Saladin 5e Extended Outline

Chapter 16

Sense Organs

I. Properties and Types of Sensory Receptors (pp. 587–589)
A. A sensory receptor is any structure specialized to detect a stimulus; a sensory organ is a structure composed of nervous tissue along with other tissues that enhance its response to a certain type of stimulus. (p. 587)
B. In terms of general properties, the fundamental purpose of any sensory receptor is transduction, the conversion of one form of energy to another, such as light into nerve signals. (p. 587–589)
1. The initial effect of a stimulus on a sensory cell is a receptor potential.
a. In many cases, the sensory cell is a neuron, and if the receptor potential is strong enough, the neuron fires a volley of action potentials, generating a nerve signal to the CNS.
b. In other cases, the sensory cell is not a neuron but an epithelial cell; it has synaptic vesicles, however, and releases a neurotransmitter that stimulates an adjacent neuron.
2. When sensory signals reach the brain, we may experience a sensation, a subjective awareness of the stimulus; however, most sensory signals produce no conscious sensation at all.
3. Sensory receptors transmit four kinds of information: modality, location, intensity, and duration.
a. Modality refers to the type of stimulus or the sensation it produces; examples of sensory modalities are vision, hearing, and taste.
i. The brain distinguishes signals by assuming that if a signal comes from the retina, for example, it must be a visual signal.
ii. It is as though the nerve pathways are “labeled” to identify their origins, and this theory of sensory interpretation is called the labeled line code.
b. Location is also encoded by which nerve fibers are issuing signals to the brain.
i. A sensory neuron detects stimuli within an area called its receptive field.
ii. The size of the receptive field varies depending on the region of the body.
iii. For example, in the sense of touch, if the skin is touched in two places in the same receptive field, it feels like a single touch. (Fig. 16.1a)
iv. In areas like the fingertips, there is much finer two-point touch discrimination, as little as 2 mm. (Fig. 16.1b)
v. Sensory projection is the ability of the brain to identify the site of stimulation; the pathways followed by sensory signals to their destinations in the CNS are called projection pathways.
c. Intensity refers to whether a sound is loud or soft, or a light is bright or dim, etc.; intensity is coded in three ways:
i. As stimulus intensity rises, the firing frequencies of sensory nerve fibers rise. (Fig. 12.28)
ii. Intense stimuli recruit greater numbers of nerve fibers to fire.
iii. Weak stimuli activate only the most sensitive nerve fibers, whereas strong stimuli can activate a less sensitive group of fibers with higher thresholds.
d. Duration, or how long a stimulus lasts, is encoded by changes in the firing frequency of a nerve fiber with the passage of time.
i. All receptors exhibit the property of sensory adaptation, by which the firing of a neuron decreases as a stimulus is prolonged.
ii. Phasic receptors generate a burst of action potentials upon initial stimulation, then quickly adapt and reduce or stop signaling even if the stimulus continues.
iii. Tonic receptors adapt more slowly and generate nerve signals more steadily; examples are the proprioceptors—the brain must always be aware of body position, muscle tension, and joint motion.
C. Receptors can be classified by several overlapping systems. (p. 589)
1. Classifying by stimulus modality:
a. Thermoreceptors respond to heat and cold.
b. Photoreceptors respond to light
c. Nociceptors are pain receptors that respond to tissue injury or potentially damaging situations.
d. Chemoreceptors respond to chemical, including odors, tastes, and body fluid composition.
e. Mechanoreceptors respond to physical deformation caused by vibration, touch, pressure, stretch, or tension, such as sound waves, touch, gravity, and movements.
2. Classifying by the origin of the stimulus:
a. Exteroceptors sense stimuli external to the body.
b. Interoceptors detect stimuli in the internal organs.
c. Proprioceptors sense the position and movements of the body or its parts.
3. Classifying by the distribution of receptors in the body:
a. General (somesthetic)senses employ receptors widely distributed in the skin, muscles, tendons, joint capsules, and viscera.
b. Special sense are limited to the head and employ complex sense organs.
II. The General Senses (pp. 589–595)
A. Receptors for the general senses are structurally simple and consist of one or a few sensory nerve fibers and usually a sparse amount of connective tissue. (p. 590) (Fig. 16.2) (Table 16.1)
B. Unencapsulated nerve endings are sensory dendrites that are not wrapped in connective tissue. (p. 590)
1. Free nerve endings include warm receptors, cold receptors, and nociceptors.
a. They are bare dendrites with no special associations with accessory cells or tissues.
b. They are most abundant in the skin and mucous membranes.
2. Tactile (Merkel) discs are tonic receptors for light touch and also textures, edges, and shapes.
a. They are flattened nerve endings that terminate adjacent to tactile (Merkel) cells in the basal layer of the epidermis.
3. Hair receptors (peritrichial endings) monitor the movements of hairs.
a. They consist of a few dendrites entwined around the base of a hair follicle and respond to any light touch that bends a hair.
b. They adapt quickly so that we are not constantly stimulated by clothing, but are very sensitive to light touch.
C. Encapsulated nerve endings are nerve fibers wrapped in glial cells or connective tissue; most are mechanoreceptors for touch, pressure, and stretch. (pp. 590–591)
1. Tactile (Meissner) corpuscles are phasic receptors for light touch and texture.
a. The consist of two or three nerve fibers meandering upward through a mass of flattened Schwann cells.
b. They occur in the dermal papillae of the skin and are especially concentrated in sensitive hairless areas.
2. End bulbs (Krause end bulbs, Krause corpuscles) are ovoid bodies composed of a connective tissue sheath around a sensory nerve fiber.
a. They occur in mucous membranes of the lips and tongue, in the conjunctiva on the anterior surface of the eye, and in the epineurium of large nerves.
3. Lamellar (pacinian) corpuscles are phasic receptors for deep pressure, stretch, tickle, and vibration.
a. They are large receptors, 1 or 2 mm long, and look like a sliced onion in cross section.
b. A single dendrite travels through the center of the corpuscle, which consists of concentric layers of fibroblasts with flattened Schwann cells comprising the innermost layers.
c. They occur in the periosteum of bone, in joint capsules, in some viscera, and deep in the dermis.
4. Bulbous (Ruffini) corpuscles are tonic receptors for heavy touch, pressure, skin stretching, and joint movements.
a. They are flattened, elongated capsules with a few nerve fibers, located in the dermis, subcutaneous tissue, and joint capsules.

D. Somesthetic projection pathways consist of three neurons, called the first-order, second-order, and third-order neurons. (pp. 591–592)
1. The first-order fibers for touch, pressure, and proprioception are large, myelinated, and fast, whereas those for heat and cold are small, unmyelinated or lightly myelinated,, and slower.
2. Somesthetic signals from the head, such as facial sensations, travel by way of several cranial nerves (especially the trigeminal nerve, CN V) to the pons and medulla oblongata.
a. In the brainstem, the first-order fibers of these neurons synapse with second-order neurons that decussate and end in the contralateral thalamus.
b. Third-order neurons then complete the route to the cerebrum.
c. Proprioceptive signals are an exception, as second-order fibers carry these to the cerebellum.
