Nerve Stimulants

These are remedies which increase the nervous activity of the cerebro-spinal system. They are subdivided into those which act on the cerebrum, or cerebral stimulants, and those which affect the spinal cord, or spinal stimulants. Spinal stimulants have been already discussed (p. 181).

Cerebral Stimulants

In popular language, the name of stimulant is generally applied to drugs which have the power to increase the activity of the brain. From their producing a feeling of comfort and mirth they are also called exhilarants. The functional activity of the brain, like that of other organs, depends upon the tissue-change which goes on in the cells and fibres which compose it, and the amount of tissue-change is regulated to a great extent by the quantity and quality of the blood supplied to the organ. A free supply of blood to the brain may be obtained by general excitement of the circulation, i.e. more powerful and rapid action of the heart and contraction of the vessels in other parts of the body driving blood into the brain, or by local dilatation of the cerebral arteries allowing blood more ready access to the brain, or by a combination of these factors.

Free circulation through the cerebral arteries may be induced to some extent by posture : thus, some men can think best when the head is low, and almost everyone naturally assumes the sitting posture with the head bowed down and held between the hands when suffering from the effects of mental depression. This posture is not, as is often supposed, merely consequent on the depressed condition of the nerve-centres, it is voluntarily assumed because it affords an actual sense of relief. In eager conversation also the body generally stoops forward and the head is held low so as to allow of a free supply of blood to the brain.1

This effect of posture on the human brain is admirably shown by a tracing taken from a patient with an aperture in the skull by Francois-Franck and Brissaud (Fig. 69).

1 Francois-Franck et Brissaud, Marey's Travaux, 1877, tome iii. p. 147.

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Fig. 69.- Tracing showing the increased circulation in the brain caused by inclining the head and body forwards. The tracing was taken by Brissaud and Francois-Franck, from the parietal region of a woman who had lost a large piece of bone from syphilis.1

Local dilatation of the arteries of the brain appears to be produced in animals by the movements of mastication (Fig. 70) and probably also by savoury food or irritating substances in the mouth.

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Fig. 70. - Tracing to show the increased rapidity of circulation in the carotid of a horse during mastication. (After Marey.)

It is probably on this account that so many substances are chewed for their stimulant action, such as tobacco, betel nut, cola nut, and raisins. The effect of smoking is probably to a great extent due also to its action on the cerebral circulation through the stimulating effect of the smoke on the nerves of the mouth and nares, and so is the use of alcohol in sips by men, such as jour-

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Fig. 71. - Pulsations of the fontanelle (F) in an infant six weeks old while sucking. R shows a simultaneous tracing of the thoracic respiration. The breast was offered to the child at the beginning of the tracing. At the time indicated by the third respiratory wave, which has a flattened top, the child began to take the breast. It will be noticed that the line of the tracing F rises, indicating increased circulation on the brain. (After Salathe.)1 nalists, who are engaged in writing. It is probable that tea and coffee also cause local dilatation of the arteries supplying the brain. Suction also causes an increased supply of blood to the brain (Fig. 71).1

1 Marey's Travaux for 1877, p. 147.

2 Salathe, Marey's Travaux, 1876, p. 354.

The effect of local dilatation of the cerebral vessels is very greatly increased, if in addition to it the general circulation is increased and the blood-pressure raised by contraction of the arterioles in the body generally, or by more vigorous action of the heart.

General excitement of the circulation is induced by exercise short of fatigue, and a brisk walk will sometimes remove a condition of low spirits. Sometimes the supply of blood to the brain is but slightly increased during continuous exercise, as a large portion of the blood is then diverted to the muscles, but after the exertion is over the excitement of the circulation continues for some time, and then the supply to the brain is increased. In some persons a cold wind acts as an exhilarant, causing contraction of the vessels, with consequent increase in the general blood-pressure and increased circulation in the brain. In persons who are debilitated and feeble, on the contrary, the cold may have an opposite effect, by depressing the action of the heart.

Some men can think best when walking about, on account of the excitement in the circulation which the exertion produces; but many such people, when they come to a very difficult point, will stand still or sit down, so as to allow the blood to flow more to the head and less to the muscles.

