Introduction (Psychology and Mental Health)
The reflex is undoubtedly the simplest form of behavior that has been studied widely by psychologists and neuroscientists. Reflexes involve two separate yet highly related events: the occurrence of an eliciting stimulus and the production of a specific response. Most organisms are capable of displaying a variety of complex behaviors; however, because these behaviors are complex, it has been very difficult, if not impossible, to understand biological or psychological processes involved in generating or modifying the variety of complex behaviors that most organisms can display. In attempts to study these complex behaviors, a number of researchers have adopted a strategy of studying simpler behaviors, such as reflexes, that are thought to make up, contribute to, or serve as a model of the more complex behavior.
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Spinal Reflex (Psychology and Mental Health)
A number of reflexes can be generated in the mammalian spinal cord even after it has been surgically isolated from the brain. The stretch reflex is an example of a spinal reflex. When a muscle is stretched, such as when a tendon is tapped or when an attempt is made to reach for an object, sensory “detectors” or receptors within the muscle are activated to signal the muscle stretch. These receptors are at the end of very long nerve fibers that travel from the muscle receptor to the spinal cord, where they activate spinal motor neurons. The motor neurons control the same muscle on which the stretch receptor that initiated the stretch signal is located. When activated, the spinal motor neurons signal the muscle, causing it to contract. In this manner, when a muscle stretch is detected, the stretch reflex ensures that a contraction is generated in the muscle to counteract and balance the stretch. This type of reflex is referred to as a monosynaptic reflex because it involves only one synapse: the synapse between the sensory receptor neuron and the motor neuron (where a synapse is the junction between two neurons).
Another example of a spinal reflex is the flexion or withdrawal reflex. Anyone who has accidentally touched a hot stove has encountered this reflex. Touching a hot stove or applying any aversive stimulus to the skin activates pain receptors in the skin. These receptors are at the end of long sensory fibers that...
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Musculature Reflexes (Psychology and Mental Health)
Reflexes are not limited to the spinal cord. Responses involving the musculature of the face and neck can also be reflexive in nature. For example, a puff of air that strikes the cornea of the human eye elicits a brisk, short-latency eyelid closure. Like the polysynaptic spinal reflexes, this eyeblink reflex appears to involve three elements: a sensory nerve, called the trigeminal nerve, that carries information from receptors in the cornea of the eye to the trigeminal nucleus (a cranial nerve nucleus); interneurons that connect the trigeminal nucleus with several other brain-stem neurons; and a motor nerve that originates from brain-stem motor neurons and contracts the muscles surrounding the eye to produce the eyeblink. This reflex is defensive in nature because it ensures that the eyeball is protected from further stimulation if a stimulus strikes the cornea.
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Use of Autonomic Nervous System (Psychology and Mental Health)
Not all reflexes involve activation of skeletal muscles. For example, control of the urinary bladder involves a spinal reflex that activates smooth muscles. In addition, temperature regulation is partially the product of a reflexive response to changes in external or internal environments. Many of these types of reflexes engage the autonomic nervous system, a division of the nervous system that is involved in regulating and maintaining the function of internal organs.
Not all reflexes involve simple, local, short-latency responses. The maintenance of posture when standing upright is a generally automatic, reflexive system that one does not think about. This system includes neurons in the spinal cord and brain stem. The body’s equilibrium system (the vestibular or balance system) involves receptors in the middle ear, brain-stem structures, and spinal motor neurons, while locomotion requires the patterned activation of several reflex systems. Finally, a number of behavioral situations require a rapid response that integrates the motor system with one of the special senses (such as quickly applying the car brakes when a road hazard is seen). These are generally referred to as reaction-time situations and require considerable nervous system processing, including the involvement of the cerebral cortex, when engaged. Nevertheless, these responses are considered reflexive in nature because they involve an eliciting...
