VIEWS: 76 PAGES: 19 POSTED ON: 5/21/2010
JOHNSON COUNTY COMMUNITY COLLEGE Human Physiology Ateegh Al-Arabi, Ph.D. Sensations A. Characteristics of Sensations For a sensation to occur, four prerequisites must be met: (1) a stimulus, or changing input of energy in the environment, capable of initiating a response by the nervous system, must be present; (2) a receptor or sense organ must receive the stimulus and convert it to a nervous impulse; (3) the impulse must be conducted along a nervous pathway from the receptor or sense organ to the brain; (4) a region of the brain must translate the impulse into a sensation. A sense receptor or sense organ may be viewed as specialized nervous tissue that exhibits a high degree of sensitivity to specific internal or external conditions. A receptor might be very simple, such as the dendrites of a single neuron that detect pain in the skin, or it may be a complex organ, such as the eye, that contains highly specialized neurons, epithelium, and connective tissues. All sense receptors contain the dendrites of sensory neurons, exhibit a very high degree of excitability, and possess a low threshold stimulus. Furthermore, the majority of sensory impulses are conducted to the sensory areas of the cerebral cortex, for it is only in this region of the body that a stimulus can produce conscious feeling. We see with our eyes, hear with our ears, and feel pain in an injured part of our body only because the cortex interprets the sensation as coming from the stimulated sense receptor. One of the characteristics of sensations, that of projection, describes the process by which the brain refers sensations to their point of learned origin of the stimulation. A second characteristic of many sensations is adaptation, that is, the disappearance of a sensation even though a stimulus is still being applied. For example, when you first get into a tub of hot water, you might feel an intense burning sensation. But after a brief period of time the sensation decreases to one of comfortable warmth, even though the stimulus (hot water) is still present. Another characteristic is after images, that is the persistence of a sensation after the stimulus has been removed. One common example of after image occurs when you look at a bright light and then look away. You will still see the light for several seconds afterward. The fourth characteristic of sensations is modality, that is, the possession of distinct properties by which one sensation may be distinguished from another. For example, pain, pressure, touch, body position, equilibrium, hearing, vision, smell and taste are all distinctive because the body perceives each differently. B. Classification of Sensations One convenient method of classifying sensations is to categorize them according to the location of the receptor. On this basis, receptors may be classified as exteroceptors, visceroceptors, and proprioceptors. Exteroceptors, which are located near the surface of the body, provide information about the external environment. They receive stimuli from outside the body and transmit sensations of hearing, sight, touch, pressure, temperature and pain on the skin. Visceroceptors, which are located in blood vessels and viscera, provide information about the internal environment. This information arises from within the body and may be felt as pain, taste, fatigue, hunger, thirst and nausea. Proprioceptors, which are located in muscles, tendons, and joints, allow us to feel sensations of position, movement, equilibrium and tension of muscles and joints through the stretching or movement of parts where these receptors are located. C. Cutaneous Sensations Touch, pressure, cold, heat and pain are known as the cutaneous sensations. The receptors for these sensations are located in skin, connective tissue, and the ends of the gastrointestinal tract. Inasmuch as the sensation of pain is not limited to cutaneous receptors, we will consider pain under a separate heading (Section D). The cutaneous receptors are not randomly distributed over the body surface; some parts of the skin are densely populated with receptors and other parts contain only a few, widely separated ones. Areas of the body that have few cutaneous receptors are relatively insensitive, whereas those regions that contain large numbers of cutaneous receptors are quite sensitive. This difference can be demonstrated by the two-point discrimination test for touch. In the following tests, students can work in pairs, with one acting as subject and the other as experimenter. The subject will keep his or her eyes closed during the experiments. 1. Two-Point Discrimination Test In this test, the two points of a measuring compass are applied to the skin and the distance in millimeters between the two points is varied. The subject indicates when he or she feels two points and when he or she feels only one. The compass can be placed on the tip of the tongue, an area where receptors are very densely packed. The distance between the two points can then be narrowed to 1.4 mm. At this distance, the points are able to stimulate two different receptors, and the subject feels that he or she is being touched by two objects. If, however, the distance is decreased to less than 1.4 mm, he or she feels only one point, even though both points are touching the tongue. Only one point is felt because the points are so close together that they reach only one receptor. The compass can then be placed on the back of the neck, where receptors are relatively few and far between. Here the subject feels two distinctly different points only if the distance between them is 36 mm or more. The two-point discrimination test shows that the more