Anatomy & Physiology

Anatomy & Physiology Study Guide

©2018 Achieve Page 186 , or Pacinian corpuscles, are sensitive to deep pressure. Ruffini corpuscles are also sensitive to pressure and distortion of the skin, but they are located in the reticular (deep) dermis. Baroreceptors identify pressure changes in the walls of blood vessels and organs of the urinary, digestive, and reproductive tracts. A baroreceptor consists of free nerve endings that branch within the elastic tissues in the wall of a distensible organ, such as a blood vessel or a portion of the of 368 16.2 General Sensory Receptors The general senses have receptors scattered throughout the body and are relatively simple in structure. They can be divided into exteroceptors, proprioceptors, and interoceptors. Exteroceptors provide information about the external environment; proprioceptors report the positions of skeletal muscles and joints; interoceptors monitor visceral organs and functions. A more detailed classification system divides the general sensory receptors into four types by the nature of the stimulus that excites them: nociceptors (pain), mechanoreceptors (physical distortion), thermoreceptors (temperature), and chemoreceptors (chemical concentration). Each class of receptors has distinct functional and structural characteristics. Nociceptors Pain receptors, or nociceptors, are especially common in joint capsules, in the superficial portions of the skin, within the periostea of bones, and around the walls of blood vessels. Most visceral organs and deep tissues have few nociceptors. Pain receptors are comprised of free nerve endings with large receptive fields. This large field, therefore, makes it difficult to pinpoint the exact source of a painful sensation. Nociceptors may be stimulated by extremes of temperature, mechanical damage, and dissolved chemicals. Thermore eptors Temperature receptors, or thermoreceptors, are free nerve endings located in the dermis, in the liver, in skeletal muscles, and in the hypothalamus. Cold receptors are about four times more numerous than warm receptors. Structural differences between warm and cold thermoreceptors have not been identified. The same pathways that carry pain sensations also conduct temperature sensations. The nerve impulses are sent to the reticular formation, the thalamus, and (to a lesser extent) the primary sensory cortex. Mechanoreceptors Mechanoreceptors are sensitive to stimuli that contort their plasma membranes. These membranes contain mechanically-gated ion channels whose gates open or close in response to stretching, compression, twisting, or other distortions of the membrane. There are three classes of mechanoreceptors. Tactile receptors provide the closely related sensations of touch, pressure, and vibration. Numerous types of tactile receptors are within the skin: free nerve endings recognize touch and pressure and are situated between epidermal cells, Merkel (tactile) discs sense fine touch and pressure, distortions and movements across the body surface are monitored by the nerve endings of root hair plexus, and Meissner corpuscles perceive sensations of fine touch and pressure and low- frequency vibration. Lamellated corpuscles