3. For signals below the head, first-order fibers enter the posterior horn of the spinal cord.
a. Signals ascend via the spinothalamic and other pathways.
b. These pathways decussate at or near the point of entry into the spinal cord or in the brainstem, so the primary somesthetic cortex in each hemisphere receives contralateral signals.
c. Proprioceptive signals from below the head travle up the spinocerebellar tracts to the cerebellum.
d. Signals from the thoracic and abdominal viscera travel to the medulla oblongata by way of sensory fibers in the vagus nerve (CN X).
E. Pain is a discomfort caused by tissue injury or noxious stimulation, and typically leading to evasive action. (pp. 592–595)
1. Pain is an adaptive and necessary sensation that has its own specialized nerve fibers, the nociceptors.
2. Although no nociceptors occur in the brain, they do occur in the meninges, where they have a role in headaches.
3. There are two types of nociceptors corresponding to different pain sensations.
a. Myelinated pain fibers conduct at speeds of 12 to 30 m/sec and produce the sensation of fast (first) pain—a sharp, localized pain perceived at the time of injury.
b. Unmyelinated pain fibers conduct at speeds of 0.5 to 2 m/sec and produce the slow (second) pain that follows—a longer-lasting, diffuse feeling.
4. Pain from the skin, muscles, and joints is called somatic pain; pain from the viscera is called visceral pain.
5. Injured tissues release several chemicals that stimulate the nociceptors.
a. Bradykinin is the most potent pain stimulus known.
b. Serotonin, prostaglandins, and histamine also stimulate nociceptors, as do K+ and ATP released from injured cells.
6. Projection pathways for pain are diverse and complex, and the sensation can originate anywhere along any of the routes.
7. Pain signals travel by two main pathways, but have multiple subroutes.
a. Pain signals from the head travel to the brainstem by way of four cranial nerves: trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X).
i. Trigeminal fibers enter the pons and descend to synapses in the medulla, where pain fibers of the other three cranial nerves also terminate.
ii. Second-order neurons arise in the medulla and ascend to the thalamus, and are relayed to the cerebral cortex.
b. Pain signals from the neck down travel by way of three of the ascending spinal cord tracts: the spinothalamic tract, the spinoreticular tract, and the gracile fasciculus. (Fig. 16.3)
i. The spinothalamic tract is the most significant and carries most of the somatic pain signals that ultimately reach the cerebral cortex.
ii. The spinoreticular tract carries pain signals to the reticular formation of the brainstem, and these are ultimately relayed to the hypothalamus and limbic system; these signals activate visceral, emotional, and behavioral reactions to pain such as nausea, fear, and some reflex responses.
iii. The gracile fasciculus has only recently been recognized as a pain pathway; it carries signals to the thalamus for visceral pain.
c. The thalamus relays most pain signals through third-order neurons to their final destination in the postcentral gyrus of the cerebrum.
i. Most of the gyrus receives somatic pain, but a deep region receives the signals of visceral pain conveyed by the gracile fasciculus. (Fig. 14.22)
d. Pain in the viscera is often mistakenly thought to come from the skin or other superficial sites, a phenomenon called referred pain; it results from convergence of neural pathways in the CNS. (Fig. 16.4)
8. The CNS has analgesic mechanisms that allow modulation of pain.
a. Having discovered the receptors for opioid drugs in the brain, physiologists wondered what substances normally bind to these receptors; they found enkephalins, which have 200 times the potency of morphine.
b. Larger analgesic neuropepties, the endorphins and dynorphins, were discovered later.
c. All three groups are known as endogenous opioids—internally produced opium-like substances.
9. Endogenous opioids are secreted by the CNS, pituitary gland, digestive tract,a nd other organs in states of stress or exercise.
a. In the CNS they are found in the central gray matter of the midbrain and the posterior horn of the spinal cord.
b. They act as neuromodulators that can block pain signals and produce feelings of pleasure and euphoria.
c. They may be responsible for the “second wind” and for gravely wounded soldiers’ reports of not feeling pain; their secretion rises sharply in women giving birth.
10. For pain to be perceived, signals must travel beyond the posterior horn of the spinal cord to the brain; opioids block pain through spinal gating mechanisms that stop the signals at the posterior horn.
11. Two mechanisms block the sensation of pain.
a. The normal route of pain transmission is as follows: (Fig. 16.5)
i. A nociceptor stimulates a second-order nerve fiber whose neurotransmitter is substance P.
ii. The second-order nerve fiber transmits signals up the spinothalamic tract to the thalamus.
iii. The thalamus relays the signals through a third-order neuron to the cerebral cortex.
b. The pathway for blocking or modulation, the descending analgesic fibers, is as follows: (Fig. 16.5)
i. Signals from the hypothalamus and cerebral cortex feed into the central gray matter of the midbrain.
ii. The midbrain relays signals to nuclei in the reticular formation of the medulla oblongata.
iii. Signals are sent from the medulla through the descending, serotonin-secreting analgesic fibers to the spinal cord; the fibers travel the reticulospinal tract and terminate in the posterior horn at all levels.
iv. In the posterior horn, some fibers synapse on short spinal interneurons that in turn synapse on second=order pain fibers; the interneurons secrete enkephalins, causing postsynaptic inhibition on the second-order fibers.
v. Some fibers from the medulla also exert presynaptic inhibition, synapsing on axons of nociceptors and blocking substance P release.
c. A second mechanism for pain blocking or modulation involves mechanoreceptors.
i. When a bumped or injured area is rubbed, mechanoreceptors are stimulated; these stimulate spinal interneurons that secrete enkephalins.
III. The Chemical Senses (pp. 595–600)
A. Taste (gustation) results from actions of chemicals on the taste buds. (pp. 595–597)
1. Anatomically, the tongue exhibits four types of surface protrusions called lingual papillae. (Fig. 16.6a)
a. Filiform papillae are tiny spikes without taste buds; they are the most abundant papillae on the human tongue.
i. They play no role in taste perception but serve in perception of texture of food.
ii. They are important to many mammals for grooming fur.
b. Foliate papillae form parallel ridges on the sides of the tongue about two-thirds back from the tip; their taste buds degenerate by the age of 2 or 3 years.
c. Fungiform papillae are shaped somewhat like mushrooms. Each has about three taste buds; they are widely distributed but especially concentrated at the tip and sides of the tongue.
d. Vallate (circumvallate) papillae are arranged in a V at the rear of the tongue; there are only 7 to 12 of them but they contain up to half of all taste buds (250 each). (Fig. 16.6b)
2. All taste buds look alike: lemon-shaped groups of 40 to 60 taste cells, supporting cells, and basal cells. (Figs. 16.6c, d)
a. Taste (gustatory) cells are more or less banana shaped and have a tuft of apical microvilli called taste hairs.
i. The hairs project into a pit called a taste pore on the epithelial surface of the tongue.
ii. Taste cells are epithelial cells but synapse with sensory nerve fibers at their base and are able to release neurotransmitters.
iii. A taste cell lives for only 7 to 10 days.
b. Basal cells are stem cells that multiply and replace taste cells that have died.
c. Supporting cells resemble taste cells but have no taste hairs or sensory role.