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Role in Learning and Memory (Psychology and Mental Health)
Reflexes have also been widely studied by psychologists and biologists interested in learning and memory. Russian physiologists Ivan Sechenov and Ivan Petrovich Pavlov have generally been credited with the first attempts to study systematically how reflexes could be used to examine relationships between behavior and physiology. Pavlov in particular had a huge influence on the study of behavior. Most students are familiar with the story of Pavlov and his successful demonstration of conditioned salivation in dogs produced by pairing a bell with meat powder. Over the years, the Pavlovian conditioning procedure (also known as classical conditioning) has often been used to study the behavioral principles and neural substrates of learning. The conditioning of a variety of reflexes has been observed, including skeletal muscle responses such as forelimb flexion, hindlimb flexion, and eyelid closure, as well as autonomic responses such as respiration, heart rate, and sweat gland activity.
One of the most widely studied classical conditioning procedures is classical eyelid conditioning. This reflex conditioning procedure has been studied in a variety of species, including rabbits, rats, cats, dogs, and humans. Mostly because of the research efforts of Isadore Gormezano and his colleagues, which began in the early 1960’s, much is known about behavioral aspects of classical eyelid conditioning in rabbits. In this paradigm,...
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Innate Reflexes (Psychology and Mental Health)
The study of reflexes has not been limited to learning and memory. Developmental psychologists have studied a variety of innate reflexes that are generated by newborn infants. Sucking is a very prominent reflex that is readily observed in newborns. Related to feeding is the rooting reflex, which can be elicited when the cheek of an infant is stroked softly. The skin stimulation causes the infant to open his or her mouth and turn toward the point of stimulation. This reflex has obvious applications in helping the infant locate food. The infant’s ability to hold on to objects is, in part, attributable to the presence of the grasp reflex. When an object touches the palm of a newborn’s hand, the newborn’s fist will close immediately around the object, thus allowing the infant to hold the object for a short period of time. The infantile reflexes disappear within a few months after birth and are replaced by voluntary responses. Most developmental researchers believe that the infantile reflexes are temporary substitutes for the voluntary responses. Apparently, the voluntary responses are not present during the first few months of life because various parts of the infant nervous system, including the cerebral cortex, have not matured sufficiently to support the behavior. Therefore, the disappearance of the infantile reflexes serves as an important marker of neural and behavioral development.
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Contributions to Psychology (Psychology and Mental Health)
The study of reflexes has played a prominent role in shaping the field of psychology. During the late nineteenth century and early twentieth century, Sir Charles Sherrington, a British physiologist, conducted an extensive series of studies concerned with spinal reflexes. He showed that a number of skin stimulations, such as pinching or brushing, produced simple responses even when a spinal transection separated the spinal cord from the rest of the nervous system. From these experiments, he argued that the basic unit of movement was the reflex, which he defined as a highly stereotyped, unlearned response to external stimuli. This work created a flurry of activity among physiologists and psychologists, who tried to trace reflexes throughout the nervous system and assemble them into more complex behaviors.
Early in the twentieth century, many psychologists and physiologists, including Sherrington and Pavlov, adopted the reflex as the basic unit of behavior to study, in part because of the relative simplicity of the behavior and in part because of the ease with which the behavior could be reliably elicited by applying external stimuli. Based on his research, Sherrington believed that complex behaviors were produced by chaining together simple reflexes in some temporal order. This basic idea provided the framework for much of the physiological and behavioral work completed early in the twentieth century. Sechenov and...
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Sources for Further Study (Psychology and Mental Health)
Carlson, Neil R. Foundations of Physiological Psychology. 7th ed. Boston: Allyn & Bacon, 2008. A very up-to-date textbook on the neuroscience of behavior.
Domjan, Michael, and Barbara Burkhard. The Principles of Learning and Behavior. 5th ed. Belmont, Calif.: Thomson/Wadsworth, 2006. This text is widely used by students interested in learning and behavior. The sections on the history of the reflex and its use in the learning research field are particularly applicable to the present discussion.
Fancher, Raymond E. Pioneers of Psychology. 3d ed. New York: W. W. Norton, 1996. This book provides biographies of several prominent psychologists who have had an impact on the field. Included is a chapter detailing the experiments and theories of Ivan Petrovich Pavlov. Valuable for understanding how the study of the reflex fits into the history of psychology.
