Lecture Outline - Chapter 19

19.1. Receptors and Sensations

1. Receptors are sensitive to specific stimuli; they are classified by these stimuli and sensation produced in brain.
2. Chemoreceptors
   • a. Sensitive to chemical substances in immediate vicinity.
   • b. Taste and smell detect external stimuli.
   • c. Concentration of H⁺ in blood is monitored; lower pH reflecting increased carbon dioxide triggers breathing increase.
3. Mechanoreceptors
   • a. Stimulated by mechanical forces, usually pressure.
   • b. Sense of touch detects strong and slight pressures.
   • c. Baroreceptors in lungs sense degree of lung inflation.
   • d. Hearing depends on mechanoreceptors sensitive to pressure waves in inner ear fluid.
   • e. Pressure receptors in inner ear also assist equilibrium.
4. Proprioreceptors
   • a. Sense movement of ligaments, stretch of tendons, degree of muscle contraction.
   • b. Muscle spindles (Fig. 19.1)
     • i. Several modified muscle fibers are wrapped in sensory nerve fibers.
     • ii. Relaxed muscle bulges and stretches muscle spindles that send nerve impulse to spinal cord.
     • iii. CNS responds to maintain amount of contraction for positioning limb.
     • iv. Golgi tendon organs in tendons work opposite to muscle spindles; stretch when muscle contracts.
5. Thermoreceptors
   • a. Stimulated by changes in temperature: heat receptors detect rise in temperature and separate cold receptors detect fall in temperature.
   • b. Skin contains surface thermoreceptors.
   • c. Hypothalamus thermoreceptors monitor internal temperature.
6. Pain Receptors
   • a. Also called nociceptors.
   • b. Naked dendrites respond to chemicals released from damaged tissues and excess stimuli of heat or pressure.
   • c. Pain is vital sense critical in limiting our use of tissues, responding to infection, etc.
7. Photoreceptors
   • a. Detect light.
   • b. Eyes lined with photoreceptors send nerve impulses perceived in brain as sense of vision.
8. Types of Sensations
   • a. Somatic senses are associated with the skin, muscles, joints, and internal organs.
   • b. Specialized senses involve organs for taste, smell, vision, equilibrium, and hearing.

1. Skin contains receptors for temperature, touch, pressure, and pain. (Fig. 19.2)
2. Distribution of receptors varies; fingertips are rich in touch receptors.
3. Pacinian corpuscles detect pressure from deep in dermis.
4. Meissner corpuscles and Merkel disks detect touch close to surface.
5. Pain receptors are in skin and internal organs.
6. Adaptation occurs when receptors become accustomed to stimuli and stop generating impulses, even though stimulus is still present; example is clothes against our skin.
7. Pain
   • a. Pain receptors do not exhibit adaptation.
   • b. Acute pain fiber conduct impulses faster than chronic pain fibers.
   • c. Visceral pain is usually transmitted along chronic pain fibers.
   • d. Referred pain appears to be coming from a different part of the body; due to sharing of nerve pathways, for example heart pain seems to come from left shoulder.
   • e. If blood flow is interrupted or mechanical damage occurs, pain-causing chemicals accumulate.
   • f. Bradykinin and prostaglandins stimulate pain receptors and stimulate substantia gelatinosa (SG) cells; aspirin interferes with prostaglandin production.
   • g. Pain is alleviated if both large and small diameter fibers carry impulses to SG cells: the pain gate theory.
   • h. Heroin-like opiate drugs fill receptors and stop pain; the body produces the natural opiates called endorphins and enkephalins.

1. Breathing Rate
   • a. Aortic arch contains chemoreceptors sensitive to pH of blood.
   • b. Increase in CO₂ causes pH to drop.
   • c. Chemoreceptors signal brain and breathing rate increases.
   • d. Expiration of CO₂ raises pH of blood.
2. Taste Buds: For Tasting
   • a. Located on tongue, mostly along walls of raised papillae; also on palate, pharynx, and epiglottis.
   • b. Taste buds are pockets of cells extending through epidermis; open at a taste pore.
   • c. Receptor sites on microvilli of receptor cells are sensitive to various chemicals.
   • d. Binding of molecule to receptor protein causes changes in cell membrane; associated nerve fiber generates impulse to parietal lobe of cerebrum.
   • e. Taste sensations are located in regions of receptors:
     • i. sweet: near tip of tongue.
     • ii. sour: located primarily along margins of tongue.
     • iii. salt: tip and upper front portion of tongue.
     • iv. bitter: toward the back of the tongue.
3. Olfactory Epithelium: For Smelling
   • a. Smell neurons are located high in roof of nasal cavity.
   • b. Each neuron ends in a tuft of five cilia that bear receptor proteins for various chemicals.
   • c. Molecules bind to receptor proteins and nerve impulses pass to olfactory bulb in front of brain.
   • d. Some processing occurs before olfactory information sent to temporal lobe where sensation of smell is produced.
   • e. About 1,000 different odor receptor proteins discovered; an odor can activate a combination of neurons with information pooled by olfactory lobe.
   • f. Olfactory receptors also adapt to consistent stimuli and no longer generate nerve impulses.
   • g. Sense of taste and smell supplement each other, creating a combined effect when interpreted by the cerebral cortex.

