tremor

tremor With sensitive recording systems it can be shown that all parts of the living body are vibrating; the only time when we are truly still is when we are dead! Most of these micro-vibrations cannot, however, be seen by the naked eye. For most studies of tremor, the motion is recorded by the use of small devices sensitive to acceleration taped onto the fingers. Such instability may often be seen in normal people when they hold out their arms with their fingers gently separated and straight. The oscillation that may then be seen is called physiological tremor. There is a very wide degree of variation in the size of this, and a range of about a hundred-fold has been observed. Heavily pressurized students may have particularly high levels; also, there are statistical variations in the amount of tremor according to occupation.

The extent to which the fingers shake varies with the person's psychological circumstances; there are many references in the Bible and in Shakespeare to the association with fright and strong emotion. This is due to the liberation of the hormone adrenaline at these times. Musicians and others making public appearances may suffer from stage fright, and excessive tremor is then often seen. High levels have been observed in air squadron cadets learning to fly light aircraft, and unusually high levels have been observed after bungee jumping.

Physiological tremor is usually rhythmic, the peak of the oscillations being at about 10 cycles per second. Whilst its size may vary widely from moment to moment, the rate remains much the same. Each muscle is controlled by the nerve impulse activity in a number of nerve fibres, each of which is responsible for controlling the contraction of a number of individual muscle fibres. The number of muscle fibres controlled by a single nerve fibre (forming a motor unit) varies considerably in different muscles. In those of the trunk it is large, but is smaller in the small muscles of the hand where high precision of control is needed. The nerve impulses occur usually at a rate of 5–20 cycles per second. This is not fast enough to cause the muscle elements to produce a smooth contraction; these facts about the behaviour responsible for our posture and its disturbances have been established by animal and human experiments. Our posture is thus based on many small, jerky sources of force. We are not usually aware of these discontinuities, because the different motor units are operating out of step with one another so that overall there is a large measure of smoothing in the force acting through the tendons. An analogue would be soldiers falling out of step when marching over a bridge. These bumpy forces are applied across our joints.

In general, limbs are under-damped, that is, they oscillate preferentially at a certain rate: the resonant frequency. It so happens that the resonant frequency of the human wrist is about 10 cycles per second for small movements — close to the rate of discharge of the nerve fibres to the muscles of the arm. It is the combination of these two factors which is mainly responsible for physiological tremor.

In disease the discharges of the individual nerve fibres may become unusually closely timed to one another; they are then said to be synchronized. There is then a prominent tremor. This is the situation in pathological tremors such as that seen in Parkinsonism. This lower frequency tremor at 4–6 cycles per second can be widespread, involving the forearms and even the head and trunk.

After a stroke, the foot on the affected side may at times jerk rhythmically, in response to ankle displacement, in a powerful and disturbing manner. Strictly speaking this is classed as clonus rather than tremor.

Movements and posture are regulated by the cerebellum. This part of the brain is responsible for organizing the motor programmes. The correct timing of the multiplicity of muscle actions needed in performing everyday tasks is a problem of great magnitude. If the cerebellum or its pathways are not capable of fully normal action, as in Freidrich's Ataxia, movements will be poorly co-ordinated and there will be an ‘intention tremor’: when picking something up, the hand may oscillate to and fro at the end of the motion, under-reaching and over-reaching, instead of coming to rest neatly in the correct position. These pathways are also after impaired in multiple sclerosis. Some people, who are otherwise quite healthy, have unusually prominent shaking. This is called essential tremor. Sometimes it is hereditary. In northern Sweden the population moved little for hundreds of years, and very good church records exist. From this region and certain other areas very extensive family trees have been prepared, which show the mode of inheritance of some forms of the condition. If severe, essential tremor can be a serious disability as it interferes with skilled movements such as writing. This can be socially embarrassing, and the person may falsely be believed to be an alcoholic — whereas small amounts of alcohol may actually suppress tremor. Tremor associated with alcoholism occurs the morning after rather than at the time of the drinking. With excessive intake the person may show widespread shaking, the Latin name for this being delirium tremens — a very serious condition. Many other drugs also can give rise to shaking. Adolph Hitler had a prominent tremor, perhaps due to the stimulant amphetamine (purple hearts), which he was taking. Some people find that coffee brings out tremor; this is due to the presence of caffeine, which is an excitant; these effects are not, however, universal.

As to treatment, there is no panacea. For tremors associated with anxiety, drugs which block the action of adrenaline can be of use, but there may be side actions. Very severe and incapacitating tremors are sometimes treated by surgical operations on the brain. Important and interesting human and animal research is currently being conducted on tremor mechanisms and the possibilities of refining treatment.

E. Geoffrey Walsh

Bibliography

Walsh, E. G. (1992). Muscles, masses and motion. Mackeith/Cambridge University Press.

See also cerebellum; movement; skeletal muscle.