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reflexes

reflexes The term ‘reflex’ was first used to describe an automatic, almost immediate movement in response to a stimulus, involving a nerve circuit that traverses the spinal cord. It is now applied also to other types of automatic response to a stimulus, including those involving the brain. A reflex requires sensory receptors that detect the stimulus, sensory nerve fibres that conduct the information to the central nervous system (CNS), neurons in the CNS itself, nerve fibres conducting the command away from the CNS, and the effector. Sir Charles Sherrington (1857–1952) was the first to introduce the word ‘reflex’, taking the view that sensory 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 of structures — receptor, conductor, and effector — as a reflex arc.

The study by Sherrington and colleagues of the spinal reflex provided an understanding of the basis of the simplest neural circuits in the central nervous system, an understanding on which subsequent advances in neuroscience relied. Sherrington wished to remove the element of consciousness and consciously-guided movement, so that he could study the nature of the behaviour repertoire of the spinal 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 the effects of consciousness could be achieved in experimental animals by surgically interrupting influences from higher centres. This afforded the means of unravelling features of the activity of the cord that had hitherto escaped analysis.

Spinal reflexes

Although simple manifestations of activity of the central nervous system, spinal reflexes are meaningful, in that each reflex subserves an obvious function. For example, the reflex withdrawal of the 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 to which 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 mass on 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 muscle spindle receptors. In animal experiments, Sherrington showed that the adequate stimulus for this reflex 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 nerve fibres form connections directly with the nerve cells that send out motor nerve fibres to innervate the effector muscle. The testing of these reflexes, together with a knowledge of the different levels of the spinal cord responsible for each of them, provides a clinical method of examining the integrity of a reflex 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 lipid cell membrane. This membrane has a high electrical resistance. Conduction of nerve impulses along nerve fibres is subserved 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 information from the sensory nerve fibres to the motor nerve cells within the spinal cord. In consultation with Classics colleagues in the University of Liverpool, in 1897 Sherrington introduced into our language the noun synapse to describe those areas of functional contact, between nerve cells, that are specialized for transmission of nerve impulses. He deduced that it was synaptic transmission that conferred the reflex with the property of directionality. In the reflex arc, information entered the cord along sensory nerve fibres to elicit activity leaving the cord in motor nerve fibres but, because of the special properties of the synapse, information could not flow in the opposite direction. Synaptic transmission was subsequently shown to be subserved by a chemical mechanism. Action potentials in the sensory nerve fibres cause the release of a neurotransmitter chemical that diffuses to attach to specific recognition sites on the motor nerve cells. This attachment changes the electrical excitability of 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 basis of the integrative activity of the nervous system. Modulation of synaptic transmission underlies the mechanism of action of most drugs, both therapeutic and drugs of abuse, that act on the brain.

Evolutionary aspects

In simple vertebrates the spinal cord and lower brain stem dominate, there being little or no developed forebrain. As higher centres have developed in the course of evolution, they have come to exert many of 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 by another clinically useful test. When a firm stroke is applied to the sole of the foot, the primitive spinal reflex response, when influences from higher centres are absent, is withdrawal of the foot from the mildly noxious stimulus. The response of a normal human adult to this same stimulus, however, is a thrust, to push the stimulus away. This latter response is part of the complicated mechanism that allows us to stand; the pressure on the soles of our feet elicits a continuous muscular effort to keep the feet pushing against the ground to prevent us from falling. If a human adult suffers damage to the higher motor centres in the brain, the reflex reverts from its normal thrust to the more primitive withdrawal response. Doctors refer to the reflex as the Babinski response, named after the neurologist in Paris who first described its significance in 1896. It is a clinically useful indicator of the integrity of the higher motor centres together with the tracts projecting down from these centres to the motor nerve cells in the spinal cord. Normal new-born babies, in whom the higher central control of posture has 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 from higher motor centres become functional.

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

Reflexes interact with each other. The reflex response to a stimulus which is severely threatening to the 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 that utilize the same muscles.

