A&P 2 Ch. 16

Deafness

Deafness means any hearing loss, from mild and temporary to complete and irreversible. Conductive deafness results from any condition that interferes with the transmission of vibrations to the inner ear. Such conditions include a damaged tympanic membrane, otitis media, blockage of the auditory canal, and otosclerosis. Otosclerosis is fusion of the auditory ossicles to each other, or fusion of the stapes to the oval window. Either way, it prevents the bones from vibrating freely. Sensorineural (nerve) deafness results from the death of hair cells or any of the nervous elements concerned with hearing. It is a common occupational disease of factory and construction workers, musicians, and other people exposed to frequent or sustained loud sounds. Deafness leads some people to develop delusions of being talked about, disparaged, or cheated. Beethoven said his deafness drove him nearly to suicide.

Somatic pain

Pain from the skin, muscles, and joints is called somatic pain, and pain from the viscera is called visceral pain. The latter often results from stretch, chemical irritants, or ischemia, and is often accompanied by nausea.

Taste (gustatory) cells

Regardless of location and sensory specialization, all taste buds look alike (fig. 16.6c, d). They are lemon-shaped groups of 50 to 150 taste cells, supporting cells, and basal cells. Taste (gustatory) cells are more or less banana-shaped and have a tuft of apical microvilli called taste hairs, which serve as receptor surfaces for tastants. The hairs project into a pit called a taste pore on the epithelial surface of the tongue. Taste cells are epithelial cells, not neurons, but they synapse with sensory nerve fibers at their base and have synaptic vesicles for the release of neurotransmitters. A taste cell lives for only 7 to 10 days. Basal cells are stem cells that multiply and replace taste cells that have died, but they also synapse with sensory nerve fibers of the taste bud and may play some role in the processing of sensory information before the signal goes to the brain. Supporting cells resemble taste cells but have no synaptic vesicles or sensory role.

Equilibrium

The sense of equilibrium is divided into static equilibrium, the perception of the orientation of the head in space (whether it is erect or tilted in any direction), and dynamic equilibrium, the perception of motion or acceleration.

Vestibulocochlear nerve

cochlear nerve. This nerve joins the vestibular nerve, discussed later, and the two together become the vestibulocochlear nerve (cranial nerve VIII).

Color blindness

color blindness. The most common form is red-green color blindness, which results from a lack of either L or M cones and causes difficulty distinguishing these and related shades from each other. For example, a person with normal trichromatic color vision sees figure 16.42 as showing the number 74, whereas a person with red-green color blindness sees no number. Red-green color blindness is a sex-linked recessive trait. It occurs in about 8% of males and 0.5% of females.

Lacrimal apparatus

lacrimal apparatus consists of the lacrimal (tear) gland and a series of ducts that drain the tears into the nasal cavity

Vision/Light

Vision (sight) is the perception of objects in the environment by means of the light they emit or reflect. Light is visible electromagnetic radiation. Human vision is limited to wavelengths ranging from about 400 to 700 nm. Most solar radiation of shorter and longer wavelengths is filtered out by ozone, carbon dioxide, and water vapor in the atmosphere. Therefore, the radiation that reaches the surface of the earth falls within that range, and vision is adapted to take advantage of the wavelenths available to us. The ultraviolet (UV) radiation just below 400 nm and the infrared (IR) radiation just above 700 nm are invisible to us. Furthermore, radiation in the ultraviolet range has such high energy that it destroys macromolecules rather than producing the controlled chemical reactions needed for vision, and radiation in the infrared range has such low energy that it merely warms the tissues, also failing to energize chemical reactions.

Sensory receptor

A sensory receptor is any structure specialized to detect a stimulus. Some receptors are simple, bare nerve endings, such as the receptors for heat and pain, whereas others are true sense organs. A sense organ is a structure that combines nervous tissue with other tissues that enhance its response to a certain type of stimulus. The accessory tissues may include epithelial, muscular, or connective tissue. Sense organs can be as complex as the eye and ear or as microscopic and simple as a dendrite wrapped in a little bit of connective tissue.

Perilymph

Between the bony and membranous labyrinths is a cushion of fluid, the perilymph (PER-ih-limf), similar to cerebrospinal fluid. Within the membranous labyrinth is a fluid called endolymph, similar to intracellular fluid. The bony and membranous labyrinths form a tube-within-a-tube structure, somewhat like a bicycle inner tube within the tire.

