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reflexesThe term reflex wasfirst usedto describe anautomatic, almostimmediatemovement in response to astimulus, involving a nervecircuit that traverses thespinal cord. It is now applied also to other types of automatic response to astimulus, including those involving thebrain. A reflex requiressensory receptorsthat detect thestimulus, sensory nerve fibres that conduct the information to thecentral nervous system(CNS),neurons in the CNS itself, nerve fibres conducting the command away from the CNS, and the effector.

Sir Charles Sherrington (18571952) was the first to introduce the word reflex, taking the view thatsensory information going into the cord was reflected out again along the motor nerve fibres,analogous to a beam of light being reflected by a mirror. Sherrington referred to the chain ofstructures receptor, conductor, and effector as a reflex arc.

The study by Sherrington and colleagues of the spinal reflex provided an understanding of the basisof the simplest neural circuits in thecentralnervous system, an understanding on which subsequentadvances in neuroscience relied. Sherrington wished to remove the element ofconsciousnessandconsciously-guided movement, so that he could study the nature of the behaviour repertoire of thespinal cord. The experimental results prompted Sherrington to define the term reflex and, with this,the implicit assumption that a reflex response is independent of consciousness. Elimination of theeffects of consciousness could be achieved in experimental animals by surgically interruptinginfluences from higher centres. This afforded the means of unravelling features of the activity of the

cord that had hitherto escaped analysis.Spinal reflexesAlthough simple manifestations of activity of the central nervous system, spinal reflexes aremeaningful, in that each reflex subserves an obvious function. For example, the reflex withdrawal ofthe hand from a noxious object minimizes the damage inflicted on the organism by the noxious agent.

One of the best-known reflexes is the tendon jerk reflex. When a tendon is tapped, the muscle towhich it is attached gives a twitch. An example is the knee-jerk reflex; a tap to the patellar tendon(just below the front of the knee) causes a reflex twitch in the quadriceps muscles (the muscle masson the front of the thigh). This twitch may be sufficiently powerful to extend the lower leg at the knee.We now know that this reflex response is initiated from the class of sensory receptors called musclespindle receptors. In animal experiments, Sherrington showed that the adequate stimulus for thisreflex was a mere 0.01 mm elongation of the quadriceps muscle.

The tendon jerk reflex is the simplest reflex; within the central nervous system, the sensory nervefibres form connections directly with the nerve cells that send out motor nerve fibres to innervate theeffector muscle. The testing of these reflexes, together with a knowledge of the different levels of thespinal cord responsible for each of them, provides a clinical method of examining the integrity of areflex arc involving particular peripheral nerves and segments of the spinal cord. Also, since tendon

jerks are normally partly suppressed by nerve impulses descending from higher levels of the CNS,their exaggeration is a valuable sign of damage above the relevant spinal segments.Transmission of information in the reflex arc

As with other cells in the body, each nerve cell is surrounded by its own thin lipidcell membrane. Thismembrane has a high electrical resistance. Conduction of nerve impulses along nerve fibres issubserved by an electrical mechanism. The nerve fibre acts as a cable with a conducting core(the cell sap) surrounded by its insulating membrane. Nerve impulses can propagate in either

direction along the nerve fibre.

The study of spinal reflexes allowed early workers to deduce properties of transmission of informationfrom the sensory nerve fibres to the motor nerve cells within the spinal cord. In consultation withClassics colleagues in the University of Liverpool, in 1897 Sherrington introduced into our languagethe nounsynapseto describe those areas of functional contact, between nerve cells, that arespecialized for transmission of nerve impulses. He deduced that it was synaptic transmission thatconferred the reflex with the property of directionality. In the reflex arc, information entered the cordalong sensory nerve fibres to elicit activity leaving the cord in motor nerve fibres but, because of thespecial properties of the synapse, information could not flow in the opposite direction. Synaptictransmission was subsequently shown to be subserved by a chemical mechanism. Action potentialsin the sensory nerve fibres cause the release of aneurotransmitterchemical that diffuses to attach tospecific recognition sites on the motor nerve cells. This attachment changes the electrical excitabilityof the nerve cells and may initiate nerve impulses in the motor nerve fibres. Synaptic transmission,the fundamental properties of which were initially revealed by the study of the spinal reflex, is the