3. Physiologically, there are five primary taste sensations, which are sensed when molecules dissolve in saliva and floor the taste pore.
a. Salty taste is produced by metal ions such as sodium and potassium, which are critical nutrients.
b. Sweet taste is produced by many organic compounds, especially sugars.
c. Sour taste is usually associated with acids in foods such as citrus fruits.
d. Bitter is associated with spoiled foods and alkaloids; alkaloids are often poisonous, and plants produce them in their leaves to deter animals from eating them.
e. Umami taste is a “meaty” taste produced by amino acids such as aspartic and glutamic acid.
4. Flavors we perceive are influenced not only by the five primary tastes, but also by texture, aroma, temperature, appearance, and state of mind.

a. Some flavors, such as hot pepper, are due to stimulation of free endings of the trigeminal nerve rather than taste buds.
b. Texture is referred to by food technologists as mouth feel.
c. Filiform and fungiform papillae are innervated by the lingual nerve (branch of trigeminal) and are sensitive to texture.
d. All of the primary tastes can be detected throughout the tongue, but some regions are more sensitive to one taste than to another; the tip of the tongue is most sensitive to sweet tastes, and the lateral margins for salty and sour tastes.
e. Sugars, alkaloids, and glutamate stimulate taste cells by binding to receptors on the membrane surface that then activate G proteins and second-messenger systems.
f. Salts and acids penetrate into the cell and depolarize it directly.
5. Projection pathways include the cranial nerves CN VII, CN IX, and CN X.
a. The facial nerve (CN VII) collects sensory information from the taste buds of the anterior two-thirds of the tongue.
b. The glossopharyngeal nerve (CN IX) collects information from the posterior one-their of the tongue.
c. The vagus nerve (CN X) collects information from the taste buds of the palate, pharynx, and epiglottis.
d. All taste fibers project to a site in the medulla oblongata called the solitary nucleus.
i. Second-order neurons from the solitary nucleus relay signals to two destinations: nuclei in the hypothalamus and amygdala that activate reflexes such as salivation, gagging, and vomiting; and the thalamus, which relays signals to the insula and postcentral gyrus, where we become conscious of the taste.
ii. Processed signals are further relayed to the orbitofrontal cortex where they are integrated with signals from the nose and eyes.
B. Smell (olfaction) is detected by receptor cells in a patch of epithelium, the olfactory mucosa, in the roof of the nasal cavity. (pp. 597–599) (Fig. 16.7)
1. Anatomically, the olfactory mucosa covers about 5 square cm of the superior concha, cribriform plate, and nasal septum of each nasal fossa.
a. It consists of 10 to 20 million olfactory cells as well as epithelial supporting cells and basal stem cells; the rest of the cavity is lined by a nonsensory respiratory mucosa.
b. Olfactory cells are neurons shaped like little bowling pins, with the widest part, the neurosoma, containing the nucleus and the neck and head consisting of a modified dendrite with a swollen tip. (Fig. 16.7c)
c. The head bears 10 to 20 cilia called olfactory hairs that have binding sites for odor molecules.
d. The basal end of each cell tapers to become an axon, and these axons collect into small fascicles that leave the nasal cavity through cribriform foramina in the ethmoid bone.
i. Collectively, the fascicles are regarded as CN I (olfactory nerve).
e. Olfactory cells are the only neurons directly exposed to the environment and they have a life span of only 60 days; however, unlike most neurons they are replaceable.
2. Physiologically, humans have a poorer sense of smell than most mammals; humans have about half as much olfactory mucosa as a house cat.
a. Smell is nevertheless much more sensitive than the sense of taste, and odor concentrations as low as a few parts per trillion are detectable.
i. Women are on average more sensitive to odors than men are, and their sensitivity to some odors vary with the menstrual cycle.
ii. Most people can distinguish 2,000 to 4,000 odors, and some up to 10,000.
Insight 16.1 Human Pheromones
d. Rats and mice have up to 1,200 functional genes coding for receptor proteins that bind odor molecules, but in humans, two-thirds of the olfactory genes have mutated to become inoperative pseudogenes.
e. Each olfactory cell has only one receptor type and therefore binds only one odorant.
f. The first step in smell is that an odorant molecule binds to a receptor.
i. Hydrophilic odorants diffuse freely through the mucus of the olfactory epithelium to bind to a receptor.
ii. Hydrophobic odorants are transported by odorant-binding proteins in the mucus.
g. When the receptor binds an odorant, a G protein is activated and through it, the cAMP second-messenger system.
i. The second-messenger system opens ion channels to admit cations and depolarize the cell.
ii. Action potentials are transmitted to the brain.
h. Some odorants act on nociceptors of the trigeminal nerve, among them ammonia, menthol, chlorine, and capsaicin.
3. Projection pathways for smell begin with olfactory fibers passing through the roof of the nose and entering a pair of olfactory bulbs. (Fig. 14.27)
a. In the olfactory bulbs, the fibers synapse with dendrites of two types of neurons, mitral cells and tufted cells, in spherical clusters called glomeruli. (Fig. 16.7 b)
i. Each glomerulus is dedicated to a particular odor.
ii. Higher brain centers interpret complex odors by decoding signals from a combination of odor-specific glomeruli.
b. Axons from the tufted and mitral cells form bundles called olfactory tracts that course posteriorly along the underside of the frontal lobes.
c. Most fibers from these tracts end in neighboring regions of the inferior surface of the temporal lobe; these are collectively considered the primary olfactory cortex. (Fig. 16.8)
d. Olfactory signals can reach the cerebral cortex directly, without first passing through the thalamus, which is not true of any other senses; however, some signals continue to the thalamus on their way to association areas elsewhere.
e. From the primary olfactory cortex, signals travel to several other secondary destinations in the cerebrum and brainstem, among them the insula and orbitofrontal cortex.
i. The orbitofrontal cortex integrates input from both taste and smell.
ii. Other secondary destinations include the hippocampus, amygdala, and hypothalamus; these are responsible for the strong emotional and visceral reactions to different scents.
f. Most areas of olfactory cortex also send fibers back to the olfactory bulbs by way of granule cells, neurons that can inhibit the mitral and tufted cells.
IV. Hearing and Equilibrium (pp. 600–613)
A. Hearing is a response to vibrating air molecules and equilibrium is the sense of motion, body orientation, and balance; both senses reside in the inner ear. (p. 600)
B. Sound is any audible vibration of molecules; it is transmitted through water, solids, or air, but not through a vacuum. (pp. 600–601)
1. Pitch is our sense of whether a sound is “high” (treble) or “low” (bass), and it is determined by the frequency at which the sound source, eardrum, and other parts of the ear vibrate.
a. A single movement of a vibrating object back and forth is called a cycle, and the number of cycles per second (hertz, Hz) is called frequency.
i. The most sensitive human ears can hear frequencies from 20 to 20,000 Hz.
ii. Infrasonic frequencies below 20 Hz are not detected by the ear, but are sensed through vibrations of the skull and skin.
iii. Ultrasonic frequencies are inaudible vibrations above 20,000 Hz.
b. Humans are most sensitive to frequencies from 1,500 to 5,000 Hz at low energy (volume); sounds above or below this range must be louder to be audible. (Fig. 16.9)
2. Loudness is the perception of sound energy, intensity, or the amplitude of vibration; it is expressed in decibels (dB) with 0 being the threshold of human hearing.
a. Every 10 dB step up the scale represents a sound with 10 times greater intensity.
b. The threshold of pain is about 120 to 140 dB; prolonged exposure to sounds greater than 90 dB can cause permanent hearing loss.
