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reflex:see nervous systemnervous system,
network of specialized tissue that controls actions and reactions of the body and its adjustment to the environment. Virtually all members of the animal kingdom have at least a rudimentary nervous system.
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A simple, unlearned, yet specific behavioral response to a specific stimulus. Reflexes are exhibited by virtually all animals from protozoa to primates. Along with other, more complex stimulus-bound responses such as fixed action patterns, they constitute much of the behavioral repertoire of invertebrates. In higher animals, such as primates, where learned behavior dominates, reflexes nevertheless persist as an important component of total behavior.
The simplest known reflexes require only one neuron or, in the strictest sense, none. For example, ciliated protozoa, which are single cells and have no neurons, nevertheless exhibit apparently reflexive behaviors. However, most reflexes require activity in a large sequence of neurons. The neurons involved in most reflexes are connected by specific synapses to form functional units in the nervous system. Such a sequence begins with sensory neurons and ends with effector cells such as skeletal muscles, smooth muscles, and glands, which are controlled by motor neurons. The central neurons which are often interposed between the sensory and motor neurons are called interneurons. The sensory side of the reflex arc conveys specificity as to which reflex will be activated. The remainder of the reflex response is governed by the specific synaptic connections that lead to the effector neurons. A familiar reflex is the knee-jerk or stretch reflex. It involves the patellar (kneecap) tendon and a group of upper leg muscles. Other muscle groups show similar reflexes.
in painting and, less commonly, in the other graphic arts, light and color represented as reflected from one object (or the sky) onto another object. The term “reflex” applies in this sense to both real objects and their depictions. Accurate and subtle depiction of reflex helps to convey the three-dimensionality and the wealth of colors of real objects.
a response of an organism mediated by the central nervous system after stimulation of receptors by internal or external environmental agents; it is manifested by the occurrence of or change in the functional activity of individual organs or the body as a whole. The term “reflex,” adopted from the physical sciences, emphasizes the fact that nervous activity is “reflected,” that is, it is a response to influences from the external or internal environment. The structural mechanism of a reflex is the reflex arc, which includes receptors, a sensory (afferent) nerve that conducts excitation from receptors to the brain or spinal cord, a nerve center located in the brain and spinal cord, and an efferent nerve, which conducts excitation from the brain or spinal cord to effector organs, that is, muscles, glands, and internal organs. The biological significance of reflexes consists in the regulation of the work of organs and their functional interactions to maintain the stability of the organism’s internal environment (homeostasis) while preserving its integrity and ability to adapt to the external environment. The reflex activity of the nervous system assures the organism’s functional integrity and controls the organism’s interaction with the external environment, that is, its behavior.
History of the study of reflexes. The concept of reflexes was first conceived by the French philosopher Descartes. The ancient physicians, for example, Galen in the second century, divided human motor actions into voluntary actions, which require the participation of consciousness in their execution, and involuntary actions, which are performed without the participation of consciousness. Descartes’s teaching on the reflex principle of nervous activity was based on the mechanism of involuntary movements. The entire process of nervous activity, characterized by automatism and involuntariness, consists in stimulation of the sensory apparatus and conduction of the apparatus’ impulses along peripheral nerves to the brain and from the brain to the muscles. As examples, Descartes cited blinking in response to the sudden appearance of an object before one’s eyes and withdrawal of a limb after the sudden application of a painful stimulus. He described the impulses conducted along peripheral nerves by the term “animal spirits,” which he borrowed from the ancient physicians. Despite the spiritual aura surrounding the term, Descartes attached to it actual and, for his time, completely scientific significance based on ideas from mechanics, kinematics, and hydraulics.
The studies of such 18th-century physiologists and anatomists as A. von Haller and G. Prochaska freed Descartes’s ideas from metaphysical terminology and mechanicism and applied them to the activity of the internal organs (several reflexes specific to various organs were found). C. Bell and F. Magendie made very important contributions to the understanding of reflexes and the reflex apparatus by showing that sensory (afferent) fibers enter the spinal cord as part of the posterior roots, while efferent fibers, such as motor ones, leave it as part of the anterior roots. This discovery enabled M. Hall, a British physician and physiologist, to advance clear-cut ideas on the reflex arc and make extensive clinical use of the theory of reflexes and the reflex arc.
