Ch12 Somatic and Special Senses.docx

CHAPTER 12: SOMATIC & SPECIAL SENSES OBJECTIVES: 1. Define the terms sensation and perception and differentiate between the two. 2. Name the site where most conscious sensation occurs. 3. Define the term sensory receptor and classify them according to stimulus type. 4. Explain what is meant by the term sensory adaptation, and name the only type of sensory receptor that does not undergo sensory adaptation. 5. Name the three groups of somatic senses. 6. Discuss the three types of receptors responsible for the senses of touch and pressure. 7. Distinguish between the two types of thermoreceptors. 8. Name the ultimate function of pain receptors. 9. Explain the phenomenon of referred pain. 10. Compare and contrast acute and chronic pain. 11. Explain how a rhizotomy could relieve chronic pain. 12. Compare the two types of stretch receptors. 13. Name the five special senses. 14. Define the term olfaction, name the type of sensory receptors involved in olfaction, name the location of those receptors, and identify the responsive portion of those receptors. 15. Once an olfactory chemoreceptor is stimulated, track the nerve impulse to its site of interpretation in the brain. 16. Define gustation, name the type of sensory receptors involved in gustation, name the location of those receptors, and identify the responsive portion of those receptors. 17. Sketch a tongue and locate the four different types of taste buds. 18. Once a gustatory chemoreceptor is stimulated, track the nerve impulse to its site of interpretation in the brain. 19. Name the three parts of the ear, list the specific components within each, and give a general function for the structures. 20. Describe the tympanic reflex, and explain its significance. 21. Name the opening between the middle and inner ear (i.e. the entrance to the inner ear). 22. Name the tube that connects the middle ear to the nasopharynx and explain its significance. 23. Describe the structure of the inner ear labyrinth in detail, and give a function for each of the three major portions. 24. Name the fluid that fills each compartment in the inner ear. 25. In the cochlea, give the specific names for the bony and membranous labyrinths, name the fluid that fills each, and name the membranes that separate the three chambers. 26. Describe the structure, location and function of the Organ of Corti. 27. Trace the pathway of sound from where sound waves reach the auricle to where to its interpretation site in the brain. 28. In the vestibule, give the specific names for the bony and membranous labyrinths, and name the fluid that fills each of them. 29. Describe the structure and function of the macula in the vestibule. 30. Name the enlarged portion at the end of each semi-circular canal, and describe the significance of what each canal contains. 31. Once the mechanoreceptors in the macula or crista ampullaris are stimulated, trace the nerve impulse to its site of interpretation. 32. Discuss the accessory organs of the eye in terms of their names, location, and functions. 33. Explain the function of the enzyme lysozyme. 34. Name the three tunics of the eye, give a general function for each, and name the specific components of each tunic. 36. Describe the structure and function of the cornea & sclera. 37. Name the fibers that connect the components of the ciliary body, and describe what happens to them when focusing on a close versus a distant object (i.e. describe accommodation). 38. Explain how the eye is divided into cavities and chambers, and name the fluid that fills each. 39. Name the instrument used to observe the retina of the eye. 40. Compare and contrast the two types of photoreceptors present in the retina of the eye. 41. Explain why we possess a blind spot. 42. Trace a photon of light from where it penetrates the cornea of the eye to where it's interpreted in the brain. I. RECEPTORS AND SENSATIONS A. Receptor Types = specialized structures at the end of peripheral nerves that respond to stimuli; can be classified according to their location in the body, stimulus type and structure. 1. Classification is by Stimulus Type: a. Chemoreceptors respond to changes in chemical concentrations; b. Nociceptors respond to extreme (harmful) stimuli by producing the sensation of pain (i.e. all types under extreme stimuli). c. Thermoreceptors are sensitive to temperature change; d. Mechanoreceptors respond to a change in pressure; (i.e. touch, pressure, vibrations, stretch); e. Photoreceptors (in retina of eye) respond to light energy; B. Sensory Impulse All senses work in basically the same fashion. Special sensory receptors collect information from the environment and stimulate neurons to send a message to the brain. There the cerebral cortex forms a perception, a person's particular view of the stimulus. C. Sensations: 1. Sensation = the conscious or unconscious awareness of external or internal stimuli. 2. Perception = the conscious awareness and interpretation of sensations. 3. See Table 12.1, page 423 to distinguish between sensation & perception. D. Sensory Adaptation a. The process by which a sensory receptor becomes less stimulated following continuous stimuli. b. All sensory receptors, except nociceptors, adapt to continuous stimuli (i.e. undergo sensory adaptation). II. SOMATIC SENSES A. Introduction Receptors associated with skin, muscles, joints, and viscera provide somatic senses. B. Three groups: 1. Exteroceptive Senses: a. detect changes at the body's surface: touch pressure temperature 2. Proprioceptive Senses: a. detect changes in muscles, tendons, and body position: 3. Visceroceptive Senses: a. detect changes in viscera: only pain will be discussed here. C. Touch and Pressure Senses: See Fig 12.1, page 425. 1. Employ three types of receptors: a. free nerve endings (naked dendritic); in epithelium, CT; b. Meissner's Corpuscles are encapsulated dendritic endings; surrounded by CT wrapping; mechanoreceptors; detect light touch; abundant in the hairless portions of skin (i.e. lips, fingertips, palms, soles, nipples, external genitalia). c. Pacinian Corpuscles are also encapsulated dendritic endings: surrounded by CT wrapping; mechanoreceptors; detect heavy pressure; abundant in deep subcutaneous tissues of hands, feet, penis, clitoris, urethra, breasts. II. SOMATIC SENSES D. Temperature Senses 1. Two types that respond to temperature change; a. Heat receptors sensitive to temps above 25oC (77oF); unresponsive at temps above 45oC (113oF). * Pain receptors are also triggered as this temperature approaches producing a burning sensation. b. Cold receptors sensitive to temps between 10oC (50oF) and 20oC (68oF); below 10oC, pain receptors are triggered producing a freezing sensation. 2. Both undergo rapid sensory adaptation! E. Sense of PAIN 1. Introduction Free nerve endings are the receptors that detect pain. They are widely distributed throughout the skin and internal tissues, with the exception of the nervous tissue of the brain. 2. Pain Receptors (Nociceptors) a. function is protection against further tissue damage; b. many stimuli may trigger them (i.e. temperature, pressure, chemoreceptors); c. generally do not adapt to continual stimuli. 3. Visceral Pain: See Fig 12.2, page 426. a. only visceral receptors that produce sensations; b. stretch receptors are stimulated by pressure and/or a decrease in oxygen levels; c. may feel as if its coming from another area of the body = referred pain. may derive from common nerve pathways. See Fig 12.3, page 427. II. SOMATIC SENSES E. Sense of PAIN (continued) 4. Pain Nerve Pathways: a. Acute pain occurs rapidly (0.1 sec); is not felt in deep tissues; sharp, fast, pricking pain; conducted on myelinated fibers; ceases when stimulus is removed. Artificial OTC pain relief usually adequate b. Chronic pain begins slowly and increases in intensity over a period of several seconds or minutes; dull, aching, burning, throbbing pain; can occur anywhere; conducted on unmyelinated fibers; may continue after stimulus is removed. Natural pain relief (see below) or narcotics are needed 5. Regulation of Pain Impulses See Clinical Application 12.1, page 428, concerning cancer pain & chronic pain. a. Inappropriate pain = when pain sensations are not warning about impending tissue damage; b. Analgesics are used to reduce inappropriate pain. natural pain relief (produced by CNS) Neuropeptides 1. enkephalins 2. Serotonin 3. endorphins Inhibit pain nerve pathways in the Spinal Cord Stop pain signal from reaching brain = no perception II. SOMATIC SENSES E. Sense of PAIN (continued) 5. Regulation of Pain Impulses b. Analgesics Artificial pain relievers OTC drugs