Somatosensory receptors located within the skin of the dog provide him with the ability to discriminate touch stimulation. There are five categories of these receptors. The nociceptors, associated with pain, proprioceptors, sensitive to body movement and position, thermoreceptors, sensitive to hot and cold, chemoreceptors, sensitive to chemical stimulation and mechanoreceptors, sensitive to pressure due to physical changes of the body.
Mechanoreceptors are the most abundant and located at the base of each hair follicle. These receptors become activated when the hair is disturbed by external movements that cause the surrounding tissue to stretch or bend. In addition to the mechanoreceptors found in the skin of mammals, scientists have identified other receptors. Mammalian skin has two layers known as the dermis and epidermis. Within the epidermis scientist have discovered a pressure-sensitive and slowly adapting receptor identified as Merkel’s receptor. These receptors respond to pressure near the skins surface. Also identified are Meissner’s corpuscles that are responsive to touch and low-frequency vibrations (50 Hz). These corpuscles are very receptive, discriminative and rapidly adaptive. Pacinian corpuscles are found much deeper within the dermis and respond depending on the amount of pressure. They have a large receptive field, stimulated at a higher vibration frequency, respond quickly, and rapidly adapt to this type of stimulation. The last mechanoreceptor known is Ruffini’s corpuscles also located in the dermis. They respond to a large receptive field, but are slower in adapting to continuous stimulation over long periods.
Nociceptors are bare (unmyelinated) nerve endings responsive to harmful stimulation that threatens body tissue. These receptors are associated with pain and tend to stimulate escape mechanisms in animals. There are four types of nociceptors and identified according to the source and type of stimulation. They are mechanical, thermal, chemical and polymodal. The result from nociceptive stimulation causes a debilitating effect on most of the bodies major organs. Additionally, localized tissue damage may result with a quick release of “pain-enhancing hormones” known as prostaglandin. The secretion sensitizes the nerve endings to histamine which is an inflammatory by-product of cell damage.
Pain information is relayed along two pathways: a fast pain system and a slow pain system. Fast pain system provides immediate information related to trauma and the slow pain system maintains the painful feelings after the stimulus is removed. The fast pain system is associated with the cerebral cortex, which receives the stimulus via two thalamic nuclei. The slow pain system relays stimuli via the reticular formation to the hypothalamus and the limbic system where emotional responses are interpreted and motivate “flight-freeze” responses. The fast pain system is limited to surface nociception and more recent evolutionary development than the slow pain system associated with the limbic system considered to have evolved out of primitive structures involved in analysis and interpretation of olfactory information. Stimulation of the slow pain system produces a side effect in the release of endorphins. These endorphins, when activated affect opioid receptor sites alleviating pain and allowing an animal the ability to escape or fight.
Proprioceptors are located in the muscles and joints and responsible for determining the body’s position and movements. They are controlled by various areas of the brain, including the sensory motor cortex and cerebellum. These receptors relay information about the body’s movements and its orientation relative to the location of its different parts. The two common receptors are muscle spindles and Golgi tendon organs. The muscle spindles are responsive to the rate and stretching of a working muscle. The Golgi tendon organs measure force exerted by the muscle on the tendon. Additionally, these type receptors are responsible for providing information relative to physical changes in joints and sensory information received from manipulation of objects and balance.
Thermoreceptors are different. It seems dogs and humans are different when it comes to perceiving heat stimuli. Dogs possess only cold–sensing temperature receptors with heat sensors located only around their noses. However, this does not mean dogs are unable to feel warmth, it serves to point out they perceive it differently from humans. As a result, dogs are unable to adapt to hot environmental conditions that may affect their well-being.
Science of touch
Comfort seeking behavior
There were studies made on comfort-seeking behavior not only with dogs, but also with rhesus monkeys. The result of these studies concluded that both species preferred soft items, over wire ones even those wire objects providing milk. In addition, researchers found that separation distress vocalization was reduced by providing soft comforting objects and food and hard objects had no effect. Additionally, the provision of mirrors helped reduce distress, speculating their images helped provide some sort of comfort. Therefore, it is not hard to understand the importance touch has on the development of normal emotional and social behavior in animals, including dogs.
Gantt et al performed the first studies on the calming effects of dogs in 1966. What they observed were dogs in distress calmed to petting resulting in decreasing effects on heart and respiratory rates during this contact. He dubbed this phenomenon as the “effect of person”.
More importantly, researchers reported that dogs’ heart rates were reduced by petting during pre-shock and post-shock stimulations during classical conditioning. In addition, Tuber (1986) suggests, that training dogs to relax should be just as important as other training activities. Still other researchers suggest the way in which petting is performed will determine the best outcome, suggesting an influence on cortisol levels associated with stress and aversive emotional arousal.
More recent studies have confirmed a reciprocal experience on humans through direct tactile contact with dogs and reduction in blood pressure and heart rate. Despite these psychological and physiological benefits gained from human and animal interaction, the majority of these studies are of the “nongeneralizable statistical variety” providing only limited validation for the hypothesized beneficial therapeutic effects of animal companionship.
It is perceived that the combined effect of Ruffini’s corpuscles with the importance of human tactile contact is the basis of the biological explanation of the mechanics of all medical pressure garments. Ruffini’s corpuscles inhibit stress while increasing carbon dioxide while exhaling, resulting in a calm breathing pattern and a more relaxed animal.