Somatosensory Systems Research Paper

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The somatosensory systems receive and process somatic, or bodily, information. Like all the other senses (vision, hearing, taste, and smell), the somatosensory systems reveal qualities of stimuli present in the external world (exteroceptive information). However, the somatosensory systems play an additional role, unique among their counterparts: They provide interoceptive information—data regarding internal conditions, such as core temperature and internal pain.

As this discussion implies, somatosensation involves the operation of several distinct systems. In this research-paper, we discuss several of these systems, including touch, pain, proprioception (i.e., information about bodily position), and temperature regulation. As you read further, however, keep in mind that these systems, though distinct, interact with one another. Together, the various somatosensory systems offer a rich description regarding states of both the internal and external environments.

Touch

If you are like most individuals, you are acutely aware of the importance of your vision and hearing for everyday life—the thought of blindness or deafness is devastating. But when is the last time you considered the importance of your sense of touch? Upon awakening in a dark room in the middle of the night, your awareness of your surroundings depends primarily on tactile sensations—the feel of the linens against your skin, or the furry dog underfoot as you attempt to rise from the bed. Imagine finding your ringing cell phone in the bottom of your purse or school-bag in the absence of touch. By the time you searched through the clutter using vision alone, the ringing would have stopped. The sense of touch reveals not only the presence of an object but also information about its shape, size, firmness, and texture, all important qualities for our interactions with objects in the environment.

The sense of touch may serve another important purpose as well. Though we often consider vision to be the most informative sense, we probably should consider touch to be the most reliable sense. Imagine reaching toward a seen object, only to feel nothing but thin air. Upon which of your senses would you rely? In the face of such conflict, we typically trust our sense of touch; the information revealed by direct contact with an object (or lack thereof) cannot be easily denied.

Receptors for Touch

Your sense of touch involves several types of receptors, called mechanoreceptors, located in the skin. The mechanoreceptors are so named because they respond directly to mechanical stimulation: pressure or deformation of the skin.

Two types of mechanoreceptors, the Meissner corpuscles and the Merkel disks, reside immediately beneath the skin’s surface. These two receptor types have small, punctate receptive fields: They respond to touch information over a small, distinct area of the skin. As such, these receptor types are responsible for processing fine details of a tactile stimulus, including texture. To appreciate the importance of these receptors, consider that the density of Meissner corpuscles in the fingertips declines steadily from 40-50 per square millimeter of skin during late childhood to around 10 per square millimeter by age 50. This decline predicts accurately the loss of sensitivity to detailed tactile information that older people experience (Thornbury & Mistretta, 1981).

The other two types of mechanoreceptors, the Pacinian corpuscles and the Ruffini endings, inhabit deeper positions in the skin. These receptors have large, diffuse receptive fields: They respond to touch information over a much larger, indistinct region, and provide more general, “big picture” information about the nature of a touch stimulus. For all four receptor types, receptive field size—the area of skin over which a touch stimulus can produce a response—varies over the body. Receptive field sizes are much smaller, and thus provide more precise information, in regions of the body of evolutionary importance for processing touch information: the fingertips and lips.

It should be noted that the mechanoreceptors also differ in their rate of adaptation. The Merkel’s disks and Ruffini endings adapt slowly; they respond relatively steadily to continuously applied pressure. By contrast, both the Meissner’s corpuscles and the Pacinian corpuscles adapt very rapidly; they respond vigorously when a tactile stimulus first contacts the skin and when the stimulus is removed, but little in between. If you are wondering about the relevance of fast and slow adaptation, consider the following situation. It’s a cold winter morning and you hurry to pull on your favorite sweatshirt. For a very brief time you may be aware of the feel of your sweatshirt against your skin, but you soon become unaware of any pressure of your clothing (unless you consciously focus on it). This scenario reflects the response of the rapidly adapting receptors. Initially, both slowly and rapidly adapting receptors respond; then, after a brief period (e.g., 300-600 milliseconds), the Meissner and Pacinian corpuscles adapt and your tactile experience changes. The importance of rapidly adapting touch receptors can best be experienced by closing your eyes and trying to identify an object by only touching it. Can you identify the object without keeping your hands in constant motion? No—you need the constant motion to keep the rapidly adapting touch receptors responding and providing you with information.