1. How the Eye Looks (Fig. 19.6; Table 19.2)
   • a. Elongated sphere about 2.5 cm in diameter.
   • b. Sclera
     • i. Outermost layer.
     • ii. White fibrous layer.
     • iii. Used for protection.
     • iv. Transparent cornea forms window of eye.
   • c. Choroid
     • i. Middle layer.
     • ii. Thin, dark brown layer.
     • iii. Contains many blood vessels.
     • iv. Absorbs stray light.
     • v. In front of eye, choroid thickens and forms:
       - ciliary body that contains ciliary muscle; controls shape of lens for focusing for near and far vision.
       - iris that is circular, muscular diaphragm that regulates size of pupil through which light enters.

       vi. Lens divides eye cavity.

       - vitreous humor is behind lens; very viscous.

       - aqueous humor in front of lens is liquid, secreted by ciliary body.

       vii. Glaucoma:

       - results from blockage of ducts that drain aqueous humor.

       - increased pressure prevents blood flow to the retina.

       - lack of nutrients for retina leads to partial or full blindness.
2. Retina: Three Layers of Cells (Fig. 19.7) (p. 351)
   • a. Innermost layer of eye has three layers of cells.
     • i. Rods and cones are closest to choroid; generates nerve impulses in response to light.
     • ii. Middle layer contains bipolar cells.
     • iii. Innermost layer contains ganglionic cells; axons form optic nerve.
   • b. 150 million rods communicate with bipolar cells.
   • c. Bipolar cells communicate with one million ganglionic cells; reduction in nerve fibers involves mixing and integration of impulses.
   • d. Impulses are relayed to thalamus and on to occipital lobe of cerebrum.
   • e. Blind spot is the region where the optic nerve passes through the retina, lacks rods and cones, and vision is impossible.
   • f. Fovea centralis is region of retina that contains only cones; has most acute vision.
3. Focusing: Bending Light (Fig. 19.8)
   • a. Light rays must be bent (refracted) to focus image on retina.
   • b. As light passes through cornea, lens and humors, image is focused upon retina.
   • c. Accommodation is ability of lens to adjust in order to see close or distant objects. (Fig. 19.8b, c)
     - For distant objects, lens is flat and ciliary muscle is relaxed, which causes suspensory ligaments to be taut.
     - For close objects, lens becomes round for greater refraction as ciliary muscle contracts; because close work requires contraction of ciliary muscle, it often causes eyestrain.

     d. With aging, lens loses some elasticity and is unable to accommodate.

     e. Lens is subject to cataracts where lens becomes opaque and unable to transmit light; research indicates it may be due to oxidized crystalline proteins in lens.
4. The Upside-Down Image (p. 353)
   • a. Image formed on retina is upside down but righted in brain by experience; the brain learns to see it "right side up."
5. Stereoscopic Vision (Fig. 19.9)
   • a. Stereoscopic vision is due to each eye forming an image from a slightly different angle.
   • b. Some optic nerves cross-over at optic chiasma; one-half of brain receives information from both eyes about a visual object.
   • c. Images on each half of brain interpreted as a whole.
6. Seeing Uses Chemistry
   • a. Rods detect dim light and motion.
   • b. Rhodopsin
     • i. Located in membrane of disks (lamellae) in outer segment of rod.
     • ii. Composed of opsin protein and retinal (derivative of vitamin A).
     • iii. Retinal absorbs light energy, changes shape and activates rhodopsin.
     • iv. Activation of rhodopsin reduces cGMP and closes some sodium ion channels in rod plasma membrane.
     • v. Increased negativity of rod interior causes electrochemical changes in bipolar and ganglionic cell layers and sends nerve impulse to brain.
     • vi. One molecule of rhodopsin causes cleavage of many cGMP molecules, thereby amplifying the stimulus.
   • c. Cones: (p. 354)
     • i. Detect fine detail and color of an object and located primarily in fovea.
     • ii. Color vision depends on three kinds of cones containing pigments sensitive to either blue, green, or red light.
     • iii. Each pigment is made of retinal and opsin, but with different opsin structure.
     • iv. Combinations of cones are stimulated by in-between shades of color to send combined nerve impulses interpreted by brain as particular color.
7. Color Blindness (Fig. 19.11) (p. 356)
   • a. Complete color blindness in humans is very rare.
   • b. Usually one particular type of cone is lacking or deficient in number.
   • c. Lack of red or green cones is most common, about 5 - 8% of human males.
   • d. Lack of red cones accents green, etc.
8. Corrective Lenses
   • a. 20/20 vision is ability to see at 20 feet what average person can see at 20 feet.
   • b. Nearsighted person:
     • i. can see close objects well but not at a distance.
     • ii. has eyeball elongated so image of a distant object is brought to focus in front of retina.
     • iii. corrected by using concave lenses to diverge light rays to focus on retina.
     • iv. new treatment called radial keratotomy cuts cornea and results in flatter cornea; some side effects noted.
   • c. Farsighted person:
     • i. can see distant objects.
     • ii. has eyeball shortened so image of a close object is brought to focus behind retina.
     • iii. corrected by using a convex lens to increase bending of light rays.
   • d. Astigmatism:
     • i. occurs when cornea or lens is uneven.
     • ii. image is fuzzy since light rays do not focus evenly.
     • iii. can be corrected by using unevenly ground lens.
   • e. Aging normally sees loss of lens' ability to change shape; bifocals common.