Gleitman, Henry, Alan J. Fridlund, and Daniel Weisberg. Psychology. 7th ed. New York: W. W. Norton, 2007. This text provides broad coverage of the field of psychology. The chapters on development, learning, and memory should provide the reader with additional information concerning reflexes and other simple behaviors.
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Reflexes (Encyclopedia of Psychology)
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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Reflexes (Encyclopedia of Nursing & Allied Health)
Reflexes are the body's automatic reaction to some type of sensory stimuli. They involve nerve impulses passing from a receptor to a nerve center and then outward to, for example, a nerve or a gland.
Reflexes are classified as unconditioned and conditioned.
Unlike most human behaviors, unconditioned reflexes occur without specific learning or experience. They are considered involuntary acts, because a response occurs automatically when a stimulus (for example, a pinprick) takes place.
Unconditioned reflexes that protect us from harm are called nociceptive reflexes. For example, sneezing, coughing, and gagging are automatic responses to foreign bodies in the nose and throat. Eye blinking or winking helps protect the eye from harm. Reacting quickly to touching a hot stove is yet another example of a nociceptive reflex.
Most reflex acts are very complicated. However, in simple reflexes four events are involved: reception, conduction, transmission, and response. The stimulation is received by receptors, or sensitive nerve endings. These may be in the eye, ear, nose, tongue, or skin. Energy from the stimulus is changed into nerve impulses and conducted from the receptor to the central nervous system. From there, the nerve impulses are transmitted to the motor nerves, which control muscle action. The motor nerves conduct the impulses to the muscles and glands, causing them to respond or act. For example, touching a hot stove stimulates receptors in the skin of the finger. This creates a nerve impulse that travels along a sensory nerve to the spinal cord. In the spinal cord, the sensory nerve fibers interlace with motor nerve fibers. The nerve impulse passes from the sensory fibers to the motor fibers, which relay it to the muscles, causing them to contract. When the muscles contract, the person's hand jerks back.
People have many reflex reactions to such emotional stimuli as anger or fear, including changes in blood pressure and respiration. Lie detectors measure specific physical reactions to emotional stimuli.
Conditioned reflexes are acquired as the result of experience. When an action is performed repeatedly, the nervous system learns to react automatically. Walking, running, and typing are examples of learned conditioned activities that require a large number of complex muscular coordinations.
Conditioned reflexes work by association. For example, a dog's mouth begins to water when the animal smells food. The Russian physiologist Ivan P. Pavlov showed that the flow of saliva, originally an automatic reaction to the smell of food, may become a conditioned reflex. Pavlov rang a bell each time he brought food to a dog. Eventually, the dog's mouth began to water when Pavlov merely rang the bell without food being present. The dog associated the ringing of the bell with the food, just as it associated the odor with the food.
In a simple reflex, a sensory receptor initiates a nerve impulse in an afferent sensory nerve fiber that 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 that passes along the impulse to the appropriate muscle, producing the reflex.
Afferentonveying impulses toward a nerve center, such as the brain or spinal cord.
Efferentonveying nervous impulses away from the central nervous system to nerves or neurons.
Nociceptive stimulus that causes pain or injury.
Synaptic gaphe space between neurons across which a nerve impulse is transmitted by a neuro-transmitter. Also referred to as a synaptic cleft.
Role in human health
Nerve cells are sensitive to disturbances caused by tumors, trauma, circulatory problems, metabolic disorders, and a host of other diseases that can be diagnosed by determining which reflexes show abnormalities. Abnormal reflexes may suggest the presence of significant central nervous system or peripheral nerve problems.
Reflex tests measure the presence and strength of a number of reflexes to help assess the integrity of the nerve circuits involved. Reflex tests are performed as part of a neurological exam to quickly confirm the integrity of the spinal cord, or to diagnose the presence and location of spinal cord injury or neuromuscular disease.