Emmetropia & Near Response

Emmetropia is a state in which the eye is relaxed and focused on an object more than 6 m (20 ft) away, the light rays coming from that object are essentially parallel, and the rays are focused on the retina without effort. If the gaze shifts to something closer, light rays from the source are too divergent to be focused without effort. In other words, the eye is automatically focused on things in the distance unless you make an effort to focus elsewhere. For a wild animal or our prehistoric ancestors, this arrangement would be adaptive because it allows for alertness to predators or prey at a distance. The near response, or adjustment to close-range vision, involves three processes to focus an image on the retina:

Encapsulated nerve endings

Encapsulated nerve endings are nerve fibers wrapped in glial cells or connective tissue. Most of them are mechanoreceptors for touch, pressure, and stretch. The connective tissue either enhances the sensitivity of the nerve fiber or makes it more selective with respect to which modality it responds to

More receptors

Exteroceptors sense stimuli external to the body. They include the receptors for vision, hearing, taste, smell, and cutaneous sensations such as touch, heat, cold, and pain. Interoceptors detect stimuli in the internal organs such as the stomach, intestines, and bladder, and produce feelings of stretch, pressure, visceral pain, and nausea. Proprioceptors sense the position and movements of the body or its parts. They occur in muscles, tendons, and joint capsules.

Vestibular nuclei

Fibers of the vestibular apparatus lead to a complex of four vestibular nuclei on each side of the pons and medulla. Nuclei on the right and left sides of the brainstem communicate extensively with each other, so each receives input from both the right and left ears. They process signals about the position and movement of the body and relay information to five targets

Free nerve endings

Free nerve endings include warm receptors, which respond to rising temperature; cold receptors, which respond to falling temperature; and nociceptors for pain. They are bare dendrites that have no special association with specific accessory cells or tissues. They are most abundant in the skin and mucous membranes. You can locate some of your cold receptors by gently touching your skin with the point of a graphite pencil, which conducts heat away from the skin. In spots where these receptors are located, this produces a sensation of cold.

General senses

General (somatosensory, somesthetic) senses employ widely distributed receptors in the skin, muscles, tendons, joints, and viscera. These include touch, pressure, stretch, heat, cold, and pain, as well as many stimuli that we do not perceive consciously, such as blood pressure and composition. Their receptors are relatively simple—sometimes nothing more than a bare dendrite.

Gustation

Gustation begins with the chemical stimulation of sensory cells clustered in about 4,000 taste buds. The chemical stimuli are called tastants. Most taste buds are on the tongue, but some occur inside the cheeks and on the soft palate, pharynx, and epiglottis, especially in infants and children

Refraction

If light traveling through air strikes a medium of higher refractive index at a 90° angle of incidence, it slows down but does not change course—the light rays are not bent. If it strikes at any other angle, however, the light ray changes direction—it is refracted (fig. 16.31a). The greater the difference in refractive index between the two media, and the greater the angle of incidence, the stronger the refraction is.

Rod and Cone cells

In a rod, the outer segment is cylindrical and resembles a stack of coins in a wrapper—there is a plasma membrane around the outside and a neatly arrayed stack of about 1,000 membranous discs inside. Each disc is densely studded with globular proteins—the visual pigment rhodopsin. The membranes hold these pigment molecules in a position that results in the most efficient light absorption. Rod cells are responsible for night (scotopic55) vision and produce images only in shades of gray (monochromatic vision). A cone cell is similar except that the outer segment tapers to a point, and the discs are not detached from the Page 613plasma membrane but are parallel infoldings of it.

Photopsin

In cones, the pigment is called photopsin. Its retinal moiety is the same as that of rhodopsin, but the opsin moiety has a different amino acid sequence that determines which wavelengths of light the pigment absorbs. There are three kinds of cones, which are identical in appearance but optimally absorb different wavelengths of light. These differences enable us to perceive different colors.

Intensity

Intensity refers to whether a sound is loud or soft, a light is bright or dim, a pain is mild or excruciating, and so forth. It is encoded in three ways: (a) As stimulus intensity rises, the firing frequencies of sensory nerve fibers rise (see fig. 12.29); (b) intense stimuli recruit greater numbers of neurons to fire; and (c) weak stimuli activate only the most sensitive neurons, whereas strong stimuli can activate a less sensitive group of neurons with higher thresholds. Thus, the brain can distinguish intensities based on which neurons are firing, how many are doing so, and how fast they are firing.

