Review of Key Concepts

Properties and Types of Sensory Receptors (p. 568)

1. Sensory receptors range from simple nerve endings to complex sense organs.

2. Sensory transduction is the conversion of stimulus energy into a pattern of action potentials.

3. Transduction begins with a receptor potential which, if it reaches threshold, triggers the production of action potentials.

4. Receptors transmit four kinds of information about stimuli: modality, location, intensity, and duration.

5. Receptors can be classified by modality as chemoreceptors, thermoreceptors, nociceptors, mechanoreceptors, and photoreceptors.

6. Receptors can also be classified by the origins of their stimuli as interoceptors, proprioceptors, and exteroceptors.

7. General (somesthetic) senses have receptors widely distributed over the body and include the senses of touch, pressure, stretch, temperature, and pain. Special senses have receptors in the head only and include vision, hearing, equilibrium, taste, and smell.

The General Senses (p. 588)

1. Unencapsulated nerve endings are simple sensory nerve fibers not enclosed in specialized connective tissue; they include free nerve endings, tactile discs, and hair receptors.

2. Encapsulated nerve endings are nerve fibers enclosed in glial cells or connective tissues that modify their sensitivity. They include muscle spindles, Golgi tendon organs, tactile corpuscles, Krause end bulbs, lamellated corpuscles, and Ruffini corpuscles.

3. Somesthetic signals from the head travel the trigeminal and other cranial nerves to the brainstem, and those below the head travel up the spinothalamic tract and other pathways. Most signals reach the contralateral primary somesthetic cortex, but proprioceptive signals travel to the cerebellum.

4. Pain is a sensation that occurs when nociceptors detect tissue damage or potentially injurious situations.

5. Fast pain is a relatively quick, localized response mediated by myelinated nerve fibers; it may be followed by a less localized slow pain mediated by unmyelinated fibers.

6. Somatic pain arises from the skin, muscles, and joints, and may be superficial or deep pain. Visceral pain arises from the viscera; it is less localized and is often associated with nausea.

7. Injured tissues release bradykinin, serotonin, prostaglandins, and other chemicals that stimulate nociceptors.

8. Pain signals travel from the receptor to the cerebral cortex by way of first-through third-order neurons. Pain from the face travels mainly by way of the trigeminal nerve to the pons, medulla, thalamus, and primary somesthetic cortex in that order. Pain from lower in the body travels by way of spinal nerves to the spinothalamic tract, thalamus, and somesthetic cortex.

9. Pain signals also travel the spinoreticular tract to the reticular formation and from there to the hypothalamus and limbic system,

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630 Part Three Integration and Control producing visceral and emotional responses to pain.

10. Referred pain is the brain's misidentification of the location of pain resulting from convergence in sensory pathways.

11. Enkephalins, endorphins, and dynorphins are analgesic neuropeptides (endogenous opioids) that reduce the sensation of pain. Pain awareness can also be reduced by the spinal gating of pain signals.

The Chemical Senses (p. 592)

1. Taste (gustation) results from the action of chemicals on the taste buds, which are groups of sensory cells located on some of the lingual papillae and in the palate, pharynx, and epiglottis.

2. Foliate, fungiform, and vallate papillae have taste buds; filiform papillae lack taste buds but sense the texture of food.

3. The primary taste sensations are salty, sweet, sour, bitter, and umami. Flavor is a combined effect of these tastes and the texture, aroma, temperature, and appearance of food. Some flavors result from the stimulation of free nerve endings.

4. Some taste chemicals (sugars, alkaloids, and glutamate) bind to surface receptors on the taste cells and activate second messengers in the cell; sodium and acids penetrate into the taste cell and depolarize it.

5. Taste signals travel from the tongue through the facial and glossopharyngeal nerves, and from the palate, pharynx, and epiglottis through the vagus nerve. They travel to the medulla oblongata and then by one route to the hypothalamus and amygdala, and by another route to the thalamus and cerebral cortex.

6. Smell (olfaction) results from the action of chemicals on olfactory cells in the roof of the nasal cavity.

7. Odor molecules bind to surface receptors on the olfactory hairs of the olfactory cells and activate second messengers in the cell.

8. Nerve fibers from the olfactory cells assemble into fascicles that collectively constitute cranial nerve I, pass through foramina of the cribriform plate, and end in the olfactory bulbs beneath the frontal lobes of the cerebrum.

9. Olfactory signals travel the olfactory tracts from the bulbs to the temporal lobes, and continue to the hypothalamus and amygdala. The cerebral cortex also sends signals back to the bulbs that moderate one's perception of smell.

Hearing and Equilibrium (p. 597)

1. Sound is generated by vibrating objects. The amplitude of the vibration determines the loudness of a sound, measured in decibels (db), and the frequency of vibration determines the pitch, measured in hertz (Hz).

2. Humans hear best at frequencies of 1,500 to 4,000 Hz, but sensitive ears can hear sounds from 20 Hz to 20,000 Hz. The threshold of hearing is 0 db and the threshold of pain is about 140 db; most conversation is about 60 db.

3. The outer ear consists of the auricle and auditory canal. The middle ear consists of the tympanic membrane and an air-filled tympanic cavity containing three bones (malleus, incus, and stapes) and two muscles (tensor tympani and stapedius). The inner ear consists of fluid-filled chambers and tubes (the membranous labyrinth) including the vestibule, semicircular ducts, and cochlea.

