Sensory Abilities: Somatosensory System

By | May 19, 2010

The dog’s body is equipped with a variety of receptors sensitive to stimuli impinging on the skin or arising from within the body itself. Specific receptors have evolved for the detection and measurement of pressure, vibration, heat and cold, chemicals, and various noxious stimuli. In addition, internal receptors sensitive to joint location, muscle stretch, and tendon tension provide kinesthetic information about the relative location, direction, and action of the body. In combination, these highly specific sensory organs provide a tremendous amount of information about the external and internal environment and a dog’s moment-to-moment orientation within it.

Dogs exhibit significant differences with respect to their individual responses to somatosensory stimulation. Some dogs are much more sensitive to touch than are others. Thresholds for stimulation are profoundly affected by an individual dog’s emotional state, general physical condition, and past experience (learning). For example, fearful or hypervigilant dogs will likely respond to nociceptive stimulation at a much lower level of intensity than dogs that are relaxed and confident. Fearful dogs are also more likely to exhibit emotionally reactive behavior when stimulated. Similarly, dogs suffering from disease or deprivation may show significant changes in their relative responsiveness to certain kinds of stimulation. Hypothyroidism, for example, may cause affected dogs to seek warmth and avoid cold areas. Likewise, hungry dogs are more alert to stimuli associated with the acquisition of food. Somatosensory responsiveness is also significantly influenced by experience. A dog’s response to stimulation may be decreased or increased depending on the presence or absence of previous habituating or sensitizing exposure to the evoking stimulus. The amount of past socialization received by a dog will also influence how that dog interprets and responds to tactile stimulation. Well-socialized dogs, for example, will more likely accept and respond in a friendly way to petting and hugs, whereas undersocialized dogs may only begrudgingly tolerate such tactile contact — if at all. Although the way in which dogs ultimately interpret and respond to sensory input is highly variable and dependent on many factors, the manner in which sensory input is obtained from the impinging external and internal environment follows a regular pattern of processing.

Mechanoreceptors

The largest sensory organ in the body is the skin, which contains numerous receptors adapted and specialized for the reception of specific sensory input. There are five basic categories of somatosensory receptors in the skin: nociceptors (associated with noxious or painful stimulation), proprioceptors (sensitive to body movement and position), thermoreceptors (responsive to heat and cold), chemoreceptors (sensitive to chemical stimulation), and mechanoreceptors (sensitive to physical changes, twisting, stretching, and pressure). Mechanoreceptors are the most numerous receptors in skin. At the base of each hair follicle, for example, is a group of pressure-sensitive hair-follicle receptors that are activated whenever the hair is disturbed by external movements that cause the surrounding tissue to stretch or bend. Follicle receptors of special importance to dogs are those associated with the vibrissae or whiskers located at various points on the face. The vibrissae provide dogs with information about nearby objects, coordinate the movement of the muzzle and mouth toward nearby objects, and may serve an important protective function against ocular injury by avoiding accidental collisions. In addition to direct mechanical stimulation, the vibrissae are responsive to vibrations and the subtle movement of air currents. The sensory information from the vibrissae is especially important for rats and cats. Welker (1973) categorized rats as feelers, stemming from their extraordinary reliance on their whiskers for survival. In addition to indicating the presence of a nearby object to rats while in darkness, the vibrissae also appear to provide supplemental information about its shape, texture, and distance. An interesting possible cause of reflexive aggressive behavior occasionally exhibited by some dogs to a puff of air blown into their face may be related to a species-typical defensive reaction mediated by vibrissae. During combat between dogs, vibrissae may provide information about the opponent’s close location and movements, perhaps mediating some measure of defense through the reflexive organization of combative behavior. Motile vibrissae on the muzzle quickly flare and reorient in a forward direction when a dog is aggressively aroused, suggesting that they play some functional role. Sensory information originating in the face, including receptors associated with the vibrissae, is conducted by the trigeminal nerve. In addition to providing mechanoceptive and proprioceptive information about the face and jaw, the trigeminal nerve is an important conduit for the transmission of chemoceptive information resulting from the chemical stimulation of the nasal and oral mucosa (e.g., the nonolfactory sensation of alcohol vapor to the nose or the burning sensation it produces if placed on the tongue).

A number of other mechanoreceptors have been identified in the skin of mammals. The skin is composed of two layers: the dermis and the epidermis. In the epidermal layer, a pressure-sensitive and slowly adapting receptor known as Merkel’s receptor is found. Merkel’s receptors respond to indentations produced near the surface of the skin. In humans, specialized pressure and vibration receptors are located in the elevated ridges of the epidermis, forming fingerprints. These Meissner’s corpuscles are responsive to both touch and low-frequency vibrations (50 Hz). Meissner’s corpuscles exhibit an extremely small receptive field and are employed to form fine tactile discriminations. Unlike Merkel’s receptors, Meissner’s corpuscles are rapidly adapting.

