Nociceptive sensation. Somatic sensory analyzer
Physiological importance of pain
► According to moderotion, pain is subjective perception of systemic processes, which include information about tissue damage. Activation of pain receptors leads to starting different protective reflexes to avoid tissue damage. However, pain is unpleasant sense and involve to pain reaction wide net of regulative and homeostatic systems of human organism.
► The important specialty in reaction to pain in human is participation of brain cortex and limbic system, which leads to severe emotional experience and autonomic reactions.
Changes in human organism following pain
► Nociceptive reactions are accompanied by motion reactions of entire body towards avoidance the pain.
► In human organism such motion reactions for the some part presented by unconditioned reflexes with short reflector arc formed by neurons of spinal cord and brain stem. But majority of that are behavioral and emotional reactions, which based on conditional reflexes.
► That is why reflector arc includes besides neurons of spinal cord and brain stem structures of limbic-reticular complex and brain cortex.
► Also different changes in human organism followed by pain are observed: increase of muscle tone, accelerated heartbeat, increase of blood pressure, intensification of sweating, dilatation of pupils and elevation of glucose and cuprum level in plasma, activation of hemostasis.
► It considered to cause the majority of both visceral and biochemical reactions by excitation of sympathetic nervous system, which is presented by neurons of hypothalamus, hypophisis and cells in medullar substance of adrenal glands.
Pain and stress reaction
► In fact tissue damage and pain triggers the stress reaction – common reaction of an organism, which leads to stimulation all the functions, especially motion and that is why blood circulation due to cardiovascular system, metabolism, transport of gases due to activation of breathing. Stimulation of pituitary-adrenal axis increases secretion of adrenocorticotropic hormone from the anterior pituitary, and thus there is increased secretion of glucocorticoides from the adrenal cortex. That is why functions of organism activate to defend one.
► But long lasting stress reaction is rather dangerous for organism. Adrenalin in high concentration may produce decreasing of blood supply in visceral organs, which leads to metabolic disorders and disturbances of its function. Besides that nociceptive nerve endings in damaged tissues produce a lot of nervous impulses, which spreading into central nervous system activates wide net of nervous cells. This considerable excitation leads to disturbances iervous regulation of all functions in human organism.
Pain and diagnosing
► Pain gives useful information about cause of tissue damage. When determining the origin of pain it is important to special fiches of pain.
► It is necessary pay attention for location, character, duration, motion and visceral reactions and subjective sensations, following the pain.
► It is considered, after diagnosing pain must be removed.
Ranks of pain
► According to location of pain: somatic and visceral.
► According to time of appearance of pain after tissue damage: early and late pain.
► According to subjective sensation (or character): acute or burning, dull or spread.
► According to duration: sort or prolonged pain attacks.
► Subjective sensations in pain may be presented emotional experience as terror, worry, visual hallucination, dizziness, which appears before of followed the pain.
Reflected pain
► Reflected pain is caused by irritation of visceral organs. Such events as strong constriction of smooth muscles; disorders of blood supply; tension of vessels, stomach, intestines result in pain in certain parts of body. It is determined sensor neurons to connect through interneurons with autonomic and motor neurons in spinal cord. In such a way, viscerosomatic autonomic reflexes are realized.
► Due to mentioned intracellular contacts, human capable to locate nociceptive sensation. These zones of human body where impulses from certain visceral organs are reflected called as Zacharjin-Ged zones. For example, in stomach disorders a human fells pain around navel. Acute pain caused by blood supply disorders in heart muscle reflected to the left shoulder, left shoulder blade and left epigastria.
Pain reception
► Damage stimuli perception created by the brain from electrochemical nerve impulses delivered to it from sensory receptors. These receptors transfuse (or change) different influences of both internal processes in organism and surrounding environment into the electric impulses.
► Pain receptors are specific. Pain does not appear in hyperstimulation of improper receptors. On other hand, adequate stimuli are not so specific as for other sensations. That is why pain receptors maybe stimulated by different kind of irritations.
► Pain receptors may react also to electric, mechanic and especially chemical energy.
Nociceptive structures in central nervous system
► Information about the pain from head, face and mouth cavity ascend to central nervous system by sensory fibers of cranial nerves, for instance facial, glossopharyngeal, vagus and trigeminus nerves.
► Central nociceptive neurons lay iucleus of thalamus, hypothalamus, midencephalon central gray substance, reticular formation and somatosensoric fields of brain cortex.
Anti-nociceptive system
► To antinociceptive neuro-endocrine system belong nervous structures, which are concentrated, obviously, in brain stem.
► High intensity of pain stimuli permits activation of these structures, which contain neurons capable to release endogenous opioids.