sensitive the area, the closer the compass points can be placed and still be felt separately. The following order, from greatest to least sensitivity, has been established from the test: tip of tongue, tip of finger, side of nose, back of hand, and back of neck. Test these areas and record your results. Part of Body Least Distance at Which Two Points Can Be Detected Tip of tongue ______________________________________________ Tip of finger ______________________________________________ Side of nose ______________________________________________ Part of Body Distance Between Points Touched by Chalk Palm of Hand ______________________________________________ Arm ______________________________________________ Forearm _____________________________________________ Back of neck ______________________________________________ IDENTIFYING COLD AND HEAT RECEPTORS The cutaneous receptors for the sensation of cold (cold spots) are widely distributed in the dermis and subcutaneous connective tissue, and are also located in the cornea of the eye, tip of the tongue, and external genitals. The cutaneous receptors for heat (heat spots) are deeply embedded in the dermis and are less abundant than cold receptors. Draw a 1 inch square on the back of the wrist. Place a forceps or other metal probe in ice-cold water for a minute, dry it quickly and, with the dull point, explore the area in the square for presence of cold spots. Keep the probe cold and mark with ink the position of each spot that you find. Mark each corresponding place in Square 2 with the letter “c.” Immerse the forceps in hot water so that it will give a sensation of warmth when removed and applied to the skin, but avoid having it too hot. Proceeding as before, locate the position of the warm spots in the same area of the skin. Mark these spots with ink of a different color, and then mark each corresponding place in Square 2 with the letter “h.” Repeat the entire procedure, using both cold forceps and warm forceps on the back of the hand and the palm of the hand, respectively, and mark Squares 3 and 4 as you did Square 2. Square 2. Cold and heat, Square 3. Cold and heat, Square 4. Cold and heat, back of wrist. back of hand. palm of hand. D. Pain Sensations The receptors for pain are simply the branching ends of the dendrites of certain sensory neurons. Pain receptors are found in practically every tissue of the body and adapt only slightly or not at all. They may be stimulated by any type of stimulus. Excessive stimulation of any sense organ causes pain. For example, when stimuli for other sensations such as touch, pressure, heat and cold reach a certain threshold, they stimulate pain receptors as well. Pain receptors, because of their sensitivity to all stimuli, have a general protective function of informing us of changes that could be potentially dangerous to health or life. Adaptation to pain does not readily occur. This lack of adaptation is important, because pain indicates disorder or disease. If we became used to it and ignored it, irreparable damage could result. Using the same 1 inch square of the forearm that was previously used for the touch test in Section C-2, perform the following experiment. Apply a piece of absorbent cotton soaked with water to the area of the forearm for 5 minutes to soften the skin. Add water to the cotton as needed. Place the point of a needle to the surface of the skin and press enough to produce a sensation of pain. Explore the marked area systematically. Using dots, mark the places in Square 5 that correspond to the points that give pain sensation when stimulated. Distinguish between sensations of pain and touch. Are the areas for touch and pain identical? ___________________________________________________________________________ Part of Body Least Distance at Which Two Points Can Be Detected Back of hand ______________________________________________ Back of neck ______________________________________________ 2. Identifying Touch Receptors Cutaneous receptors for touch include Meissner’s corpuscles, Merkel’s discs, and root hair plexuses. Meissner’s corpuscles (see Figure 5-1) and Merkel’s discs are most numerous in fingertips, palms of hand, and soles of feet. They are also abundant in eyelids, tip of tongue, lips, nipples, clitoris and tip of penis. Root hair plexuses are dendrites arranged in networks around the roots of hairs. Touch and pain receptors are variously distributed throughout the surface of the skin. These receptors are very small, but they can be located by using various techniques. Using a colored felt marking pen, draw a 1 inch square on the back of the forearm and divide the square into 16 smaller squares. With the subject’s eyes closed, press a Von Frey’s hair or a bristle against the skin, just enough to cause the hair to bend, once in each of the 16 squares. The pressure should be applied in the same manner each time. The subject should indicate when he or she experiences the sensation of touch, and the experimenter would make dots in Square 1 at the places corresponding to the points at which the subject feels the sensations. The subject and the experimenter switch roles and repeat the test. Square 1. Touch, forearm. The pair of students working as a team should examine their 1 inch squares after the test is done. They should compare the number of positive and negative responses in each of the 16 small squares, and see how uniformly the touch receptors are distributed throughout the entire 1 inch square. Other general areas used for locating touch and pain receptors are the arm and the back of the hand. 3. Identifying Pressure Receptors Sensations of pressure are longer lasting and have less variation in intensity