The word 'reflex' comes from the idea that nerve impulses are 'reflected' in the central nervous system. Descartes instances the constriction of the pupil when a light is shone in the eye. In the 18th century, Robert Whytt and Stephen Hales showed that the integrity of the central nervous system is, indeed, essential for 'reflection' to occur. In the next century the subject was greatly clarified by the discovery of Magendie and Bell that the nervous system uses separate channels (nerve fibres) for input and output so that 'reflection' must occur centrally. With minor exceptions ('axon reflexes') the dorsal spinal nerve roots are exclusively sensory and the ventral roots exclusively motor in function. Detailed knowledge of the connections between the sensory and motor nerves in the grey matter of the spinal cord dates only from 1951, when Eccles obtained records from a microelectrode inside a motor nerve cell — the first time this had been achieved with any central neuron.
The tendon jerk is the simplest and fastest mammalian reflex known and its neuronal mechanism (although not its function in everyday life) is still the best understood. Endless other reflexes exist of greater complexity and longer latency. Commonly instanced are responses to injury or irritation: sneezing and coughing, the withdrawal of a foot in a frog or quadruped, the scratching of a dog. There are very many reflexes concerned in the vital functions: blood pressure is reflexly affected by pressure receptors in the walls of the aorta, breathing by reflexes from stretch receptors in the diaphragm, and so forth. Reflexes from receptors in the muscles of the limbs and trunk (of which the knee jerk is one) are a large class, of still controversial function in the control of bodily movement.
Although the experimental investigation of reflexes in animals is traditionally carried out on the spinal cord after severing it from the brain, or on the lower parts of the nervous axis after removing the cerebral hemispheres (decerebrate preparation), there are many reflexes whose pathway is through the cerebral cortex. The involuntary blink to a threatening gesture is one. And the elaborate learned responses called 'conditioned reflexes' (see conditioning) are cortical or usually so.
C. S. Sherrington, to whom we owe much of our knowledge of reflex action, regarded the reflex as the unit of nervous action and suspected that complex sequential acts, such as walking, were in the nature of chain reflexes, in which one element reflexly caused the next: in walking, for example, the movement of the leg forward excited receptors in the leg which reflexly caused it to move back again, and so on. There is now evidence that the nervous mechanism for performing such acts as walking or breathing exists in the central nervous system and can function, after a fashion, without reflex inputs, but that, normally, reflexes modify and regulate these actions and adapt them to changing circumstances.
The point at which an animal's responses to stimuli cease to be regarded as reflex and are called deliberate or voluntary, or by some similar term, is ill defined. A mild cough can be suppressed by an effort of will during a concert, but such coughing would be regarded as reflex. A similar suppression of the urge to pass water is more easily achieved and passing water is normally to be considered a deliberate act; the underlying reflex element is dominant only in infancy or when self-control is impaired.
(Published 1987)
— P. A. Merton
- Bibliography
- Eccles, J. C. (1957). The Physiology of Nerve Cells.
- Liddell, E. G. T. (1960). The Discovery of Reflexes.
- Merton, P. A. (1979). 'The central nervous system'. In Lippold, O. C. J., and Winton, F. R. (eds.), Human Physiology.
- Sherrington, C. S. (1900). 'The spinal cord'. In Schäfer, E. A., Textbook of Physiology, vol. ii.
- — — (1906). The Integrative Action of the Nervous System (new edn. 1947).