Haptic Perception

This discussion highlights a general principle of perception: To glean the best information regarding an object or event, we actively explore it using whichever senses are available to us. Think about what you would do if asked to describe, in detail, the appearance of an object sitting in front of you. Most likely, you wouldn’t simply glance at the object; you would systematically move your eyes over its surface, examining every detail. Similarly, to fully appreciate an object’s tactile properties, you move it around with your hands, exploring it in a variety of ways. Researchers refer to active exploration using the sense of touch as haptic perception.

Lederman and Klatzky (1987, 1990) discovered that people engage in a series of predictable, ritualized actions, deemed exploratory procedures, when examining objects with their hands. Each exploratory procedure reveals different types of information about the object in question. For example, “lateral motion” of the fingers across an object’s surface yields texture information, whereas “contour following” provides data regarding the object’s shape. As long as use of these and other exploratory procedures is not constrained, people can identify objects by touch alone with remarkable speed and accuracy (Klatzky, Lederman, & Metzger, 1985; Lederman & Klatzky, 2004).

As just suggested, haptic perception yields much more information about the nature of an object than passive touch. Attempts to identify an object by simply grasping it, with no exploration, often lead to gross errors in perceptual judgment (Rock & Victor, 1964). No doubt, the superiority of haptic perception must be attributed, in part, to the ongoing activity of the detail-oriented, rapidly adapting touch receptors. However, haptic perception trumps passive touch even after taking adaptation into account; moving the object across your stationary hand, which fully engages rapidly adapting receptors, does not produce the same level of perceptual knowledge as active exploration. The reason for the difference in utility between active and passive touch can be simply explained when you recall that all of the somatosensory systems interact with one another. Tactile input alone provides ambiguous data regarding the nature of an object. Feeling a sharp point, for example, reveals little about an object if you don’t know where the point is positioned relative to other features, or what actions your fingers made to discover the point. To make sense of what your mechanoreceptors signal, you must take into account the relative positions and movements of your fingers. In other words, haptic perception involves integrating tactile information with sensory information regarding proprioception and kinesthesis. We return to these aspects of somatosensation later in the chapter.

Dermatomes

Touch receptors synapse with neurons that carry the touch information to the spinal cord and then to the brain for processing. It is noteworthy that the neurons from distinct, identifiable “strips” of the body enter the spinal cord together and ascend to the brain as a group. Researchers call the bodily strips that send groups of neurons coursing upward to the brain dermatomes. The dermatomes on the left side of the body mirror the dermatomes on the right side of the body. Damage to the axons entering the spinal cord at a single level can cause a loss of tactile sensation from a single dermatome of the body.

Ascending Somatosensory Pathways

Two major neural pathways ascend the spinal cord and carry somatosensory information to the brain. The dorsal-column medial lemniscus system carries touch information and ascends in the dorsal (back) portion of the spinal cord. The anterolateral system, also called the spinothalamic tract, conveys information about pain and temperature (discussed later) and ascends in the anterior (front) portion of the spinal cord. It is noteworthy that the dorsal-column medial lemniscus system is a contralateral system; that is, the neurons from the left side of the body cross from the left side of the spinal cord to the right side and send their information to the right hemisphere of the brain. On the other hand, the anterolateral system is an ipsilateral system. That is, the majority of the neurons in this system do not cross as they ascend the spinal cord; for example, information originating on the left side of the body is processed in the left hemisphere of the brain.

