Reflexes

• TYPE OF PSYCHOLOGY: Biological bases of behavior
• A reflex is one of the most basic types of behavior that can be elicited. Psychologists and physiologists have studied the behavioral and biological processes associated with reflex production to better understand the principles and processes involved in generating simple and complex behaviors, such as learning, memory, and voluntary movement.

Introduction

The reflex is among the simplest forms of behavior 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 can display a variety of complex behaviors; however, because these behaviors are complex, it has been very difficult, if not impossible, to understand the 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.

Spinal Reflex

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 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 . 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 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 project to neurons in the spinal cord. The spinal neurons that receive input from the sensory fibers are not motor neurons, as in the stretch reflex, but rather very small neurons called spinal . The interneurons make synaptic contact on other interneurons as well as on motor neurons that innervate flexor muscles. When activated, the flexor muscles typically cause limb withdrawal. The flexor reflex ensures that a relatively rapid withdrawal of one’s hand from a hot stove will occur if it is accidentally touched. The flexor reflex is an example of a polysynaptic reflex because there are two or more synapses involved in the reflex (the presence of at least one synapse between a sensory neuron and an interneuron and a second synapse between the interneuron and a motor neuron).

One functional difference between monosynaptic and polysynaptic reflexes is the amount of information processing that can take place in the two reflex systems. The monosynaptic reflex is somewhat limited because information flow involves only the synapse between the sensory and motor neurons. This type of reflex is ideal for quick adjustments that must be made in muscle tension. Conversely, polysynaptic reflexes typically involve a number of levels of interneurons. Hence, and divergence of information can occur as information flows from sensory to motor elements. In essence, the polysynaptic system, in addition to having and efferent components, has a “processor” of sorts between the sensory and motor elements. In intact organisms, the integration that takes place within the processor allows information to be shared by other regions of the . For example, some of the interneurons send information upward to the brain. When a hot stove is touched, the brain is informed. This sensory experience is likely to be evaluated and stored by the brain, therefore making it less likely that the hot stove will be touched a second time.

Musculature Reflexes

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.

Use of Autonomic Nervous System

Not all reflexes involve the 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 . The body’s equilibrium system (the vestibular or balance system) involves receptors in the middle ear, brainstem 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 , when engaged. Nevertheless, these responses are considered reflexive in nature because they involve an eliciting stimulus and a well-defined, consistent response.

Role in Learning and Memory

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 ) 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 1960s, much is known about behavioral aspects of classical eyelid conditioning in rabbits. In this paradigm, a mild electric shock or air puff is presented to elicit reliably a reflexive blink from the rabbit. The blink is typically measured by means of devices that are attached to the nictitating membrane, a third eyelid that is present in a variety of species, including the rabbit. During training sessions, a neutral stimulus such as a tone or light is delivered 0.3 to 1.0 second prior to the air puff. After about one hundred of these tone and air-puff pairings, the rabbit learns to blink when the tone or light is presented (the rabbit begins to use the tone to signal the impending air-puff presentation).

This preparation has yielded a wealth of data concerning the parameters of behavioral training that produce the fastest or slowest learning rates (such as stimuli intensities, time between stimuli, and number of trials per day). Furthermore, this simple reflexive learning situation has been used to study how the brain codes simple forms of learning and memory. A number of researchers (most notably Richard F. Thompson) have studied the activity of a variety of brain structures during learning and performance of the classically conditioned eyelid response. These studies have shown that discrete brain regions, such as the and alter their activity to generate or modify the . In brief, these researchers have used the conditioning of a very simple reflex to advance the understanding of how the brain might code more complex learning and memory processes.

Innate Reflexes

The study of reflexes has not been limited to learning and memory. Developmental psychologists have studied a variety of innate reflexes in 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 their 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 infantile reflexes are temporary substitutes for voluntary responses. Apparently, 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.

Contributions to Psychology

The study of reflexes has played a prominent role in 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 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 Pavlov also believed that the concept of the reflex could explain more complex behaviors. Pavlov, for example, showed that not all reflexes were innate; rather, new reflexes could be established by associating a “neutral” stimulus (a stimulus that did not initially produce a reflex) with a stimulus that reliably elicited a reflex. As a result of this demonstration, Pavlov proposed an elaborate theory of reflex learning that involved forming associations between stimuli in the cerebral cortex.

In the latter half of the twentieth century, many psychologists interested in studying overt behavior and physiologists interested in studying nervous system function adopted the study of reflexes as a means of simplifying behavior or nervous system activity. Psychologists such as Gormezano, Robert Rescorla, and Allan Wagner, who have studied classical conditioning phenomena, helped develop a comprehensive understanding of the learning process that occurs when simple paradigms such as classical conditioning are used. Gormezano's classical conditioning experiments on rabbits improved the scientific understanding of the influence of reinforcement schedules and stimulus timing on classical conditioning and provided insight into the neural pathways involved. Rescorla and Wagner used this knowledge to create the Rescorla-Wagner model, which was widely used in neuroscience research. The theory emphasizes the importance of the contingency between the conditioned and unconditioned stimuli in learning and proposes the idea of prediction error.

Behavioral neuroscientists and neurobiologists, such as Thompson and Eric R. Kandel, who study nervous system function, have used reflexes as the basic behavior unit to investigate nervous system function when a fairly simple behavioral response is being generated and modified by learning experiences. In both cases, using the reflex as the unit of behavior simplifies the experimental situation. Indeed, researchers are not likely to understand complex behavioral processes without first understanding how simpler behaviors and nervous system functions are generated, modified, and maintained.

Reflex research continues to provide important insight into the complexities of the nervous system and its link to physical and mental phenomena. Research concerning primitive reflexes—adaptive reactions present in infancy that diminish over time—indicates there may be a link between longer retention of these reflexes and some conditions, such as attention deficit hyperactivity disorder and learning disabilities. From a behavioral and biological standpoint, the study of reflexes provides a valuable approach to understanding human behavior and how the nervous system generates activity to produce the behavior.

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