C. Anatomically, the ear consists of three sections, the outer, middle, and inner ear, of which the first two are concerned only with transmitting sound to the inner ear. (pp. 601–605)
1. The outer (external) ear is essentially a funnel for conducting airborne vibrations to the tympanic membrane (eardrum).
a. The fleshy auricle (pinna) on the side of the head, is shaped and supported by elastic cartilage except for the earlobe and directs sound into the auditory canal. (Fig. 16.10)
b. The auditory canal is the passage leading from the external acoustic meatus (external ear opening) through the temporal bone to the tympanic membrane. (fig. 16.11)
i. The outer end of the canal is protected by stiff guard hairs.
ii. The canal has ceruminous and sebaceous glands whose secretions mix with dead skin cells and form cerumen (earwax).
iii. These block foreign particles and pests from entering the auditory canal
iv. Cerumen also contains lysozyme and has a low pH, which inhibit bacterial growth; it also waterproofs the canal and keeps the tympanic membrane pliable.
2. The middle ear is located in the tympanic cavity of the temporal bone.
a. The tympanic membrane (eardrum) closes the inner end of the auditory canal, separating it from the middle ear.
i. The membrane is about 1 cm in diameter and is suspended in a ring shaped groove in the temporal bone.
ii. It is enervated by sensory branches of the vagus and trigeminal nerves.
b. Posteriorly, the tympanic cavity is continuous with the mastoidal air cells in the mastoid process.
i. It is filled with air that enters by way of the auditory (eustachian) tube, which connects to the nasopharynx.
ii. This tube allows equalization of air pressure on both sides of the tympanic membrane.
c. The tympanic cavity contains the three small auditory ossicles and two small muscles.
i. The malleus has an elongated handle attached to the innder surface of the tympanic membrane; a head suspended by a ligament from the tympanic cavity; and a short process that articulates with the next ossicle.
ii. The incus has a roughly triangular body that articulates with the malleus; a long limb that articulates with the stapes; and a short limb suspended by a ligament from the tympanic cavity.
iii. The stapes has an arch and a footplate and is shaped like a stirrup; the footplate is held by a ringlike ligament in the oval window, where the inner ear begins.
iv. The muscles of the middle ear are the stapedius, which inserts on the stapes, and the tensor tympani, which inserts on the malleus.
Insight 16.2 Middle-Ear Infection
3. The inner (internal) ear is housed in a maze of temporal bone passageways called the bony (osseous) labyrinth, which lined by a system of fleshy tubes called the membranous labyrinth. (Fig. 16.12)
a. Between the bony and membranous labyrinths is a cushion of fluid called perilymph, similar to cerebrospinal fluid.
b. Within the membranous labyrinth is a fluid called endolymph, similar to intracellular fluid.
c. The two labyrinths form a tube-within-a-tube structure.
d. The labyrinths begin with a chamber called the vestibule, which contains organs of equilibrium.
e. The organ of hearing is the cochlea, a coiled tube that arises from the anterior side of the vestibule.
i. In other vertebrates, the cochlea is straight or slightly curved, but in most mammals it forms a snail-like spiral; its apex points anterolaterally.
ii. The cochlea winds for about 2.5 coils around a screwlike axis of spongy bone called the modiolus.
f. The cochlea has three fluid filled chambers separated by membranes.
i. The superior chamber is the scala vestibule and the inferior one is the scala tympani.
ii. The scala vestibuli begins near the oval window and spirals to the apex; from there, the scala tympani spirals back down to the base and ends at the round window, covered by the secondary tympanic membrane. (Fig. 16.12)
iii. The middle chamber is a triangular space, the cochlear duct (scala media), separated from the scala vestibuli by a thin vestibular membrane and from the scala tympani by the basilar membrane; it is filled with endolymph rather than perilymph.
g. Within the cochlear duct is the spiral organ (acoustic organ or organ of Corti), which converts vibrations into nerve impulses. (Fig. 16.13c)
i. The spiral organ has an epithelium composed of hair cells and supporting cells.
ii. The hair cells have long, stiff microvilli called stereocilia on their apical surfaces, and resting on top of them is a gelatinous tectorial membrane.
h. The spiral organ has four rows of hair cells along its length. (Fig. 16.14)
i. 3,500 of these are inner hair cells (IHCs) arranged in a row on the medial side of the basilar membrane; each has a cluster of 50 to 60 stereocilia graded from short to tall.
ii. 20,000 outer hair cells (OHCs) are neatly arranged in three rows across from the inner hair cells; each has about 100 stereo cilia about 100 stereocilia arranged in the form of a V.
iii. all that we hear comes from the IHCs, which supply 90% to 95% of sensory fibers of the cochlear nerve; the function of OHCs is to adjust the response to different frequencies.
iv. Hair cells are not neurons, but synapse with nerve fibers at their bases: OHCs with sensory and motor neurons and IHCs with sensory neurons only.
D. Physiologically, sound waves that enter the auditory canal on one side must be transformed into nerve signals that exit the inner ear on the other. (pp. 605–609)
1. The middle ear and its bones and muscles play important roles in sound transmission.
a. The tympanic membrane moves quite easily, whereas the stapes footplate at the oval window must push against the fluid of the inner ear, which is resistant.
b. The tympanic membrane has 18 times the area of the oval window, so the bones of the middle ear create a greater force per unit area at the oval window; however, no amplification of sound occurs.
c. The ossicles can serve to lessen the transfer of energy to the inner ear.
i. In response to loud noise, the tympanic reflex muffles the transfer of vibrations; the tensor tympani pulls the tympanic membrane inward and tenses it, while the stapedius reduces the motion of the stapes.
ii. The tympanic reflex has a latency of about 40 msec, which is not quick enough to protect against sudden artificial noises such as gunshots.
iii. It also does not protect adequately against sustained loud noises such as factory noise or loud music, which can irreversibly damage the hair cells of the inner ear.
d. The middle ear muscles also help coordinate speech with hearing.
i. Without their effects, the sound of your own speech would be so loud it could damage your inner ear.
ii. Just as you are about to speak, the brain signals these muscles to contract; this dampens the sense of hearing in phase with the inflections of your own voice and makes it easier to hear other people while speaking.
