An Introduction to the Special Senses

An Introduction to the Special Senses
Learning Outcomes 17-1 Describe the sensory organs of smell, trace the olfactory pathways to their destinations in the brain, and explain the physiological basis of olfactory discrimination. 17-2 Describe the sensory organs of taste, trace the gustatory pathways to their destinations in the brain, and explain the physiological basis of gustatory discrimination. 17-3 Identify the internal and accessory structures of the eye, and explain the functions of each.

Learning Outcomes 17-4 Explain color and depth perception, describe how light stimulates the production of nerve impulses, and trace the visual pathways to their destinations in the brain. 17-5 Describe the structures of the external, middle, and internal ear, explain their roles in equilibrium and hearing, and trace the pathways for equilibrium and hearing to their destinations in the brain.

Five Special Senses Olfaction Gustation Vision Equilibrium Hearing

17-1 Smell (Olfaction) Olfactory Organs
Provide sense of smell located in nasal cavity on either side of nasal septum Made up of two layers Olfactory epithelium: Olfactory receptors, supporting cells , and basal (stem) cells Lamina propria: Areolar tissue , blood vessels , Nerves , and olfactory glands

Figure 17-1a The Olfactory Organs.
Olfactory Pathway to the Cerebrum Olfactory epithelium Olfactory nerve fibers (N I) Olfactory bulb Olfactory tract Central nervous system Cribriform plate Superior nasal concha a The olfactory organ on the right side of the nasal septum.

Figure 17-1b The Olfactory Organs.
Basal cell: divides to replace worn-out olfactory receptor cells To olfactory bulb Olfactory gland Cribriform plate Olfactory nerve fibers Lamina propria Developing olfactory receptor cell Olfactory receptor cell Olfactory epithelium Supporting cell Mucous layer Knob Olfactory dendrites: surfaces contain receptor proteins (see Spotlight Figure 17–2) Substance being smelled b An olfactory receptor is a modified neuron with multiple cilia-shaped dendrites.

17-1 Smell (Olfaction) Olfactory Glands :Secretions coat surfaces of olfactory organs Olfactory Receptors: Highly modified neurons Olfactory reception involves detecting dissolved chemicals as they interact with odorant-binding proteins

17-1 Smell (Olfaction) Olfactory Pathways
Axons leaving olfactory epithelium Collect into 20 or more bundles Penetrate cribriform plate of ethmoid Reach olfactory bulbs of cerebrum where first synapse occurs

17-1 Smell (Olfaction) Olfactory Pathways
Axons leaving olfactory bulb: Travel along olfactory tract to reach olfactory cortex, hypothalamus, and portions of limbic system Arriving information reaches information centers without first synapsing in thalamus

17-1 Smell (Olfaction) Olfactory Discrimination
Can distinguish thousands of chemical stimuli CNS interprets smells by the pattern of receptor activity Olfactory Receptor Population Considerable turnover Number of olfactory receptors declines with age

Figure 17-2 Olfaction and Gustation (Part 4 of 10).
Olfaction and gustation are special senses that give us vital information about our environment. Although the sensory information is diverse and complex, each special sense originates at receptor cells that may be neurons or specialized receptor cells that communicate with sensory neurons. Stimulus Dendrites Specialized olfactory neuron Stimulus removed Action potentials −70 mV mV Stimulus Threshold Generator potential to CNS Time (msec)

Olfactory reception occurs on the surface membranes of the olfactory dendrites. Odorants—dissolved chemicals that stimulate olfactory receptors—interact with receptors called odorant-binding proteins on the membrane surface. In general, odorants are small organic molecules. The strongest smells are associated with molecules of either high water or high lipid solubilities. As few as four odorant molecules can activate an olfactory receptor. 1 The binding of an odorant to its receptor protein leads to the activation of adenylyl cyclase, the enzyme that converts ATP to cyclic AMP (cAMP). 2 The cAMP opens sodium ion channels in the plasma membrane, which then begins to depolarize. 3 If sufficient depolarization occurs, an action potential is triggered in the axon, and the information is relayed to the CNS. MUCOUS LAYER Odorant molecule Closed sodium channel + + + Depolarized membrane Receptor protein cAMP cAMP Inactive G protein Active G protein + + adenylate cyclase RECEPTOR CELL cAMP cAMP Sodium ions enter ATP +