than do sensations of touch. Moreover, whereas touch is felt in a small “pinprick” area, pressure is felt over a much larger area. The pressure receptors are called Pacinian corpuscles and are found in the deep subcutaneous tissues that lie under mucous membranes, in serous membranes of the abdominal cavity, around joints and tendons, and in some viscera. The experimenter touches the skin of the subject (whose eyes are closed) with a point of a piece of colored chalk. With eyes still closed, the subject then tries to touch the same spot with a piece of differently colored chalk. The distance between the two points is then measured. Proceed using various parts of the body, such as palm of hand, arm, forearm, and back of neck. Record your results. At the end of the test, compare your squares as you did in Section C-2. Square 5. Pain, forearm. Perform the following test to demonstrate the phenomenon of referred pain. Place your elbow in a large shallow pan of ice water, and note the progression of sensation that you experience. At first, you will feel some discomfort in the region of the elbow. Later, pain sensations will be felt elsewhere. Where do you feel the referred pain? _______________________________________________________________________ E. Proprioceptive Sensations An awareness of the activities of muscles, tendons, and joints is provided by the proprioceptive, or kinesthetic, sense. It informs us of the degree to which tendons are tensed and muscles are contracted. The proprioceptive sense enables us to recognize the location and rate of movement of one part of the body in relation to other parts. It also allows us to estimate weight and to determine the muscular work necessary to perform a task. With the proprioceptive sense, we can judge the position and movements of our limbs without using our eyes when we walk, type, play a musical instrument, or dress in the dark. Proprioceptive receptors are located in muscles, tendons, and joints, and in the connective tissue that surrounds muscle fibers. Proprioceptors adapt only slightly. This slight adaptation is beneficial because the brain must be appraised of the status of different parts of the body at all times so that adjustments can be made to ensure coordination. 1. Face a blackboard close enough so that you can easily reach to mark it. Mark a small “X” on the board in front of you and keep the chalk on the X for a moment. Now close your eyes, raise your right hand above your head and then, with your eyes still closed, mark a dot as near as possible to the X. Repeat the procedure by placing your chalk on the X, closing your eyes, raising your arm above your head, and then marking another dot as close as possible to the X. Repeat the procedure a third time. Record your results by estimating or measuring how far you missed the X for each trial. first trial ________________ second trial _______________ third trial _____________ 2. Write the word “physiology” on the left line that follows. Now, with your eyes closed, write the same word immediately to the right. How do the two samples of writing compare? ___________________________________ ___________________________________ Explain your results: ______________________________________________________ ________________________________________________________________________ 3. The following experiments demonstrate that the kinesthetic sensations make it possible to repeat more easily certain acts involving muscular coordination. Students will work in pairs for these experiments. a. The experimenter asks the subject to carry out certain movements with his or her eyes closed, for example, pointing to the middle finger of the subject’s left hand with the index finger of the subject’s right hand. b. With his or her eyes closed, the subject extends his or her right arm as far as possible behind the body, and then brings the index finger quickly to the tip of his or her nose. How accurate is the subject in doing this? __________________________________ c. Ask the subject, with eyes shut, to touch the named fingers of one hand with the index finger of the other hand. How well does the subject carry out the directions? _____________________________________________________________________ F. Olfactory Sensations 1. Olfactory Receptors The receptors for the olfactory sense are found in the nasal epithelium in the superior portion of the nasal cavity on either side of the nasal septum. The nasal epithelium consists of two principal kinds of cells. The supporting cells are columnar epithelial cells of the mucous membrane that lines the nose. The olfactory cells are bipolar neurons. Their cell bodies lie between the supporting cells. The distal (free) end of each olfactory cell contains six to eight dendrites, called olfactory hairs. The unmyelinated axons of the olfactory cells unite to form the olfactory nerves, which pass through foramina in the cribriform plate of the ethmoid bone. The olfactory nerves terminate in paired masses of gray matter, the olfactory bulbs. They lie beneath the frontal lobes of the cerebrum on either side of the crista galli of the ethmoid bone. The first synapse of the olfactory neural pathway occurs in the olfactory bulbs between the axons of the olfactory nerves and the dendrites of neurons inside the olfactory bulbs. Axons of these neurons run posteriorly to form the olfactory tract. From here, impulses are conveyed to the olfactory area of the cerebral cortex. In the cortex, the impulses are interpreted as odor and give rise to the sensation of smell. Adaptation happens quickly, especially the adaptation to odors. For this reason, we become accustomed to some odors and are also able to endure