2. The stimulation of cochlear hair cells is the next step in hearing, and it occurs through movements of inner-ear fluids and the basilar membrane, while the tectorial membrane remains relatively still. (Fig. 16.15)
a. Inward movement of the tympanic membrane pushes the ossicles inward, and the stapes pushes on the perilymph in the scala vestibuli.
b. the resulting pressure pushes the vestibular membrane downward, which pushes on the endolymph in the cochlear duct.
c. The endolymph pushes down on the basilar membrane, which in turn pushes on the perilymph of the scala tympani.
d. Finally, the secondary tympanic membrane bulges outward.
e. Meanwhile, the hair cells of the basilar membrane bob up and down as the basilar membrane moves.
f. The tips of the hair cells are bathed in the endolymph, which is a high-potassium fluid.
i. Potassium is secreted into the endolymph by cells around the circumference of the cochlear duct.
ii. Relative to the perilymph, the endolymph has an electrical potential of about +80 mV, and the interior of the hair cell about –40 mV.
iii. This electrical gradient provides the potential energy that enables the hair cells to work.
g. Each stereocilium of the inner hair cells has a single transmembrane protein at its tip that functions as a mechanically regulated ion gate.
i. A fine, stretchy protein filament called a tip link extends from the ion channel of one stereocilium to the sidewall of the stereocilium next to it. (Fig. 16.16)
ii. The stereocilia increase in height progressively, so that every stereocilium but the tallest one has a tip link leading to the next taller one beside it.
h. The tectorial membrane is anchored to the core of the cochlea and remains relatively still as the hair cells move up and down.
i. Each time the basilar membrane rises toward the tectorial membrane, the hair cell stereocilia are pushed against the membrane and tilt toward the tallest one.
ii. As each taller stereocilium bend over, it pulls on the attached tip link.
i. The tip link, connected to the ion gate of the next shorter stereocilium, pulls the gate open and allows K+ ions to flood into the cell.
i. The hair cell depolarizes while the gate is open, and is briefly hyperpolarized when the gate closes.
ii. While depolarized, the hair cell releases a burst of neurotransmitter from its base, exciting the sensory dendrite with which it synapses.
j. A signal is generated in the cochlear nerve and transmitted to the brain.
3. Sensory coding refers to the response of the cochlea to sounds having different amplitude (loudness) and frequency.
a. The intensity of cochlear vibration varies depending on loudness; a soft sound produces relatively slight up-and-down movements.
b. Frequency discrimination is based on a structural gradient in the basilar membrane.
i. A standing wave is produced in the basilar membrane by a sound of a given pitch.
ii. Low-frequency sounds create a peak amplitude near the distal end of the membrane, and high-frequency sounds near the proximal end.
c. When the brain receive signals mainly from inner hair cells, it interprets the sound as low-pitched; if signals come from the proximal end, it interprets the sounds as high-pitched. (Fig. 16.17)
4. The cochlea is able to become “tuned” in order to receive some frequencies better than others.
a. The OHCs are supplied with a few sensory fibers, but more importantly, they receive motor fibers.
b. In response to sound, the OHCs send nerve signals to the medulla, and the pons sends signals immediately back by way of the motor neurons.
c. In response they shorten about 10% to 15%, reducing the basilar membrane’s mobility.
i. Some regions of the cochlea then send fewer signals to the brain, so that the brain can better distinguish between the more active and less active hair cells and sound frequencies.
ii. When OHCs are experimentally incapacitated, the IHCs respond much less precisely to difference in pitch.
d. In another tuning mechanism, the pons sends efferent fibers to the cochlea that synapse with the sensory nerve fibers near the base of the IHCs.
i. The efferent fibers can inhibit the sensory fibers from firing in some areas, and thus enhance the contrast between signals.
ii. This sharpens the tuning of the cochlea and our ability to discriminate sounds of different pitch.
Insight 16.3 Deafness
5. With regard to the auditory projection pathway, the sensory nerve fibers at the bases of hair cells belong to bipolar sensory neurons.
a. These neurons’ somas form a coil, the spiral ganglion, around the modiolus. (Fig. 16.13)
b. Their axons lead away from the cochlea as the cochlear nerve.
c. The cochlear nerve joins the vestibular nerve, and the two together become the vestibulocochlear nerve (CN VIII).
d. Each ear sends nerve fibers to both sides of the pons where they end in the cochlear nuclei.
i. Here they synapse with second-order neurons that ascend to the nearby superior olivary nucleus of the pons. (Fig. 16.18)
ii. By way of CN VIII, the superior olivary nucleus issues the efferent fibers back to the cochlea involved in cochlear tuning.
iii. By way of cranial nerves CN V3 and CN VII, the superior olivary nucleus also issues motor fibers to the tensor tympani and stapedius muscles, respectively.
e. The superior olivary nucleus also functions in binaural hearing—comparing signals from the right and left ears to identify direction of sound.
f. Other fibers from the cochlear nuclei ascend to the inferior colliculi of the midbrain.
i. These regions help to locate the origin of a sound in space, to process fluctuations in pitch, and to mediate the startle response in reaction to loud or sudden noises.
g. Third-order neurons begin in the inferior colliculi and lead to the thalamus.
h. Fourth-order neurons complete the pathway from the thalamus to the primary auditory cortex.
i. The presence of fourth-order neurons is unique among the senses; the other senses have only three neurons from receptor to cerebral cortex.
i. The primary auditory cortex lies in the superior margin of the temporal lobe deep within the lateral sulcus; this is the site of conscious perception of sound. (Fig. 14.21)
j. Because of extensive decussation, damage to the right or left auditory cortex does not cause a unilateral loss of hearing.
E. Equilibrium—coordination, balance, and orientation in three-dimensional space—was the original function of the ear in vertebrate evolution. (pp. 609–613)
1. In humans, the receptors for equilibrium constitute the vestibular apparatus, consisting of three semicircular ducts and two chapters, the anterior saccule and posterior utricle. (Fig. 16.12 b, c)
2. The sense of equilibrium includes static equilibrium, the perception of the orientation of the head when the body is stationary, and dynamic equilibrium, the perception of motion or acceleration.
a. Linear acceleration is a change in velocity in a straight line, as when a car accelerates down a straight road.
b. Angular acceleration is a change in the rate of rotation, as when a car turns a corner.
c. The saccule and utricle detect static equilibrium and linear acceleration; the semicircular ducts detect only angular acceleration.
3. The saccule and utricle each contain a 2 by 3 mm patch of hair cells and supporting cells called a macula.
a. The macula sacculi lies nearly vertically on the wall of the saccule; the macula utriculi lies nearly horizontally on the floor of the utricle. (Fig. 16.19a)
b. Each hair cell of a macula has 40 to 70 stereocilia and one true cilium called a kinocilium.
i. The tips of the stereocilia and kinocilium are embedded in a gelatinous otolithic membrane.
ii. The otolithic membrane is weighted with calcium carbonate–protein granules called otoliths that enhance the sense of gravity and motion. (Fig. 16.19b)
c. As an example, with the head erect, the otolithic membrane bears directly down on the hair cells of the macula utriculi; when the head is tilted, the heavy otolithic membrane sags and bends the stereocilia, stimulating the hair cells. The macula sacculi works in a similar way.
d. Any orientation of the head causes a combination of stimulation to the utricles and saccules of the two ears.
i. The brain interprets head orientation by comparing these inputs to each other and to input from the eyes and stretch receptors in the neck.
e. The inertia of the otolithic membranes is especially important in detecting linear acceleration.
i. The macula utriculi detects forward and backward acceleration changes, as when you stop and go at a traffic light.
ii. The macula sacculi detects vertical acceleration and deceleration, as when you go up or down in an elevator.
4. Rotary movements are detected by the three semicircular ducts housed in the semicircular canals of the temporal bone. (Fig. 16.20)
a. The anterior and posterior semicircular ducts are oriented vertically and right angles to each other.
b. The lateral semicircular duct is about 30° from the horizontal plane.
c. The ducts are filled with endolymph, and each one opens into the utricle and has a dilated sac at one end called the ampulla.
i. With the ampulla is a mound of hair cells and supporting cells called the crista ampullaris.
ii. The hair cells have stereocilia and a kinocilium embedded in the cupula, a gelatinous cap that extends from the crista to the roof of the ampulla.
d. When the head turns, the semicircular ducts rotate, but the endolymph lags behind, pushing the cupula, which bends the stereocilia to stimulate the hair cells.
e. After 25 to 30 seconds of continuous rotation, the endolymph catches up and stimulation ceases.