Cortical Processing of Somatosensory Information

What happens when the somatosensory information carried by the two ascending pathways reaches the brain? To answer this question, we need to examine the classic research conducted by neurosurgeon Wilder Penfield and his colleagues. During brain surgery, these investigators electrically stimulated the brains of their human patients, who were awake and could verbalize, and recorded the responses that these stimuli produced (see Penfield & Boldrey, 1937; Penfield & Rasmussen, 1950). Upon stimulation of the frontmost part of the parietal lobe, the postcentral gyrus, patients reported experiencing tactile sensations. As the region of the brain most central to somatosensory processing, the postcentral gyrus is commonly referred to as primary somatosensory cortex.

Further investigations by Penfield and his colleagues revealed two important characteristics of primary somatosensory cortex. First, different regions along the postcentral gyrus correspond to different parts of the body. Adjacent regions of the body (e.g., the hand and the arm) receive representations in adjacent regions of cortex. Second, the size of areas on this cortical map, which is consistent from person to person, varies according to the relative sensitivity of specific areas of the body to tactile stimulation. The greater the density of mechanoreceptors in a given region, the more sensitive that region, and the larger the cortical representation tends to be. Areas like the fingers and lips, which are especially important for adaptation to and interaction with the environment, occupy greater cortical space than do areas of less adaptive importance, such as the feet and back.

What sort of experience would you expect if the somatosensory cortex fails to process tactile information completely or adequately? Although rare, failure of the brain to process touch stimuli can result in astereognosia, also called tactile agnosia. Individuals suffering from astereognosia experience basic tactile sensations, but cannot identify objects using the sense of touch.

Plasticity in Somatosensory Cortex

As we’ve seen, the cortex devotes considerable processing capacity to areas of the body of evolutionary importance for tactile exploration: in humans, the hands and the mouth. This finding implies that, over time, the brain has adapted to the needs of the organism. Importantly, however, adaptation also occurs on a much shorter timescale. Converging evidence suggests that the brain shows remarkable plasticity—flexibility in its representation—with regard to somatosensory processing.

Work by Merzenich and colleagues first demonstrated that increased input to a particular region of skin could cause expansion of the cortical representation of that body region. In monkeys, three months of increased tactile stimulation of a single finger resulted in a vast expansion of the already large cortical representation of the digit in question (Buonomano & Merzenich, 1998). In humans, it has since been demonstrated that the somatosensory cortices of musicians who play stringed instruments contain unusually large representations of the overstimulated fingers of the left hand (Elbert, Pantev, Wienbruch, Rockstroh, & Taub, 1995).

Blind individuals who read Braille with their hands provide further insight into the plasticity of somatosensory representations. Consistent with the research described above, the brains of blind individuals contain significantly larger representations of the dominant Braille-reading finger(s) than of any other fingers (see Hamilton & Pascual-Leone, 1998, for a review). Furthermore, functional imaging studies demonstrate that, in congenitally blind individuals, Braille reading activates areas of the occipital lobe normally used for visual processing (Sadato et al., 1996). When transcranial magnetic stimulation (TMS) momentarily disrupts processing in “visual” processing areas, blind individuals remain aware of the tactile stimulation induced by touching the Braille letters, but have difficulty identifying the letters being touched; no such effects occur in sighted people (Cohen et al., 1997; Kupers et al., 2007). Thus, it appears that somatosensory processes can “hijack” otherwise unused areas of the brain.

What happens when the unused brain tissue actually resides in the somatosensory cortex itself? Under extreme circumstances, such as an automobile accident, a natural disaster, or warfare, doctors may need to amputate a limb.

A vast majority of amputees experience a phantom limb; their missing arm or leg continues to “feel” as if it’s still present (Melzack, 1992). The phenomenon, documented for centuries, can best be explained as the perceptual consequence of the lingering brain tissue that represents the amputated limb; when this tissue becomes active, amputees experience sensation in the phantom. More recently, Ramachandran and colleagues (e.g., Ramachandran & Hirstein, 1998) discovered a number of patients who, when their faces are touched, experience not only sensation in their faces but also referred sensation in the phantom arm. Though puzzling, this finding can be explained when we note that the representation of the face within somatosensory cortex lies immediately adjacent to the representation of the hand. Over time, the cortical map adapts, such that tactile input to the face activates not only the face region but also the idle tissue nearby.