Suspensory ligament

It is suspended behind the pupil by a ring of fibers called the suspensory ligament, which attaches it to the ciliary body. Tension on the ligament somewhat flattens the lens so it is about 9.0 mm in diameter and 3.6 mm thick at the middle.

Receptive field

Location is also encoded by which nerve fibers issue signals to the brain. Any sensory neuron detects stimuli within an area called its receptive field. In the sense of touch, for example, a single sensory neuron may cover an area of skin as large as 7 cm in diameter. No matter where the skin is touched within that field, it stimulates the same neuron. The brain may be unable to determine whether the skin was touched at "point A" or at some other point 1 or 2 cm away. If the skin is simultaneously touched at two places in the same field, it can feel like a single touch

Loudness/Amplitude

Loudness is the perception of sound energy, intensity, or the amplitude of vibration. In the speaker example, amplitude is a measure of how far forward and back the cone vibrates on each cycle and how much it compresses the air molecules in front of it. Loudness is expressed in decibels (dB), with 0 dB defined by a sound energy level that corresponds to the threshold of human hearing. Every 10 dB step up the scale represents Page 590a sound with 10 times greater intensity. Thus, 10 dB is 10 times threshold, 20 dB is 100 times threshold, 30 dB is 1,000 times threshold, and so forth. Normal conversation has a loudness of about 60 dB. At most frequencies, the threshold of pain is 120 to 140 dB, approximately the intensity of a loud thunderclap. Prolonged exposure to sounds greater than 90 dB can cause permanent loss of hearing.

Modality

Modality refers to the type of stimulus or the sensation it produces. Vision, hearing, and taste are examples of sensory modalities, as are subcategories of these such as red or blue light, bass or treble sounds. The action potentials for vision are identical to the action potentials for taste or any other Page 577modality—so how can the brain tell a visual signal from a taste signal? Modality is determined by where the sensory signals end in the brain. Hypothetically, if we could rewire the brain so signals from the ear were routed to the visual cortex of the occipital lobe, we would perceive acoustic signals as light rather than sound.

Sensation

Not all sensory signals go to the brain, but when they do, we may experience a sensation—a subjective awareness of the stimulus.

Olfaction

Olfaction The sense of smell; is a response to airborne chemicals called odorants. These are detected by receptor cells in a patch of epithelium, the olfactory mucosa, in the roof of the nasal cavity (fig. 16.7). This location places the olfactory cells close to the brain, but it is poorly ventilated; forcible sniffing is often needed to identify an odor or locate its source.

Referred pain

Pain in the viscera is often mistakenly thought to come from the skin or other superficial sites—for example, the pain of a heart attack is felt "radiating" along the left shoulder and medial side of the arm. This phenomenon, called referred pain, results from the convergence of neural pathways in the CNS. In the case of cardiac pain, for example, spinal cord segments T1 to T5 receive input from the heart as well as from the chest and arm. Pain fibers from the heart and skin in this region converge on the same spinal interneurons, then follow the same pathway from there to the thalamus and cerebral cortex. The brain cannot distinguish which source the arriving signals are coming from. It acts as if it assumes that they are most likely from the skin, since skin has more pain receptors than the heart and suffers injury more often. Knowledge of the origins of referred pain is essential to the skillful diagnosis of organ dysfunctions

Pain

Pain is discomfort caused by tissue injury or noxious stimulation, typically leading to evasive action. Few of us enjoy pain and we may wish that no such thing existed, but it is one of our most important senses and we would be far worse off without it. We see evidence of its value in such diseases as leprosy and diabetes mellitus, where the sense of pain is lost because of nerve damage (peripheral neuropathy). The absence of pain makes people unaware of minor injuries. They neglect them, so the injuries often turn gangrenous and cost people their fingers, toes, feet, or entire limbs.