block formation of prostaglandins, which stimulate nociceptors: 1. aspirin (acetylsalicylic acid) 2. Tylenol (acetaminophen); 3. Motrin (ibuprofen) Narcotics mimic natural pain relief by blocking nerve impulses 1. morphine 2. vicodin 3. demerol c. Surgery may be necessary: Cordotomy = severing the sensory nerve; Rhizotomy = cutting of spinal posterior (sensory) nerve roots. F. Stretch Receptors 1. Introduction Stretch receptors are proprioceptors that send information to the spinal cord and brain concerning the length and tension of muscles. There are two main types: a. Muscle Spindles: See Fig 12.4a, page 429. located in skeletal muscles near their junction with tendons; This sensory receptor is stimulated when the skeletal muscle relaxes and therefore the spindle is stretched; Action produced is called the "stretch reflex"; helps maintain the desired position of a limb despite other forces tending to move it. b. Golgi tendon organs: See Fig 12.4b, page 429. found in tendons close to their muscle attachment; each is connected to a set of skeletal muscle fibers and is innervated by a sensory neuron; These receptors have a high threshold and are stimulated by increased tension; stimulate a reflex with an opposite effect as above; helps maintain posture, prevents tearing of tendons. * See Table 12.2, page 429 to summarize the different types of somatic receptors. I. Introduction A. SPECIAL SENSES are senses whose sensory receptors are located in large, complex organs in the head. B. The five special senses are 1. vision, 2. hearing, 3. equilibrium, 4. taste, and 5. smell. II. Sense of Smell = Olfaction See Fig 12.5 and Fig 12.6, page 431. A. Olfactory Receptors: 1. chemoreceptors that are located in the upper nasal cavity; a. sensitive portion is cilia-like dendrites on bipolar neurons b. chemicals must be dissolved in solution to be detected; c. undergo rapid sensory adaptation. B. Olfactory Organ = olfactory epithelium in upper nasal cavity of nose (on the superior nasal concha) C. Olfactory Nerve Pathways: 1. Primary Neuron = Olfactory receptor cell; Axons pass through cribriform plate of ethmoid; synapse in Olfactory bulb; 2. Secondary Neuron in olfactory bulb (CN I) a. Axons reach to cerebral cortex; b. do not pass through thalamus. D. Olfactory Stimulation 1. Theory is smell stimulates many receptors 2. certain combinations = specific smells 3. Sometimes “sniffing” is needed to bring odorant molecules up to olfactory epithelium 4. Olfactory receptors are continually replaced by stem cells * See blue box on page 431 concerning our keen sense of olfaction. III. Sense of Taste (Gustation) A. Organ = taste buds on tongue. See Fig 12.7, and Fig 12.8, page 433. B. Taste Receptors = 1. chemoreceptors that are located in taste buds; a. Sensitive portion is a "taste hair" which protrudes out of a "taste pore", which is an opening in a "taste cell", which makes up the "taste bud"; b. Chemicals must be dissolved in saliva to be detected; c. undergo rapid sensory adaptation; C. Taste Sensations 1. most taste buds are far posterior near the base of the tongue 2. the rest of the taste buds provide sensations based on location: a. sweet = tip of tongue, b. sour = lateral tongue, c. salt = perimeter of tongue, d. bitter = posterior tongue. 3. Taste varies from person to person D. Taste Nerve Pathways: 1. Two pathways: a. CN VII ---> anterior 2/3 of tongue; b. CN IX ----> posterior 1/3 of tongue; 2. Once chemoreceptors in these areas are stimulated, a gustatory impulse passes to the a. medulla, b. thalamus, c. gustatory cortex within parietal lobe. * See blue box on pages 432 concerning the inheritance of taste and smell. * See CA 12.3, page 435 and Table 12.3, page 434 concerning taste and smell disorders. IV. SENSE OF HEARING Intro: The organ of hearing is the Organ of Corti, which is present in the cochlea of the inner ear. The sensory receptors are called mechanoreceptors. Once these mechanoreceptors are stimulated, the impulse travels on the cochlear branch of the vestibulocochlear (CN VIII) nerve, which leads to the primary auditory cortex (temporal cortex) of the cerebrum. A. EAR STRUCTURE: See 12.9, page 435. 1. External Ear: a. Auricle = outer ear (cartilage); Function = collection of sound waves. b. External auditory meatus = ear canal; Function = starts vibrations of sound waves and directs them toward tympanic membrane. 2. Middle Ear: Function = to amplify and concentrate sound waves. a. Tympanic membrane = eardrum. * Tympanic (attenuation) Reflex = protective mechanism for hearing mechanoreceptors; Loud noises cause 2 muscles associated with the tympanic membrane to contract; This decreases amplification effect of ossicles (see below). * See blue box on page 437. b. Tympanic cavity = air-filled space behind eardrum; separates outer from inner ear. c. Auditory ossicles = 3 tiny bones in middle ear: See Fig 12.10, page 436. Malleus (hammer) is connected to tympanic membrane; Incus (anvil) connects malleus to stapes; Stapes (stirrup) connects incus to the * Oval window = the entrance to inner ear. d. Auditory (Eustachian) tube = passageway which connects middle ear to nasopharynx (throat). Function = to equalize pressure on both sides of the tympanic membrane, which is necessary for proper hearing. * See blue box on page 437 concerning otitis media. IV. SENSE OF HEARING A. EAR STRUCTURE (continued) 3. Inner Ear: See Fig 12.11, page 438. a. The inner ear consists of a complex system of intercommunicating chambers and tubes called a labyrinth. Actually, two labyrinths compose the inner ear: Osseous labyrinth = bony canal in temporal bone; Membranous labyrinth = membrane within osseous labyrinth. b. Two types of fluid fill the spaces in the labyrinths: Perilymph fills the space between the osseous and membranous labyrinth; Endolymph fills the membranous labyrinth. c. The inner ear labyrinth can further be divided into three regions (cochlea, vestibule & semi-circular canals), each with a specific function: Cochlea = snail shaped portion; Function = sense of hearing. Semi-circular canals = three rings; Function = dynamic equilibrium. Vestibule = area between cochlea and semi-circular canals; Function = static equilibrium. d. The osseous labyrinth of the cochlea can be divided into two compartments: See Fig 12.12 & 12.13, page 439–440. Scala vestibuli = upper compartment which extends from oval window to apex; Scala tympani = lower compartment which extends from apex to round window. * Both compartments are filled with perilymph. e. Between the two bony compartments, we find the membranous labyrinth = cochlear duct. The cochlear duct is filled with endolymph. IV. SENSE OF HEARING A. EAR STRUCTURE (continued) 3. Inner Ear: See Fig 12.11, page 438. f. There are membranes that separate the cochlear duct from the bony compartments: Vestibular membrane separates the cochlear duct from the scala vestibuli; Basilar membrane separates the cochlear duct from the scala tympani; g. The mechanoreceptors responsible for the sense of hearing are contained in the Organ of Corti = 16,000 hearing receptor cells located on the basilar membrane. See Fig 12.14, page 440 and Fig 12.14, page 441. The receptor cells are called "hair cells"; The hair cells are covered by the tectorial membrane, which lies over them like a roof. B. Auditory Nerve Pathways See Table 12.4 and Fig 12.16, page 442. Pathway of sound waves from outside to the Organ of Corti 1. auricle 2. external auditory meatus 3. tympanic membrane 4. malleus 5. incus 6. stapes 7. oval window 8. perilymph of scala vestibuli 9. endolymph of cochlear duct 10. hair cells in Organ of Corti. Once these mechanoreceptors are stimulated, a sensory impulse is triggered and then travels on the 11. cochlear branch of vestibulocochlear nerve (CN VIII) to the 12. thalamus for direction to the 13. primary auditory cortex (temporal lobes) of cerebrum for interpretation. * See CA 12.4, page 443 concerning Hearing Loss. V. SENSE OF EQUILIBRIUM A. Static Equilibrium functions to sense the position of the head and help us maintain posture while motionless. 1. The vestibule of the inner ear contains the two membranous chambers responsible for static equilibrium. Fig 12.17, a. The utricle communicates with the semi-circular page 444. canals; b. The saccule communicates with the cochlear duct. c. Each of these chambers contains a macula = organ of static equilibrium. Fig 12.18, pg 445 The macula is composed of "hair cells" (see Fig 12.19, page 445) that are in contact with a jelly-like fluid containing calcium carbonate crystals (otolith). When the head is moved, the gelatin sags due to gravity and the hair cells bend. This triggers a sensory impulse, which travels on the vestibular branch of the VC nerve to the pons, which directs the impulse to the cerebellum for interpretation. B. Dynamic Equilibrium functions to prevent loss of balance during rapid head or body movement. 