Approximately half of all amputees also experience pain from the phantom limb. The exact cause of this pain remains unclear. Some researchers have speculated that this pain is caused by some type of irritation of the nerves at the point where the limb was amputated. However, if this explanation is correct, then damaging the neural pathway that carries information from the point of amputation to the cortex, thus leaving the brain without pain signals to process, should yield a permanent, long-lasting decrease in phantom limb pain. Unfortunately, such surgery has produced, at best, only a very transitory period of relief (Melzack, 1992). We turn now to the general topic of pain.

Pain

Sometimes the stimuli applied to the skin produce tissue damage. In cases of potential or actual skin damage, in addition to the sense of touch or pressure, we experience pain. Though aversive, the experience of pain serves an adaptive purpose. Acute pain functions as an early warning system; it allows us to act quickly when a stimulus poses an imminent threat to our well-being. More prolonged pain reduces the temptation to touch a damaged region of skin or move an injured joint, supporting the healing process. Some individuals cannot feel pain, despite experiencing basic tactile stimuli, a condition referred to as idiopathic analgesia. Though you may be tempted to believe that such individuals are lucky, they actually have a reduced life expectancy.

In their study of the sensation we call pain, researchers have unearthed several intriguing research questions. For example, you have heard athletes talk about “playing through” the pain of a major injury that they have suffered. How do humans use emotional or cognitive processes to completely suppress or block pain? Researchers (e.g., Roland, 1992) also have been puzzled by the fact that when the brain processes information received from the pain receptors, this activity is not localized to discrete areas in the same manner as the processing of touch information in the primary somatosensory cortex. We discuss research findings related to these, and other, questions in this section.

Pain Receptors, Nerves, and Neurotransmitters

The somatosensory receptors responsible for pain are known as nociceptors. Different nociceptors react to different types of painful stimuli: sharp touch, extremes of temperature, and chemical stimulation. The body’s primary nociceptors can be found among free nerve endings located just beneath the skin’s surface and in the underlying layer of fat.

Two specialized types of nerve fibers carry pain messages from the nociceptors to the spinal cord. Messages regarding strong touch (e.g., a kick to the shin) or intense heat travel along fast-conducting, AS nerve fibers. Activation of these fibers results in a sense of sudden, acute pain, such as when you accidentally touch a hot stove. This kind of pain serves a protective function, causing you to withdraw from the painful stimulus. Messages regarding less intense heat or chemical stimulation—including the neurochemicals released by our own bodies upon injury to the skin— travel along more slowly conducting, C nerve fibers. If your C fibers become active, you might experience a more prolonged, “achy” pain, such as that associated with an injured back. These two types of nerve fibers work together in forming the pain experience. Imagine hitting your thumb with a hammer; you will likely experience a sudden, sharp pain upon first impact, followed by a longer-lasting aching. Moreover, the C fibers (and the AS fibers, to a lesser extent) become potentiated upon injury, such that normally innocuous stimuli (e.g., a gentle touch) feel painful.

Upon reaching the spinal cord, the AS and C nerve fibers, when active, release two neurotransmitters: glutamate and substance P. If the pain stimulus is mild and short-lived (i.e., causing AS activity only), then only glutamate is released; however, if the pain stimulus is strong (i.e., causing both AS and C fiber activity), then both glutamate and substance P are released (Cao et al., 1998). DeFelipe and colleagues (1998) experimentally verified that the release of substance P precedes the experience of intense pain. Their study utilized “knockout” mice that lacked receptors for substance P. Because these animals could not react to substance P when it was released, the researchers predicted that the mice would react to intense pain stimuli as if they were mild annoyances. Their results supported this prediction.