1. Ear functions for equilibrium (balance) and hearing.
2. Both receptors located in inner ear as hair cells with cilia that respond to mechanical stimulation.
3. How the Ear Appears (Fig. 19.13, Table 19.3)
   • a. Ear consists of three divisions: outer, middle, inner.
   • b. Outer ear:
     • i. pinna is external flap.
     • ii. auditory canal has modified sweat glands that secrete earwax to protect against foreign material.
   • c. Middle ear:
     • i. Begins at the tympanic membrane (eardrum).
     • ii. Contains three ossicles (small bones): malleus-hammer, incus-anvil, and stapes-stirrup.
     • iii. Vibrations transferred from tympanic membrane to malleus to incus to stapes to oval window.
     • iv. Eustachian tube extends from each middle ear to nasopharynx to permit equalization of air pressure.
   • d. Inner ear: (Fig. 19.14a) (p. 359)
     • i. Filled with fluid; two regions for equilibrium, one for hearing.
     • ii. Semicircular canals are arranged in all three dimensions of space; base of each canal is enlarged ampulla will hair cells with cilia inserted in ampullae; provide sense of balance.
     • iii. Vestibule:
       - Is chamber between semicircular canals and cochlea.
       - Contains two membranous sacs: utricle and saccule.
       - Both contain hair cells with cilia protruding into gelatinous material that has otoliths (calcium carbonate granules).

       iv. Cochlea:

       - Spirals resembling shell of a snail.

       - Contains three canals: vestibular canal, tympanic canal, and cochlear canal.

       - Cochlear canal contains the organ of Corti which is sense organ of hearing composed of hair cells plus the tectorial membrane.

       - Nerve impulses from here go to brain stem and to temporal lobe of cerebrum and are interpreted as sound.
4. Balance: Two Kinds
   • a. Sense of balance divided into:
     • i. Dynamic equilibrium required when the body is moving.
       - fluid in semicircular canals causes bending of hair cell cilia within ampullae which generates a nerve impulse.
       - Continuous movement causes one form of motion sickness.

       ii. Static equilibrium is required when head or body moves horizontally or vertically.

       - Otoliths in vestibule of utricle and saccule are displaced.

       - Bending the cilia generates a nerve impulse that travels to brain.
   • Hearing: Rubbing Cilia (Figs. 19.15 - 16) (p. 362 - 363)
     • a. Sound waves are vibrating molecules.
     • b. Sound causes eardrum to vibrate slightly.
     • c. Malleus-incus-stapes relay vibrations to oval window with 20-fold increase in pressure.
     • d. Vibrations at oval window are passed to fluid within inner ear.
     • e. As the basilar membrane moves up and down, cilia of hair cells of organ of Corti rub against the tectorial membrane initiating a nerve impulse that passes via auditory nerve to temporal lobe of brain.
     • f. At tip of cochlear canal, organ of Corti is wide and responds to low pitches; at base of cochlear canal, hair cells are narrow and respond to high pitches (frequencies).
     • g. Nerves along canal lead to slightly different regions of brain which determines pitch heard.
     • h. Volume (loudness) is a function of the amplitude of the sound waves (basilar membrane moves up and down to a greater extent).
     • i. Tone is believed to be an interpretation of the brain based on distribution of hair cells stimulated.
   • Deafness: Two Main Kinds (p. 363)
     • a. Conduction deafness:
       • i. May be due to a congenital defect, as when pregnant woman contracts German measles during first trimester.
       • ii. Ear infections may fuse ossicles together.
     • b. Nerve deafness:
       • i. Most often occurs when cilia on sense receptors are worn away.
       • ii. Occurs with aging or listening to loud music amplified to 130 decibels.
       • iii. Cochlear implants may help.