At the ends of the axon are small thickenings – nerve endings. In them lies the answer to the question of how electrical impulses are transmitted from one neuron to another. When the microscopes were created, which made it possible to clearly see the neurons, an amazing thing came to light: most of the endings of one neuron do not come into close contact with the dendrites of the next, as it has been assumed so far.
The space that separates them is called a synapse (shown in Figure 3-2). Of course, the question arises, how is the electric current conducted from one neuron to another, if they do not touch? It is now known that when the current reaches the nerve ending, the chemicals (neurotransmitters) in it are released into the synapse, and it is they who activate the adjacent neuron.
Thus, the transmission of nerve impulses is an electrochemical process: electric, as long as current flows along an axon, and chemical at the synapse. This is important, since it can be assumed that drugs act on the nervous system through the synapse, because here there are chemical processes of information transfer. Indeed, most psychoactive substances produce their main effect through the synapse. Therefore, it is appropriate to consider in detail the chemical processes occurring in the synapse.
To describe the process of transmission of nerve impulses, we use the analogy with the key and lock. Special endings are scattered across the entire surface of the dendrites and the cell body – receptors. They can be compared with locks that protect the neuron from excitation. For excitation, you need to open the locks, and this is done by neurotransmitters released into the synapse. Molecules of neurotransmitters – the keys. The mechanism for opening the lock is shown in Figure 3-2. Receptors are depicted as circular depressions on the surface of the dendrite, neurotransmitters – balls released from the nerve ending. The idea is simple – in order to trigger the nerve impulse transmission mechanism, the key must go to the lock.
In fact, neurotransmitter molecules and receptors have a much more complex chemical structure than can be seen from the figure, and the analogy with the key and lock does not fully explain the process. The mediators and their receptors are electrically charged, and therefore attract each other, and when the mediator key enters the receptor lock, they bind. When a mediator molecule enters the receptor in a neuron, a reaction occurs causing its excitation. It is important to note that there are many types of neurotransmitters and their corresponding receptors. In brain tissue, there are chemically encoded paths along which various neurotransmitters move.