Deeper within the dermis are other pressure receptors called pacinian corpuscles. These onionlike structures are composed of several concentric layers of connective tissue that variably respond according to the amount of pressure applied to them. Pacinian corpuscles are responsive to a large receptive field involving both pressure and vibration but in a higher frequency range than observed in Meissner’s corpuscles, approximately 200 to 300 Hz. They respond quickly and rapidly adapt to continuous stimulation. Other mechanoreceptors located in the dermis are Ruffini’s corpuscles. Like pacinian corpuscles, Ruffini’s corpuscles exhibit a relatively large receptive field. Unlike pacinian corpuscles, however, Ruffini’s corpuscles are much slower to adapt to long periods of continuous stimulation.

Nociceptors

Nociceptors are free, unmyelinated (bare) nerve endings in the skin and body that respond to noxious stimulation that either damages or threatens to damage body tissue. The subjective experience of nociception is pain. Painful stimulation elicits species-typical escape reactions that serve to separate the organism from the source of noxious stimulation. Nociceptors are divided into four types, depending on the source of stimulation: mechanical (responds to sharp pressure), thermal (extremes of burning heat or freezing cold), chemical (stinging sensation of ammonia or pepper), and polymodal (nociceptors that combine sensitivity to a combination of mechanical, thermal, and chemical stimuli).

Pain results from the stimulation of nociceptive nerve endings terminating on the skin’s surface and enervating most of the body’s major organ systems. In addition to the direct stimulation of these specialized receptors, traumatic stimulation may also cause local tissue damage and the rapid release of pain-enhancing hormones, such as prostaglandin. The secretion of prostaglandin sensitizes nociceptive nerve endings to histamine — an inflammatory by-product of cell damage. Aspirin and other anti-inflammatory medications produce their analgesic effects by disrupting the production of prostaglandins. Pain information is relayed along two pathways: a fast pain system and a slow pain system. The fast pain system informs the brain immediately of the traumatic event (“Yelp!”) followed by the slow pain system (throbbing, aching, and burning sensations), which maintains the feeling of constant painful sensation — even though the original stimulus has been removed. The fast pain system terminates in two thalamic nuclei: the ventrobasal complex (also associated with touch and pressure) and the posterior nucleus. From these thalamic nuclei, the impulse is relayed to the cerebral cortex. The slow pain system passes through the reticular formation and projects to the hypothalamus and the limbic system (amygdala) — areas involved in the emotional interpretation of pain and the motivation of flight-freeze-fight reactions. The fast pain system is limited to surface nociception (the skin and mucosa) and is a more recent evolutionary development than the slow pain system, which services all bodily tissue except the brain, which is not sensitive to pain.

One effect of the slow pain system is the production of endorphins (a contraction of endogenous morphine). Endorphins are pep-tides (short protein molecules) produced by the brain in response to slow pain, pressure, and touch. Endorphins are also produced by the pituitary gland (beta-endorphins), which are released into the bloodstream together with other hormones such as adrenocorticotropic hormone (ACTH) as part of the general adaptation stress response. Endorphins circulate throughout the brain to various opioid receptor sites, including the hypothalamus, amygdala, and intralaminar thalamic nuclei. Interestingly, the fast pain system bypasses the emotional and motivational centers associated with avoidance learning and aggression. The fast pain system is a pure pain/startle reaction relayed directly to the cerebral cortex. It is not affected by endorphin activity or the effect of morphine. Naloxone, a molecule resembling morphine in many details, is an active antagonist of morphine and endorphins. Naloxone has little obvious effect on an animal, but it binds with opioid receptors in the brain. Consequently, the complementary pain-reducing and pleasure-enhancing effects of increased opioid activity are impeded. Naloxone is commonly used as a medication for the temporary management of some compulsive behavior disorders, presumably based on the assumption that such disorders are, at least partially, mediated by the endogenous opioid system.

Proprioceptors

Proprioceptive sensitivity is essential for the smooth locomotor functioning of the body. The perception of the body’s orientation in space and its coordinated movements are under the control of various brain centers, including the sensory motor cortex and cerebellum. Sensory information mediating this process is produced by proprioceptors located in the muscles and joints. These receptors provide fast moment-to-moment information about the body’s movements and its orientation relative to the location of its different parts. There are two common proprioceptors: muscle spindles and Golgi tendon organs. Muscle spindles respond to the rate and amount of stretching that the working muscle undergoes. (Incidentally, stretch-sensitive receptors in the detrusor muscle of the bladder send signals indicating that the bladder is full and needful of evacuation.) Golgi tendon organs measure the amount of force being exerted by the muscle on the tendon. In addition, many other mechanoreceptors located in the surrounding connective tissue provide information about physical changes in the joint, including angle and velocity of movement. Besides providing information about the body’s orientation and movement, proprioceptors also provide sensory information about the external world resulting from the physical manipulation of objects.