Cortical Involvement in Pain

The release of glutamate and substance P in the spinal cord sends impulses coursing up the anterolateral system to the brain. Unlike nonpainful tactile stimuli, which result in localized activity in the primary somatosensory cortex, PET scans (see Chapter 16) indicate that painful stimuli cause distributed activity over many regions of the brain (Talbot et al., 1991). For example, regions of somatosensory cortex (specifically, cortical maps that parallel the representations for basic touch sensation) appear to process the sensory aspects of the pain experience and allow the painful stimulus to be localized. By contrast, pain-related activity in an area of the cortex known as the anterior cingulate gyrus plays a role in determining the emotional reaction to pain. The contributions of many other pain-processing regions are not, as yet, well understood.

At this juncture, it should be noted that researchers often speak of the multimodal nature of pain. The experience of pain cannot be captured adequately by describing the raw sensation, such as “too hot.” Pain has a subjective, emotional component as well. Pain both influences and is influenced by one’s mental state. Upon taking this complex, multimodal nature into account, it should not be surprising that a variety of cortical areas play a role in processing pain information.

Gate-Control Theory Of Pain

In 1965, Melzak and Wall proposed the influential gate-control theory of pain. Importantly, this theory reflects pain’s multimodal nature. Specifically, the gate-control theory aims to explain the fact that the experience of pain depends not only on the strength of a stimulus but also on a variety of cognitive and emotional factors.

In the gate-control theory, pain sensations begin with the activation of nociceptors and a message sent along the AS and/or C nerve fibers, as described above. However, in order for the pain signals to reach the brain, and thus be consciously experienced, they must pass through open “gates” in the spinal cord.

Two different aspects of the gate-control circuitry can cause the gates to close and block the perception of pain. First, the pain signal’s transmission is inhibited when the input from the fast nerve fibers carrying basic (nonpainful) tactile sensations exceeds the input from the slower nerve fibers carrying the pain sensations. Do you immediately rub your knee after you’ve banged it against the underside of your desk? If so, the gate-control theory may explain why; by activating the mechanoreceptors, you can actually prevent (or at least reduce) the pain signal being sent to your brain. Some researchers also hypothesize that acupuncture reduces pain by stimulating these same touch fibers.

Second, gate-control theory proposes that pain can be blocked by top-down influences. That is, messages that originate within the brain descend to the spinal cord and inhibit the transmission of pain signals to the brain. As an illustration of top-down control over pain experiences, numerous studies indicate that patients taught relaxation techniques before surgery require subsequently require lower doses of painkillers. Moreover, there appears to be real truth in the saying that “laughter is the best medicine.” Laughter and other positive emotions can significantly reduce the experience of pain (e.g., R. Cogan, D. Cogan, Waltz, & McCue, 1987), apparently through top-down influences over spinal cord pain transmission circuits. For further details on the gate-control theory, see Melzak and Wall (1965).

Descending Pain-Control System

Although our understanding of ascending pain pathways remains in its infancy, research conducted since the proposal of the gate-control theory has successfully identified several specific structures involved in the descending pain-control system. Activation of neurons in the periaqueductal gray—the gray matter surrounding the cerebral aqueduct—initiates the pain-suppression circuit. The importance of this region first became apparent in the late 1960s, when Reynolds (1969) reported that stimulation of the periaqueductal gray reduces the reaction to painful stimuli (i.e., produces analgesia).

By the late 1970s, Basbaum and Fields (1978; see also Fields & Basbaum, 1984) had proposed a complete descending pain-suppression circuit. Activation of the periaqueductal gray results in stimulation of the raphe nucleus, a distinct cluster of neurons located in the depths of the medulla in the hindbrain (brain stem). Stimulation of the raphe nucleus, in turn, inhibits pain-sensitive neurons in the spinal cord, blocking the experience of the incoming pain stimulus.