1. The three semi-circular canals contain the organ responsible for dynamic equilibrium. Fig 12.17, a. Each semi-circular canal ends in an enlargement called the page 444. ampulla. Fig 12.20, pg. 446. b. Each ampulla houses a sensory organ for dynamic equilibrium called the crista ampullaris, which contains a patch of "hair cells" in a mass of gelatin. Fig 12.21, c. When the head is moved, the gelatin stays put due to inertia, page 446. causing the hair cells to bend. This triggers a sensory impulse, which travels on the vestibular branch of the VC nerve to the pons, which directs the impulse to the cerebellum for interpretation. * See blue box on page 446 concerning motion sickness. VI. SENSE OF SIGHT: Vision Introduction: The organ of vision is the retina of the eye. The sensory receptors are called photoreceptors. When photoreceptors are stimulated, impulses travel within the optic nerve (CN II) to the visual (occipital) cortex for interpretation A. Visual Accessory Organs: See Fig 12.23, page 448 and Fig 12.24, page 449. 1. Eyelids = protective shield for the eyeball. a. Conjunctiva= inner lining of eyelid; = red portion around eye. * See blue box on page 448 concerning pink eye. 2. Lacrimal apparatus = tear secretion & distribution. a. Lacrimal gland = tear secretion; located on upper lateral surface Tears contain an enzyme called lysozyme, which functions as an anti-bacterial agent. b. Nasolacrimal duct = duct which carries tears into nasal cavity (drainage) 3. Extrinsic muscles hold eyeball in orbital cavity and allow for eye movement. (Name the cranial nerves that innervate these muscles). a. superior rectus muscle b. inferior rectus muscle c. lateral rectus muscle d. medial rectus muscle e. inferior oblique muscle f. superior oblique muscle * See Table 12.5, page 449 for a summary of the muscles associated with the eyelids and eye. VI. SENSE OF SIGHT: Vision B. Structure of the Eye: See Fig 12.25, page 450. The eye is composed of three distinct layers or tunics: 1. The Outer Tunic (fibrous) = protection. Cornea = transparent anterior portion; Function: helps focus (75%) incoming light rays. * See blue box, page 451 on cornea transplant. Sclera = white posterior portion, which is continuous with eyeball except where the optic nerve and blood vessels pierce through it in the back of eye. Functions: 1. protection 2. attachment (of eye muscles) 2. The Middle tunic (vascular) (uvea)= nourishment... a. Choroid coat = membrane joined loosely to sclera containing many blood vessels to nourish the tissues of the eye. b. Ciliary body = anterior extension from choroid coat, which is composed of 2 parts: See Fig 12.28, page 452. Ciliary muscles which control the shape of the lens (i.e. Accommodation); Ciliary processes which are located on the periphery of the lens. 1. Suspensory ligaments extend from the ciliary processes on the lens to the ciliary muscles (i.e. they connect above structures), and function to hold the lens in place. * Accommodation = the process by which the lens Fig 12.29, page 452. changes shape to focus on close objects. 1. The lens is responsible (with cornea) for focusing incoming light rays. 2. If light rays are entering the eye from a distant object, the lens is flat. 3. When we focus on a close object, the ciliary muscles contract, relaxing the suspensory ligaments. Accordingly, the lens thickens allowing us to focus. VI. SENSE OF SIGHT: Vision B. Structure of the Eye: See Fig 12.25, page 450. 2. The Middle tunic (vascular) (uvea) Iris = colored ring around pupil; thin diaphragm muscle; lies between cornea and lens; The iris separates the anterior cavity of the eye into an anterior chamber and posterior chamber. See Fig 12.26, page 451. The entire anterior cavity is filled with aqueous humor, which helps nourish the anterior portions of the eye, and maintains the shape of the anterior eye. 3. The Inner tunic (nervous, sensory) a. Retina = inner lining of the eyeball; site of photoreceptors. . A picture of the retina can be taken with a camera attached to an ophthalmoscope as seen in Fig 12.34, page 455. The optic disc is the location on the retina where nerve fibers leave the eye & join with the optic nerve; the central artery & vein also pass through this disk. No photoreceptors are present in the area of the optic disk = blind spot. See Fig 12.34, page 455. The posterior cavity of the eye is occupied by the lens, ciliary body, and the retina. The posterior cavity is filled with vitreous humor, which is a jelly-like fluid, which maintains the spherical shape of the eyeball. VI. SENSE OF SIGHT: Vision C. Light Refraction: 1. Intro: Incoming light rays are refracted (bent) onto the retina due to the convex surface of both the cornea and the lens. See Fig 12.36 and Fig 12.37, page 456. 2. Pathway of Light Through Eye: a. cornea b. aqueous humor c. lens d. vitreous humor e. photoreceptors in retina. * Once the rods and/or cones are stimulated, a sensory impulse is carried 3. Visual Receptors a. There are two types of visual receptors (photoreceptors) in the retina: See Fig 12.32, page 454. 1. Cones = photoreceptors for color vision; produce sharp images. 2. Rods = photoreceptors for night vision; produce silhouettes of images 3. Macula lutea –yellow spot of mostly cones 4. Fovea centralis – depression of ALL cones, sharpest vision 4. Visual Pigments a. found in membrane sacs in rods and cones b. sensitive to light energy c. Rods – rhodopsin contains protein (opsin) attached to pigment (retinal) light causes retinal to change shape releasing it from opsin chain reaction of events results in closing of Na+ channels resulting hyperpolarization slows the tonic firing of AP’s dark adapted – all opsin and retinal is together, therefore rods are VERY sensitive, vision possible even in dark light adapted – most opsin and retinal decomposes cones take over sharp, color vision results VI. SENSE OF SIGHT: Vision C. Light Refraction: 4. Visual Pigments d. Cones – Iodpsins similar to rhodopsin 3 types of pigment 1. erythrolabe = red cones 2. chlorolabe = blue cones 3. cyanolabe = green cones combination of stimulation = different colors 5. Stereoscopic Vision a. produced because humans have binocular vision b. each eye produces a slightly different image for the brain to interpret c. visual cortex puts together as 3-D image 6. Visual Nerve Pathway Once the rods and/or cones are stimulated, a sensory impulse is carried on the: a. optic nerve (CN II) which crosses at the b. optic chiasma forming optic tracts that carry the impulse to the c. thalamus for direction to the d. primary visual cortex (occipital lobe) for interpretation. VII. LIFESPAN CHANGES Diminished senses are often one of the first noticeable signs of aging. Hearing loss can be due to Organ of Corti damage Degeneration of pathways to brain Tinnitus. Visual problems include: dry eyes Floaters and light flashes Presbyopia Glaucoma Cataracts Retinal detachment Macular degeneration. VIII HOMEOSTATIC IMBALANCES OF THE SPECIAL SENSES Color blindness. See introduction on page 422. Cancer and Chronic Pain. See Clinical Application 12.1, page 428. Mixed-up Senses. See Clinical Application 12.2, page 430. Smell and Taste Disorders. See Clinical Application 12.3, page 435. Hearing Loss. See Clinical Application 12.4, page 443. Refraction Disorders. See Clinical Application 12.5, page 457. IX. Clinical Terms Related to the Senses See page 462-463. Chapter 12: Somatic and Special Senses I. Introduction A. Introduction 1. Special senses are senses that have receptors in large sensory organs of the head. 2. Sensory receptors collect information from the environment and send impulses to the brain. 3. The cerebral cortex forms perceptions II. Receptors and Sensations A. Receptor Types 1. Five general groups of sensory receptors are chemoreceptors, pain receptors, thermoreceptors, mechanoreceptors, and photoreceptors. 2. Chemoreceptors respond to changes in chemical concentrations. 3. Pain receptors respond to tissue damage. 4. Thermoreceptors respond to temperature changes. 5. Mechanoreceptors respond to mechanical forces. 6. Proprioceptors sense changes in the tension of muscles and tendons. 7. Baroreceptors detect changes in changes in blood pressure. 8. Stretch receptors respond to stretch. 