Balance

In addition to proprioceptive information, the ability to coordinate bodily movement and balance is made possible by sensory information provided by two vestibular structures in the inner ear: the semicircular canals and the vestibular sacs. The semicircular canals are composed of three tubular structures extending from the cochlea and set at 90 degree angles to one another. The canals are filled with a fluid substance called endolymph that shifts in a direction opposite to the body’s movement. The displacement of cochlear fluid during rotational movement causes hairlike receptors to bend, thereby generating a nerve impulse. During linear movement or while standing still, balance is controlled by information from the vestibular sacs (the utricle and saccule). These sacs contain a jellylike substance in which otoliths or tiny stones are suspended. Gravity pulls the otoliths against receptor hair cells that, in turn, produce signals about the relative position of the head to the line of gravity. Information from the semicircular canals and vestibular sacs is gathered in the vestibular nerve and relayed to the cerebellum and sensory motor cortex, where balance is finely coordinated.

Effects of Touch

Many studies have confirmed the enormous importance of touch for the ontogeny of normal emotional and social behavior. Harlow and Zimmerman (1959), for example, studied the comfort-seeking behavior of rhesus monkeys: Infant monkeys who had been separated from their biological mothers shortly after birth were offered two surrogate mother alternatives, one made of carpet and the other made of wire. The researchers found that the infant monkeys preferred surrogate mothers made of soft carpeting material and shunned artificial mothers made of wire, despite the fact that the wire surrogate provided milk whereas the carpet one did not. When Igel and Calvin (1960) replicated this experiment with puppies, they discovered that puppies also preferred a cloth nonlactating surrogate mother over a wire one that provided milk. In a series of experiments studying separation distress in puppies, Pettijohn and coworkers (1977) compared the effect of various stimulus conditions on the amount of distress vocalization exhibited by puppies that were briefly isolated from their mother and littermates. They found that separation distress vocalization was reduced by soft comfort objects (e.g., a piece of cloth), but food (novel and familiar) or hard play toys had no discernible effect on separation-related behavior. Curiously, the researchers also observed a decrease in distress when a mirror was put inside of the holding pen. Ostensibly, the puppies were comforted by viewing the image of themselves, and some even rubbed up against the mirror, apparently in a futile effort to make physical contact with the image.

The first systematic effort to quantify the calming effect of touch on dogs was performed by Gantt and coworkers (1966). Gantt observed that dogs in distress are calmed by social contact, exhibiting a significant decrease in both heart and respiratory rates while being petted. He referred to this phenomenon as the effect of person. Lynch and McCarthy (1969) reported that shock-elicited aversive arousal (as indicated by heart and respiratory rates) was reduced by petting. Also, they found that during classical conditioning, if the dogs were continuously petted during the preshock and postshock periods, heart rates were strongly dampened immediately before (anticipatory arousal) and after the shock was delivered. Tuber (1986) noted the usefulness of massage, or what he calls the “soft exercise,” for promoting calmness in dogs. He advised that training dogs to relax should be just as important as other training activities. Recently, Hennessy and colleagues (1998) reported evidence suggesting that it is not only petting but the way in which petting is done that yields the best effect on objective measures (e.g., cortisol levels) associated with reduced stress. The best results were obtained by utilizing deep muscle massage or long firm strokes of petting from the head to hindquarters. These findings underscore the value of massage for reducing stress in dogs. Massage and relaxation training have many applications in the management of dog behavior, especially in situations involving aversive emotional arousal.

Since Gantt’s discovery, subsequent studies have shown that the effect of person is reciprocal, with humans also experiencing pronounced cardiovascular benefits from tactile contact with dogs. Vormbrock and Grossberg (1988) confirmed previous studies indicating that petting causes a reduction of blood pressure in humans. In addition, they found that these physiological effects are not due to cognitive or conditioned associations but depend on direct tactile interaction between a person and a dog. The mere physical presence of a dog is insufficient; the dog must also be the object of petting to lower blood pressure.

Animals handled early in life exhibit many lasting benefits as the result of such exposure. Experiments with rats show that a minimum amount of preweaning handling results in increased vitality and activity levels, more confidence, and greater resistance to disease; handled subjects are larger and more socially dominant; and, finally, handling has a significant positive impact on learning and problem-solving abilities, as well as reducing reactive emotionality. Puppies handled early in life appear to obtain many similar benefits.

Touch mediates a great deal of social communication between a dog and others with whom the dog comes into contact. Most training efforts exploit hedonically pleasurable or aversive responses mediated by touch receptors. Dogs learn to value gentle petting as a reward and rough handling as punishment. Touch is also an important modality of canine emotional expressiveness, whether it be a gentle lick on the chin, a casual pawing movement for attention, or a hard bite on the leg — the dog, too, understands the power of touch. Consequently, touch provides a basic medium for direct communication and intimate exchange based on analogous experiences of pleasure and pain shared by the human and the dog. Through the agency of touch, we develop an intuitive appreciation of dogs as emotional beings. Dogs react to our handling (whether positive or negative) in ways that are comparable to our own reactions undergoing similar stimulation. Humans and dogs appear to share an empathetic appreciation of one another through the modality of touch and tactile communication. Dogs cannot speak about how they feel, but they are, perhaps, more direct and transparent than virtually any human can be when communicating how they feel through the agency of physical posture, gesture, and various subtle movements and expressions of touch.