Neurotransmitter

The neurotransmitters are stored in tiny sac-like structures called vesicles at the end of axons. When an impulse, or nerve signal, reaches the end of the axon, the vesicles release a neurotransmitter into the small space between the adjoining cells (synaptic gap). Neurotransmitters diffuse across the synapse and bind to receptors in the receiving cell that are specific for the neurotransmitter. The kinds of reactions that occur in the receiving cell may vary. The cell may be excited and an action potential is set up and the impulse is transmitted; or the opposite effect may occur, and the cell will be inhibited from transmitting the impulse because it becomes hyperpolarized. Neurotransmitters function by changing the permeability of the cell membrane to various ions such as sodium and potassium. If an excess of sodium ions flow into the nerve cell, an impulse is generated. If an excess of potassium ions flow out, the impulse is inhibited. Sometimes the receiving cell receives a number of different messages at the same time, and they can cancel one another out. By their effects on the receiving cells, neurotransmitters coordinate behavior. The number and kind of neurotransmitter molecules received by the receptor cell, as well as the kind of receptor, determines whether the effect will be to stimulate or to inhibit. If neurotransmitters were allowed to operate over a long period of time, the results would be disastrous for the organism since there would be a constant overload of messages being sent. One way in which the problem is solved is through enzymes which break down the neurotransmitter very rapidly. One enzyme, acetylcholinesterase, can split one acetylcholine molecule every 40 microseconds. Many organic phosphates, which are poisons, operate by inhibiting acetylcholinesterase which leads to prolonged muscle contractions. Other neurotransmitters, such as norepinephrine and dopamine, are removed when they are taken up again by the vesicles in the axon endings after their release, or they are broken down by monamine oxidase. Some of the first anti-depressant drugs created were monoamine oxidase inhibitors (MAO inhibitors) such as phenelzine or tranylcypromine. MAO inhibitors inhibit the action of monoamine oxidase, thus allowing neurotransmitters, like norepinephrine, to have longer action. In this way, MAO inhibitor antidepressants are believed to alleviate symptoms of depression. Significant side effects resulting from the alteration of neurotransmitter degradation has led to a decline in the use of MAO inhibitors in the treatment of clinical depression.