unpleasant ones. Rapid adaptation also accounts for the failure of a person to detect gas that accumulates slowly in a room. 2. Olfactory Adaptation The subject should close his or her eyes after plugging one of his or her nostrils with cotton. Hold a bottle of oil of cloves, or other substance having a distinct odor, under the open nostril. The subject breathes in through the open nostril, and exhales through the mouth. Note the time required for the odor to disappear, and repeat with the other nostril. As soon as olfactory adaptation has occurred, test an entirely different substance. Compare results for the various materials tested. Olfactory stimuli, such as pepper, onions, ammonia, ether, and chloroform, are irritating and may cause tearing because they stimulate the receptors of the trigeminal nerve as well as the olfactory neurons. G. Gustatory Sensations 1. Gustatory Receptors The receptors for gustatory sensations, or sensations of taste, are located in the taste buds. Although taste buds are most numerous on the tongue, they are also found on the soft palate and in the pharynx. Taste buds are oval bodies consisting of two kinds of cells. The supporting cells are a specialized epithelium that forms a capsule. Inside each capsule are 4 to 20 gustatory cells. Each gustatory cell contains a hairlike process (gustatory hair) that projects to the surface through an opening in the taste bud called the taste pore. Gustatory cells make contact with taste stimuli through the taste pore. Taste buds are located in some connective tissue elevations on the tongue called papillae. They give the upper surface of the tongue its rough texture and appearance. Circumvallate papillae are circular and form an inverted V-shaped row at the posterior portion of the tongue. Fungiform papillae are knoblike elevations found primarily on the tip and sides of the tongue. All circumvallate and most fungiform papillae contain taste buds. Filiform papillae are threadlike structures that cover the anterior two-thirds of the tongue. Have your partner protrude his or her tongue and examine its surface to identify the shape and position of the papillae. 2. Identifying Taste Zones For gustory cells to be stimulated, the substances we taste must be in solution in the saliva so that they can enter the taste pores in the taste buds. Despite the many substances we taste, there are basically only four taste sensations: sour, salt, bitter and sweet. Each taste is due to a different response to different chemicals. Certain regions of the tongue react more strongly than other regions to particular taste sensations. To identify the taste zones for the four taste sensations, perform the following steps: 1. The subject thoroughly dries his or her tongue (use absorbent cotton). The experimenter places some granulated sugar on the tip of the tongue and notes the time. The subject indicates when he or she tastes sugar by raising his or her hand. The experimenter notes the time again and records how long it takes for the subject to taste the sugar. 2. Repeat the experiment, but this time use a drop of sugar solution. Again record how long it takes for the subject to taste the sugar. How do you explain the difference in time periods? 3. The subject rinses his or her mouth again. The experiment is then repeated using the quinine solution (bitter taste), and then the salt solution. 4. After rinsing yet again, the experiment is repeated using the acetic acid solution or vinegar placed on the tip and sides of the tongue. Record the results in Table 10-1 by inserting a “+” (taste detected) or a “–” (taste not detected) where appropriate. TABLE 10-1. AREAS OF TONGUE IN WHICH BASIC TASTES ARE DETECTED SOUR SALT BITTER SWEET Tip of Tongue Back of Tongue Sides of Tongue 3. Testing for Visual Activity The acuteness of vision may be tested by means of a Snellen Chart. It consists of letters of different sizes which are read at a distance normally designated at 20 feet. If the subject reads to the line that is marked “50,” he or she is said to possess 20/50 vision in that eye, meaning that he or she is reading at 20 feet what a person who has normal vision can read at 50 feet. If he or she reads to the line marked “20,” he or she has 20/20 vision in that eye. The normal eye can sufficiently refract light rays from an object 20 feet away to focus a clear object on the retina. Therefore, if you have 20/20 vision, your eyes are perfectly normal. The higher the bottom number, the larger the letter must be for you to see it clearly, and of course, the worse or weaker your eyes are. Have the subject stand 20 feet from the Snellen Chart and cover the right eye with a 3” X 5” card. Instruct the subject to slowly read down the chart until he or she can no longer focus the letters. Record the number of the last line (20/20, 20/30, or whichever) that can be successfully read. Repeat this procedure covering the left eye. Now the subject should read the chart using both eyes. Record your results and change places. Visual acuity, left eye ___________________________________ Visual acuity, right eye __________________________________ Visual acuity, both eyes __________________________________ 5. Testing for the Blind Spot Hold this page about 20 inches from your face with the cross in the following diagram directly in front of your right eye. You should be able to see the cross and the circle when you close your left eye. Now, keeping the left eye closed, slowly bring the page closer to your face while fixing the right eye on the cross. At a certain distance the circle will disappear from your fiend of vision because its image falls on the blind spot. 6. Image Formation The formation of an