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basis of the integrative activity of the nervous system. Modulation of synaptic transmission underliesthe mechanism of action of most drugs, both therapeutic and drugs of abuse, that act on the brain. Evolutionary aspectsIn simple vertebrates the spinal cord and lowerbrain stemdominate, there being little or no developedforebrain. As higher centres have developed in the course of evolution, they have come to exert manyof their effects by controlling and modifying the pre-existing spinal reflex mechanisms, not by

replacing them. An example of modification of primitive cord activity by higher centres is afforded byanother clinically useful test. When a firm stroke is applied to the sole of the foot, the primitive spinalreflex response, when influences from higher centres are absent, is withdrawal of the foot from themildly noxious stimulus. The response of a normal human adult to this same stimulus, however, is athrust, to push the stimulus away. This latter response is part of the complicated mechanism thatallows us to stand; the pressure on the soles of our feet elicits a continuous muscular effort to keepthe feet pushing against the ground to prevent us from falling. If a human adult suffers damage to thehigher motor centres in the brain, the reflex reverts from its normal thrust to the more primitivewithdrawal response. Doctors refer to the reflex as the Babinskiresponse, named after theneurologist inPariswho first described its significance in 1896. It is a clinically useful indicator of theintegrity of the higher motor centres together with the tracts projecting down from these centres to themotor nerve cells in the spinal cord. Normal new-born babies, in whom the higher central controlofposturehas yet to develop, show the primitive withdrawal response. This reverses to the normal

adult response at the age of about 6 months. This is the time in development at which the tracts fromhigher motor centres become functional.

For different reflexes, the reflex responses range from simple to complex. The tendon jerk reflex isrelatively simple and involves a relatively small region of the spinal cord. This contrasts withcomplicated, repeated movements, such as those occurring in a limb of a dog showing a scratchreflexto dislodge an insect biting its flank. For these more complicated reflexes, extensive regions ofthe cord are involved and the reflex circuits are correspondingly elaborate. Whereas the tendon jerkreflex is executed by a direct connection in the cord between the sensory nerve fibres and motornerve cells, a scratch reflex depends on long pathways involving multiple synaptic relays, and thetriggering into action of a rhythm generator responsible for the frequency and vigour of the scratchingmovements.

Reflexes interact with each other. The reflex response to a stimulus which is severely threatening tothe well-being, or even to the life, of an animal, will, whilst commanding its own response,simultaneously switch off any other interfering reflexes that are less important in survival and thatutilize the same muscles.Reflexes mediated by cranial nervesThe cranial nerves (the nerves that arise from the brain rather than the spinal nerves that arise fromthe cord) provide the pathways to and from the central nervous system for reflexes utilizing themuscles of the head, such as those controlling movements of the eyeball, face, and tongue. Thenerve cells giving rise to the cranial motor nerve fibres lie in clusters (nuclei) in the brain stem; theyrepresent an upward extension of the homologous groups of nerve cells in the spinal cord. Examplesof reflexes involving the cranial nerves are the closure of the eyelids when the cornea is stimulated, orgagging when the back of the throat is irritated.Autonomic reflexesThese reflexes produce effects such as: changing the rate or force of contraction of the heart;

contraction or relaxation of smooth muscle; glandular secretion. The reflexes are mediated by the

sympathetic or parasympathetic nerves of theautonomic nervous system, in response to information

reaching the central nervous system from a variety of receptors in the organs and tissues. For

example, when a light shines in theeye, there is constriction of the pupil produced by contraction of

the circular smooth muscle of the iris; when a person rises rapidly from bed or bath, the heart rate

promptly increases in response to a fall inblood pressure; in response to the taste of a lemon, there is

an outpouring ofsaliva.

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