5. With regard to projection pathways, hair cells of the maculae and the semicircular ducts synapse at their bases with fibers of the vestibular nerve.
a. This nerve and the cochlear nerve merge to form the vestibulocochlear nerve (CN VIII).
b. Fibers of the vestibular apparatus lead to four vestibular nuclei on each side of the pons and medulla.
c. Nuclei on the right and left sides of the brain stem communicate extensively with each other, so each receives input from both ears.
d. The nuclei process signals and relay information to five targets. (Fig. 16.21)
i. The cerebellum, which integrates the input to control head movements, eye movements, muscle, tone, and posture.
ii. Nuclei of the oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves, which produce eye movements that compensate for movements of the head (the vestibulo-ocular reflex)
iii. The reticular formation, which is thought to adjust breathing and blood circulation.
iv. The spinal cord, where fibers descend the vestibulospinal tracts on each side and synapse on motor neurons that innervate the extensor (antigravity) muscles to keep balance.
v. The thalamus, which relays signals to two areas of the cerebral cortex: the inferior postcentral gyrus (conscious awareness of body position and movement); and the area rostral to this at the inferior end of the central sulcus (motor control of the head and body).
V. Vision (pp. 614–632)
A. Vision (sight) is the perception of objects in the environment by means of the light that they emit or reflect. (p. 614)
1. Light is visible electromagnetic radiation.
2. Human vision is limited to the range of wavelengths from about 400 to 700 nm, with the ultraviolet (UV) region just below 400 nm and the infrared (IR) just above 700 nm.
3. Vision in animals takes advantage of the radiation range that is most available at the earth’s surface.
B. Accessory structures surrounding the orbit of the eye include eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles. (pp. 614–616) (Figs. 16.22, 16.23)
1. The eyebrows probably serve mainly to enhance facial expressions and nonverbal communication but may also protect eyes from glare and perspiration.
2. The eyelids, or palpebrae, block foreign objects from the eye, prevent visual stimuli from disturbing our sleep, and blink to moisten the eyes with tears and sweep debris from the surface.
a. The separation between the eyelids is termed the palpebral fissure.
b. The eyelids meet at the corners called the medial and lateral commissures (canthi).
c. The eyelid consists largely of the orbicularis oculi muscle covered with skin. (Fig. 16.23a)
d. The eyelid also contains a supportive fibrous tarsal plate, thickened along the margin of the eyelid; within the plate are 20 to 25 tarsal glands that open along the edge of the eyelid and secrete an oil that lubricates the eye and prevents evaporation of tears.
e. The eyelashes are guard hairs that help keep debris from the eye; touching the eyelashes triggers the blink reflex.
3. The conjunctiva is a transparent mucous membrane that covers the inner surface of the eyelid and anterior surface of the eyeball, except for the cornea.
a. The primary purpose of the conjunctiva is to secrete a thin mucous film that prevents the eyeball from drying.
b. It is highly sensitive to pain and is very vascular.
c. Because it is vascular and the cornea is not, the conjunctiva heals more readily than the cornea.
4. The lacrimal apparatus consists of the lacrimal (tear) gland and a series of ducts that drain the tears into the nasal cavity.
a. The lacrimal gland is nestled in a shallow fossa of the frontal bone in the superolateral corner of the orbit.
b. About 12 short ducts lead from the lacrimal gland to the surface of the conjunctiva.
c. Tears, cleanse and lubricate the eye surface, deliver oxygen and nutrients to the conjunctiva, and prevent infection by means of lysozyme.
d. Tears collect near the medial commissure and flow into a tiny pore, the lacrimal punctum, on the margin of each eyelid.
e. The punctum opens into a short licrmal canal, which leads to the lacrimal sac in the medial wall of the orbit.
f. From this sac, a nasolacrimal duct carries the tears to the inferior meatus of the nasal cavity, from which they normally flow back to the throat and are swallowed.
5. The extrinsic eye muscles are the six muscles attached to the walls of the orbit and to the external surface of the eyeball; they move the eye. (Fig. 16.24)
a. The six muscles include four rectus muscles and two oblique muscles.
b. The superior, inferior, medial, and lateral rectus muscles originate on the posterior wall of the orbit and insert on the anterior region, just beyond the visible “white of the eye.”
c. The superior oblique travels along the medial wall of the orbit; its tendon passes through a fibrocartilage ring, the trochlea, and inserts on the superolateral aspect of the eyeball.
d. The inferior oblique extends from the medial wall of the orbit to the inferolateral aspect of the eye.
e. The superior oblique is innervated by the trochlea nerve (CN IV), the lateral rectus by the abducens (CN VI), and the rest of these muscles by the oculomotor nerve (CN III).
6. The eye is surrounded on the sides and back by orbital fat that cushions the eye, allows it to move freely, and protects blood vessels and nerves as they pass through the rear of the orbit.
C. Anatomically, the eyeball itself is a sphere about 24 mm in diameter with three principle components: the three tunics; optical components that admit and focus light; and neural components including the retina and optic nerve. (pp. 616–620) (Fig. 16.25)
1. The retina is also part of the inner tunic, and the cornea is part of the outer tunic as well as an optical component.
2. The three tunics of the eyeball are as follows:
a. The outer layer is the fibrous layer (tunica fibrosa) and is divided into two regions, the sclera and the cornea.
i. The sclera, or white of the eye, covers most of the eye surface and consists of dense collagenous tissue perforated by blood vessels and nerves.
ii. The cornea is the anterior transparent region of modified sclera that admits light.
b. The middle layer is the vascular layer (tunica vasculosa), also called the uvea; it contains three regions.
i. The choroid is a highly vascular, deeply pigmented layer behind the retina.
ii. The ciliary body forms a muscular ring around the lens; it supports the iris and lens and secretes aqueous humor.
iii. The iris is an adjustable diaphragm that controls the diameter of the pupil; the pigmented posterior pigment epithelium blocks stray light, while chromatophores in the pigmented anterior border layer is responsible for eye color.
c. The inner layer (tunica interna) consists of the retina and beginning of the optic nerve.