image on the retina requires four basic processes, all concerned with focusing light rays. These basic processes are: (1) refraction of light rays, (2) accommodation of the lens, (3) construction of the pupil, and (4) convergence of the eyes. The lens of the eye has the unique ability to change the focusing power or the eye by becoming moderately curved at one moment and greatly curved the next. When the eye is focusing on a close object, the lens curves greatly in order to bend the rays toward the central fovea of the eye. This increase in the curvature of the lens is called accommodation. a. Testing for Near-Point Accommodation The following test determines your near-point accommodation. Using any card that has a letter printed upon it, close one eye and focus on the letter. Measure the distance of the card from the eye using a ruler or a meter stick. Now slowly bring the card as close to your open eye as possible, and stop when you no longer see a clear, detailed letter. Measure and record this distance. This value is your near-point accommodation. Repeat this procedure three times and then test your other eye. Check Table 10-3 to see whether the near point for your eyes corresponds with that recorded for your age group. (Note: Use a letter that is the size of typical newsprint.) TABLE 10-3. CORRELATION OF AGE AND NEAR-POINT ACCOMMODATION AGE INCHES CENTIMETERS 10 2.95 7.5 20 3.54 9.0 30 4.53 11.5 40 6.77 17.2 50 20.67 52.5 60 32.80 83.3 I. Auditory Sensations and Equilibrium Tests for Hearing Impairment To test for hearing impairment, the subject places a cotton plug in one ear and closes the eyes. The student partner then holds a watch next to the other ear and slowly moves it away. The subject indicates when he or she can no longer hear the watch. Measure and record this distance. Repeat this test three times and calculate the average of the distances. The other ear is tested in a similar manner. Compare your results with those of your partner. Left ear (1) ________ (2) ________ (3) ________ Average ________ Right ear (1) ________ (2) ________ (3) ________ Average ________ The inner ear is located within the temporal bone of the cranium. Therefore, if the stimulus has a high enough intensity, any bone of the cranium can conduct sound to the cochlea. Strike a tuning fork with a rubber mallet and place the vibrating fork upon the following bones: (1) temporal, (2) parietal, (3) frontal, (4) occipital. Keep the vibrating fork on these bones until you can barely hear it, and then put the fork next to your ear. Notice whether sound is conducted better in bone, or in air. 3. Equilibrium Apparatus The term equilibrium has two meanings. One kind of equilibrium, called static equilibrium, refers to the orientation of the body (mainly the head) relative to the ground. The second kind of equilibrium, called dynamic equilibrium, is the maintenance of the position of the body (mainly the head) in response to sudden movements or to a change in the rate or direction of movement. The receptor organs for equilibrium are the saccule, utricle, and semicircular ducts. The utricle and saccule each contain within their walls sensory hair cells that project into the cavity of the membranous labyrinth. The hairs are coated with a gelatinous layer in which particles of calcium carbonate, called otoliths, are embedded. When the head tips downward, the otoliths slide with gravity in the direction of the ground. As the particles move, they exert a downward pull on the gelatinous mass which, in turn, exerts downward pull on the hairs and makes them bend. The movement of the hairs stimulates the dendrites at the base of their hair cells. The three semicircular ducts are positioned at right angles to each other in three planes– frontal (the superior duct), sagittal (the posterior duct), and lateral (the lateral duct). This positioning permits correction of an imbalance in three planes. In the ampulla, the dilated portion of each duct, there is a small elevation called the crista. Each crista is composed of a group of hair cells covered by a mass of gelatinous material called the cupula. When the head moves, the endolymph in the semicircular ducts flows over the hairs and bends them as water in a stream bends the plant life growing at its bottom. The movement of the hairs stimulates sensory neurons, and the impulses pass over the vestibular branch of the vestibulocochlear nerve. The impulses then reach the temporal lobe of the cerebrum before they are sent to the muscles that must contract to maintain body balance in the new position. 4. Tests for Equilibrium You can test equilibrium by using a few simple procedures. Test balance by having the subject stand perfectly still with his or her hands at his or her sides and the feet close together. Note any swaying movements. It might be helpful if the subject stands in front of a light, so that swaying movements can be detected by observing the subject’s shadow. Now have the subject repeat this test with the eyes closed. Note any swaying movements. The next test serves to evaluate the semicircular canals. The subject sits on a stool, legs up on the stool rung, and the stool is revolved for a few seconds and suddenly stopped. The subject will experience the sensation that the stool is still rotating, which means that the semicircular canals are functioning properly. A cold test also serves to evaluate the semicircular canals. When a cold swab is placed in one ear, it increases the density of the endolymph of the semicircular canal adjacent to that ear. This increase in the density of the endolymph stimulates the hair cells within the semicircular canals, causing a sensation of rotation called nystagmus. Place a cotton swab in an ice bath for several minutes and then carefully insert it in one of your ears, noting the results. The Pain Sensation Pain is defined as the unpleasant sensory and emotional experience associated with either an actual or potential damage to the tissue. It makes the person aware of the danger and forces him to respond to this danger. This means it always leads to a protective reflex such as scratch, withdrawal, or abdominal guarding reflexes. The Purpose of Pain Sensation: 1. Warning of a threat (usually leads to withdrawal reflex). 