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									                         JOHNSON COUNTY COMMUNITY COLLEGE
                                    Human Physiology
                                  Ateegh Al-Arabi, Ph.D.

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

     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             ______________________________________________


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

   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

            ___________________________________            ___________________________________

            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
   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.


                            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.)


            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
                                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

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.


           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

     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

        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

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.

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