9. Photoreceptors respond to light energy. B. Sensory Impulses 1. Sensory receptors can be ends of neuron or other kinds of cells located close to them. 2. Stimulation of sensory receptors causes local changes in their membrane potential, generating a graded electric current that reflects the intensity of stimulation. 3. If a receptor is a neuron and the change in membrane potential reaches threshold, an action potential is generated. 4. If the receptor is another type of cell, its receptor potential must be transferred to a neuron to trigger and action potential. C. Sensations 1. A sensation is a feeling that occurs when the brain interprets sensory impulses. 2. Sensations depend on which region of the cerebral cortex receives the impulse. 3. Projection is a process in which the cerebral cortex projects a sensation back to its apparent source. D. Sensory Adaptation 1. Sensory adaptation is the adjustment of sensory receptors. 2. An example of a receptor that undergoes sensory adaptation is a smell receptor. III. Somatic Senses A. Introduction 1. Somatic senses are those whose sensory receptors are associated with the skin, muscles, joints, and viscera. 2. Three groups of somatic senses are exteroceptive senses, proprioceptive senses, and visceroceptive senses. 3. Exteroceptive senses include senses of touch, pressure, temperature, and pain. 4. Proprioceptive senses include senses associated with changes in muscles and tendons and in body position. 5. Visceroceptive senses include senses associate with changes in viscera. B. Touch and Pressure Senses 1. Three kinds of touch and pressure receptors are free nerve endings, Meissner’s corpuscles, and Pacinian corpuscles. 2. Free nerve endings are located in epithelial tissues and are associated with sensations of touch and pressure. 3. Meissner’s corpuscles are located in hairless portions of skin and are associated with the sensation of light touch. 4. Pacinian corpuscles are located in deeper subcutaneous tissues of the hands, feet, penis, clitoris, urethra, breasts, and tendons and ligament and are associated with heavy pressure and vibrations. C. Temperature Senses 1. Two types of temperature receptors are warm and cold receptors. 2. Warm receptors respond to temperatures between 25oC and 45oC. 3. Cold receptors respond to temperatures between 10oC and 20oC. 4. Temperatures above 45oC and below 10oC activate pain receptors. D. Sense of Pain 1. Introduction a. Receptors that consist of free nerve endings sense pain. b. Pain receptors are distributed widely throughout the skin and internal tissues, except in the nervous tissue of the brain. 2. Pain Receptors a. Pain receptors can be stimulated by damaged tissue. b. Pain receptors adapt very little, if at all. 3. Visceral Pain a. Visceral pain receptors respond differently to stimulation than those of surface tissues. b. Pain in visceral organs result from stimulation of mechanoreceptors and from decreased blood flow accompanied by lower tissue oxygen levels and accumulation of pain-stimulating chemicals. c. Referred pain is a phenomenon is which visceral pain may feel as if it is coming from some part of the body other than the part being stimulated. d. Referred pain may come from common nerve pathways that sensory impulses coming both from skin areas and from internal organs use. e. During a heart attack, the cerebral cortex may incorrectly interpret the source of the impulses as coming from the left arm. 4. Pain Nerve Pathways a. Two main types of pain fibers are acute pain fibers and chronic pain fibers. b. Acute pain fibers are thin, myelinated nerve fibers. c. Acute pain fibers are associated with the sensation of sharp pain. d. Chronic pain fibers are thin, unmyelinated nerve fibers that conduct impulses more slowly. e. Impulses from chronic pain fibers cause dull, aching pain sensations. f. Acute pain is usually sensed as being from a local area of skin and chronic pain is likely to be felt in deeper tissues as well as the skin. g. Pain impulses that originate from tissues of the head reach the brain on sensory fibers of fifth, seventh, ninth, and tenth cranial nerves. h. All other pain impulses travel on sensory fibers of spinal nerves and they pass into the spinal cord by way of dorsal roots. i. Upon reaching the spinal cord, pain impulses enter the gray matter of the posterior horn, where they are processed. j. Within the brain, most pain fibers terminate in the reticular formation and from there are conducted on fibers to the thalamus, hypothalamus, and cerebral cortex. 5. Regulation of Pain Impulses a. Awareness of pain occurs when pain impulses readh the level of the thalamus. b. The cerebral cortex judges the intensity of pair and locates its source. c. Enkephalins and serotonin can suppress pain impulses. d. Endorphins are natural pain controlling substances in the pituitary gland and hypothalamus. E. Stretch Receptors a. Stretch receptors are proprioceptors that send information to the spinal cord and brain concerning the lengths and tensions of muscles. b. Two main kinds of stretch receptors are muscle spindles and Golgi tendon organs. c. Muscle spindles are located in skeletal muscles near their junctions with tendons and function to detect stretch. d. Golgi tendon organs are located in tendons cose to their attachments to muscles and function to detect increased tension. e. The stretch reflex is an action that opposed the lengthening of a muscle and helps maintain the desired position of a limb in spite of gravitational or other forces tending to move it. IV. Special Senses A. Introduction 1. Examples of special senses are sight, smell, hearing, and taste. 2. Special senses are those whose sensory receptors are within sensory organs of the head. B. Sense of Smell 1. Olfactory Receptors a. Olfactory receptors are used to sense smell and are chemoreceptors. b. Taste is a combination of smell and taste sensations. 2. Olfactory Organs a. Olfactory organs contain the olfactory receptors. b. Olfactory organs are located in the upper pars of the nasal cavity, the superior nasal conchae and a portion of the nasal septum. c. The olfactory receptor cells are bipolar neurons surrounded by columnar epithelial cells. d. Cilia of olfactory receptor cells project into the nasal cavity. e. Smell impulses are generated when odorants enter the nasal cavity, dissolve in fluids, and bind to receptors proteins on cilia that are part of the cell membranes of the olfactory receptor cells. 3. Olfactory Nerve Pathways a. Once olfactory receptors are stimulated, nerve impulses travel along their axons to synapse with neurons in the olfactory bulbs. b. The olfactory bulbs function to analyze the sensory impulses. c. From olfactory bulbs, impulses travel to olfactory tracts. d. From olfactory tracts, impulses travel to portions of the limbic system and cerebral cortex. e. The limbic system functions to put an emotion with the smell information. f. The olfactory cortex is located in the temporal lobes and interprets the smell sensations. 4. Olfactory Stimulation a. The olfactory code is a particular combination used by the brain to determine the smell sensation. b. The intensity of a smell drops about 50% within a second because olfactory receptors adapt quickly. c. The olfactory receptor neurons are the only example of damaged neurons that are regularly replaced from stem cells in the olfactory bulb. C. Sense of Taste 1. Introduction a. Taste buds are special organs of taste. b. Papillae of the tongue are tiny elevations. c. Taste buds are located on papillae of the tongue. 2. Taste Receptors a. Taste cells are modified epithelial cells that function as receptors. b. A taste pore is an opening in a taste bud. c. Taste hairs are tiny projections from the surface of taste cells. d. The mechanism of tasting probably involves combination of chemicals binding specific receptors on taste hair surfaces, altering membrane polarization, and thereby generating sensory impulses on nearby nerve fibers. 3. Taste Sensations a. The four primary taste sensations are sweet, sour, salty, and bitter. b. Spicy foods activate pain receptors. c. Responsiveness to a sweet stimulus peaks at the tip of the tongue. d. Responsiveness to a sour stimulus is greatest at the margins of the tongue. e. Receptors that are responsive to salt are widely distributed. f. Sweet receptors are usually stimulated by carbohydrates. g. Acids stimulate sour receptors. h. Salt receptors are stimulated by ionized inorganic salts. i. Bitter receptors are stimulated by a variety of chemicals. j. Taste receptors, like olfactory receptors, undergo adaptation. 