2. Basis for learning not to touch or get near dangerous objects. 3. Forces the person to rest the whole body or part of the body which gives chance to the defense mechanism to contain and repair the damage. Absence of Pain in Some Individuals: It has been described that some individuals lack the pain sensations. Although these individuals did not exhibit any anatomical abnormalities, the cause of this was obvious in some cases. The absence of pain sensation in these cases was either familial or a result of acquired disease which results in the destruction of central mechanisms subserving pain sensation. People with this kind of abnormality lack the learning ability to recognize danger and consequently they lack some protective reflexes. Therefore, cutting and burning of the skin takes place without the subject being aware, which usually results in chronic skin ulceration, tissue damage and infection, and damage to the joints. When the joint surfaces become damaged and ligaments stretched, the joint becomes unstable and unable to carry weight; this condition is known as Charcot joint. In this group of people, acute appendicitis is usually unnoticed until peritonitis has developed and becomes a threat to life. Types of Pain: Based on the speed of pain onset after the appearance of the stimulus, physiologists as well as clinicians classify pain sensations into two major types. 1. Fast (Acute) Pain: This type of pain occurs very rapidly (within 0.1 second) after the stimulus is applied and is felt only in the superficial layers of the skin. This type of pain is also known as sharp, pricking, and electric pain. Acute pain can be elicited only by thermal or mechanical stimuli and transmitted by A fibers. 2. Slow (Chronic) Pain: This pain begins after a second or more and then gradually increases in intensity over a period of several seconds or minutes. It can occur both in the skin and deeper tissues or in the viscera. Chronic pain is also known as burning, throbbing and aching pain (e.g., toothache, stomach ache and headaches). It can be due to chemicals, mechanical or thermal stimuli and transmitted by unmyelinated C fibers. Pain Receptors: All pain receptors are free nerve endings. They are non-adapting; instead they can become hypersensitive as the stimulus continues. This condition is called hyperalgesia. Pain receptors are widespread in the superficial layers of the skin and also in certain areas of the internal tissues (e.g., periosteum, the atrial walls, the joint surfaces, and the falx and tentorium of the cranial vault). Most of the other internal tissues are weakly supplied. Stimuli Which Elicit Pain: Tissue injury is the only cause of pain, but the potential for producing pain depends on the nature of the damaging energy. 1. Infection and mechanical damage (very painful) 2. Ultraviolet, X-ray, and malignant disease (no pain until irremediable damage has occurred) 3. In some conditions, pain occurs in the absence of any detectable tissue damage Generally three types of stimuli: mechanical, thermal and chemical that cause pain in the pain receptors. Some of the Chemical Substances that Excite the Chemical Type of Pain Receptors: 1. Bradykinin 2. Serotonin 3. Histamine 4. Potassium ions 5. Acids 6. Acetylcholine 7. Proteolytic enzymes While bradykinin is the most painful one, Prostaglandins do not cause pain, instead they enhance the sensitivity of the pain receptors (by facilitation). Bradykinin not only excites the chemosensitive receptors but also greatly reduces the threshold of the others (mechanosensitive and thermosensitive). Other chemicals do not show this effect. Ischemic Muscle Pain: When a pressure is exerted on the skeletal muscle, pain sensation starts to appear (it appears faster if exercise was applied). What is the cause of this pain? There are more than one cause: 1. Mechanical stimulus because of the pressure which excites the mechanosenstive receptors. 2. Accumulation of large amounts of lactic acid. This is because the oxygen supply to the muscle was reduced to almost zero and as a result the muscle switches its metabolic activity from aerobic to anaerobic. 3. Other chemicals such as bradykinin (as a result of lactic acid release), proteolytic enzymes, potassium ion, etc., which are released due to cell damage. Pain Due to Muscle Spasm: Muscle spasm usually leads to an ischemic muscle because the stretch of the muscle puts some pressure on the blood vessels and causes their constriction which diminishes the blood supply and causes lack of oxygen. The stretch effect also excites the mechanosensitive pain receptors. Peripheral Pain Fibers: Two types of nerve fibers that are involved in transmitting pain: fast pain fibers (A fibers 6 to 30 m/sec.) and slow pain fibers (C 0.5 to 2 m/sec.). Blocking the A fibers by moderate compression of the nerve trunk without blocking the C fibers results in the disappearance of the acute pain while blocking of the C fibers by means of low concentration of anesthesia without blocking the A fibers results in the disappearance of the chronic pain. This means while both pain modalities (acute and chronic) start to