General Senses

General Senses

The general senses of touch, pressure, ­temperature, and pain are spread throughout the body via muscle,­ joint, skin, and visceral receptors. They are also known as the somatic senses and involve relatively simple receptors. General sensory receptors are nerve ­endings of two types: nonencapsulated (free) or encap-sulated. The senses of touch and pressure utilize both of these types.

NonEncapsulated (Free) Nerve Endings

Nonencapsulated (free) nerve endings of the sensory neurons are very common in epithelia and connective tissues. They are mostly non-myelinated with group C fibers that are small in diameter. Their distal endings called sensory terminals often have small, knob-like swellings. They mostly respond to temperature and painful stimuli in the skin and internal tissues, except for the brain. They may also respond to tissue movements that are influenced by pressure. Pain receptors are stimulated by tissue damage but adapt to improving conditions poorly. They may send pain impulses persistently to the CNS regardless of whether tissue damage is ­continuing, ­making pain persist. Pain is triggered by releases of certain chemicals and deficiency of oxygen­-rich blood (a condition­ known as ischemia) or the ­stimulation of certain mechanoreceptors.

Temperature is sensed via warm receptors and cold receptors . In the superficial dermis, nerve end-ings respond to cold temperatures. Cold receptors are most sensitive to temperatures between 50°F (10°C) and 68°F (20°C) to produce a freezing sen-sation. These receptors work rapidly, and sensation begins to fade away after approximately one minute of continuous stimulation. Deeper in the dermis are nerve endings that respond to warmer temperatures. Warm receptors are most sensitive to temperatures above 77°F (25°C), becoming unresponsive to tem-peratures above 113°F (45°C) . At this temperature, pain receptors are stimulated to produce a burning sensation.

Any temperatures outside the range of thermo-receptors activate the nociceptors and is perceived as painful. The nociceptors also respond to chemi-cals that are released from damaged tissues and to pinching of the skin. A plasma membrane protein called the vanilloid receptor is important in the detection of painful stimuli. This protein is actu-ally an ion channel. It is opened by heat, low pH, and chemicals such as capsaicin, which is found in hot peppers.

Free nerve endings may extend between epithe-lial cells and control the sensation of itching. The itch receptor of the dermis has an extremely thin diameter, and was not discovered until long after other types of receptors were recognized. The nerve endings related to itching are activated by many different chemicals, but mostly histamine, which is present in areas of inflammation. Other types of nonencapsulated nerve endings include:

■■ Tactile (Merkel) discs: Within the deepest epidermal layer, they are receptors for light touch. They are formed by certain free nerve endings associated with enlarged tactile or Merkel cells.

■■ Hair follicle receptors: Wrapping around hair follicles, these are light touch receptors that detect the bending of hairs, which may occur when an insect lands on the skin.

Encapsulated Nerve Endings

Encapsulated nerve endings are those that contain one or more fiber terminals of sensory neurons. These neurons are enclosed in connective tissue capsules. Almost all encapsulated receptors are mechanorecep-tors, though there are wide variations in distribution over the body, shape, and size. Encapsulated nerve endings include:

■■ Meissner’s (tactile) corpuscles: Oval yet flattened connective tissue cells, with two or more fibers spiraling into each corpuscle to end in small knobs; located in hairless skin (fingertips, lips, palms, soles, external genitalia, and nipples), they respond to objects that lightly touch the skin. They are small receptors surrounded by Schwann cells, and then thin, egg-shaped connective capsules.

■■ Pacinian (lamellar) corpuscles: Scattered deep in the dermis and subcutaneous tissue, they are mechanoreceptors stimulated by deep pressure, but only when it is first applied. This makes them able to monitor vibrations as on/off pressure stimuli. These corpuscles are the longest in size, sometimes more than 3 mm long and 1.5 mm wide. They can be seen by the naked eye as white, egg-shaped structures. They have a single dendrite surrounded by a capsule of as many as 60 layers of collagen fibers with flat supporting cells.

■■ Bulbous corpuscles (Ruffini endings): They are tactile receptors found in the dermis, subcutaneous tissue, and joint capsules, their receptor endings are enclosed by flattened capsules. They look similar to tendon organs and function to monitor changes in dense connective tissues, responding to deep and continuous pressure.

■■ Muscle spindles: Fusiform (spindle-shaped) pro-prioceptors throughout the skeletal muscle per-imysium. Each of them has a bundle of modified intrafusal fibers within a connective tissue capsule. Muscle spindles function to detect muscle stretch-ing. They initiate a reflex that resists this stretching.