3. The optical components of the eye are transparent elements that admit light, bend its rays, and focus images on the retina; they include the cornea, aqueous humor, lens, and vitreous body.
a. The aqueous humor is a serous fluid secreted by the ciliary body into a space between the iris and lens called the posterior chamber. (Fig. 16.26)
i. The fluid flows through the pupil into the anterior chamber between the cornea and iris.
ii. From the anterior chamber it is reabsorbed by a ringlike blood vessel called the scleral venous sinus (canal of Schlemm).
b. The lens is composed of flattened, tightly compressed, transparent cells called lens fibers.
i. It is suspended behind the pupil by a ring of fibers called the suspensory ligament that attaches to the ciliary body. (Figs. 16.25, 16.27)
ii. Tension on the ligament somewhat flattens the lens.
c. The vitreous body (vitreous humor) is a transparent jelly that fills a large space called the vitreous chamber behind the lens.
i. A channel through this body called the hyaloid canal is the remnant of a hyaloid artery present in the embryo. (Fig. 16.25)
Insight 16.4 Cataracts and Glaucoma
4. the neural components are the retina and optic nerve.
a. The retina forms from a cup-shaped outgrowth of the diencephalons and is actually a part of the brain—the only part that can be viewed without dissection.
i. 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 eye, and its scalloped anterior margin, the ora serrata.
ii. The vitreous body presses the retina smoothly against the rear of the eyeball; insufficient pressure or a blow to the head may cause a detached retina that can lead to blindness if it is separate from the chorion for too long.
iii. Examination with an ophthalmoscope reveals a patch of cells called the macula lutea, directly posterior to the center of the lens. (Fig. 16.28)
iv. In the center of the macula is a tiny pit, the fovea centralis, where the most finely detailed images are produced.
v. About 3 mm medial to the macula lutea is the optic disc, where nerve fibers from the retina converge and exit the eye to form the optic nerve, and blood vessels enter and exit the eye.
vi. The optic disc produces a blind spot in the visual field of each eye. (Fig. 16.29)
vii. Visual filling occurs when the brain fills in the blank area of the blind spot using the image surrounding the spot.
D. The process of forming an image begins when light rays enter the eye, focus on the retina, and produce a tiny inverted image. (pp. 620–622)
1. When fully dilated, the pupil admits five times as much light as when it is fully constricted; its diameter is controlled by two sets of contractile elements in the iris:
a. The pupillary constrictor, smooth muscle cells that encircle the pupil, narrows the pupil and admits less light when stimulated by the parasympathetic nervous system.
b. The pupillary dilator, a spokelike arrangement of myoepithelial cells widen the pupil and admit more light when stimulated by the sympathetic nervous system. (Fig. 15.9)
c. Pupillary constriction and dilation occur in two situations: when light intensity changes and when we shift our gaze between distant and nearby objects.
i. Pupillary constriction in response to light is called the photopupillary reflex or consensual light reflex because both pupils constrict even if only one is illuminated.
ii. The reflex is mediated by a parasympathetic reflex arc in which signals are transmitted to the pretectal region of the upper midbrain, which then sends impulses through preganglionic parasympathetic fibers to ciliary ganglia, and thence to the eye to stimulate the pupillary constrictor.
iii. Sympathetic innervation originates in the spinal cord, as with all other sympathetic efferents; preganglionic fibers from the thoracic cord lead to superior cervical ganglia and thence into the head, ending at the pupillary dilator.
2. Image formation depends on refraction, the bending of light rays.
a. Light slows down slightly in air, water, glass, and other media, and the refractive index of a medium is a measure of this effect.
b. As light passes from air into a more refractive medium at an angle of greater or less than 90°, the light ray changes direction, that is, it is refracted. (Fig. 16.30a)
c. The refractive index of air is set at 1.00, while the refractive index of the cornea is 1.38, and that of the lens is 1.40; therefore most refraction is accomplished by the cornea, while the lens fine-tunes the image. (Fig. 16.30b)
3. The near response is an adjustment to close-range vision.
a. Emmetropia is a state in which the eye is relaxed and focused on an object more than 6 m (20 ft) away; light rays from the object are focused on the retina without effort.
b. If the gaze shifts to something closer, light rays from the source are too divergent to be focused without effort.
c. The near response involves three processes to focus an image on the retina: (Fig. 16.31)
i. Convergence of the eyes. Convergence orients the visual axis of each eye toward the object in order to focus its image on each fovea; diplopia is double vision that occurs when the eyes cannot converge accurately.
ii. Constriction of the pupil. Constriction screens out peripheral light rays that may be blurry due to spherical aberration at the edges of the cornea.
iii. Accommodation of the lens. Accommodation is a change in the curvature of the lens that enables focus on a near object; The ciliary muscle surrounding each lens contracts, narrowing the diameter of the ciliary body and relaxing the fibers of the suspensory ligament—the lens can then relax into a more convex shape.
iv. The closest an object can be and still come into focus is the near point of vision; it depends on the flexibility of the lens, which stiffens with age.
d. Some common defects in image formation are described in Table 16.2.
E. Visual sensory transduction, the conversion of light energy into action potentials, occurs in the retina. (pp. 623–627) (Fig. 16.34)
1. The most posterior part of the retina is the pigment epithelium, a darkly pigmented layer that absorbs stray light.
2. The neural components of the retina consist of three principal cell layers, progressing from the rear of the eye forward: photoreceptor cells, bipolar cells, and ganglion cells.
a. The photoreceptor cells absorb light and generate a chemical or electrical signal; they consist of rods and cones.
i. Rods and cones are derived from the same stem cells as ependymal cells of the brain.
ii. Each rod or cone has an outer segment that points toward the wall of the eye and an inner segment facing the interior. (Fig. 16.35)
iii. The two segments are separated by a narrow constriction containing nine pairs of microtubules.
iv. The outer segment is a highly modified cilium specialized to absorb light; the inner segment contains mitochondria and other organelles; at its base is a cell body with nucleus and processes that synapse with retinal neurons.
v. In a rod, the outer segment is cylindrical and resembles a stack of coins in a paper roll; the stacked discs are studded with the visual pigment rhodopsin.
vi. Rod cells are responsible for night (scotopic) vision.
vii. A cone cell is similar except that the outer segment tapers to a point and the discs are not detached from the plasma membrane; cones are responsible for day (photopic) vision as well as color (trichromatic) vision.
b. Rods and cones synapse with the dendrites of bipolar cells, the first-order neurons of the visual pathway; these in turn synapse with the ganglion cells. (Fig. 16.34b)
c. Ganglion cells are the largest neurons of the retina, arranged in a single layer close to the vitreous body.
i. They are the second-order neurons of the visual pathway.
ii. Most receive input from multiple bipolar cells, and their axons form the optic nerve.
iii. Some ganglion cells absorb light directly and transmit signals to brainstem nuclei to affect pupillary diameter and circadian rhythms; their sensory pigment is melanopsin.
d. There are about 130 million rods and 6.5 million cones per retina, but only 1.2 million nerve fibers in the optic nerve, a ratio of 114 receptor cells per optic nerve fiber.
i. This ratio is an example of neural convergence; multiple rod or cone cells synapse with one bipolar cell, and multiple bipolar cells feed into one ganglion cell.
e. Other retinal cells include horizontal cells and amacrine cells that form connections among rod, cone, and bipolar cells, enhancing contrast, edges of objects, and changes in intensity; also present in the retina are astrocytes and other glial cells.
3. The visual pigment of the rods is called rhodopsin or visual purple.
a. Each molecule consists of two moieties: a protein called opsin and a vitamin A derivative called retinal (retinene). (Fig. 16.36)
b. Opsin is embedded in the disc membranes of the rod’s outer segment.
c. All rod cells contain a single kind of rhodopsin with a peak absorption at 500 nm; they cannot distinguish color.
4. The visual pigment in cones is photopsin (iodopsin).
a. Retinal is the same as in cones, but the opsin moiety has a different amino acid sequence that determines the wavelengths absorbed.
b. The three kinds of cones absorb different wavelengths of light, enabling color vision.