► To such structure belong, for instance, prefrontal cortex, hypothalamus, central gray substance, medial thalamic nuclei and limbic system.
Role of opioid peptides
► In brain and digestive tract are located receptors, which bind to morphine. Investigation endogenous ligands of these receptors give ability to reveal two similar pentapeptides, called encephalines, which bind to opioid receptors: met-encephalin and ley-encepfalin. Such chemicals are known as opioid peptides.
Encephalines are containing ierve endings of digestive tract and many parts in brain. It function as neurotransmitters. These peptides are present in gelatinous substance.
In injecting into brain stem, opioid peptides manifestate analgetic effect. Encephalines also may slow down intestines peristaltic.
Touch
Our sense of touch is actually four senses—pressure, warmth, cold, and pain—that combine to produce other sensations, such as “hot.” One theory of pain is that a “gate” in the spinal cord either opens to permit pain signals traveling up small nerve fibers to reach the brain, or closes to prevent their passage. Because pain is both a physiological and a psychological phenomenon, it often can be controlled through a combination of physical and psychological treatments.
While vision, hearing, smell, and taste provide you with important information about your environment, another group of senses provides you with information that comes from a source much closer to home: your own body. In this section, we’ll first consider the skin senses, which provide essential information about your physical status and your physical interaction with objects in your environment. We’ll next consider the body senses, which keep you informed as to your position and orientation in space. own body. In this section, we’ll first consider the skin senses, which provide essential information about your physical status and your physical interaction with objects in your environment.
We’ll next consider the body senses, which keep you informed as to your position and orientation in space. We usually don’t think of our skin as a sense organ. But the skin is in fact the largest and heaviest sense organ. The skin of an average adult covers about
One important receptor involved with the sense of touch, called the Pacinian corpuscle, is located beneath the skin. When stimulated by pressure, the Pacinian corpuscle converts the stimulation into a neural message that is relayed to the brain. If a pressure is constant, sensory adaptation takes place. The Pacinian corpuscle either reduces the number of signals sent or quits responding altogether (which is fortunate, or you’d be unable to forget the fact that you’re wearing underwear). Sensory receptors are distributed unevenly among different areas of the body, which is why sensitivity to touch and temperature varies from one area of the body to another. Your hands, face, and lips, for example, are much more sensitive to touch than are your back, arms, and legs. That’s because your hands, face, and lips are much more densely packed with sensory receptors.
Pain
Pain is important to our survival. It provides us with important information about our body, telling us to pay attention, to stop what we are doing, or to pull away from some object or stimulus that is injuring us. A wide variety of stimuli can produce pain—the sensation of discomfort or suffering. Virtually any external stimulus that can produce tissue damage can cause pain, including certain chemicals, electric shock, and extreme heat, cold, pressure, or noise. Pain can also be caused by internal stimuli, such as disease, infection, or deterioration of bodily functions. Some areas of the body are more sensitive to pain than are other areas.
The most influential theory of pain is the gate-control theory, developed by psychologist Ronald Melzack and anatomist Patrick Wall (1965, 1996). The gate-control theory suggests that the sensation of pain is controlled by a series of “gates” that open and close in the spinal cord. If the spinal gates are open, pain is experienced. If the spinal gates are closed, no pain is experienced.
Taste
Taste, a chemical sense, is likewise a composite of five basic sensations—sweet, sour, salty, bitter, and umami—and of the aromas that interact with information from the taste buds. The influence of smell on our sense of taste is an example of sensory interaction.
Our sense of taste, or gustation, results from the stimulation of special receptors in the mouth. The stimuli that produce the sensation of taste are chemical substances in whatever you eat or drink. These substances are dissolved by saliva, allowing the chemicals to activate the taste buds. Each taste bud contains about 50 receptor cells that are specialized for taste.
The surface of the tongue is covered with thousands of little bumps with grooves in between. These grooves are lined with the taste buds. Taste buds are also located on the insides of your cheeks, on the roof of your mouth, and in your throat (Oakley, 1986). When activated, special receptor cells in the taste buds send neural messages along pathways to the thalamus in the brain. In turn, the thalamus directs the information to several regions in the cortex (O’Doherty & others, 2001b). There were long thought to be four basic taste categories: sweet, salty, sour, and bitter. Recently, the receptor cells for a fifth basic taste, umami, were identified (Chaudhari & others, 2000). Loosely translated, umami means “yummy” or “delicious” in Japanese. Umami is the distinctive taste of monosodium glutamate and is associated with protein-rich foods and the savory flavor of Parmesan and other aged cheeses, mushrooms, seaweed, and meat. Each taste bud shows maximum sensitivity to one particular taste, and lesser sensitivity to other tastes. Most tastes are complex and result from the activation of different combinations of basic taste receptors. Taste is just one aspect of flavor, which involves several sensations, including the aroma, temperature, texture, and appearance of food.