appear at two different times they can be transmitted simultaneously. The fibers, after they enter the spinal cord, will either ascend or descend one to three segments in the Lissaure’s tract and terminate on neurons in the dorsal horns. Then the signals will be transmitted to the brain through two different processing systems (the neospinothalamic tract and paleospinothalamic tract). Pain Pathways: In the spinal cord pain signals are transmitted through two different pathways to the brain. Neospinothalamic pathways for transmitting fast (acute) pain and the paleospinothalamic pathway for transmitting slow (chronic) pain. The Neospinothalamic Pathway: This pathway transmits the sharp pain signals (caused by mechanical or thermal stimuli) carried by the peripheral A fibers with a conduction velocity between 6 and 30 m/sec. These fibers terminate mainly in lamina I (lamina marginalis) of the dorsal horn where they excite second order neurons of the neospinothalamic tract. The second order neurons make up the tract and immediately cross to the opposite side of the spinal cord through the anterior commissure and ascend to the brain in the anterolateral columns. Some of these fibers terminate in the reticular areas of the brain stem but most of them terminate in the ventrobasal complex of the thalamus. The Paleospinothalamic Pathway: The paleospinothalamic pathway is much more understood and is known to transmit chronic pain signals carried mainly by the peripheral C fibers, 0.5 to 2 m/sec. (also some A fibers). The peripheral fibers terminate mainly in lamina II and III (together called substantia gelatinosa). The signals are passed to short local fibers and in turn these fibers extend their axons to lamina V. From lamina V, neurons give rise to long axons joining the axons of the neospinothalamic pathway by passing to the opposite side through the anterior commissure (few of these fibers do not cross, instead they ascend ipsilaterally) and ascend in the anterolateral columns. In contrast to the neospinothalamic pathway fibers, only few (10- 25%) fibers of the paleospinothalamic pathways terminate in the thalamus; instead they terminate mainly in one of the following areas of the brain stem: 1. The reticular nuclei of the medulla, pons, and mesoencephalon. 2. The tectal area of the mesoencephalon deep to the superior and inferior colliculi. 3. The periaqueductal gray region surrounding the aqueduct of Sylvius. From there pain signals are relayed by short neurons through the interlaminar nuclei of the thalamus and also into certain regions of the hypothalamus and adjacent region of the basal brain upward to the sensory cortex. Substance P is believed to be the neurotransmitter for the C fibers. • Processing of pain signals in general is done by the thalamus and lower centers of the brain, while the cerebral cortex is responsible for the quality of pain. • Localization of pain in general is poor, but it is better in the case of the neospinothalamic pathway. It would be easier to pinpoint the pain source if the tactile receptors were stimulated. Pain Relief: There are many methods used to relieve pain. Here are some of them: 1. Pharmacological Method: The use of drugs to relieve pain is well known. These drugs are usually in the form of antagonists to the pain transmitters or to excite the natural analgesia system (explained later). 2. Surgical Method: Surgery is also used to interrupt one part or another of the pain pathways in patients with terminal diseases such as cancer. This is done either by: Cordotomy: The spinal cord is sectioned contralaterally (in the upper thoracic region) almost entirely through its anterolateral quadrant which relieves pain for a few weeks to a few months. This is not always successful either because some fibers ascend ipsilaterally and do not cross the spinal cord until they reach the brain or because of the development of other pain pathways (after several months) partly caused by the fibrous tissue stimulating other pain fibers. OR Lesioning the Interlaminar Nuclei in the Thalamus: This procedure often helps in the relief of chronic pain, while it does not get rid of the acute pain. The reason for the appearance of the acute pain is because the processing of the signals carried by the A fibers takes place mainly in the spinal cord. 3. Alternative Medicine: Chiropractors as well as physical therapists help to relieve the pain by working on the reason for having pain signals as well as stimulating the natural analgesia system. 4. Natural Analgesia System: The reason that people react differently to pain is the ability of the brain itself to control the input of pain sensations by activating a pain control system called the analgesia system. The analgesia system is spread between the brain and the spinal cord and composed of three major components (and some other components). a. The Periaqueductal Gray Area: This is found in the mesoencephalon and upper pons surrounding the aqueduct of Sylvius. Axons of the neurons in this area descend to terminate in the second component where they release the neurotransmitter enkephalin. b. The Raphe Magnus Nucleus: It is a thin midline nucleus located in the lower pons and the upper medulla. When the nerve terminal of the periaqueductal gray area releases enkephalins in this nucleus, it leads to the excitation of serotonergic neurons in this area. Axons of these neurons descend all the way to the dorsal horn of the spinal cord where they release serotonin, which leads to the excitation of a set of local neurons called the pain inhibitory complex. c. The Pain Inhibitory Complex: This is composed of a special set of neurons located in the dorsal horn of the spinal