PHYSIOLOGY OF SUBSTANCE ABUSE

Normal function of the nervous system makes it possible to coordinate our
system together. This coordination is made possible through the integrative,
controlling action of the billions of nerve cells throughout the body. Our
behaviour is as a result of the brain interpretation and interaction of all incoming
messages transmitted via the nerve cells. These impulses or messages can be
depressed, intensified or distorted by chemical substances – substances known
as drugs. A basic facet of the study of substance abuse is the action of these
substances on the central nervous system. Crucial to the physiological basis of
drug action are nerve cells and synaptic transmission.

THE NERVE CELL
The nervous system, like every other system in the body, is composed of
specialized cells. The specialized cell of the nervous system is the nerve cell, or
the neuron. The neuron is as an electrochemical unit with its action dependent
on the constant flow of chemical-carrying electrical charges. The actions of many
drugs can be explained merely by their presence inside the nerve cell. The
chemical similarity between some of the currently popular drugs and natural body
chemicals may explain how drugs get inside the nerve cell. Nerve cells vary in
shape and size according to their location and basic neural function. A typical
neuron consist of a number of impulse-receiving branches called dendrites and of
one impulse-sending branch, the axon. The body of the cell also receives stimuli
from other axons and one nerve cell may receive impulses from hundred of
different axons, some of them excitatory and some of them inhibitory.
Summation of like impulses (excitatory or inhibitory) then causes the nerve cell to
“fire” or remain inactive.

SYNAPTIC TRANSMISSION OF NERVE IMPULSE
A unique feature of nerve cells is that they do not come into direct physical
contact with one another. They are separated by a microscopic space. This
space, known as the synapse, prevents the continuous flow of impulses and
becomes the focal point of the discussion of drug action on the nervous system.
Psychoactive drugs exert their behavioural effects by altering the delicate
balance of excitation and inhibition generated by the processes of synaptic
transmission. When information arrives at the synapse from the axon, a chemical
transmitter is released, diffuses across a small fluid-filled gap (the synaptic cleft)
and alters the dendrite membrane. If the transmitter chemical affects the
membrane in such a manner that the membrane depolarizes, the neuron
becomes excitable. This depolarization is referred to by physiologists as an
excitatory postsynaptic potential (EPSP). Some transmitter chemicals instead of
depolarizing the dendrites, induce a hyperpolarization of the dendritic membrane
and the action exerted are inhibitory in nature. This hyperpolarization potential is
referred to as an inhibitory postsynaptic potential (IPSP). Thus, IPSP opposes
the action of EPSP, stabilizes the neuron and sends to prevent the generation of
action potentials. Most psychoactive drugs exert their effects not on the axon,
but at the synaptic junction between the axon of one cell and dendrites of
another. The synapse is the prime site of action of most, if not all, psychoactive
drugs.

DRUGS AND NERVE TRANSMISSION
Even though we generally understand that each part of the brain serves a
specific function and we are beginning to understand what some of those
functions are, there are two large pieces of the puzzle that need to be in place in
order to understand CNS functioning. One is how these various parts of the brain
relate or communicate, and two is how they are activated or inhibited. The latter
is of primary interest to the study of the behavioural effect of drugs. Generally
speaking, drugs act on the CNS because of their ability of mimic or displace the
naturally occurring neurotransmitters. Understanding the presence, nature and
action of neurotransmitter and the “look-alike” drugs should help in understanding
the behavioural effects of taking specific drugs. Neurotransmitters are naturally synthesized in small pockets called vesicles, located at the end of the axon.
Some of these substances that have been identified to date are acetylcholine,
norepinephrine, serotonin, dopamine, and gamma amino butyric acidor GABA. In
addition to these neurotransmitters, there are a number of CNS hormones that
may act as neurohormonal transmitters as well as carry out their roles as
hormones.
Since the synaptic events are chemical in nature, they are vulnerable to
foreign chemicals, such as drugs. Look up at the synapse, one may consider
what would happen if a drug could (a) inhibit the production of neurohormone (b)
cause the neurohormone to be broken down more rapidly than normal, or (c)
alter the postsynaptic membrane so that neurohormones would not affect it. In
any of these cases, it is apparent that the nerve cell action would be depressed
because either there was no neurohormone or it was not allowed to work
normally. It appears that this is the action of the depressant drugs (such as
alcohol, narcotics, barbiturates) when they come in contact with nerve cells in
special parts of the brain. On the other hand, consider the drugs that cause
excess production and release of neurohormonal transmitter substances, or have
the ability to mimic the action of neurohormone or not allow its reuptake. In these
cases, neurons involved would be stimulated at a greater than normal rate. This
is the action of the stimulatory drugs on nerve cells within the brain.
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