Animal reflexes

That there are immediate motor responses to sensory stimulation is a fact that has been apparent in the thinking and writing of many scholars for centuries. Yet the term “reflex” actually did not appear until the eighteenth century. Georg Prochaska of Vienna, one of the first to use the term, wrote in 1784, “The reflexion of sensorial into motor impressions, which takes place in the sensorium commune [common sensory center] . . . may take place either with consciousness or without consciousness.”

The word “reflex,” then, originates from the idea that nerve impulses are “reflected” in the central nervous system. Modern use of the term dates from the earliest experimental investigations of the role of the spinal cord in mediating muscle responses to sensory stimuli. These studies, conducted in the 1820s by Charles Bell in England and François Magendie in France, first established that sensory fibers are contained in the dorsal (upper) roots and motor fibers in the ventral (underside) roots of the spinal cord.

Response to Stimuli

Reflexes are types of action consisting of relatively simple segments of behavior usually occurring as direct and immediate responses to particular stimuli uniquely correlated with them. In addition to other, more complex stimulus-bound responses such as fixed action patterns, reflexes account for many of the behavioral patterns of invertebrates. In higher animals such as primates, learned behavior dominates, but reflexes remain a significant component of total behavior.

Reflexes are genetically determined. No effort is needed to acquire reflexes; they simply occur automatically, usually taking place beneath the conscious level. Although heart rate and ventilation rate, for example, are constantly regulated by reflexes, most people are unaware of these modifications. Similarly, pupillary diameter and blood pressure are regulated reflexively, without conscious knowledge. Other reflex responses, such as perspiring, shivering, blinking, and maintaining an upright position, are more apparent, although they also occur without conscious intervention.

Only one neuron, if that, is required for the simplest known reflexes. Ciliated protozoa, single-celled animals having no neurons, exhibit what appear to be reflex actions. When a paramecium collides with an object, it reverses the stroke of the cilia (hairlike outgrowths), backs away a short distance, turns, and again moves forward. When touched caudally (at or near the tail), the animal moves forward. In this instance, the animal’s cell membrane itself serves as the receptor of the stimulus, and the cilia act as effectors for directed movement. Very simple reflexes also occur in higher animals. For example, when human skin is injured sufficiently to stimulate a single pain neuron, an unknown substance is released, causing small local blood vessels to dilate. In contrast to these simple responses, however, most reflexes require a vast sequence of neurons.

In most reflexes, the neurons involved 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, all controlled by motor neurons. Interneurons, or central neurons, are often interposed between the sensory and motor neurons. This sequence of neurons is called a reflex arc. The sensory aspect of the reflex arc conveys specificity regarding the particular reflex to be activated. That is, the sensory cells themselves determine which environmental change is sensed, either inside or outside the body. The remainder of the reflex response is regulated by the specific synaptic connections that lead to the effector neurons.

Types of Reflexes

One visible and familiar reflex is the knee-jerk or stretch reflex. This reflex involves the patellar (kneecap) tendon and a group of upper leg muscles. While other muscle groups exhibit similar reflexes, this one is described here because of its widespread clinical use. This reflex involves a relatively simple reflex arc in which the terminals of the sensory cells synapse directly on the effector neurons. It is thus an example of a monosynaptic reflex. When the patellar tendon is tapped, a brief stretch is imposed on the attached muscles (quadriceps femoris), including the muscle spindles that are embedded within the quadriceps. This stretch causes the sensory neurons of the spindles to fire impulses, which return to the spinal cord. Within the spinal cord, excitatory synapses are then activated on the specific motor neurons supplying the same muscles that were stretched (the quadriceps). A series of impulses occurs in the motor neurons, which activate a brief contraction of the quadriceps, which in turn extends the leg. Another component of this reflex causes the central inhibition of the flexor motor neurons. A possible function of stretch reflexes is to oppose sudden changes in the length of skeletal muscle.