5. Generation of an optic nerve signal begins with a change in the shape of retinal.
a. In the dark, retinal molecules in rods have a bent shape called cis-retinal; when light is absorbed, the shape changes to trans-retinal and breaks away from the opsin, a process called bleaching. (Fig. 16.37)
b. For a rod to continue functioning, trans-retinal has to be converted back to the cis form and reunited with opsin.
i. Fifty percent of the bleached rhodopsin is regenerated in about 5 minutes.
ii. Cones are faster, and their photopsin is 50% regenerated in about 90 seconds.
c. In the dark, rods release glutamate; when they absorb light, glutamate secretion ceases. (Figs. 16.38a, b)
d. The bipolar cells, next in line, are sensitive to the changes in glutamate; some are inhibited by glutamate and others are excited by it.
e. As the eye scans a scene, areas of less and greater brightness form a rapidly changing pattern of bipolar cell responses as the image is projected on the retina.
f. Ganglion cells are stimulated by the bipolar cells, either directly via synapses or indirectly via amacrine cells; only ganglion cells produce action potentials.
g. The action potentials travel via the optic nerve to the brain.
F. Light and dark adaptation are adjustments in vision to light conditions. (p. 627)
1. Light adaptation occurs when you go from a dark or dimly lit area into brighter light.
a. Pupils quickly constrict to reduce the intensity of stimulation.
b. Color vision and visual acuity remain below normal for 5 to 10 minutes—the time needed for pigment bleaching to adjust retinal sensitivity.
c. Rods bleach quickly, and rod vision is nonfunctional even in typical indoor light.
2. Dark adaptation occurs when going from bright light to a dark area.
a. Rod pigment that was bleached by the light now regenerates faster than it bleaches, and scotopic vision begins to function after a minute or two.
b. After 20 to 30 minutes, scotopic vision is at its maximum sensitivity; dilation of the pupils also helps by admitting more light.
G. The rods and cones constitute a dual visual system. (p. 627)
1. The duplicity theory of vision holds that a single type of receptor system cannot produce both high sensitivity and high resolution—each requires a different type of cell and neural circuit.
2. Rods are highly sensitive in dim light, partly due to the extensive neural convergence.
a. Up to 600 rods converge on each bipolar cell, and many bipolar cells converge on each ganglion cell.
b. The result is a high degree of spatial summation in the scotopic system. (Fig. 16.39a)
c. Weak stimulation of rod cells can produce an additive effect on one bipolar cell, and several bipolar cells can collaborate to excite on ganglion cell, producing a response to dim light.
d. A shortcoming is that the system cannot resolve finely detailed images.
3. Visual acuity decreases rapidly as the image falls away from the fovea centralis; peripheral vision is a low-resolution system that alerts us to motion to the periphery and stimulates us to look in that direction.
4. The fovea is occupied by about 4,000 tiny cones and no rods, and there is no neural convergence.
a. Each tiny cone synapses with only one bipolar cell, and each bipolar cell with only one ganglion cell.
b. Each foveal cone has a “private line to the brain,” and each ganglion reports a receptive field of just 2 μm2 of retinal area. (Fig. 16.39b)
c. Cones distant from the fovea exhibit some neural convergence, but not as much as rods do.
d. The cone system is therefore less sensitive to light because of the lack of spatial summation.
H. Color vision is especially well developed in primates, although many vertebrates have both cone and rod cells. (pp. 627–628)
1. Color vision is based on three kinds of cones named for the absorption peaks of their photopsins.
a. Short-wavelength (S) cones have a peak sensitivity at 420 nm; they were formerly termed blue cones.
b. Medium-wavelength (M) cones have peak sensitivity at 531 nm; they were formerly called green cones.
c. Long-wavelength (L) cones have peak sensitivity at 558 nm; they were formerly called red cones.
2. Color perception is based on a mixture of nerve signals representing cones with different absorption peaks; the brain interprets the mixture of signals as different colors. (Fig. 16.40)
3. A hereditary alteration or lack of one or more photopsins results in color blindness.
a. The most common form is red–green color blindness, which results from a lack of either L or M cones. (Fig. 16.41)
b. Red–green color blindness is a sex-linked recessive trait that occurs in about 8% of males and 0.5% of females.
I. Stereoscopic vision (stereopsis) is depth perception—the ability to judge how far away objects are. (pp. 628–629)
1. Depth perception requires having two eyes with overlapping visual fields, such that each eye looks at an object from a different angle.
2. Mammals such as rodents and horses have panoramic vision, in which eyes are on opposite sides of the head.
3. Panoramic vision has a larger visual field and thus is suited to perception of predators approaching, but it lacks in depth perception.
4. When you fixate on something within 30 m (100 ft) each eye views it from a slightly different angle and focuses its image on the fovea centralis; this is the called the fixation point.
a. Objects farther away than the fixation point cast an image more medially, whereas objects closer cast images more laterally.
b. The brain uses information on the distance of an image from the two foveas to judge the position of other points relative to the fixation point.
J. In the visual projection pathway, the first-order neurons are the bipolar cells of the retina, and the second-order neurons are the retinal ganglion cells, whose axons are the fibers of the optic nerve. (pp. 629–631)
1. The optic nerves leave each orbit through the optic foramen and then converge to form the optic chiasm inferior to the hypothalamus and anterior to the pituitary.
2. The fibers continue beyond the chiasm as a pair of optic tracts.
3. Within the chiasm, half of the fibers from each optic nerve cross over to the opposite side of the brain, called hemidecussation. (Fig. 16.43)
a. The right cerebral hemisphere sees object in the left visual field, because their images fall on the right half of each retina.
b. Conversely, the left hemisphere sees objects in the right visual field.
c. Because the right brain controls motor responses on the left side of the body, and vice versa, each side of the brain needs to see what is on the side of the body where it exerts motor control.
d. In animals with panoramic vision, nearly 100% of optic nerve fibers of the right eye decussate to the left brain, and vice versa.
4. The optic tracts pass laterally around the hypothalamus, and most axons end in the lateral geniculate nucleus of the thalamus, where the third-order neurons arise.
a. The third-order neurons form the optic radiation of fibers in the white matter of the cerebrum and project to the primary visual cortex of the occipital lobe, where conscious visual sensation occurs.
b. A lesion in the occipital lobe can cause blindness even if the eyes are fully functional.
5. A few optic nerve fibers project to the midbrain and terminate in the superior colliculi and pretectal nuclei.
a. The superior colliculi control the visual reflexes of the extrinsic eye muscles.
b. The pretectal nuclei are involved in the photopupillary and accommodation reflexes.
6. Visual information processing in the brain is highly complex, involving connections between the primary visual cortex in the occipital lobe and association areas in the posterior parietal lobe and inferior temporal lobe.
a. The association areas process retinal data and also store visual memories and allow the brain to identify what is being seen.
Insight 16.5 Anesthesia—From Ether Frolics to Modern Surgery

Cross References
Additional information on topics mentioned in Chapter 16 can be found in the chapters listed below.

Chapter 1: Evolution of color vision and stereoscopic vision in primates
Chapter 12: Neural coding
Chapter 13: Muscle spindles and tendon organs
Chapter 13: Spinothalamic tract, spinoreticular tract, and gracile fasciculus
Chapter 14: Sensory homunculus