cord. These neurons get excited when they receive serotonin from the terminals of the serotonergic neurons of the Raphe magnus nucleus and release enkephalin. Enkephalins are believed to block the transmission of the pain signals at the terminals of the peripheral (both A and C) fibers by causing presynaptic inhibition, which blocks the calcium channels in the terminals of the presynaptic neurons, resulting in the reduction of the pain neurotransmitter release. Thus the analgesia system can block the pain signals at their initial point of entry to the spinal cord. It is also possible that this analgesia system can block pain signals at other points of the pain pathways. It is worth noticing that the analgesia system does not only block pain signals, but also blocks many of the local reflexes that result from pain signals. The Brain’s Natural Opiates There are several natural opiates (pain-relieving substances) which are produced by the human body. They are all breakdown products of three large protein molecules: a. Proopiomelanocortin b. Proenkaphalin c. Prodynophin These proteins lead to at least five important opiates: a. -endorphin–found mainly in the hypothalamus b. met-enkephalin–located in the periaqueduct and lamina I and II of the doral horn c. leu-enkephalin–located in the periaqueduct and lamina I and II of the dorsal horn d. dynorphin–least potent e. neoendorphin–newly discovered opiate All of the above opiates include in their structure a special sequence of amino acids, Tyr-Gly- Gly-Phe, which seems to be recognized by their receptors. Three different receptors were found in the nervous system; mu ( ) and delta ( ) for enkephalins and kappa ( ) for dynorphin. The receptor is more abundant in the nervous system and especially located in the periaqueductal area and the superficial layers of the dorsal horn and is used by the drug morphine. 5. Inhibition of Pain Transmission by Tactile Sensory Signals: Stimulation of large sensory fibers from the peripheral tactile receptors depresses the transmission of pain signals from the same area of the body or even from areas sometimes located many segments away; presumably, this results from a type of local lateral inhibition. This can be done by mechanical stimulus such as rubbing (massage), acupuncture, etc., or by chemical stimuli such liniments which are often useful in the relief of pain. These methods are usually accompanied by a simultaneous psychogenic excitation of the natural analgesia system. 6. Treatment of Pain by Electrical Stimulation: This method is also based on the stimulation of large sensory neurons. Electrodes are placed in selected areas of the skin (occasionally implanted over the spinal cord to stimulate the dorsal sensory columns). In few cases the electrodes are implanted stereotaxically in the interlaminar nuclei of the thalamus or in the periventricular or periaqueductal areas of the mesoencephalon. With a few minutes of stimulation of these areas (controlled by the patient), pain relief usually lasts for as long as 24 hours. Electrical stimulation of the periaqueductal area or the Raphe magnus nucleus can almost completely suppress many very strong pain signals entering by way of the dorsal spinal roots. Also stimulation of areas at still higher levels of the brain that in turn excites the periaqueductal area (especially the periventricular nuclei in the hypothalamus and to a lesser extent the medial forebrain bundle also in the hypothalamus) can suppress pain. Referred Pain: This type of pain is produced in one part (visceral) of the body but is referred to another part (could be on the skin or another part). This is due to the fact that branches of visceral pain fibers are shown to synapse in the spinal cord with some of the second order neurons that receive pain from the skin. This pain is usually a true visceral pain. True Visceral Pain: The true visceral pain is the pain generated due to a damage to the visceral organs (the viscera, and not all, has only pain receptors, i.e., no tactile receptors). The main difference between true visceral pain and surface pain is that while highly localized damage to the viscera rarely cause severe pain, any stimulus that causes diffuse stimulation of pain nerve endings throughout a viscera causes pain that can be extremely severe. Causes of True Visceral Pain: a. Ischemia: Pain in this case results from the production of acids (due to the switch from aerobic to anaerobic metabolic activity of the muscle), bradykinin (due to the presence of acids), proteolytic enzymes (due to damage of the tissue). b. Chemical stimuli: This usually appears in the case of gastric or duodenal ulcer where damaging substances leak from the gastrointestinal tract into the peritoneal cavity (the pain as a result of proteolytic acidic gastric juice). c. Spasm of the smooth of a hollow viscus (cramp): This may take place in the gut, gallbladder, bile duct, or the ureter, also in the case of gastroenteritis or constipation. Pain in this case is due to mechanical stimulation of the pain nerve endings or diminished blood flow in the muscle which results in a relative ischemia. This type of pain usually appears in rhythmic cycles due to the rhythmic contractions. d. Overdistension of a hollow viscus: The pain in this case is due to either stimulation of the mechanosensitive pain receptors, as a result of the stretch, or ischemia as a result of the collapse of the blood vessels. Some Insensitive Viscera: Parenchyma of the liver and alveoli of the lungs are insensitive to pain. However, the liver capsule, the bile ducts, the bronchi, and the parietal pleura are very sensitive to pain resulting from direct trauma or due to stretch.