Brain And Nervous System - part 3

Antiparkinsonism medicaments

Antiparkinsonism medicaments are used in the treatment of parkinsonism. This is the general term used to describe shaking of the head and limbs, muscular stiffness, an expressionless face, and inability to control or initiate movement. It is caused by an imbalance between the chemicals dopamine and acetylcholine in the brain. These chemicals transmit nerve signals in the part of the brain that coordinates movement. They have opposing actions and are normally finely balanced. In parkinsonism there is a reduction in the action of dopamine, so that the effect of acetylcholine is increased.

Parkinsonism has a variety of causes, but the most common is degeneration of the dopamine-producing cells in the brain, known as Parkinson's disease. Other causes include the side effects of certain medicaments, notably antipsychotics, brain damage, and narrowing of the blood vessels to the brain.

Antiparkinsonism medicaments restore the balance between dopamine and acetyl-choline. They fall into two main groups: those medicaments that act by reducing the effect of acetylcholine (anticholinergic medicaments) and those that act by boosting the effect of dopamine.

Each type of medicament relieves some symptoms of parkinsonism better than others, although it is difficult to control symptoms in the more advanced stage of the disease. Anticholinergics improve stiffness more than shaking or the inability to initiate movement, and benefit is felt within a few days. They also reduce excessive salivation: dribbling is often a problem in Parkinson's disease.

Levodopa often produces a dramatic improvement in all symptoms. Side effects of levodopa include nausea, vomiting, and flushing. When given in excess it can also cause involuntary movements in the face and the body. Although these problems may be alleviated by reducing the dose, as the disease progresses it becomes increasingly difficult to give sufficient levodopa to improve symptoms without causing side effects. Also, in the later stage of the disease, the effect of each dose wears off before the next one is taken, and it may be necessary to take the medicament more frequently.

Amantadine relieves all symptoms of parkinsonism in people with mild to moderate cases. It has few side effects but the beneficial effect may wear off over a few months. The most common side effects of bromocriptine and selegiline are the same as those produced by levodopa.

Nervous system stimulants

A person's state of mental alertness varies throughout the day and is under the control of chemicals in the brain, some of which are depressant (causing drowsiness) and others that are stimulant (heightening awareness).

It is thought that an increase in the activity of the depressant chemicals may be responsible for a rare condition called narcolepsy, a tendency to fall asleep for no obvious reason. Nervous system stimulants are given to increase wakefulness. They include the amphetamines and related medicaments, notably methylphenidate. Respiratory stimulants, including caffeine (found in coffee, tea, and cola), are used to improve breathing.

The level of wakefulness is controlled by a part of the brain stem called the reticular activating system (RAS). Activity here depends on the balance between chemicals, some of which are excitatory (including norepinephrine) and some inhibitory (such as gamma-aminobutyric acid). Stimulants promote the release of norepinephrine, increasing activity in the RAS and other parts of the brain, so raising the level of alertness.

In adults, central nervous system stimulants taken in the prescribed dose for narcolepsy increase wakefulness allowing normal concentration and thought processes to occur. They may also reduce appetite and cause tremors. In hyperactive children they reduce the general level of activity to a more normal level and increase the attention span.

Some people, especially the elderly or those with previous psychiatric problems, are particularly sensitive to stimulants and may experience adverse effects, even when the medicaments are given in comparatively low doses. They are used with caution in children because they can retard growth if taken for prolonged periods. In a child, an excess of these medicaments depresses the nervous system, producing drowsiness or possibly loss of consciousness. Palpitations may occur.

These medicaments reduce the level of natural stimulants in the brain, so that after a few weeks' regular use a person may become physically dependent on them for normal function. If they are then abruptly withdrawn, the excess of natural inhibitory chemicals in the brain depresses activity in the central nervous system, producing withdrawal symptoms. These may include lethargy, depression, and increased appetite.

If used by adults in excess or inappropriately, stimulants can produce over-activity in the brain, resulting in extreme restlessness, sleeplessness, and feelings of nervousness or anxiety. They also stimulate the sympathetic branch of the autonomic nervous system, causing shaking, sweating, and palpitations. More serious risks of exceeding the prescribed dose are seizures and a disturbance in mental functioning that may result in delusions and hallucinations.

Migraine medicaments

Migraine is a term applied to recurrent severe headaches affecting only one side of the head and caused by changes in the blood vessels. Such headaches may be accompanied by nausea and vomiting and preceded by flashing lights or numbness and tingling in the arms. Occasionally, speech may be impaired, and the attack may be disabling. The exact cause of migraine is unknown, but an attack may be triggered by excitement, tension, shock, physical exertion, a blow to the head, some foods, and some medicaments. Victims often have a family history of migraine.

A migraine attack begins when blood vessels surrounding the brain constrict. This is thought to be caused by certain chemicals in food or produced in the body. Methysergide and propranolol block the effect of the chemicals on blood vessels and so prevent attacks.

The next stage occurs when blood vessels in the scalp and around the eyes dilate, releasing pain-producing chemicals called prostaglandins. ASA and acetaminophen relieve pain by blocking the production of prostaglandins, while codeine acts directly on the brain to alter the perception of pain. Ergotamine and sumatriptan relieve pain by narrowing the dilated blood vessels.

All these medicaments have their own side effects. Sumatriptan may cause chest tightness. Ergotamine may cause drowsiness, tingling sensations in the skin (paresthesia), cramps, weakness in the legs, and pain in the abdomen, arms, and legs.

Ergotamine should be used with caution if you have poor circulation because it can damage blood vessels through prolonged over constriction. Frequent use can lead to dependence and numerous adverse effects, including headache. You should not take more than your physician advises in any one week. Ergotamine should not be used if you have an infection because it can restrict blood flow to the site of infection and delay recovery. Sumatriptan should not be used if you are taking ergotamine or if you suffer from coronary heart disease, angina, or high blood pressure.

Methysergide can produce pain in the abdomen or lower back, and also shortness of breath due to an unusual type of damage to tissues.

Anti-emetic medicaments

Anti-emetic medicaments are used to suppress vomiting and nausea. Vomiting (emesis) is a reflex action that protects the body by expelling harmful substances. Common causes of vomiting and nausea are digestive tract infection, pregnancy, motion sickness, and vertigo. They can also occur as a side effect of a medication, or medicament or radiation therapy for cancer.

The main anti-emetic medicaments are metoclopramide, domperidone, ondansetron, the antihistamines, and phenothiazine medicaments, which are also used to treat mental illness. Nabilone, a marijuana derivative, may be used in cancer patients needing an anti-emetic.

Nausea and vomiting occur when a specialized part of the brain called the vomiting center is stimulated by signals which may arise from various points in the brain and body: from the digestive system, from the part of the brain that is responsible for consciousness, or from the inner ear. Signals may also arise from an area of the brain called the chemoreceptor trigger, which stimulates the vomiting (emetic) center if it detects any harmful substances present in the blood. Anti-emetic medicaments may act at one or more of these places in the body. In addition, these medicaments may also promote the normal emptying of the stomach contents into the intestine.

In addition to reducing or preventing vomiting and nausea, most anti-emetics may make you feel drowsy. Certain non-sedating antihistamines may therefore be preferred for the prevention of motion sickness.

Because the antihistamines block the parasympathetic system, they can produce many anticholinergic side effects, including dry mouth, blurred vision, and difficulty passing urine. Phenothiazine medicaments - which also may produce anticholinergic side effects - can produce dizziness.

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