Where the circulation is feeble, so that the heart is not much stimulated by walking about, men often find that they can think better when lying down, or sitting with their head in their hands (Fig. 69), so as to gain the advantage of the greater flow of blood to the head in these positions.

Stimulation of the mucous membrane of the nose by smelling the vapour of strong ammonia, carbonate of ammonium, or acetic acid, raises the blood-pressure generally throughout the body by reflexly stimulating the vaso-motor centre, and thus increases the circulation of blood in the brain. Smelling salts or aromatic vinegar are therefore frequently employed, not only to enable people to attend more readily to any subject in which they are engaged, and to prevent them from falling asleep, but also to arouse them from syncope. 1 Salathe, op. cit.

The action of sipping is a powerful stimulant to the circulation, for, as Kronecker has shown, the inhibitory action of the vagus on the heart is abolished while the sipping continues, and the pulse-rate is very greatly increased. A glass of cold water slowly sipped will produce greater acceleration of the pulse for a time than a glass of wine or spirits taken at a draught. Sipping cold water has been recommended to allay the craving for alcohol in drunkards endeavouring to reform, and probably its use is Owing to this stimulant action on the heart. It is sometimes said that a single glass of ale sucked through a straw will intoxicate a man, although three times the quantity would not do so if taken in large draughts. If this he true, the more rapid intoxication caused by sucking is probably due to the conjoined effects of the alcohol and of temporary paralysis of the vagus caused by the suction, possibly aided by the direct effect of suction on the cerebral circulation (Fig. 71, p. 193).

One of the most typical stimulants is alcohol. In small quantities it increases the arterial tension by locally stimulating, first the sensory nerves of the mouth, and afterwards those of the stomach, and thus causing reflex contraction of the vessels and reflex acceleration of the beats of the heart. This effect occurs before its absorption, and is best marked when the alcohol is strong, and is but slightly marked when it is diluted. It is possible that by inducing local dilatation of the cerebral arteries while the heart still continues active, it may have a stimulant action on the cerebral functions, besides that which it induces by merely exciting the circulation generally.

Any stimulant action on the brain beyond what may be explained in this way is very slight, if indeed it exist at all. Its further actions are those of paralysis exerted on the nerve-centres in the order of their development, the higher centres being paralysed first (see p. 146).

At or about this point the stimulating action ceases and the narcotic action commences. The exhilarating effect of alcohol, however, may be most marked just at this point, because just here, while the circulation in the brain generally remains increased, the restraining or inhibitory parts of it begin to be paralysed. Thus, imagination and emotion are more readily excited and expression is free and unrestrained; external circumstances are less attended to, and a boyish or childish hilarity occurs.

It is probable that some substances, such as strychnine, increase the mental powers by a direct action on the brain-tissue itself, and possibly caffeine may do so also.

Drugs which lessen the Functional Activity of the Brain. These drugs are soporifics or hypnotics; narcotics; anodynes or analgesics; and anaesthetics.

Most of the substances belonging to those classes have a certain resemblance to one another in their action. Most of them stimulate the mental functions when given in very small doses. In larger doses they have also a stimulating action at first, i.e. while a small quantity only has been absorbed, but later on they diminish or abolish the mental faculties. The same drug - as, for example, opium or alcohol - in different doses may thus act as a stimulant, narcotic, soporific, and anaesthetic.

In a certain stage of their action opium and alcohol do not merely lessen the functional activity of the brain, but they disturb the normal relations of one part to another, so as to produce disorder of the mental functions. Bromide of potassium, on the other hand, appears simply to lessen the functional activity of the brain without disturbing the relation of one part to another. We do not know what the causes of this difference in their action are, but with some degree of probability we may consider that such substances as bromide of potassium, or the normal products of tissue-waste, such as lactic acid, simply diminish the functional activity of the nerve-cells without disturbing the nervous paths by which they communicate with one another, so that we have merely a general and even diminution of the mental faculties, as in natural sleep. Such substances as alcohol, on the other hand, may be supposed not only to diminish the functional activity of the cells, but also to disturb the rate at which the impulses pass from one cell to another, or to alter the direction in which these impulses are sent, so that instead of the mental activity being lessened in degree but natural in kind, as after the administration of bromide of potassium, we have a disturbance of the functions resembling that which we find in delirium or madness.