The stretch reflex, as well as other more complex reflexes, involves the inhibition of antagonistic muscle groups. This activity requires the activation of inhibitory interneurons in the spinal cord, which synapse upon the motor neurons of antagonistic muscles. In other complex reflexes, there is a sweep of activity to higher and lower levels of the spinal cord and to the opposite side of the body. Examples of reflexes with these characteristics include the flexor reflex and accompanying crossed extensor reflex. These reflexes are commonly initiated by a painful stimulus. The returning sensory signals excite the motor neurons of the leg flexor muscles, which inhibit those of the leg extensor muscles on the same side as the stimulus. This behavior assures that the foot is quickly removed from the harmful stimulus. Balance, however, must also be maintained, which is accomplished by exciting the leg extensors of the opposite limb—thus the term “crossed extensor reflex.” The flexor reflex is a prime example of a protective response, and it takes place so rapidly that the pain is felt only after the withdrawal response is complete. Speed of response is particularly important when the severity of the injury is time-dependent, such as in response to a chemical exposure or burn.

Many thousands of neurons are usually involved in flexor and extensor responses. From only a few to many sensory neurons may activate several hundred to several thousand motor neurons, depending upon the reflex involved. Each motor neuron, in turn, branches to synapse on as many as a thousand muscle cells. In addition, in reflexes with bilateral effects, another thousand or more interneurons may become involved.

The Functions of Reflexes

The functions of reflexes are numerous and varied. Some reflexes adjust important biological functions rapidly and efficiently without conscious effort, while other reflexes are largely protective. For example, the eye and the ear, the most delicate and sensitive sensory systems, can be damaged or destroyed by overstimulation or by accident. The amount of light admitted to the retina is controlled by the pupillary reflex, which in humans can effect a change in pupillary diameter from approximately eight millimeters in darkness to two millimeters in bright light. A sudden flash of intense light can evoke the reflex closing of the eyelids, further protecting the delicate retina. The eyeball itself is protected from drying by the blink reflex and from mechanical injury by the eyelid closure reflex. The latter reflex is triggered when an object approaches the eye or when the lashes or cornea are touched. The ear is also protected from potentially damaging sounds through the reflex contraction of middle-ear muscles in response to loud noise. This reflex functions to lower the efficiency with which sound vibrations are transmitted through the bones of the middle ear and thus reduces the possibility of damage to the delicate hair cell receptor of the inner ear.

Another category of protective reflexes exhibited by many animals is stereotyped escape responses. For example, a startled squid takes evasive action by contracting its mantle muscles (the membranous flap or folds of the body wall), forcing a jet of water through its siphon. Fish respond to vibrations carried through water by contracting the muscles on one side of their bodies; the reflex contraction occurs on the side opposite the source of vibration, and the result is a sudden move away from potential danger.

Equally essential for survival are the numerous feeding reflexes exhibited by animals. For example, flies as well as many other insects possess chemoreceptors located on their feet, mouth parts, and antennae. Thus, when a hungry fly walks on a surface moistened with nutrients, a set of reflexes is triggered. A reflex extension of the proboscis occurs. If the proboscis receptors are favorably stimulated, then the animal begins to drink. Drinking continues until the crop is sufficiently distended to stimulate its stretch receptors. Finally, this stimulus initiates the reflex termination of feeding.

Instinctive or innate behaviors, such as courtship rituals, nest building, aggression, and territorial behaviors, demonstrate many similarities to reflexes. Although generally more complex, they are, like reflexes, unlearned, species specific, genetically determined, and stereotypic in nature. Importantly, fixed action patterns such as these are similar to reflexes in that they are initiated by a specific stimulus, called a sign stimulus or a releaser. Generally, both forms of behavior are also comparable in that they are thought to be controlled by specific sets of neurons that underlie each behavior. Like the more complex learned behaviors, however, the neural basis of instinctive behaviors remains largely unknown.

Modification of Response

Normally, the degree of reflexive response depends upon the intensity of the stimulus; many reflexes tend to weaken gradually if the stimulus is applied repeatedly. This phenomenon, termed habituation, allows an animal to ignore a familiar or repeated stimulus. If a strong, unexpected stimulus is introduced along with a reflex-evoking stimulus, however, the strength of the reflex is often enhanced. This process, called sensitization, causes the animal to detect a potentially threatening situation by responding forcefully to its environmental cues.

Similarly, reflexes may be modified by classical (Pavlovian) conditioning, a form of associative learning. For example, in dogs, the sight and smell of food causes the reflexive secretion of saliva. A light or the sound of a bell does not. Ivan Pavlov demonstrated that linking the sight and smell of food (unconditioned stimuli) with a light or bell caused the animal to associate the light or bell with food. After several training sessions, introduction of the light or bell alone produced salivation. Whether classical conditioning contributes to a significant portion of learned behavior is still a controversial issue.