4. Taste Nerve Pathways a. The three cranial nerves that carry taste sensations are the facial, glossopharyngeal, and the vagus nerves. b. Cranial nerves conduct taste sensations to the medulla oblongata. c. From the medulla oblongata, taste sensations go to the thalamus and the gustatory cortex. d. The gustatory cortex is located in the parietal lobes. D. Sense of Hearing 1. Introduction a. The organ of hearing is the ear. b. The three parts of the ear are external, middle, and inner. c. The ear also provides the sense of equilibrium. 2. External Ear a. The auricle of the ear is an outer, funnel-like structure and functions to collect sounds waves. b. The external auditory meatus is a canal that extends from the auricle to the tympanic membrane and functions to deliver sounds waves to the tympanic membrane. c. The external auditory meatus ends with the tympanic membrane. d. Ceruminous glands line the external auditory meatus and secrete cerumen. e. The tympanic membrane is semitransparent membrane and moves back and forth in response to sound waves. 3. Middle Ear a. The middle ear is an air filled space in the temporal bone. b. The middle ear contains three auditory ossicles. c. The three auditory ossicles are the malleus, incus, and stapes. d. The malleus is attached to the tympanic membrane. e. The stapes covers the oval window. f. The oval window is an opening in the wall of the tympanic cavity. g. Vibration of the stapes moves a fluid within the inner ear. h. Vibrations in the inner ear stimulate hearing receptors. i. The tensor tympani is a small skeletal muscle that is anchored to the malleus and wall of the auditory tube. j. The stapedius is a small skeletal muscle that is attached to the posterior side of the stapes and the inner wall of the tympanic cavity. k. The tympanic reflex is a reflex that causes the ear ossicles to become more rigid. l. The tympanic reflex reduces the effectiveness in transmitting vibrations to the inner ear. 4. Auditory Tube a. The auditory tube connects the nasopharynx to the middle ear cavity. b. The auditory tube functions to maintain equal pressure on both side of the eardrum. 5. Inner Ear a. The inner ear is a complex system of intercommunicating chambers and tubes. b. The osseous labyrinth is a bony canal in the temporal bone. c. The membranous labyrinth is a tube that lies within the osseious labyrinth and has a similar shape. d. Perilymph is located in osseous labyrinth. e. Endolymph is located in membranous labyrinth. f. The three parts of the labyrinths are the cochlea, semicircular canals, and vestibule. g. The cochlea functions in hearing. h. The semicircular canals provide a sense of equilibrium. i. The vestibule is a bony chamber between the semicircular canals and the cochlea. k. The cochlea is shaped like the shell of a snail. l. The scala vestibuli is the upper compartment of the bony labyrinth of the cochlea. m. The scala tympani is the lower compartment of the bony labyrinth of the cochlea. n. The round window is a membrane-covered opening of the inner ear. o. The cochlear duct is a portion of the membranous labyrinth within the cochlear that lies between the two bony compartments and is filled with endolymph. p. The vestibular membrane is the membrane that separates the cochlear duct from the scala vestibuli. q. The basilar membrane is the membrane that separates the cochlear duct from the scala tympani. r. The organ of Corti is located on the upper surface of the basilar membranes and stretches from the apex to the base of the cochlea. It contains hearing receptor cells. s. The tectorial membrane is a membrane above the hearing receptor cells and is in contact with the hair of the receptor cells. t. Different frequencies of vibration move different pars of the basilar membrane. u. A particular sound frequency causes the hairs of a specific group of receptors cells to bend against the tectorial membrane. 6. Auditory Nerve Pathway a. The cochlear branch of the vestibulocochlear nerve carries hearing impulses to the medulla oblongata. b. The medulla oblongata conveys the hearing impulses through the midbrain to the thalamus. c. From the thalamus, hearing impulses go to the temporal lobes where they are interpreted. E. Sense of Equilibrium 1. Introduction a. The organs of static equilibrium sense the position of the head, maintaining stability and posture when the head and body are still. b. The organs of dynamic equilibrium sense movements when the head and body suddenly rotate. 2. Static Equilibrium a. The organs of static equilibrium are located within the vestibule. b. The two expanded chambers within the vestibule are the utricle and saccule. c. The macula is a patch of hair cells and supporting cells of the utricle and saccule. d. When the head is upright, the hairs of the macula in the utricle project vertically and those of the saccule project horizontally. e. The otolithic membrane is a gelatinous membrane. f. Gravity stimulates hair cells to respond. g. When hair cells bend, they signal their associated nerve fibers. h. The nerve impulse generated by the bending of the hair cells travels to the brain by means of the vestibular branch of the vestibulocochlear nerve. i. The brain responds to equilibrium information by sending motor impulses to skeletal muscles. j. The maculae also participate in the sense of dynamic equilibrium. 3. Dynamic Equilibrium a. The three bony semicircular canals lie at right angles to each other and occupy three different planes in space. b. The ampulla is a swelling of the membranous labyrinth of a semicircular canal that communicates with the utricle of the vestibule. c. The crista ampullaris is a sensory organ composed of hair cells and supporting cells. d. The cupula is gelatinous mass that covers hair cells of the crista ampullaris. e. Rapid turns of the head or body stimulate the hair cells of the crista. f. The semicircular canals move with the head of body but the fluid inside the membranous canals tend to remain stationary and this bends the cupula. g. The bending of hairs stimulates the hair cells to signal their associated nerve fibers and, as a result, impulses travel to the brain. h. Parts of the cerebellum are particularly important in maintaining equilibrium F. Sense of Sight 1. Introduction a. Visual accessory organs assist the visual receptors. b. Examples of visual accessory organs are eyelids, lacrimal apparatus, and extrinsic eye muscles. 2. Visual Accessory Organs a. Each eyelid is composed of skin, muscle, connective tissue, and conjunctiva. b. The orbiculais oculi muscle functions to close the eyelids. c. The levator palpebrae muscle functions to raise the upper eyelids. d. Tarsal glands are modified sebaceous glands in eyelids. e. Conjunctiva is a mucous membrane that lines the inner surfaces of the eyelids and covers a portion of the surface of the eyeball and functions to keep the surface of the eyeball moist. f. The lacrimal apparatus consists of the lacrimal gland and a series of ducts and functions to secrete tears and to drain them into the nasal cavity. g. A lacrimal sac is a structure, which collects tears from superior and inferior canaliculi. h. A lacrimal duct is a duct, which collects tears from the lacrimal sac and empties tears into the nasal cavity. i. Tears contain water and en enzyme called lysozyme. j. The extrinsic muscles of the eye function to move the eyeball. k. The six extrinsic muscles of the eye are superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, and inferior oblique. l. The superior rectus muscle moves the eye upward and medially. m. The inferior rectus muscle moves the eye downward and medially. n. The medial rectus muscle moves the eye medially. o. The lateral rectus muscle moves the eye laterally. p. The superior oblique muscle moves the eye downward and laterally. q. The inferior oblique muscle moves the eye upward and laterally. 