A reflex is an automatic response, controlled and co-ordinated by the nervous system, to a stimulus which may be from either outside the body or within it. An example familiar to most people is the knee-jerk, or patellar, reflex in which the lower leg gives a kick in response to a tap just below the knee-cap. Many functions of the body are regulated automatically. The brain plays no conscious part in these actions. Via these internal nervous reflexes, and via the hormonal system of control and co-ordination, the body gets on with looking after its internal workings and leaves the conscious mind to deal with the relationships between the body and the outside world. Some reflexes, known as natural reflexes, are an integral part of the normal nervous system. We are born with these reflexes. Others are acquired through the repetition of experiences after birth and are known as conditioned reflexes. One of the best-known examples of a conditioned reflex was demonstrated in dogs by Ivan P. Pavlov (1849-1936), the Russian physiologist. The dogs were offered food (and so salivated) and at the same time a bell was rung. Eventually the dogs would salivate when the bell was rung, even if no food was offered. Conditioned responses occur in people, but they are greatly modified by conscious decision-making.
The reflex pathway
A reflex action begins when the stimulus is detected by a sense organ. In the example of the knee jerk the patellar tendon, which attaches the thigh muscles to the shin, contains sense organs known as stretch receptors. When the tendon is tapped just below the knee-cap it is briefly stretched; the stretch receptors detect this and respond by sending a nerve impulse along a sensory nerve fibre to the spinal cord. In the spinal cord the sensory neuron makes contact with a motor neuron which, in turn, sends an impulse back to the thigh muscle, causing it to contract in order to relieve the stretch.
The reflex pathway, involving a sensory receptor, a sensory nerve, a motor nerve and a muscle, is known as a reflex arc. The sensory neuron leading towards the spinal cord is said to form the afferent limb of the arc, whereas the motor neuron leading back to the muscle forms the efferent limb. The thigh muscle resists passive stretching by means of the reflex pathways. The same phenomenon occurs in all muscles; this tendency to resist passive elongation is known as maintaining muscle tone.
Reflex actions are beyond our voluntary control. However, our reflexes must be modified in order to allow voluntary actions to be performed. Babies make jerky, random movements over which they have liitle control, because they are dominated by their reflexes. At birth, many of the nerves which later exert influence on reflex arcs have not yet acquired their insulating and conduction-speeding covering of myelin and so do not provide adequate control of movement. Gradually, as the system matures and the baby learns, the responses of motor neurons to incoming stimuli are modified by the many other neurons which develop connections with them. Some of these are branches of neurons taking part in other reflex arcs, and some connect to ‘higher’ parts of the nervous system – for example, the cerebellum in the brain, which is responsible for co-ordinating movement. Eventually some reflexes are suppressed so that smooth, voluntary movements are not interfered with by chaotic reflex movements. An example of this process is the supression of the bladder-emptying reflex as the toddler brings it under voluntary control. In a baby, when the bladder is filled to a certain volume the muscles of the bladder wall are stretched, and a reflex contraction of the bladder muscles takes place to void the urine. This reflex is mediated by nerves passing between the bladder and the spinal cord. As the nervous system matures, however, the brain exerts a modifying influence on this reflex so that the toddler learns to postpone emptying the bladder until a convenient moment.
Other reflexes are difficult, if not impossible, to modify – indeed, it could be dangerous to do so. The conjunctival reflex involves motor nerves which arise in the brain stem at the base of the brain, and cause the eyes to blink automatically in response to a touch on the conjunctiva (the surface of the eye). Irritation in the air passages causes coughing by involving a reflex arc which also passes to and from the brain stem. Both of these can be suppressed, but with difficulty, and not for long.
Many reflex arcs regulate the internal functions of the body. Rapid loss offrom a wound causes a fall in pressure, which is recognized by baroreceptors in the walls of the large arteries. The information is relayed to the cardiovascular centre in the brain stem, and a reflex constriction of the muscles in the -vessel walls, plus an increase in heart rate, act to restore blood pressure and so a continuing supply to vital organs.
The examination of reflexes provides doctors with a means of assessing how well various parts of the nervous system are working. Because they know the parts involved in a particular reflex, any abnormality in or absence of a reflex provides information about disease or damage in those parts. For example, the constriction of the pupils in bright light is called the pupillary reflex. If the pupil fails to contract when a light is shone in the eye, this may be caused by a disorder somewhere in the reflex – in the retina, the optic nerve (afferent limb), occulomotor nerve (efferent limb), or the constrictor muscle of the pupil. In some neck or back injuries, the lower parts of the spinal cord may be isolated from the brain – which usually exerts a modifying influence on the spinal reflexes. Released from control in this way, the spinal reflexes become exaggerated and muscle tone increased. Damage to a nerve running to or from the spinal cord (the peripheral nerves) will also modify the reflex arc; in this case the reflex is abolished and the muscle tone reduced. This knowledge is useful to doctors when examining people with injuries or disorders of the spine.
As explained above, certain reflexes seen in babies disappear as the nervous system matures and the brain becomes able to exert a modifying influence on them. Disease or injury removes the influence of the brain on these ‘primitive reflexes’ and so their reappearance in older children and adults may provide clues in the diagnosis of nervous disorders. The most commonly recognized of these reflexes is the Babinski reflex, which is evoked when the sole or side of the foot is scratched. The normal response in older children and adults is for the big toe to curl downwards. In a wide variety of disorders of the central nervous system the response reverts to the infant type, in which the big toe curls upwards instead. Our reflexes, therefore, are indispensable servants of the normally functioning nervous system, but, released from strict control, they are responsible for many of the features of nervous system disease.