Photoreceptor cells

Photoreceptor cells. Photoreceptor cells absorb light and generate a chemical or electrical signal. There are three kinds: rods, cones, and certain ganglion cells. Only rods and cones produce visual images; the ganglion cells are discussed later. Rods and cones are not neurons, but are related to the ependymal cells of the brain. Each rod or cone has an outer segment that points toward the wall of the eye and an inner segment facing the interior (fig. 16.36). The two segments are separated by a constriction containing nine pairs of microtubules; the outer segment is actually a highly modified cilium specialized to absorb light. The inner segment contains mitochondria and other organelles. At its base, it gives rise to a cell body, which contains the nucleus, and to processes that synapse with retinal neurons in the next layer.

Pitch

Pitch is our sense of whether a sound is "high" (treble) or "low" (bass). It is determined by the frequency at which the sound source, eardrum, and other parts of the ear vibrate.

Salty

Salty, produced by metal ions such as sodium and potassium. Since their salts are vital electrolytes, there is obvious value in our ability to taste them and in having an appetite for salt. Electrolyte intake is also important in the osmotic regulation of the body's fluid balance. Electrolyte deficiencies can cause a craving for salt; many animals as diverse as insects, parrots, deer, and elephants seek salt deposits when necessary. Pregnancy can lower a woman's electrolyte concentrations and create a craving for salty food.

Sound

Sound is any audible vibration of molecules. It can be transmitted through water, solids, or air, but not through a vacuum. Our discussion is limited to airborne sound.

Stereoscopic vision

Stereoscopic vision (stereopsis) is depth perception—the ability to judge how far away objects are. It depends on having two eyes with overlapping visual fields, which allows each eye to look at the same object from a different angle. Stereoscopic vision contrasts with the panoramic vision of mammals such as rodents and horses, in which the eyes are on opposite sides of the head. Although stereoscopic vision covers a smaller visual field than panoramic vision and provides less alertness to sneaky predators, it has the advantage of depth perception. The evolutionary basis of our stereoscopic vision was also explained along with color vision in chapter 1

Depth perception

The Retinal Basis of Stereoscopic Vision (Depth Perception). When the eyes converge on the fixation point (F), more distant objects (D) are focused on the retinas medial to the fovea and the brain interprets them as being farther away than the fixation point. Nearby objects (N) are focused lateral to the fovea and interpreted as being closer

Aqueous humor/Post. chamber/Ant. chamber/Scleral venous sinus

The aqueous humor is a serous fluid secreted by the ciliary body into a space called the posterior chamber between the iris and lens. It flows through the pupil into the anterior chamber between the cornea and iris. From here, it is reabsorbed by a circular vein called the scleral venous sinus. Normally the rate of reabsorption balances the rate of secretion

Auditory canal

The auditory canal (external acoustic meatus) is the passage leading through the temporal bone to the tympanic membrane. It follows a slightly S-shaped course for about 3 cm (fig. 16.11). It is lined with skin and supported by fibrocartilage at its opening and by the temporal bone for the rest of its length.

Analgesic

The central nervous system (CNS) has analgesic4 (pain-relieving) mechanisms that are just beginning to be understood.

Near point of vision

The closest an object can be and still come into focus is called the near point of vision. It depends on the flexibility of the lens. The lens stiffens with age, so the near point averages about 9 cm at the age of 10 and 83 cm by the age of 60.

Ear sections

The ear has three sections called the outer, middle, and inner ear. The first two are concerned only with transmitting sound to the inner ear, where vibration is converted to nerve signals.

Semicircular ducts

The head also experiences rotary movements, such as when you spin in a rotating chair, walk down a hall and turn a corner, or bend forward to pick something up from the floor. Such movements are detected by the three semicircular ducts, housed in the bony semicircular canals of the temporal bone. The anterior and posterior semicircular ducts are oriented vertically at right angles to each other. The lateral semicircular duct is about 30° from the horizontal plane. The orientation of the ducts causes a different duct to be stimulated by rotation of the head in different planes.

Projection pathways

The pathways followed by sensory signals to their ultimate destinations in the CNS are called projection pathways.

Inner layer (tunica interna)

The inner layer (tunica interna). This consists of the retina and beginning of the optic nerve.

Vestibule

The labyrinths begin with a chamber called the vestibule, which contains organs of equilibrium to be discussed later.