Information was available by the second half of the 19th century on common elements in the mechanisms of both voluntary movements wholly related to manifestations of cerebral activity and involuntary automatic reflex actions, counterposed to cerebral activity. In his study Brain Reflexes (1863), I. M. Sechenov contended that all conscious and unconscious actions are reflex in origin. He substantiated the idea of the universal significance of the reflex principle in the functions of the spinal cord and brain for both involuntary and voluntary movements involving consciousness and cerebral activity. Sechenov’s concept enabled I. P. Pavlov to discover conditioned reflexes. Sechenov’s discovery of central inhibition is the most important aspect of the reflex theory. C. Sherrington, N. E. Vvedenskii, A. A. Ukhtomskii, and I. S. Beritashvili provided evidence that the reflexes of individual arcs are coordinated and integrated in the functional activity of organs based on the interaction of excitation and inhibition in the reflex centers.
The concept of the cellular organization of the nervous system plays an important role in elucidating the mechanisms of reflex action. The Spanish histologist S. Ramon y Cajal showed that the neuron is the structural and functional unit of the nervous system. This gave rise to the concept of the neuronal organization of reflex arcs and substantiated the concept of the synapse, the apparatus of interneuronal contact, and synaptic (that is, interneuronal) transmission of excitatory and inhibitory impulses in the reflex arcs (Sherrington, 1906).
Classification. The variety of reflexes led to the development of different classifications. Reflexes may be classified according to the anatomical arrangement of the central part of the reflex arcs, which are their nerve centers, as (1) spinal, involving neurons situated in the spinal cord, (2) bulbar, executed with the participation of medulla oblongata neurons, (3) mesencephalic, executed with the participation of midbrain neurons, or (4) cortical, executed with the participation of cerebrocortical neurons. According to the location of the reflexogenic zones, or receptive fields, reflexes are exteroceptive, proprioceptive, or interoceptive.
Reflexes can also be classified according to type and function of the effectors as motor reflexes (of skeletal muscles)—for example, flexor, extensor, locomotor, and statokinetic—or as autonomic reflexes of the internal organs—digestive, cardiovascular, excretory, and secretory. Depending on the degree of complexity of the neuronal organization of the reflex arcs, they can be subdivided into monosynaptic reflexes, whose arcs consist of an afferent neuron and an efferent neuron, such as the patellar reflex, or multisynaptic reflexes, whose arcs also contain one or more interneurons, such as the flexor reflex. With respect to their influence on effector activity, reflexes can be excitatory, that is, causing or intensifying (facilitating) effector activity, or inhibitory, that is, weakening and suppressing such activity, for example, the reflex acceleration of the heartbeat by the sympathetic nerve and retardation or cessation of the heartbeat by the vagus nerve.
Reflexes can also be classified according to their biological significance for the organism as a whole, for example, the defense (or protective), sexual, and orienting reflexes.
Pavlov justified dividing all reflexes according to origin, mechanism, and biological significance into unconditioned and conditioned reflexes. The former are hereditarily fixed and species-specific, which determines the constancy of the reflex connection between the afferent and efferent elements of their arcs. Conditioned reflexes are acquired during an individual’s lifetime as a result of a temporary connection (conditioned closure) between the various afferent and efferent apparatus of the organism. Since a conditioned temporary connection is formed in higher animals (vertebrates) with the necessary participation of the cerebral cortex, conditioned reflexes are also called cortical reflexes.
The biological function of unconditioned reflexes consists in regulating homeostasis and in preserving the integrity of the organism, whereas the function of conditioned reflexes is to ensure the most delicate adaptation possible to changing external conditions.
The term “reflex” is also applied to other reactions, even though the central nervous system is not involved, for example, axon reflexes and local reflexes executed by the peripheral nervous system.
Mechanism and properties. Reflexes are normally elicited by stimulation of the appropriate reflexogenic zones by external or internal agents, that is, by adequate stimuli of the receptors of these zones. The excitation that arises in the receptors—discharge of impulses—is conducted by afferent nerve conductors to the brain or spinal cord, where it is transmitted from an afferent neuron either directly to an efferent neuron (two-neuron arc) or through one or more interneurons (polyneuron arc). In the efferent neurons, excitation is transmitted by efferent nerve fibers in the reverse direction—from the brain or spinal cord to the various peripheral organs (effectors), for example, skeletal muscles, glands, and blood vessels—and a reflex response is induced, that is, a change in functional activity occurs.
The reflex response always lags behind the start of stimulation of the receptors. This lag time is called latency period. It varies, according to the complexity of the reflex, from a millisecond to several seconds.
