The body can sense stimuli by either somatic or visceral sensations. Somatic sensations are detected by peripheral receptors that include touch, pressure, pain, temperature, and body position. The somesthetic tracts mediate the signals from the somatosensory receptors. The name of these tracts would usually be a mashup of the two locations such as spinothalamic tract, the first part being the origin and last part being the insertion. If we added a third name, it would be where the axons synapse, such as the spinocervicothalamic tract. Specific modality neurons carry signals to areas of the thalamus which will then send those signals onward to the somatosensory cerebral cortex.
Pain is the perception of a noxious stimulus. It is usually accompanied by an unpleasant emotional response. The receptors that receive these stimuli are the nociceptors, which free nerve endings. These can be found in superficial layers of the skin and certain tissues, periosteum, arterial walls, joint surfaces, specific areas of the skull. The noxious stimulus stimulates a variety of responses and reflexes, sometimes where the animal withdraws or avoids the pain.
Pain is transmitted by two types of fibers: A(delta) fibers and C fibers. The A(delta) fibers transmit fast pain and are associated with superficial pain. The C fibers transmit slow pain and are associate with deep pain. Both fibers enter via the dorsal root inside of the spinal cord and synapse at the dorsal horn. Pain can also be sent toward the head to a variety of pathways found in all funiculi, a bundle of nerve fascicles. The multiple pathways are clinically significant: the presence/absence of deep pain can determine the prognosis for the animal’s recovery.
Pain receptors can be excited by three types of stimuli: mechanical, thermal, and chemical. All of the stimuli respond to slow pain but only mechanical and thermal respond to fast pain. Modulating the pain by opiate receptors, encephalin and endorphin, inhibits presynaptic neurons that transmit the pain sensation.
Proprioception is the conscious or unconscious knowledge of the body’s position. Skin tactile receptors and deep receptors near the joints determine the position. These receptors can include the muscle spindles, Golgi tendon, Pacinian corpuscles (deep pressure), and Ruffini’s ending (continuous pressure). Hair cells in the inner ear also supply information about head position and movement. The pathway where the information heads to the central nervous system is similar to other pathways. It takes the information via peripheral receptors, heads to the spinal cord, enters via the dorsal root, and heads to the brain. The information will either terminate in the somatosensory cortex for conscious proprioception or the cerebellum for unconscious proprioception. Conscious proprioception is the conscious awareness of the body’s position and movement of body parts. Subconscious proprioception is based around the stretch and tension of muscles, tendons, and ligaments, at rest and during movement, and spatial orientation of the body. The vestibular system sets the balance and posture of the whole animal and thus receives both kinds of stimuli for head position and movement. Conscious proprioceptive deficits can be viewed as the animal knuckling, walking on the dorsum of the paw. Subconscious proprioceptive deficits can be observed through a hopping test, where the animal has an abnormal position of the limbs concerning gravity.
Viscerosensory sensations are essential for heart rate, respiration, blood pressure, and micturition. The receptors are composed of free nerve endings that detect pain or physiological changes. The nociceptors in the viscera detect changes in the viscera caused by pathological or physical conditions. The organs are not sensitive to cutting, heat, or cold. The nociceptors respond to stretching, distension, spasm, inflammation, and ischemia. The visceral pain is poorly localized. The physiological receptors in the viscera respond to innocuous stimuli. These receptors can either be chemoreceptors or mechanoreceptors and respond to a variety of stimuli: changes in blood pressure, in pCO2 or pO2, coughing reflex, and the sense of fullness through either the stomach or urinary bladder. Viserosensory pathways are carried by the autonomic nerves. Physiological receptors follow the parasympathetic nerves and nociceptors follow the sympathetic nerves.