To fully understand the pain-suppression circuit, researchers still needed to identify the neurotransmitter that activated the periaqueductal gray. In 1973, Pert and Snyder had discovered that morphine and other pain-suppressing opiate drugs bind to receptors in this area. This discovery ultimately provided the missing link. Because it seems highly unlikely that the human brain would have developed receptors for an exogenous (external) substance, such as morphine, researchers reasoned that the brain produced it own natural opiates. Two subsequently identified brain peptides, metenkephalin and leuenkephalin, though structurally very different from opiates, do indeed stimulate the brain opiate receptors. The discovery of several natural endogenous (internal) morphines, collectively known as endorphins, lent further support to the theory.

Both aversive/painful situations, such as running a marathon, and pleasurable situations, such as sexual activity or listening to your favorite music, can trigger the release of endorphins. In sum, cognitive-emotional stimuli and situations prompt the release of endorphins, activating the descending pain-control system and suppressing the experience of pain.

Proprioception, Kinesthesis, And The Vestibular Sense

Imagine waking up in the middle of the night and realizing that you have a terrible crick in your neck. The sense of pain typically would be followed immediately by the realization that your body is twisted into a very strange posture that you must immediately adjust. This insight arises from proprioception, the perception of the position of the limbs and other body parts in space, and the closely related sense of kinesthesia, the perception of limb motion.

Though often overlooked, proprioception and kinesthesia play a critical role in everyday life. As discussed earlier in this research-paper, haptic perception—active exploration by touch—requires that tactile information be combined with knowledge of how the fingers are positioned and moving across the object. More generally, proprioception and kinesthesia, together with the vestibular sense (discussed later), provide continuous feedback that allows the brain to fine-tune posture and movements.

Two key receptors for proprioception and kinesthesia, referred to as proprioceptors, are the muscle spindles and the Golgi tendon organs. Each muscle spindle consists of a bundle of four to ten specialized muscle fibers that run parallel to the nonspecialized muscle fibers. The muscle spindles encode changes in muscle length; whenever a muscle stretches, the muscle spindles also stretch, resulting in a message being sent to the spinal cord and ultimately to the brain. Muscles that must perform precise movements, such as those in the hands and the jaw, tend to be more densely populated with muscle spindles than muscles that perform coarser movements. The Golgi tendon organs, found at the junction between the tendon and the muscle, provide complementary information about muscle tension.

The fibers of the muscle spindles communicate directly with motor neurons in the spinal cord. The activity of these proprioceptors initiates the stretch reflex; a rapid stretch of the muscle, detected by the muscle spindles, causes the motor neurons to contract the muscle and return it to the original length. The stretch reflex causes your leg to kick out when your doctor taps below your knee. More important, when a heavy object drops into your outstretched hand, the stretch reflex causes your arm to rise back immediately to its initial position.

Of course, muscle position and tension do not depend solely on reflexive spinal cord control; the brain determines the appropriate posture at any given moment. After reaching the spinal cord, information from proprioceptors ascends to the cortex via the dorsal-column medial lemiscus pathway. Ultimately, proprioceptive information reaches a distinct map within the somatosensory cortex, where its processing leads to awareness of body position. The location of the brain region for proprioceptive analysis is noteworthy for two reasons. First, the regions for proprioceptive and for tactile analysis lie nearby each other, supporting the integration necessary for haptic perception. Second, primary motor cortex resides in the precentral gyrus of the frontal lobe, immediately in front of somatosensory cortex; the proximity of motor and somatosensory cortex no doubt facilitates the brain’s use of proprioceptive information to program adjustments in movement.

The Vestibular Sense

Unless you find yourself in an unusual situation, such as trying to walk down a very steep slope, how often have you thought about your sense of balance? “Never” most likely would be your answer to that question. Like proprioception and kinesthesia, the sense of balance—the vestibular sense—is one that most people take for granted.

The vestibular system consists of two basic components within the inner ear: the otolith organs and the semicircular canals. The two otolith organs—the utricle and the sacculus—act as gravity detectors, responding to linear accelerations and reporting the head’s orientation in space. The three semicircular canals detect rotational accelerations of the head; positioned along three different planes at right angles to one another, the semicircular canals can sense rotation in any direction.