■■ Tendon organs: Proprioceptors inside tendons, near junctions between the tendons and skeletal muscle. They have small bundles of collagenous fibers inside a layered capsule. Sensory termi-nals coil between and around their fibers. When tendon fibers stretch due to muscle contraction, compression of nerve fibers activates the tendon organs (proprioceptors). A reflex is initiated that causes the contracting muscle to relax.

■■ Joint kinesthetic receptors: Proprioceptors that monitor stretching of articular capsules enclos-ing synovial joints. They have at least four recep-tor types, including free nerve endings, lamellar corpuscles, bulbous corpuscles, and receptors that look like tendon organs. Together, these receptors communicate information about joint positions and motions of which we are aware.

Somatosensory System

Sensation is the awareness of environmental changes both externally and internally. To survive, humans rely on sensation as well as how they interpret these changes (perception). How we respond to sensations is determined by our perceptions of them. The part of the sensory system that serves the limbs and wall of the body is known as the somatosensory system. Input is received from exteroceptors, interoceptors, and proprioceptors. The somatosensory system trans-mits information about various sensations. The sen-sory receptors make up the receptor level of this system, whereas processing in the ascending pathways makes up its circuit level. The processing in the cor-tical sensory areas is called its perceptual level. For sensations to occur, stimuli must excite a receptor and action potentials must reach the CNS.

Sensory neurons may be called either tonic receptors or phasic receptors. Tonic receptors are always active. The rate at which action poten-tials are generated changes when stimulus increases or decreases. Phasic receptors are normally inactive, but become active for a short period of time when a change occurs in the conditions they monitor. These receptors provide information about intensity and rates of change of a stimulus.

Adaptation is a reduced sensitivity, whereas a stimulus is consistently present. Peripheral adaptation occurs as levels of receptor activity change. The ­initial strong response subsides over time, partly because the size of the generator potential decreases gradually. This is typical of phasic receptors, and for this reason, they are also called fast-adapting receptors. The tonic receptors are called slow-adapting receptors because they show little peripheral adaptation. Pain receptors or nociceptors are examples of slow-adapting recep-tors. Central adaptation refers to inhibition of nuclei located along a sensory pathway.

Pain Receptors in Visceral Organs

Pain receptors in the visceral organs act differently from those located in surface tissues. When vis-ceral tissues are stimulated on a widespread basis, strong pain sensations can follow. This type of pain appears to be caused by mechanoreceptor stimulation, decreased oxygenated blood flow, or accumulation of pain-stimulating chemicals. Visceral pain may seem to be coming from a different area of the body from the one actually being stimulated. This is known as referred pain. Heart pain, for example, may appear to be occurring in the shoulder or upper left arm.

Referred pain may arise from different areas, including the skin and viscera. Heart pain impulses travel through the same nerve pathways as do skin pain impulses such as those from the skin of the left shoulder and upper left arm. A heart attack may, therefore, fool the cerebral cortex into interpreting pain impulses as if they are coming from the shoulder or arm instead.

Acute pain fibers are thin, myelinated nerve fibers that conduct nerve impulses rapidly and mostly produce­ sharp pain. Acute pain is usually sensed as coming from the skin. Chronic pain fibers are thin, unmyelinated nerve fibers that conduct impulses more slowly and mostly produce dull, aching pain. Chronic pain is usually sensed as coming from deeper within the body. Pain stimulation often causes both types of sensations—a sharp pain followed by a dull ache. The aching pain is often more intense, worsening as time passes, and can cause prolonged suffering.

The cranial nerves sense pain impulses origi-nating from the head. All other pain impulses travel through the spinal nerves. The spinal cord’s neurons process pain impulses in its gray matter to transmit them to the brain. Other neurons conduct impulses to the thalamus, hypothalamus, and cerebral cortex. Pain awareness occurs when pain impulses reach the thalamus; however, it is the cerebral cortex that con-trols the body’s response to pain. The midbrain, pons, and medulla oblongata regulate how pain impulses move from the spinal cord. Biochemicals are released to block pain signals by inhibiting presynaptic nerve fibers in the spinal cord.

The posterior horn of the spinal cord releases enkephalins to suppress pain impulses of various severities. Enkephalins bind to the same receptor sites on neuronal membranes as the drug morphine. ­Serotonin is also released, which helps by stimulat-ing further enkephalin release. Endorphins also have pain suppression actions and are found in the pituitary gland. Both enkephalins and endorphins are released in response to extreme pain. Endorphins can inhibit impulses initiated by nociceptors.

1. Identify which type of receptor is exemplified by thermoreceptors, mechanoreceptors, and chemoreceptors and explain to what they respond.

2. Explain the locations of sensory receptors of the general senses and the special senses.

3. Differentiate between the terms “sensation” and “perception.”

4. Explain nonencapsulated (free) nerve endings.

5. Describe encapsulated nerve endings.