Reflexes mediated by cranial nerves

The cranial nerves (the nerves that arise from the brain rather than the spinal nerves that arise from the cord) provide the pathways to and from the central nervous system for reflexes utilizing the muscles of the head, such as those controlling movements of the eyeball, face, and tongue. The nerve cells giving rise to the cranial motor nerve fibres lie in clusters (nuclei) in the brain stem; they represent an upward extension of the homologous groups of nerve cells in the spinal cord. Examples of reflexes involving the cranial nerves are the closure of the eyelids when the cornea is stimulated, or gagging when the back of the throat is irritated.

Autonomic reflexes

These 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 the autonomic 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 the eye, 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 in blood pressure; in response to the taste of a lemon, there is an outpouring of saliva.

In conclusion, the study of reflexes alone cannot solve the problems of higher neural function, of emotion, or of psychology. However, by providing a basis from which a study of higher functions could develop, the unravelling of the properties of reflexes was an historic, essential early step in the development of neuroscience.

Oliver Holmes


See also central nervous system; motor neuron; reaction time; synapse.

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reflex

re·flex / ˈrēˌfleks/ • n. 1. an action that is performed as a response to a stimulus and without conscious thought: a newborn baby is equipped with basic reflexes. ∎  (reflexes) a person's ability to perform such actions, esp. quickly: he was saved by his superb reflexes. ∎  (in reflexology) a response in a part of the body to stimulation of a corresponding point on the feet, hands, or head: [as adj.] reflex points. 2. a thing that is determined by and reproduces the essential features or qualities of something else: politics was no more than a reflex of economics. ∎  a word formed by development from an earlier stage of a language. ∎ archaic a reflected source of light: the reflex from the window lit his face. • adj. 1. (of an action) performed without conscious thought as an automatic response to a stimulus: sneezing is a reflex action. 2. (of an angle) exceeding 180°. ∎ archaic (of light) reflected. ∎  (also re·flexed) (esp. of flower petals) bent or turned backward. ∎ archaic (of a thought) directed or turned back upon the mind itself; introspective. DERIVATIVES: re·flex·ly / ˈrēˌflekslē; riˈflekslē/ adv.

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Reflexes

Reflexes

Movements or involuntary reponses initiated by an external stimulus which do not require input from the brain.

In a simple reflex, a sensory receptor initiates a nerve impulse in an afferent sensory nerve fiber which conducts it to the spinal cord. In the gray matter of the spinal cord, the afferent nerve impulse is fired over the synaptic gap to an efferent motor fiber which passes along the impulse to the appropriate muscle, producing the reflex.

There are other reflexes which involve neural pathways connected to the brain . When an ice cube is touched, cold receptors in the skin are stimulated and that afferent information is transmitted to the gray matter of the spinal cord, where it then travels via axons in the white matter to the brain. There, the sensory information is analyzed and movement such as dropping the ice cube (or keeping hold of it) may be initiated. This message is sent down the axons of the white matter to the appropriate motor nerves in the gray matter. This efferent motor information travels to the muscles which initiate the reflex.

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reflex

reflex An automatic and innate response to a particular stimulus. A reflex response is extremely rapid. This is because it is mediated by a simple nervous circuit called a reflex arc, which at its simplest involves only a receptor linked to a sensory neuron, which synapses with a motor neuron (supplying the effector) in the spinal cord. Such reflexes are known as monosynaptic spinal reflexes; an example is the stretch reflex. Other spinal reflexes involve more than one synapse (see polysynaptic reflex); an example is the withdrawal reflex of the hand from a painful stimulus (such as fire). Cranial reflexes are mediated by pathways in the cranial nerves and brain; examples are the blinking and swallowing reflexes. See also conditioning.

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reflex

reflex (ree-fleks) n. an automatic or involuntary response to a stimulus, which is brought about by relatively simple nervous circuits without consciousness being necessarily involved. See conditioned reflex, Moro reflex, patellar reflex, plantar (reflex), pupillary reflex, rooting reflex. r. arc the nervous circuit involved in a reflex, being at its simplest a sensory nerve with a receptor, linked at a synapse in the brain or spinal cord with a motor nerve, which supplies a muscle or gland. See illustration. r. sympathetic dystrophy see Sudek's atrophy.

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reflex

reflex: see nervous system.

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