For a long time, four of the above neurotransmitters were considered the only main substances acting in the process of transmitting nerve impulses. But with the development of more sophisticated research technologies, it became clear that we are still waiting for the discovery of many more neurotransmitters.
In the late seventies, substances similar in chemical properties to opiates were found in the mammalian brain tissue. Because of this similarity, they were called endorphins (short for “endogenous morphine”). Their functions in the body are varied and are not yet completely clear, but no doubt that these substances contribute to the relief of pain. For a detailed description of endorphins, see Chapter 9.
Another important neurotransmitter is gamma-aminobutyric acid (GABA). Its brain tissue contains much more than other known neurotransmitters, and it acts a little differently. The analogy of the key and the lock still works, but GABA, entering the receptor, does not open, but closes the lock, that is, it does not excite a neuron, but on the contrary, prevents it. Therefore, it is usually called the suppressive neurotransmitter (although other neurotransmitters can act in certain synapses in this capacity). If the GABAergic receptor of the neuron is activated, then in order for the neuron to be excited, a very large number of appropriate neurotransmitters are needed. A lot of drugs are now known to act like GABA. These are classic depressants: barbiturates, diazepam (Valium) and chlordiazepoxide (Librium) tranquilizers, and alcohol.
After considering the smallest parts of the nervous system and the effects of drugs at the neuron level, consider the entire nervous system as a whole. Its structure is depicted in Figure 3-3. It has two fundamentally different departments: the central nervous system and the peripheral nervous system. The central nervous system includes the brain and spinal cord. All nerve tissues outside of them belong to the peripheral nervous system. It consists of nerves (axonal ligaments) that transmit information from the sense organs to the brain (sensory nerves) and from the brain to muscles (motor nerves).
In addition to the nerve endings, the peripheral nervous system has an important regulatory system, called the autonomic nervous system. It regulates automatic reactions, and in turn is divided into two parts. The sympathetic branch of the autonomic nervous system is activated during the period of emotional recovery by the release of adrenaline and norepinephrine from the special glands. She is responsible for various physiological changes that accompany instantaneous subconscious reactions: an increase in pressure, increased heart rate and respiration, dilated pupils, perspiration, dry mouth, changes in blood flow in the body (it pours from the internal organs and rushes to the brain and large muscles). Many psychoactive substances cause the same changes in the body. Such substances are called sympathomimetics, and they include cocaine,amphetamines and some LSD-type hallucinogens. Other substances block a certain type of sympathetic norepinephrine receptors, the so-called beta receptors. They regulate blood pressure. Substances called beta-blockers (which include propranolol) are widely used in the treatment of hypertension.
The second, parasympathetic branch of the autonomic nervous system is associated with actions opposite to those of the sympathetic. It reduces pulse, blood pressure, etc. Unlike sympathetic neurons, the synapses of the neurons of this system are mostly cholinergic. Substances that act directly on the parasympathetic nervous system are usually very toxic. For example, the nervous paralytic gases zorin and soman bind acetylcholinesterase, which leads to excessive activity of this branch of the nervous system. The result is death from suffocation or cardiac arrest.