3. Structure of the Eye a. The three layers of the eyeball are outer fibrous, middle vascular, and inner nervous. b. The spaces within they eye are filled with fluids that support its wall and internal structures and help maintain its shape. c. The two parts of the outer tunic are the cornea and sclera. d. The cornea is transparent and functions to allow light to enter the eye. e. The sclera is white and opaque and functions to protect internal structures of the eyeball. f. In the back of the eye, the optic nerve pierces the sclera. g. The middle tunic of the eye includes choroid coat, ciliary body, and iris. h. The choroid coat is the posterior five-sixth of the middle layer and its functions include to supply nutrients to surrounding tissues and to absorb excess light. i. The ciliary body is the thickest part of the middle layer and forms a ring around the front of the eye and its functions include holding and moving the lens. j. Ciliary processes are radiating folds within the ciliary body. k. Ciliary muscles are muscles within the ciliary body. l. Suspensory ligaments extend from the ciliary processes and hold the lens in position. m. The lens is made up of specialized epithelial cells and lens fibers. n. When ciliary muscles relax, suspensory ligaments become tight and the lens becomes thinner. o. When ciliary muscles contract, suspensory ligaments become loose and the lens becomes thicker. p. The iris is a thin diaphragm and functions to control the amount of light that enters the eye. q. The anterior cavity of the eye is the portion of the eye in front of the lens. r. The anterior chamber of the eye is the portion of the eye in front of the iris. s. The posterior chamber of the eye is the portion of the eye between the iris and lens. t. Aqueous humor is located in the anterior cavity of the eye and functions to provide nutrients to surrounding tissues. u. The pupil is a hole in an iris. v. The size of the pupil changes in response to light intensity. w. The inner tunic of the eye consists of the retina which contains the visual receptor cells. x. The retina has distinct layers including pigmented epithelium, neurons, nerve fibers, and limit in membranes. y. The five major groups of retinal neurons are receptor cells, bipolar neuron, ganglion cells, horizontal cells and amacrine cells. z. Receptor cells, bipolar cells and ganglion cells provide a direct pathway for impulses triggered in the receptors to the optic nerve and brain. aa. The horizontal cells and amacrine cells function to modify the impulses transmitted on the fibers of the direct pathway. bb. The macula lutea is yellowish spot in the central region of the retina. cc. The fovea centralis is a depression in the center of the macula lutea. dd. The optic disc is the area where nerve fibers from the retina exit the eye. ee. The posterior cavity of the eye is the portion of the eye behind the lens. ff. Vitreous humor is located in the posterior cavity and functions to support the internal structures of the eye and helps maintain its shape. gg. Light waves entering the eye must pass through the cornea, aqueous humor, lens, vitreous humor, and several layer of the retina before they reach the photoreceptors. 4. Light Refraction a. Light refraction is the bending of light and occurs when light moves at an oblique angle from one medium to another medium of a different density. b. A convex surface causes light waves to converge. c. A concave surface causes light waves to diverge. d. Light is refracted by cornea and lens as it enters the eye. e. If the shape of the eye is normal, light wave are focused sharply onto the retina. f. The image focused on the retina is upside down and reversed from left to right. g. Divergent light waves focus behind the retina unless something increases the refracting power of the eye. h. Accommodation accomplishes the increase in refracting power by thickening of the lens. 5. Visual Receptors a. Two kinds of photoreceptor cells are rods and cones. b. Rods and cones are found in a deep layer of the retina. c. Rods and cones are stimulated when light reaches them. d. Rods are more sensitive to light than cones. e. Rods provide vision is dim light. f. Rods produce colorless vision, whereas cones detect colors. g. Cones provide sharp images, whereas rods produce broad outlines of objects. h. The area of sharpest vision is the fovea centralis. i. The concentration of cones decreases in areas farther away from the macula lutea. 6. Visual Pigments a. Rhodopsin is the light sensitive pigment of rods. b. Rhodopsin is located in membranous discs of rods. c. In the presence of light, rhodospin breaks down into molecules of opsin and retinal. d. A series of reactions cause nerve impulses to travel away from the retina, through the optic nerve, and into the visual cortex where visions are interpreted. e. In bright light cones detect colors. f. Dark adapted eyes are those whose rods have increased amounts of available rhodospin. g. Light adapted eyes are that are very sensitive to light and have decreased amounts of rhodopsin available. h. Three pigments found on cones are chlorolabe, erythrolabe, and cyanolabe. i. Erythrolabe is most sensitive to red light waves. j. Chlorolabe is most sensitive to green light waves. k. Cyanolabe is most sensitive to blue light waves. 7. Stereoscopic Vision a. Stereoscopic vision perceives distance, depth, height, and width of objects. b. Stereoscopic vision depends on vision with two eyes. c. A person with one eye is less able to judge distance and depth accurately. 8. Visual Nerve Pathways a. Axons of ganglionic cell leave eye through the optic nerve. b. The optic chiasm is where the optic nerve cross. c. Impulses leave the optic chiasm through optic tracts and most are carried to the thalamus. d. From the thalamus, visual impulses travel to the visual cortex. e. Visual impulses that do not go to the thalamus go to the midbrain and are important in visual reflexes. V. Life-Span Changes A. By age fifty the senses of smell and taste begin to diminish. B. By age sixty, a fourth of the population experiences hearing loss. C. Age-related hearing loss may be due to decades of cumulative damage to the sensitive hair cells of the organ of Corti. D. Presbycusis is hearing loss due to degeneration of auditory pathways. E. Tinnitus is an abnormal ringing in the ear. F. Vision may decline with age because of dry eyes, too few tears, crystal-like deposits in the vitreous humors, or cataracts in lens. G. “Floaters” are due to crystal-like deposits in the vitreous humors. H. Presbyopia is the inability to read small print up close I. Glaucoma is increased pressure in the eye due to accumulation of aqueous humor. J. Cataracts are eye disorders in which the lenses become clouded and somewhat opaque. K. Retinal detachment is a condition in which the retina becomes detached from the posterior surface of the middle layer of the eye. Chapter 12 Somatic and Special Senses List five groups of sensory receptors, and name the kind of change to which each is sensitive. Chemical concentration (chemoreceptors)—Stimulated by changes in the chemical concentration of substances. Tissue damage (pain receptors)—Stimulated by tissue damage. Temperature change (thermoreceptors)—Stimulated by changes in temperature. Mechanical receptors (mechanoreceptors)—Stimulated by changes in pressure or movement of fluids. There are three types of mechanoreceptors; they are: Proprioceptors—Send changes in tension of muscles and tendons Baroreceptors—Detect changes in blood pressure Stretch receptors—Sense degree of inflation Light intensity (photoreceptors)—Stimulated by light energy. Explain how sensory receptors stimulate sensory impulses. Sensory receptors can either be nerve endings or special cells located next to them. Stimulation causes local changes in their membrane potentials and generates a graded electrical current showing the intensity of the stimulation. Define sensation. A sensation is a feeling that occurs when the brain interprets sensory impulses. Explain the projection of a sensation. At the time when a sensation is created, the cerebral cortex causes the feeling to come from the stimulated receptors. It is called projection because the brain projects the sensation back to its apparent source. Define sensory adaptation. Sensory adaptation occurs when sensory receptors are subjected to continuous stimulation. As the receptors adapt, impulses leave them at decreasing rates, until finally these receptors may completely fail to send signals. Once receptors have adapted, impulses can be triggered only if the strength of the stimulus is unchanged. Explain how somatic senses can be grouped. Somatic senses can be divided into three groups: Exteroceptive senses—These senses are associated with changes at the body surface. Proprioceptive senses—These senses are associated with changes in muscle, tendons, and in body positions. Visceroceptive senses—These senses are associated with changes in the viscera. Describe the functions of free nerve endings, Meissner’s corpuscles, and Pacinian corpuscles. The free nerve endings (sensory nerve fibers) are common in epithelial tissue. They are associated with the sensations of touch and pressure. Meissner’s corpuscles are common in hairless portions of the skin. These are sensitive to touch. Pacinian corpuscles are common in the deeper subcutaneous tissues and occur in the tendons of the muscles and the ligaments of joints. These are associated with the sensation of deep pressure. Explain how thermoreceptors function. Thermoreceptors are actually two types