As a part of the labyrinthine network that also includes the cochlea (for hearing), the otolith organs and semicircular canals are filled with a thick fluid that moves as the head moves. This fluid movement causes the hair-like cilia and the ends of the vestibular receptors (called hair-cells) located within these chambers to bend; this receptor bending results in neural messages about head position and movement being sent along the vestibular branch of the eighth cranial to the brain. Several subcortical structures, including the vestibular nuclei of the brainstem and the cerebellum (which aids in the production of smooth, coordinated movements), play a role in the processing of vestibular information. Cortical regions falling adjacent to primary somatosensory cortex also receive projections from the otolith organs and semicircular canals.

This consideration of the vestibular system offers a useful reminder that our knowledge of the world depends on an interaction among all the senses. From your own experience, you may be aware that conflicts between visual and vestibular input cause dizziness and nausea. For example, most people become carsick not when looking out the window, but when reading. In this example, the nausea-inducing conflict results from the fact that your eyes, which are reading the text but not capturing the passing landscape, suggest that you are stationary whereas your vestibular system registers the movement. Those of you who have taken dance lessons may also have found that spinning in circles causes the most dizziness when you stop moving. As long as you continue to spin, your visual and vestibular systems provide complementary information about your movement. However, when you halt, the fluid in your semicircular canals, having built up momentum, continues to move for a few moments; thus, your vestibular system implies that you are still spinning, whereas your visual system argues that you’re not.

Controlling Body Temperature

Discussions of somatosensory systems often overlook introception, the processing of internal conditions. Though introceptive processes tend to occur automatically, with little or no awareness, they serve a critical function for our survival. Thus, we conclude this research-paper with a brief discussion of a representative introceptive process, the control of body temperature.

Fish and reptiles are poikilothermic—their body temperatures match the surrounding environment. On the other hand, birds and mammals are homeothermic—they have physiological mechanisms that allow them to adjust their body temperature in order to maintain optimal activity levels. Multiple mechanisms exist to achieve either additional warmth or cooling, as necessary. For example, if additional warmth is needed, people and animals (a) shiver, (b) huddle together to share body heat, and/or (c) fluff their fur (furry mammals), hair (humans), or feathers (birds) to improve insulation; when all else fails, the system diverts blood flow from the skin to the vital internal organs, thus protecting critical systems. By contrast, if it’s too hot and body temperature needs to be reduced, people and animals can (a) sweat, (b) reduce activity, and/or (c) route more blood to the skin in order to cool it.

Research indicates that the hypothalamus, a major regulatory center for numerous survival behaviors, plays a crucial role in temperature regulation. The hypothalamus is a small structure, composed of a number of subdivisions or nuclei, located at the base of the brain. Heating or cooling the preoptic area of the hypothalamus triggers the appropriate temperature-adjusting bodily reactions, suggesting that this region governs temperature regulation. Cells in the preoptic area receive neural input via the anterolateral pathway from thermoreceptors (temperature receptors) in the skin, as well as monitor their own temperature. Animals with surgical damage to the preoptic area cannot regulate their bodily temperatures.

Summary

In this research-paper, we discussed how somatosensory systems provide a plethora of information about the world in which we live, all of it crucial to our survival. Tactile input, especially when gained through active exploration, allows us to make reliable and accurate determinations about the nature of the objects within our reach. Pain, though unpleasant, signals that tissue damage has or is about to occur, protecting us from more severe injury. Proprioceptive, kinesthetic, and vestibular information provides continuous feedback about the states of our bodies, facilitating coordinated, finely tuned movements and contributing to haptic perception. Finally, interoceptive processes, such as temperature regulation, allow our bodies to maintain optimal levels of activity. When combined with the perceptual information provided by the other senses, somatosensory processing allows us to achieve a remarkable understanding of the world around us.

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