neurons release acetylcholine and the post-ganglionic neurons release norepineprine.
Figure 5 An acetylcholine synapse; the axon terminal releases acetylcholine, which binds to acetylcholine receptors. From: Gross de Nunez & Schwartz-Bloom, 1997; for full citation, see Resources
Figure 6 Acetylcholine binds to its receptors & opens Na+ channels. The influx of Na+ generates an electrical current across the cell membrane. From: Gross de Nunez & Schwartz-Bloom, 1997; for full citation, see Resources