Nerve cells do not come into direct physical contact with one another. Their unique feature is that they are separated by microscopic spaces known as a synapse. The synapse prevents the continuous flow of impulse and become the focal point of the discussion of drug action on the nervous system.
The synapse, by analogy, serves as a switch for electric current. The switch is ON, provided the conditions at the synapse are biochemically correct for the propagation of the nerve impulse. Extra nerve impulses are emitted, when some drugs turn this switch on themselves. The switch is OFF if conditions are not normal – for example because of the presence of a depressant drug or of fatigue. There are small pockets called vesicles at the end of the axon. Certain chemicals located there appear to be all important in transmitting the nerve impulse to the next nerve cell. These chemicals are known as neurohormonal transmitter substances. Acetylcholine, nor epinephrine, serotonin, dopamine, and gamma amino butyric acid, or GABA is some of these substances that have been identified to date. There are a number of CNS hormones, in addition to these neurotransmitters, that may act as neurohormonal transmitters as well as carry out their role as hormones. It has been suggested that the hypothalamic based hormones should be controlled. The neurohormone is released into the space between the presynaptic membrane and the membrane of the next cell, the postsynaptic membrane, by the vesicles, upon fusing with the presynapic membrane. The ability to alter the postsynaptic membrane is within the chemical make-up of the neurohormone. A nerve impulse of the same intensity as the one that came through the preceding axon is re-created by the electrochemical reactions, when this alteration takes place.