of free nerve endings located in the skin. The receptors responding to heat are called heat receptors. Those, which respond to cooler temperature, are called cold receptors. Heat receptors are most sensitive to temperatures above 25o C. Cold receptors are most sensitive to temperatures between 10o C and 20o C. At intermediate temperatures, the brain interprets sensory input from different combinations of these receptors. Compare pain receptors with other types of somatic receptors. Most pain receptors can react to more than one type of change. In other words, pain receptors may react directly to mechanical damage, chemical changes, by-products of metabolism, ischemia, hypoxia, or stimulation of other receptors such as mechanoreceptor. So, pain receptors are not usually limited to the specific types of stimulation that other somatic receptors are. List the factors that are likely to stimulate visceral pain receptors. Factors that stimulate visceral pain receptors include: widespread stimulation of visceral tissues, stimulation of mechanoreceptors, and decrease blood flow accompanied by lower oxygen concentration and accumulation of pain-stimulating chemicals. Define referred pain. Referred pain is a phenomenon that occurs when the pain feels as if it is coming from some part of the body other than the part being stimulated. An example would be pain that originates from the heart may actually be felt in the left shoulder or left arm. Explain how neuropeptides relieve pain. Neuropeptides called enkephalins and monoamine serotonin inhibit pain sensations by blocking the impulses from the presynaptic nerve fibers in the spinal cord. Enkephalins suppress both acute and chronic pain impulses much like morphine does. Serotonin stimulates other neurons to release enkephalins. Another group of neuropeptides are the endorphins. They are found in the pituitary gland, hypothalamus, and other regions of the nervous system. These act as pain suppressor with a morphine-like action. Distinguish between muscle spindles and Golgi tendon organs. Muscle spindles are found in skeletal muscles near their junctions with tendons. Each spindle contains one or more modified skeletal muscle fibers enclosed in connective muscle tissue. Each fiber has a non-striated region with the end of a sensory nerve fiber wrapped around it. Golgi tendon organs are found in the tendons close to their muscle attachment and each is connected to a set of muscle fibers and innervated by a sensory neuron. Explain how the senses of smell and taste function together to create the flavors of foods. Both olfactory and taste receptors are sensitive to chemical sensations. Because of this, we smell the food at the same time we taste it. Often, it is impossible to tell whether the sensation is mostly from the smell of a food or from the actual taste. Describe the olfactory organ and its function. The olfactory organs include yellowish-brown masses of olfactory receptor cells and epithelial supporting cells all wrapped by a mucous membrane. They are found in the superior parts of the nasal cavity, superior nasal conchae, and part of the nasal septum. They function to provide the sense of smell. Trace a nerve impulse from the olfactory receptor to the interpreting centers of the brain. An olfactory receptor that has been stimulated causes nerve impulses to be triggered and travel along the axons of the receptor cells that are the fibers of the olfactory nerves. These fibers lead to neurons located in the olfactory bulbs that like on either side of the crista galli of the ethmoid bone. In the olfactory bulbs, the impulses are analyzed and additional impulses are located within the temporal lobes and at the base of the frontal lobes just anterior to the hypothalamus. Explain how an olfactory code distinguishes odor stimuli. Although the mechanism is unknown, certain subsets of receptors may only be stimulated by a certain odor. The brain then will interpret those specific receptors stimulated to a specific odor. Explain how the salivary glands aid the taste receptors. Before the taste of a particular chemical can be detected, the chemical must be dissolved in the watery fluid surrounding the taste buds. This fluid is supplied by the salivary glands. Name the four primary taste sensations, and describe the patterns in which the taste receptors are roughly distributed on the tongue. Sweet as produced by table sugar. These receptors are most plentiful near the tip of the tongue. Sour as produced by vinegar. These receptors occur primarily along the margins of the tongue. Salty as produced by table salt. These receptors are most abundant in the tip and the upper front part of the tongue. Bitter as produced by caffeine or quinine. These receptors are located toward the back of the tongue. Explain why taste sensation is less likely to diminish with age than olfactory sensation. This is due to the fact that the taste cells reproduce continually and only function for about three days. Damaged olfactory neurons are not replaced. Trace the pathway of a taste impulse from the receptor to the cerebral cortex. Sensory impulses from the taste receptors located in various regions of the tongue travel on fibers of the facial, glossopharyngeal, and vagus nerves into the medulla oblongata. From there, the impulses ascend to the thalamus and are directed to the gustatory cortex, which is located in the parietal lobe of the cerebrum along a deep portion of the lateral sulcus. Distinguish among the external, middle, and inner ears. The external ear consists of two parts: an outer funnel-like structure, called the auricle or pinna, and S-shaped tube, called the external auditory meatus, which leads into the temporal bone for about 2.5 centimeters. The middle ear includes an air-filled space in the temporal bone, called the tympanic cavity, tympanic membrane (eardrum), and three small bones called auditory ossicles. These bones are the malleus (hammer), incus (anvil), and stapes (stirrup). The inner ear consists of a complex system of intercommunicating chambers and tubes called a labyrinth. There are, in fact, two such structures in each ear—the osseous labyrinth and the membranous labyrinth. Perilymph is a fluid that is between the two labyrinths. It also includes three semi-circular canals and cochlea. Trace the path of a sound vibration from the tympanic membrane to the hearing receptors. The sound waves enter the external auditory meatus. Changes of wave pressures cause the eardrum to reproduce the vibrations coming from the sound wave source. Auditory ossicles amplify and transmit the vibrations to the end of the stapes. Movement of the stapes at the oval window transmits vibrations to the perilymph in the scala vestibuli. Vibrations pass through the vestibular membrane and enter the endolymph of the cochlear duct. Different frequencies in the endolymph stimulate different sets of receptor cells. Describe the functions of the auditory ossicles. The bones form a bridge connecting the tympanic membrane to the inner ear. They function to transmit vibrations between these parts. Describe the tympanic reflex, and explain its importance. The tympanic reflex consists of two skeletal muscles associated with the middle ear that are controlled involuntarily. The reflex is elicited by long, external sounds causing the bridge of the ossicles to become more rigid, reducing its effectiveness in transmitting vibrations to the inner ear. The tympanic reflex reduces pressure from loud sounds that might otherwise damage the hearing receptors. Explain the function of the auditory tube. The function of the auditory tube (Eustachian tube) is to maintain equal air pressure on both sides of the tympanic membrane, which is necessary for normal hearing. Distinguish between the osseous and the membranous labyrinths. The osseous labyrinth is a bony canal in the temporal bone. The membranous labyrinth is a tube that lies within the osseous labyrinth and has a similar shape. Describe the cochlea and its function. The cochlea contains a bony core and a thin bony shelf that winds around the core like the threads of a screw. It functions in hearing by allowing the sound vibrations from the perilymph to travel along the scala vestibuli and pass through the vestibular membrane and into the endolymph of the cochlear duct where they cause movements in the basilar membrane. It then stimulates the organ of Corti, which contains the hearing receptors. Describe a hearing receptor. A hearing receptor cell is an epithelial cell. These cells act like a neuron. The cell membrane is polarized when the cell is at rest. Upon stimulation the cell membrane