Smell
Like taste, smell is a chemical sense, but there are no basic sensations for smell, as there are for touch and taste. Unlike the retina’s receptor cells that sense color by breaking it into component parts, the 5 million olfactory receptor cells with their 1000 different receptor proteins recognize individual odor molecules. Some odors trigger a combination of receptors. Like other stimuli, odors can spontaneously evoke memories and feelings.
The sensory stimuli that produce our sensation of an odor are molecules in the air. These airborne molecules are emitted by the substance we are smelling. We inhale them through the nose and through the opening in the palate at the back of the throat. In the nose, the molecules encounter millions of olfactory receptor cells located high in the nasal cavity. Unlike the sensory receptors for hearing and vision, the olfactory receptors are constantly being replaced. Each cell lasts for only about 30 to 60 days. In 1991, neuroscientists Linda Buck and Richard Axel identified the odor receptors that are present on the hairlike fibers of the olfactory neurons. Like synaptic receptors, each odor receptor seems to be specialized to respond to molecules of a different chemical structure. When these olfactory receptor cells are stimulated by the airborne molecules, the stimulation is converted into neural messages that pass along their axons, bundles of which make up the olfactory nerves.
So far, hundreds of different odor receptors have been identified (Mombaerts, 1999). We probably don’t have a separate receptor for each of the estimated 10,000 different odors that we can identify, however. Rather, each receptor is like a letter in an olfactory alphabet. Just as different combinations of letters in the alphabet are used to produce recognizable words, different combinations of olfactory receptors produce the sensation of distinct odors. Thus, the airborne molecules activate specific combinations of receptors, and the brain identifies an odor by interpreting the pattern of olfactory receptors that are stimulated (Buck, 2000).
Axons from the olfactory bulb form the olfactory tract. These neural pathways project to different brain areas, including the temporal lobe and structures in the limbic system (Angier, 1995). The projections to the temporal lobe are thought to be part of the neural pathway involved in our conscious recognition of smells. The projections to the limbic system are thought to regulate our emotional response to odors. The direct connection of olfactory receptor cells to areas of the cortex and limbic system is unique to our sense of smell. All other bodily sensations are first processed in the thalamus before being relayed to the higher brain centers in the cortex. Olfactory neurons are unique in another way, too. They are the only neurons that directly link the brain and the outside world (Axel, 1995). The axons of the sensory neurons that are located in your nose extend directly into your brain! As with the other senses, we experience sensory adaptation to odors when exposed to them for a period of time. In general, we reach maximum adaptation to an odor in less than a minute.We continue to smell the odor, but we have become about 70 percent less sensitive to it.
Olfactory Epithelium and Bulb
Ten million olfactory neurons are present within the olfactory epithelium. The axons of these bipolar neurons project through numerous small foramina of the bony cribriform plate to the olfactory bulbs. Olfactory tracts project from the bulbs to the cerebral cortex.
The dendrites of olfactory neurons extend to the epithelial surface of the nasal cavity, and their ends are modi.ed into bulbous enlargements called olfactory vesicles. These vesicles possess cilia called olfactory hairs, which lie in a thin mucous .lm on the epithelial surface. Airborne molecules enter the nasal cavity and are dissolved in the .uid covering the olfactory epithelium. Some of these molecules, referred to as odorants, bind to chemoreceptor molecules of the olfactory hair membranes. Although the exact nature of this interaction is not yet fully understood, it appears that the chemoreceptors are membrane receptor molecules that bind to odorants. Once an odorant has become bound to a receptor, the cilia of the olfactory neurons react by depolarizing and initiating action potentials in the olfactory neurons.
The mechanism of olfactory discrimination is not completely known. Most physiologists believe that the wide variety of detectable smells, which is about 4000 for the average person, are actually combinations of a smaller number of primary odors. Seven primary classes of odors have been proposed: (1) camphoraceous, (2)musky, (3) .oral, (4) pepperminty, (5) ethereal, (6) pungent, and (7) putrid. It’s very unlikely, however, that this list is an accurate representation of all primary odors, and some studies point to the possibility of as many as 50 primary odors.