Finally, reflexes are often modified during voluntary movement or locomotion. Motion itself stimulates many sensory receptors and often elicits reflexes that oppose the intended movement. Such reflexes are thought to be overridden or suppressed by commands from the brain in order for the desired movements to be executed.

Early Reflex Studies

Seventeenth-century French philosopher, mathematician, and scientist René Descartes combined the physiological discoveries of the second-century Greek physician Galen with a conception of the body as a machine to provide the first notion of reflex action. Descartes thought that within a nerve there were thin threads attached at one end to the sense organs. External stimuli pulled on the threads to open small gates to the ventricles, thereby allowing pneuma (the breath of life; soul or spirit) to flow back out of the ventricles (reflected) through the same hollow nerves. This activity then caused movement by the pineal gland (a small, cone-shaped gland in the brain of all vertebrates having a cranium), extending from the midline into the ventricles. He observed that in animals this process was strictly mechanical. In humans, who unlike animals have souls, however, the soul was thought to interact with the body at the pineal gland and thus could influence the flow of the pneuma to the muscles.

Anatomist, physician, and Oxford professor Thomas Willis advanced Galen’s idea one step further and related it to actual brain structures. His Cerebri Anatome (1664), illustrated by Sir Christopher Wren, was the most complete description of the brain to date. Sense impressions, Willis speculated, were carried by pneuma within the nerves to the sensus communis in the corpus striatum (two striated ganglia in front of the thalamus in each half of the brain) and then on to the corpus callosum (a mass of white, transverse fibers connecting the cerebral hemispheres in higher mammals) and the cerebral cortex (a layer of gray matter over most of the brain), where they were perceived and remembered. Some were “reflected” back to the muscles via the cerebellum (the section of the brain behind and below the cerebrum, regarded as the coordinating center for muscular movement). Voluntary movement was thus controlled by the cerebrum (upper, main part of the brain of vertebrates, consisting of two equal hemispheres—the largest part of the human brain and believed to control both conscious and voluntary processes) and involuntary, or “reflex” movement, by the cerebellum.

The first actual experiments on neural mechanisms of reflexes were conducted by Robert Whytt of Edinburgh in the eighteenth century. Using frogs, Whytt demonstrated that only a segment of the spinal cord was necessary for reflex responses to skin stimulation. He also showed that the pupillary reflex was contingent upon the midbrain. More than Descartes, he stressed the protective function of reflexes. While movement was strictly mechanical for Descartes, Whytt believed that it was dependent on the “sentient principle,” even when involuntary or reflex, an idea that persisted into the nineteenth century.

Modern Reflex Studies

The modern concept of a reflex largely began with the nineteenth-century English physiologist and physician Marshall Hall, who used Charles Bell and François Magendie’s distinction of sensory and motor roots to develop the notion of the reflex arc. Reflexes were, by then, by definition dependent on the spinal cord, independent of the brain, and strictly unconscious and involuntary. Hall also described the excitation or inhibition of reflex movements by various drugs. Finally, he was the first to use reflexes in medical diagnosis and treatment.

The next significant development was advanced by Charles Scott Sherrington in approximately 1890. Sherrington’s work on reflexes, which led to the concept of the synapse, was built on two foundations: he conducted a meticulous anatomical analysis of the nerves to various muscles, and then he used this knowledge to analyze quantitatively the reflex properties of specific nerves and muscle groups. This painstaking work provided the means for obtaining the first clear conception of the reflex as a combined structural and functional unit, and it established the reflex arc as a subject for further anatomical and physiological analysis by many twentieth-century scientists. In addition, Sherrington emphasized the importance of the reflex as an elementary unit of behavior and thus laid one of the cornerstones for the modern studies of animal behavior.