Common diseases and disorders
Some of the more common reflex-related diseases and disorders include stroke, traumatic brain or spinal cord tumors or injury, multiple sclerosis, Wernicke-Korsakoff syndrome, cerebral palsy, and diabetic neuropathy.
Stroke is a brain disorder involving loss of brain functions due to interruption of the brain's blood supply.
Brain and spinal cord injury
Brain and spinal cord injuries most commonly result from motor vehicle accidents, falls, sports injuries, industrial accidents, gunshot wounds, and criminal assault. Damage to the spinal cord affects all nerve function at and below the level of the injury, including muscle control and sensation.
Brain and spinal tumors
Brain and spinal cord tumors are abnormal growths of tissue found inside the skull or the spinal column. The word tumor is used to describe both abnormal growths that are new (neoplasms) and those present at birth (congenital).
Multiple sclerosis involves inflammation within the central nervous system, followed by demyelination, which is a loss of the protective myelin sheaths that surround nerve fibers. When the myelin is damaged, nerve impulses are not transmitted quickly and efficiently. As a result of the inflammatory process, lesions develop in the brain and spinal cord, causing a variety of neurologic symptoms, such as vision loss, numbness or tingling, weakness, unsteady gait, double vision, fatigue, heat intolerance, partial or complete paralysis, and electric shock sensations when bending the neck. These symptoms may cease or may persist after an attack. Symptoms may become progressively worse over time. For individuals with progressive forms of multiple sclerosis, these symptoms may gradually worsen over time without rapid or abrupt changes.
Wernicke-Korsakoff syndrome usually affects people between 40 and 80 years old. The onset is gradual. The syndrome is actually two disorders that may occur independently or together. Wernicke's disease involves damage to multiple nerves in both the central nervous system and the peripheral nervous system. It may also include symptoms caused by alcohol withdrawal. The cause is generally attributed to malnutritionspecially lack of vitamin B1 (thiamine), which commonly accompanies habitual alcohol use or alcoholism.
Korsakoff syndrome, or Korsakoff psychosis, involves impairment of memory and such intellectual/cognitive skills as problem-solving or learning, along with multiple symptoms of nerve damage. The most distinguishing symptom is confabulation (fabrication), during which the person makes up detailed, believable stories about experiences or situations to cover the gaps in their memory. Korsakoff psychosis involves damage to areas of the brain.
Cerebral palsy is a persistent qualitative motor disorder caused by nonprogressive damage to the brain. Although manifested primarily by motor dysfunction, the disorder also may involve sensory deficits and impairment of the intellect. The majority of cases are caused during labor and delivery or during the first month of infancy. Cerebral palsy may be caused by premature birth, prolonged labor, or traumatic delivery. Any situation that interferes with fetal oxygen supply can produce brain damage and cerebral palsy.
Diabetic neuropathy is a nerve disorder caused by diabetes. Symptoms of neuropathy include numbness and sometimes pain in the hands, feet, or legs. Nerve damage caused by diabetes can also lead to problems with such internal organs as the digestive tract, heart, and sexual organs, causing indigestion, diarrhea or constipation, dizziness, bladder infections, and impotence.
Schwartz, J. H., ed. Principles of Neural Science. Stamford, CT: Appleton & Lange, 2000.
Simon, R. Clinical Neurology. 4th ed. Stamford, CT: Appleton & Lange, 1998.
American Academy of Neurology. 10890 Montreal Avenue, St. Paul, MN 55116. (651) 695-1940. <<a href="http://www.aan.com/">http://www.aan.com/>.
National Institute of Neurological Disorders and Stroke. <<a href="http://accessible.ninds.nih.gov/about_ninds/labs/72.htm">http://accessible.ninds.nih.gov/about_ninds/labs/72.htm>.
National Stroke Association. 9707 E. Easter Lane, Englewood, CO 80112. (800) STROKES. (303) 649-9299. <<a href="http://www.stroke.org/">http://www.stroke.org/>.
Spinal Cord Injury Resource Center. <<a href="http://www.spinalinjury.net/">http://www.spinalinjury.net/>.
Bill Asenjo, PhD, CRC