depolarizes and the ion channels open. This makes the membrane more permeable to calcium. The cell releases a neurotransmitter that stimulates the nearby sensory nerve fibers, and they transmit impulses along the cochlear branch of the vestibulocochlear nerve to the auditory cortex of the temporal lobe of the brain. Explain how a hearing receptor stimulates a sensory neuron. The cell releases a neurotransmitter that stimulates the nearby sensory nerve fibers, and they transmit impulses along the cochlear branch of the vestibulocochlear nerve to the auditory cortex of the temporal lobe of the brain. Trace a nerve impulse from the organ of Corti to the interpreting centers of the cerebrum. Once the sound is converted into a nerve impulse at the organ of Corti, it travels along the cochlear branch of the vestibulocochlear nerve to the auditory cortex of the temporal lobe of the brain. On the way, some of the nerve branches cross over, so that impulses from both ears will be interpreted by both sides of the brain. Describe the organs of static and dynamic equilibrium and their functions. The organs of static equilibrium are located within the vestibule, a bony chamber between the semicircular canals and cochlea. The membranous labyrinth inside the vestibule consists of two expanded chambers—an utricle and a saccule. The macula is on the anterior wall of the utricle that contains numerous hair cells and supporting cells. These sense the positions of the head and help in maintaining the stability and posture of the head and body when these parts are motionless. The organs of dynamic equilibrium are located within the three semicircular canals. Suspended in the perilymph of the bony portion of each semicircular canal is a membranous canal that ends in a swelling called the ampulla. The sensory organs are located here. These organs are called the crista ampullaris and contain a number of hair cells and supporting cells. These function to detect motion of the head and aid in balancing the head and body when they are moved suddenly. Explain how the sense of vision helps maintain equilibrium. The eyes detect the body’s position relative to its surroundings. For this reason, the eyes help the brain maintain equilibrium, especially if the other organs of equilibrium are damaged. List the visual accessory organs, and describe the functions of each. Eyelids—protection of the eye Lacrimal apparatus—secretions keep the surface of the eye and the lining of the lids moist and lubricated. An enzyme within the tears functions as an antibacterial agent, reducing the chance of eye infections. Extrinsic muscles—movement of the eye Name the three layers of the eye wall, and describe the functions of each. Outer fibrous tunic—serves as the cornea and sclera. The cornea functions as a window to the eye and helps focus entering light rays. The sclera provides protection and serves as an attachment for the extrinsic muscles Middle vascular tunic—includes the choroid coat, which helps to absorb excess light so the inside of the eye is dark; the ciliary body, which forms the ciliary muscles and processes; and the iris, whose smooth muscle control the pupil size. Inner nervous tunic—consists of the retina, which contains the visual receptor cells. Describe how accommodation is accomplished. Accommodation results from the suspensory ligaments either relaxing or pulling due to decreased or increased tension from the suspensory muscles. This allows people to see objects either close up or more distant. Explain how the iris functions. The smooth muscle fibers of the iris are arranged into two groups, a circular set and a radial set. The circular muscles act as a sphincter to decrease pupil size. The radial muscles contract to increase the diameter of the pupil. Distinguish between aqueous humor and vitreous humor. The aqueous humor fills the space between the cornea and lens, helps to nourish these parts, and aids in maintaining the shape of the front of the eye. The vitreous humor fills the posterior cavity, and together with some collagenous fibers, comprises the vitreous body. This supports the internal parts of the eye and helps maintain its shape. Distinguish between the macula lutea and optic disk. The macula lutea is a small spot on the retina. In its center is a depression called the fovea centralis, which has the sharpest vision. The optic disk is just medial to the macula lutea and is the area where the optic nerve begins and blood vessels enter and exit. This area has no receptors, and is known as the blind spot of the eye. Explain how light waves focus on the retina. As light enters the eye, it is refracted, or bent, by the cornea and lens and is focused sharply on the retina. The image is projected upside down and reversed from left to right. The visual cortex somehow corrects this when it interprets the images. Distinguish between rods and cones. Rods are receptor cells that have long thin projections at their terminal ends. Rods are hundreds of times more sensitive to light than cones. This enables people to see in relatively dim light. Rods produce colorless vision. Cones are receptor cells that have short, blunt projections. Cones allow for increased visual acuity—the sharpness of images perceived. Cones make up the entire fovea centralis. Cones also detect colors of light. Explain why cone vision is generally more acute than rod vision. When the images are being sent from the rods and cones to the brain, the nerve fibers of the rods converge and send all of the images on a single tract. The brain can interpret the images, but cannot tell specifically from which rod a particular part came from. The cones converge their nerve fibers to a much lesser degree. This allows the brain to interpret the images more accurately and sharply. Describe the function of rhodopsin. Rhodopson (visual purple) is the light-sensitive substance in the rods. Bright light decomposes rhodopsin rapidly so the sensitivity in the rods is greatly reduced. Thus, this pigment allows us to see in dim light. Explain how the eye adapts to light and dark. Light comes in the form of many different wavelengths. The color perceived is an interpretation of its specific wavelength. The shortest visible wavelengths are perceived as violet. The longest is perceived as red. The cones are designed to be stimulated by one of three different visual pigments. An erythrolabe cone is sensitive to red light waves, a chlorolabe to green light waves, and a cyanolabe to blue light waves. In this way the combinations of light waves entering the eye can be detected by the cones and relayed to the brain. Describe the relationship between light wavelengths and color vision. The wavelength of a particular kind of light determines the color perceived. The cones in the eyes each have a certain pigment that is stimulated by certain wavelengths. Some cones have erythrolabe pigment that is sensitive to red light waves, some have chlorolabe which is sensitive to green, and some have cyanolabe which is sensitive to blue. The combinations of these stimulations gives us color vision. Define stereoscopic vision. Steropsis allows a person with binocular vision (using both eyes) to perceive distance, depth, width, and height of an object. Explain why a person with binocular vision is able to judge distance and depth of close objects more accurately than a one-eyed person. This happens because the eyes are separated and see slightly more of one side than the other (the left eye sees more left and the right eye sees more right). The impulses are superimposed in the brain and gives us a three-dimensional view of the object. A person using only one eye cannot see “both sides” of the object at the same time. Thus, the object appears two-dimensional. Trace a nerve impulse from the retina to the visual cortex. The axons of the retinal neurons for the optic nerves. These nerves enter the X-shaped optic chiasma just anterior to the pituitary gland and the fibers from the medial (nasal) half cross over (the lateral or temporal sides do not). For instance, the nerves from the nasal half of the left eye cross over and join the temporal half of the right eye to form the right optic tract. The nasal half of the right eye crosses over and joins the temporal half of the left eye to form the left optic tract. The impulses travel these tracts until just before the thalamus, when some of the nerve fibers branch off and enter the areas that control visual reflexes. The rest enter the thalamus and synapse in the posterior portion (lateral geniculate body) and continue on through the optic radiations into the visual cortex in the occipital lobes.