Sherrington’s first studies involved a correlation of anatomical tracing of sensory and muscle nerves with careful observations of various reflex behaviors. He introduced methods for cutting across the brain stem of a cat at the level of the midbrain, which produced a great enhancement of tone in the extensor muscles of the limbs (those muscles responsible for keeping an animal in the standing position). In order to study the reflex basis of this activity, Sherrington and his collaborators, in 1924, began to analyze the responses to passive stretch of an extensor muscle. The results indicated that stretch of the muscle by only a few millimeters produces a large increase in tension, as measured by a strain gauge. If the muscle nerve is cut, the tension developed is low because it results only from the passive elastic properties inherent in the muscle and its tendon. Thus, the greater tension depends on a reflex pathway that passes through the spinal cord. The reflex activity produces contractions of the stretched muscle. Since the reflex feeds back specifically to the stretched muscle, it is called a “myotatic reflex” or “stretch reflex.” This is the familiar knee-jerk reflex evoked by a tap on the tendon of the knee. While most muscles, invertebrate and vertebrate, demonstrate this type of reflex, working extensor muscles best demonstrate it. Although the feedback to the muscle stretched is excitatory, there is, in addition, an inhibitory effect on muscles with antagonistic actions at a joint; thus, when a knee flexor is stretched, some of the tension in the knee extensor dissipates. This action illustrates the principle of reciprocal innervation of the muscles to a joint.

Next, it was necessary to analyze the nervous pathways involved in these and other types of reflex activity. David Lloyd at Rockefeller University began these studies in about 1940. The experiments required laborious dissection of individual peripheral nerves as well as removal of the laminae (thin layer of tissue) of the vertebral bones to expose the spinal cord so that electrodes could be placed on the dorsal and ventral roots. A single burst could then be set up in a peripheral nerve, and the response of motor neurons could be recorded in terms of the compound action potential of their axons in the ventral root. He found that stimulation of a muscle nerve produces a short-latency, brief volley in the ventral root. The input from muscles is thus carried over large, rapidly conducting axons, and there is only one, or at most two or three, synaptic relays in the spinal cord. In contrast, the ventral root response to a volley in a skin nerve has a long latency and duration. This response suggests the involvement, in skin reflexes, of shower-conducting fibers, polysynaptic pathways, and prolonged activity in the neurons in these pathways.

Reflexes and Neurology

Since reflex responses are essentially invariant in healthy individuals, their examination can often provide valuable information about neurologic disorders. Relatively simple tests, such as pupillary responses or auditory perceptions, can reveal information about complex brain functions. Scratching the skin of the foot, an activity normally leading to flexion of the toes, can demonstrate the condition of the spinal circuits that control them. For example, if the toes extend and spread in response to scratching the sole of the foot (called the Babinski response), a spinal lesion in the pyramidal tracts descending from the motor centers of the cerebral cortex is indicated. When observations of these and many other reflexes are combined with data on paralyses, spasms, or losses of sensation, it is often possible to localize the site of the lesion.

In addition to the diagnostic functions of reflexes, the reflex concept has exerted a great influence on psychological thinking and initially led to premature attempts to develop a psychology based on reflexes. Pavlov’s innovative work led to extensive research in the early twentieth century on the physiology of behavior; for some time, the conditioned reflex provided the best technique for enabling at least a part of the learning process to be investigated quantitatively and to be subjected to an exact analysis. The principles proposed by such behaviorists as Edwin Ray Guthrie, Clark Leonard Hull, and B. F. Skinner to explain psychological actions as conditioned or learned responses to external and internal stimuli were based in part on earlier reflex notions and upon the fundamental model of the conditioned reflex as demonstrated by Pavlov. It is now generally recognized, however, that the reflex relationship between stimulus and response is not nearly as simple as was previously thought. The use of the conditioned reflex as a model for learning in classical-conditioning experiments artificially isolates, to an extreme degree, part of the total learning process in higher animals and is by itself inadequate in attempting to analyze the complex physiological and mental interactions that ultimately determine the behavior of humans and other mammals.

Principal Terms

axon: the part of a nerve cell through which impulses travel away from the cell body

effector: the part of a nerve that transmits an impulse to an organ of response

fixed action pattern (FAP): a complex motor act involving a specific, temporal sequence of component acts

interneuron: central neurons often interposed between the sensory and motor neurons

motor neuron: a nerve cell body and its processes which carry impulses from the central nervous system to a muscle, producing motion

neuron: the structural and functional unit of the nervous system, each consisting of the nerve cell body and all its processes, such as the dendrites and axon

receptor: a nerve ending specialized for the reception of stimuli

reflex arc: the entire nerve path involved in a reflex action

spindle: in a muscle, the bundle of nuclear fibers formed during one stage of mitosis

synapse: the point of contact between adjacent neurons, where nerve impulses are transmitted from one to the other

Bibliography

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