The threshold for the detection of odors is very low, so very few odorant molecules are required to trigger the response. Apparently there is rather low speci.city in the olfactory epithelium. A given receptor may react to more than one type of odorant. The primary olfactory neurons have the most exposed nerve endings of any neurons, and they are constantly being replaced. The entire olfactory epithelium, including the neurosensory cells, is lost about every 2 months as the olfactory epithelium degenerates and is lost from the surface. Lost olfactory cells are replaced by a proliferation of basal cells in the olfactory epithelium. This replacement of olfactory neurons is unique among neurons, most of which are permanent cells that have a very limited ability to replicate.
Neuronal Pathways for Olfaction
Axons from the olfactory neurons (cranial nerve I) enter the olfactory bulb, where they synapse with mitral cells or tufted cells. The mitral and tufted cells relay olfactory information to the brain through the olfactory tracts and synapse with associatioeurons in the olfactory bulb. Associatioeurons also receive input from nerve cell processes entering the olfactory bulb from the brain. As a result of input from both mitral cells and the brain, associatioeurons can modify olfactory information before it leaves the olfactory bulb. Olfaction is the only major sensation that is relayed directly to the cerebral cortex without first passing through the thalamus. Each olfactory tract terminates in an area of the brain called the olfactory cortex.
Body Position and Movement
Finally, our effective functioning requires a kinesthetic sense, which notifies the brain of the position and movement of body parts, and a sense of equilibrium, which monitors the position and movement of the whole body.
Pain begins when an intense stimulus activates small-diameter sensory fibers, called free nerve endings, in the skin, muscles, or internal organs. The free nerve endings carry their messages to the spinal cord, releasing a neurotransmitter called substance P. In the spinal cord, substance P causes other neurons to become activated, sending their messages through open spinal gates to the thalamus in the brain (Turk & Nash, 1993). Other areas of the brain involved in the experience of pain are the somatosensory cortex and areas in the frontal lobes and limbic system that are involved in emotion (Hunt & Mantyh, 2001; Rainville & others, 1997). When the sensory pain signals reach the brain, the sensory information is integrated with psychological information. Depending on how the brain interprets the pain experience, it regulates pain by sending signals down the spinal cord that either open or close the gates. If, because of psychological factors, the brain signals the gates to open, pain is experienced or intensified. If the brain signals the gates to close, pain is reduced.
Anxiety, fear, and a sense of helplessness are some of the psychological factors that can intensify the experience of pain. Positive emotions, laughter, distraction, and a sense of control can reduce the perception of pain. The experience of pain is also influenced by social and cultural learning experiences about the meaning of pain and how people should react to pain (Turk, 1994; Turk & Rudy, 1992). Psychological factors also influence the release of endorphins, the body’s natural painkillers that are produced in many parts of the brain and the body. Endorphins are released as part of the brain’s overall response to physical pain or stress. In the brain, endorphins can inhibit the transmission of pain signals. In the spinal cord, endorphins inhibit the release of substance P. Finally, a person’s mental or emotional state can influence other bodily processes that affect the experience of pain. Muscle tension, psychological arousal, and rapid heart rate can all produce or intensify pain (Turk & Nash, 1993). Today, a variety of techniques and procedures can effectively eliminate or reduce pain.
Movement, Position, and Balance
The phone rings. Without looking up from your textbook, you reach for the receiver, pick it up, and guide it to the side of your head. You have just demonstrated your kinesthetic sense—the sense that involves the location and position of body parts in relation to one another. (The word
kinesthetics literally means “feelings of motion.”) The kinesthetic sense involves specialized sensory neurons, called proprioceptors, which are located in the muscles and joints. The proprioceptors constantly communicate information to the brain about changes in body position and muscle tension. Closely related to the kinesthetic sense is the vestibular sense, which provides a sense of balance, or equilibrium, by responding to changes in gravity, motion, and body position. The two sources of vestibular sensory information, the semicircular canals and the vestibular sacs, are both located in the ear. These structures are filled with fluid and lined with hairlike receptor cells that shift in response to motion, changes in body position, or changes in gravity. When you experience environmental motion, like the rocking of a boat in choppy water, the fluids in the semicircular canals and the vestibular sacs are affected. Changes in your body’s position, such as falling backward in a heroic attempt to return a volleyball serve, also affect the fluids. Your vestibular sense supplies the critical information that allows you to compensate for such changes and quickly reestablish your sense of balance.
Maintaining equilibrium also involves information from other senses, particularly vision. Under normal circumstances, this works to our advantage. However, when information from the eyes conflicts with information from the vestibular system, the result can be dizziness, disorientation, and nausea. These are the symptoms commonly experienced in motion sickness, the bane of many travelers in cars, on planes, on boats, and even in space. One strategy that can be used to combat motion sickness is to minimize sensory conflicts by focusing on a distant point or an object that is fixed, such as the horizon.