Excitation is conducted in the reflex arcs in one direction, from the afferent neuron to the efferent one—never in the opposite direction. This property of reflex conduction is attributable to the chemical mechanism of interneuronal synaptic transmission, which consists basically in the formation and release by nerve endings of specific chemical mediators, for example, acetylcholine and epinephrine, that excite or inhibit the neurons with which the particular endings form synaptic contacts.
The properties of reflexes—intensity, duration, and dynamics—are determined both by the conditions of stimulation (adequacy, force, duration, location) and by the function state (background) of the reflex apparatus itself (excitability, impulses from other nerve centers, fatigue) and other internal factors.
Integration and coordination. Reflexes do not occur in isolation. They are combined (integrated) into complex reflex acts of definite functional and biological significance. For example, the very simple reflex response of an extremity to pain—the flexion reflex (flexing and withdrawal of an extremity)—is a complex multicomponent action involving the involuntary contraction of some muscles, inhibition of others, and changes in respiratory and cardiac activity. The organization of reflexes that control behavior, such as the orienting, food-procuring, defense, and sexual reflexes, is even more complex. Such reflexes include elements involving all the organs to some degree.
The processes responsible for the integration of reflexes are designated by the term “coordination.” Coordination entails essentially the combining of excitation and inhibition in the system of neurons that participate in the formation of reflexes of different complexities. The intimate nature of the mechanisms of these interactions is studied specifically by the technique of microelectrode intracellular recording of electrical reactions of neurons when the reflexes are elicited by stimulation of the receptors or afferent nerves. The synaptic apparatus of the neurons, which contains from a few hundred to 5,000 or 6,000 synaptic contacts per neuron, has both excitatory and inhibitory synapses. When the former are active due to the influx of impulses, a negative electrical reaction arises in the neuron and stimulates the discharge of other impulses. When the latter are active, a positive electrical reaction occurs that inhibits or blocks the transmission of excitation in the neuron. The quantitative relations of the activation of the synapses (number and intensity) determine the significance and extent of participation of the reflex center neurons in the execution of a particular reflex.
The coordination process that integrates reflexes of different complexities can be regarded as a distribution of excitation and inhibition in the neuronal systems involved in the execution of these reactions in accordance with a definite spatial and temporal program corresponding to these reactions. Biological cybernetics studies the factors that give rise to principles of shaping these programs. A high degree of coordination of movements is achieved by the feedback mechanism. The broad convergence in interneuronal relations characterized by hundreds and thousands of synaptic contacts of neurons with other neurons performing different functional roles is the basis for the assumption that the mechanisms of reflex action rest on a stochastic (probabilistic) principle rather than on a static, predetermined organization of reflex arcs.
P. A. KISELEV
Pathologic reflexes. Two types of pathologic reflexes are distinguished. The first type includes reflexes that are unusual in adults (they are sometimes peculiar to earlier stages of phylogeny or ontogeny) and that are manifested after structural or functional injury to different parts of the central nervous system. They are used in the diagnosis of neurological diseases (for example, Babinski’s reflex and the pathologic sucking reflex). The condition in which reflexes are of low intensity or absent is called hyporeflexia or areflexia, respectively. If reflexes are exaggerated or uneven, the condition is called hyperreflexia or anisoreflexia, respectively.
The second type of pathologic reflex includes inadequate and, from the biological standpoint, inappropriate responses to some, usually superstrong, internal or external stimulus.
A distinction is made between pathologic unconditioned and conditioned reflexes. Among the former are the pulmonocoronary reflex (cardiac arrest following irritation of some part of the tunica intima of the pulmonary artery by a foreign body), renorenal reflex (spasm of one ureter following irritation of the other by a calculus), and hepatocoronary reflex (spasm of coronary vessels during an attack of hepatic colic). The decisive factor in the formation of pathologic unconditioned reflexes is parabiosis, a phenomenon that develops in nerve structures as a result of superstrong stimulation and, as shown by N. E. Vvedenskii (1901) and I. P. Razenkov (1923–24), is responsible for the paradoxical nature of the responses.
Pathologic conditioned reflexes are induced by stimuli that are by nature indifferent as far as the body is concerned but are previously combined with superstrong unconditioned stimuli. For example, the coronary spasm that results from climbing a mountain in windy weather (stress stenocardia) may recur if the patient merely descends from the mountain in good weather. Pathologic conditioned reflexes differ from ordinary (physiological) conditioned reflexes in that they are formed after a single combination of stimuli and persist a long time without reinforcement. Pathologic reflexes may underlie some internal diseases.
V. A. FROLOV
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