4. The most important part of the cochlea, the organ of hearing, is the spiral organ of Corti, which includes sensory hair cells. A row of 3,500 inner hair cells generates the signals we hear, and three rows of outer hair cells tune the cochlea to enhance its pitch discrimination.

5. Vibrations in the ear move the basilar membrane of the cochlea up and down. As the hair cells move up and down, their stereocilia bend against the relatively stationary tectorial membrane above them. This opens K+ channels at the tip of each stereocilium, and the inflow of K+ depolarizes the cell. This triggers neurotransmitter release, which initiates a nerve signal.

6. Loudness determines the amplitude of basilar membrane vibration and the firing frequency of the associated auditory neurons. Pitch determines which regions of the basilar membrane vibrate more than others, and which auditory nerve fibers respond most strongly.

7. The cochlear nerve joins the vestibular nerve to become cranial nerve VIII. Cochlear nerve fibers project to the pons and from there to the inferior colliculi of the midbrain, then the thalamus, and finally the primary auditory cortex of the temporal lobes.

8. Static equilibrium is the sense of the orientation of the head; dynamic equilibrium is the sense of linear or angular acceleration of the head.

9. The saccule and utricle are chambers in the vestibule of the inner ear, each with a macula containing sensory hair cells. The macula sacculi is nearly vertical and the macula utriculi is nearly horizontal.

10. The hair cell stereocilia are capped by a weighted gelatinous otolithic membrane. When pulled by gravity or linear acceleration of the body, these membranes stimulate the hair cells.

11. Any orientation of the head causes a combination of stimulation to the four maculae, sending signals to the brain that enable it to sense the orientation. Vertical acceleration also stimulates each macula sacculi, and horizontal acceleration stimulates each macula utriculi.

12. Each inner ear also has three semicircular ducts with a sensory patch of hair cells, the crista ampullaris, in each duct. The stereocilia of these hair cells are embedded in a gelatinous cupula.

13. Tilting or rotation of the head moves the ducts relative to the fluid (endolymph) within, causing the fluid to push the cupula and stimulate the hair cells. The brain detects angular acceleration of the head from the combined input from the six ducts.

14. Signals from the utricle, saccule, and semicircular ducts travel the vestibular nerve, which joins the cochlear nerve in cranial nerve VIII. Vestibular nerve fibers lead to the pons and cerebellum.

1. Vision is a response to electromagnetic radiation with wavelengths from about 400 to 750 nm.

2. Accessory structures of the orbit include the eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles.

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Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

3. The wall of the eyeball is composed of an outer fibrous layer composed of sclera and cornea; middle vascular layer composed of choroid, ciliary body, and iris; and an inner layer composed of the retina and beginning of the optic nerve.

4. The optical components of the eye admit and bend (refract) light rays and bring images to a focus on the retina. They include the cornea, aqueous humor, lens, and vitreous body. Most refraction occurs at the air-cornea interface, but the lens adjusts the focus.

5. The neural components of the eye absorb light and encode the stimulus in action potentials transmitted to the brain. They include the retina and optic nerve. The sharpest vision occurs in a region of retina called the fovea centralis, while the optic disc, where the optic nerve originates, is a blind spot with no receptor cells.

6. The relaxed (emmetropic) eye focuses on objects 6 m or more away. A near response is needed to focus on closer objects. This includes convergence of the eyes, constriction of the pupil, and accommodation (thickening) of the lens.

7. Light falling on the retina is absorbed by visual pigments in the outer segments of the rod and cone cells. Rods function at low light intensities

(producing night, or scotopic, vision) but produce monochromatic images with poor resolution. Cones require higher light intensities (producing day, or photopic, vision) and produce color images with finer resolution.

8. Light absorption bleaches the rhodopsin of rods or the photopsins of the cones. In rods (and probably cones), this stops the dark current of Na+ flow into the cell and the release of glutamate from the inner end of the cell.

9. Rods and cones synapse with bipolar cells, which respond to changes in glutamate secretion. Bipolar cells, in turn, stimulate ganglion cells. Ganglion cells are the first cells in the pathway that generate action potentials; their axons form the optic nerve.

10. The eyes respond to changes in light intensity by light adaptation (pupillary constriction and pigment bleaching) and dark adaptation (pupillary dilation and pigment regeneration).

11. The duplicity theory explains that a single type of receptor cell cannot produce both high light sensitivity (like the rods) and high resolution (like the cones). The neuronal convergence responsible for the sensitivity of rod pathways reduces resolution, while the lack of

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convergence responsible for the high resolution of cones reduces light sensitivity.

12. Three types of cones—blue, green, and red—have slight differences in their photopsins that result in peak absorption in different regions of the spectrum. This results in the ability to distinguish colors.

13. Stereoscopic vision (depth perception) results from each eye viewing an object from a slightly different angle, so its image falls on different areas of the two retinas.

14. Fibers of the optic nerves hemidecussate at the optic chiasm, so images in the left visual field project from both eyes to the right cerebral hemisphere, and images on the right project to the left hemisphere.

15. Beyond the optic chiasm, most nerve fibers end in the lateral geniculate nucleus of the thalamus. Here they synapse with third-order neurons whose fibers form the optic radiation leading to the primary visual cortex of the occipital lobe.

16. Some fibers of the optic nerve lead to the superior colliculi and pretectal nuclei of the midbrain. These midbrain nuclei control visual reflexes of the extrinsic eye muscles, pupillary reflexes, and accommodation of the lens in near vision.

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