Middle ear

The middle ear is located in the tympanic cavity of the temporal bone. What we colloquially call the eardrum is anatomically known as the tympanic14 membrane. It closes the inner end of the auditory canal and separates it from the middle ear. The membrane is about 1 cm in diameter and slightly concave on its outer surface. It is suspended in a ring-shaped groove in the temporal bone and vibrates freely in response to sound. It is innervated by sensory branches of the vagus and trigeminal nerves and is highly sensitive to pain.

Retina and Optic nerve

The neural components are the retina and optic nerve. The retina forms from a cup-shaped outgrowth of the diencephalon called the optic vesicle (see fig. 14.4); it is actually a part of the brain—the only part that can be viewed without dissection. It is a thin transparent membrane attached to the rest of the eye at only two points: the optic disc, where the optic nerve leaves the rear (fundus) of the eye, and its scalloped anterior margin, the ora serrata.46 The vitreous body presses the retina smoothly against the rear of the eyeball. The retina can separate from the wall of the eyeball because of blows to the head or insufficient pressure from the vitreous body. Such a detached retina may cause blurry areas in the field of vision. It leads to blindness if the retina remains separated for too long from the choroid, on which it depends for oxygen, nutrition, and waste removal.

Olfactory cells

The olfactory mucosa covers about 5 cm2 of the superior concha, cribriform plate, and nasal septum of each nasal fossa. It consists of 10 to 20 million olfactory cells as well as epithelial supporting cells and basal stem cells. The rest of the nasal cavity is lined by a nonsensory respiratory mucosa.

Fibrous layer/Sclera/Cornea

The outer fibrous layer (tunica fibrosa). This is divided into two regions: sclera and cornea. The sclera (white of the eye) covers most of the eye surface and consists of dense collagenous connective tissue perforated by blood vessels and nerves. It serves as a tough fibrous protective cover for the eye and provides for attachment of the extrinsic muscles that move it. The cornea is the anterior transparent region of modified sclera that admits light into the eye. Most of it is composed of very compact layers of collagen fibrils and thin flat fibroblasts. It is covered by a thin stratified squamous epithelium anteriorly and a simple squamous epithelium posteriorly. Both epithelia pump sodium ions out of the corneal tissue. Water follows by osmosis, so this mechanism prevents the cornea from overhydrating, swelling, and losing transparency. The anterior epithelium also is a source of stem cells that give the cornea a great capacity for regeneration if it is injured.

Olfactory tracts/Primary olfactory cortex

The tufted and mitral cells carry output from the glomeruli. Their axons form bundles called olfactory tracts, which course posteriorly along the underside of the frontal lobes. Most fibers of the olfactory tracts end in various regions of the inferior surface of the temporal lobe regarded as the primary olfactory cortex.

Auditory ossicles

The tympanic cavity, a space only 2 to 3 mm wide between the outer and inner ears, contains the three smallest bones and the two smallest skeletal muscles of the body. The bones, called the auditory ossicles, connect the tympanic membrane to the inner ear

Lingual papillae

The visible bumps on the tongue are not taste buds but lingual papillae.

Visual pigment

The visual pigment of the rods is called rhodopsin (ro-DOP-sin), or visual purple. It consists of two major parts (moieties): a protein called opsin and a vitamin A derivative called retinene or retinal (rhymes with "pal") (fig. 16.37). Thus, a dietary deficiency of vitamin A can lead to night blindness, a difficulty seeing in dim light. Opsin is embedded in the disc membranes of the rod's outer segment. All rods contain a single kind of rhodopsin with an absorption peak at a wavelength of 500 nm. Rods are less sensitive to light of other wavelengths and cannot distinguish one color from another.

Receptors

Thermoreceptors respond to heat and cold. Photoreceptors, the eyes, respond to light. Nociceptors are pain receptors; they respond to tissue injury or situations that threaten to damage a tissue. Chemoreceptors An organ or cell specialized to detect chemicals, as in the carotid bodies and taste buds. Respond to chemicals, including odors, tastes, and body fluid composition. Mechanoreceptors respond to physical deformation of a cell or tissue caused by vibration, touch, pressure, stretch, or tension. They include the organs of hearing and balance and many receptors of the skin, viscera, and joints

Transducer

transduction, the conversion of one form of energy to another—light, sound, heat, touch, vibration, or other forms of stimulus energy into nerve signals. (Any device that converts one energy form to another is a transducer—whether a sense organ, gasoline engine, or lightbulb.)