At the end of the axon, the neuron must transmit the signal to another neuron or to an effector. This is done through the synapse. Most synapses involve the use of a chemical, called a neurotransmitter, to communicate the signal across a gap from one cell to another. One of the most widespread neurotransmitters is acetylcholine (ACh). This neurotransmitter is a target of many insecticides, in particular the organophosphorus and carbamate pesticides.
The axon ends in a synaptic knob (Figure 9.3), which contains as many as 1 million vesicles containing ACh. The membrane of the synaptic knob is separated from the membrane of a target neuron by a gap of about 20 nm width. When the depolarization wave from an axon reaches the synaptic knob, ACh is released and diffuses across the gap to the target neuron. There it stimulates pores to open, depolarizing the target neuron membrane and initiating a nerve signal transmission. An enzyme, acetylcholinesterase, rapidly decomposes the ACh into choline and acetate. The choline is reabsorbed by the synaptic knob and recycled into more ACh. It is the enzyme acetylcholinesterase that is affected by organophosphorus and carbamate pesticides (see Section 17.4.7).
There are many other neurotransmitters. Many hormones serve this function, including epinephrin (adrenaline), ADH, oxytocin, insulin, and glucagon. The amino acids glycine, glutamine, and aspartic acid are neurotransmitters, as are the gases carbon monoxide and nitric oxide. A group of compounds called endorphins modifies the effect of neurotrans-mitters and may be involved in mood and pain reduction. They are similar in structure to morphine. It is thought that exercise produces a natural release of endorphins. Dopamine is a central nervous system neurotransmitter that can be inhibitory or excitatory, depending on the receptor. A decline in dopamine production produces Parkinson's disease, in which the inhibitory action of dopamine is missing. As a result, the neurons that control muscle tone become overstimulated. All movement requires overcoming the tension of the opposing muscle. Dopamine cannot cross the blood-brain barrier, but the drug L-dopa can, and it is converted to dopamine in the brain, providing relief from symptoms.
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