How Drugs Affect the Brain

Addiction – How drugs work in the brain

The Central Nervous system is a network of tissue that specialises in controlling the actions and reactions of the body. This enables the body to adjust to its environment. THE CNS consists of the brain and spinal cord, receiving impulses from the skin and sense organs via afferent or sensory nerve fibres. The CNS functions by receiving signals from all parts of the body and relaying them to the spinal cord and brain, then sending signals to muscle and body organs.

In humans, there are millions of neurons (nerve cells); half of these neurons are in the brain in humans. Neurons consist of a cell body, containing a cell nucleus, dendrites and axons which are covered by a myelin sheath. A neuron works be receiving chemical signals through dendrites and sending electrical impulses along its axon.

Neurotransmitters released at the terminal fibres of the axon diffuse across a junction called a synapse and bind to dendrites of recipient neurons. Dendrites and axons are called nerve fibres. A nerve is a bundle of nerve fibres. The nervous system is split into the CNS and the peripheral nervous system. The peripheral nerves comprises the cranial nerves controlling the face, neck, spinal nerves and radiating to other parts of the body, autonomic nerves, controlling the iris of the eye, muscles of heart glands, lungs and so on.


Central Nervous System – The Brain

The cerebral cortex is the outer grey matter covering each hemisphere of the brain. The cerebral cortex contains a motor and speech centre, visual, olfactory, and auditory parts-along with areas concerned with higher mental activity such as judgement, memory, reasoning, and thought. Drugs that depress the cellular activity in the cerebrum, such as the opiates, barbiturates, and alcohol, may decrease the acuity of perception and sensations, and decrease alertness and concentration. Drugs that increase the cellular activity in the cortex may cause more vivid impulses to be received with greater awareness of surroundings and even hallucinations. Information on the awareness of pleasure or pain and other sensory impulses is received in this area of the brain from the body via the thalamus. The thalamus is the relay centre of the brain. All incoming and outgoing signals pass through this area. It can relay signals from the brain stem to all parts of the cerebral cortex and cause a generalized activation of the cerebrum. The thalamus is the centre for sensations such as the agreeableness or disagreeableness of a situation. Depression of this part of the brain from depressants such as tranquillisers and opiates may block off unpleasant sensations to the cortex and cause the person to feel good.

The hypothalamus lies below the thalamus and is directly connected to it. It contains centres that regulate body temperature, fat and carbohydrate metabolism, water balance, hunger and satiety, and pleasure and pain. There are also pain centres in the limbic system. Depressants acting in this area can cause the person to be sleepy, and substances such as aspirin can affect the heat-regulating centre. Natural sleep chemicals produced by the brain, such as delta-sleep-inducing peptides, react in this part of the brain. Amphetamines and other stimulants appear to act on this area to stimulate the satiety and the pleasure centres, causing the person to feel good, alert, and not hungry. The major pleasure centre of the brain is thought is to be located in the hypothalamus. Stimulation of this area results in such feelings as orgasm, joy, and extreme pleasure. The positive high and good feeling produced by drugs is also thought to originate here. The hypothalamus is directly linked to the autonomic nervous system.

Many psychosomatic illnesses including ulcers and hypertension are thought to result from over stimulation of this centre, which in turn activates the autonomic nervous system. It is thought that most drugs used for recreational purposes exert their major effect in this area of the brain.

The limbic system includes the thalamus, hypothalamus and other structures. The limbic system appears to be the memory site of emotions. If a certain drug is associated with pleasure, the emotional content of the experience is stored in this system and may become a stimulus for repeating this experience. The feelings of anger, joy, and remorse are also thought to occur here.

The reticular activating system receives input from all parts of the sensory system as well as the cerebrum. The major function of this system is to control the arousal level of the brain. This system secretes norepinephrine and dampens the many stimuli coming through it, so new and different stimuli can be recognized by the rest of the brain. If the stimulus is new it will be selected out; if it is not new or is routine it is usually ignored. As an example, if while you are sleeping you are getting cold because you do not have enough covers over you, this system will stimulate the cortex and wake you up. It is thought that in hyperactive children all incoming signals are sent into the rest of the brain for attention and that stimulants, such as Ritalin(R), given to these children stimulate under working cells to produce this selectivity effect. Depressants such as barbiturates depress this system and produce sedation and loss of consciousness. Stimulation of this part of the brain with amphetamines, caffeine and other stimulants can cause a person to feel alert, awake, and very good. Such stimulation can also cause the distortion of sensations. LSD and Cannabis are thought to affect this part of the brain by creating changes in sensory feelings.

The cerebellum controls balance and coordination of various body movements. It is also the centre for muscle coordination and tone and equilibrium. The cerebellum receives incoming messages from the cortex, spinal sensory nerves, and balance system in the ear, and from the auditory and visual system. Depressant drugs such as barbiturates and alcohol can depress the cells in this centre and cause uncoordination in body movement and balance. Stimulants can cause tremors.

The medulla oblongata in the brain stem is the centre for such vital functions as respiration, coughing, vomiting, and cardiac and vasomotor control. If overdoses of depressants, such as alcohol and barbiturates, are taken together, this centre may cease to function, and the individual could die of suffocation and heart stoppage. Stimulation of this area can cause vomiting.

The autonomic nervous system comprises the sympathetic nervous system (responds to short term stress e.g. increased heart rate) and the parasympathetic nervous system (acts in opposition to the sympathetic nervous system, e.g. reducing heart rate).


Drugs that affect the spinal cord and brain are used to treat several neurological (nervous system) and psychiatric problems:

  • Antiepileptic drugs reduce the activity of the over excited brain area to reduce or eliminate seizures.
  • Antipsychotic drugs are used to regulate neurotransmitters which do not function correctly in people with psychoses (major mental disorders often characterised by extreme behaviours) and hallucinations (as in schizophrenia).
  • Antidepressant drugs can help reduce mental depression.
  • Whilst anti-manic drugs can reduce excessive mood swings in people with manic-depression.
  • Anti-anxiety drugs (tranquilizers) treat anxiety by decreasing the activity in the anxiety centres of the brain.
  • Narcotics relieve pain by acting on receptors located on nerve cells in the brain or spinal cord.
  • Non-narcotic analgesics (e.g. aspirin, ibuprofen) reduce pain by inhibiting the formation of nerve impulses at the site of pain.
  • General anaesthetics depress brain activity, causing a loss of sensation throughout the body and unconsciousness.
  • Local anaesthetics are applied directly to a specific area of the body, causing a loss of sensation without unconsciousness, preventing nerves from transmitting impulses signalling pain.


The effect of drugs varies greatly, depending on the type of drug, the amount, the user's previous experience of it, what they want and expect to happen, the environment or social situation in which it's taken, and their mental state. The same person may react differently to the same drug at different times.

They may become tolerant of some drugs, which means their body gets used to having it, so that higher doses are needed to maintain the same effect. Withdrawal is the body's reaction when it doesn't get the drug it's adapted to. The effects of withdrawal can be stopped, either by taking more of the drug, or by stopping using it completely (sometimes called cold turkey), which may take up to a week.


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