17-2 Taste (Gustation) Gustation :Provides information about the foods and liquids consumed Taste Receptors (Gustatory Receptors) Are distributed on tongue and portions of pharynx and larynx which clustered into taste buds Taste Buds Associated with epithelial projections (lingual papillae) on superior surface of tongue

17-2 Taste (Gustation) Four Types of Lingual Papillae
Filiform papillae :Provide friction and do not contain taste buds Fungiform papillae :Contain about five taste buds each Vallate papillae :Contain 100 taste buds each Foliate papillae

17-2 Taste (Gustation) Taste Buds Contain: Basal cells
Gustatory receptor cells Extend taste hairs through taste pore Survive only 10 days before replacement Monitored by cranial nerves that synapse within solitary nucleus of medulla oblongata Then on to thalamus and primary sensory cortex

Figure 17-3a Gustatory Receptors.
Water receptors (pharynx) Detected by water receptors in the pharynx Umami Characteristic of beef/chicken broths and Parmesan cheese Receptors sensitive to amino acids, small peptides, and nucleotides a Location of tongue papillae.

Figure 17-3b Gustatory Receptors.
Four Types of Lingual Papillae Filiform papillae :Provide friction and do not contain taste buds Fungiform papillae :Contain about five taste buds each Vallate papillae :Contain 100 taste buds each Foliate papillae Taste buds Vallate papilla Foliate papillae Fungiform papilla Filiform papillae b The structure and representative locations of the four types of lingual papillae. Taste receptors are located in taste buds, which form pockets in the epithelium of fungiform, foliate, and vallate papillae.

Figure 17-3c Gustatory Receptors.
Taste buds Taste buds LM × 280 Transitional cell Gustatory cell Taste hairs (microvilli) Basal cell Taste pore c Taste buds in a vallate papilla. A diagrammatic view of a taste bud, showing gustatory (receptor) cells and supporting cells.

17-2 Taste (Gustation) Gustatory Discrimination
Four primary taste sensations Sweet Salty Sour Bitter

17-2 Taste (Gustation) Gustatory Discrimination
Dissolved chemicals contact taste hairs Bind to receptor proteins of gustatory cell Salt and sour receptors Chemically gated ion channels Stimulation produces depolarization of cell Sweet, bitter, and umami stimuli G proteins Gustducins

17-2 Taste (Gustation) Taste Sensitivity
Exhibits significant individual differences Some conditions are inherited For example, phenylthiocarbamide (PTC) 70 percent of Caucasians taste it but 30 percent do not Number of taste buds Begins declining rapidly at age 50

Salt and Sour Channels Sweet, Bitter, and Umami Receptors The diffusion of sodium ions from salt solutions or hydrogen ions from acids or sour solutions into the receptor cell leads to depolarization. Receptors responding to stimuli that produce sweet, bitter, and umami sensations are linked to G proteins called gustducins (GUST-doos-inz)—protein complexes that use second messengers to produce their effects. Na+ + + + Sweet, bitter, or umami H+ + + Na+ ion leak channel Membrane receptor Resting plasma membrane Inactive G protein Active G protein Depolarized membrane Depolarized membrane Active G protein + + + Active 2nd messenger Inactive 2nd messenger Depolarization of membrane stimulates release of chemical neurotransmitters. Activation of second messengers stimulates release of chemical neurotransmitters.

17-3 Accessory Structures of the Eye
Provide protection, lubrication, and support Include: The palpebrae (eyelids) The superficial epithelium of eye The lacrimal apparatus

Eyelids (Palpebrae) Continuation of skin Blinking keeps surface of eye lubricated, free of dust and debris Palpebral fissure Gap that separates free margins of upper and lower eyelids

Eyelids (Palpebrae) Medial canthus and lateral canthus Where two eyelids are connected Eyelashes Robust hairs that prevent foreign matter from reaching surface of eye Tarsal glands Secrete lipid-rich product that helps keep eyelids from sticking together

Superficial Epithelium of Eye Lacrimal caruncle: Mass of soft tissue which contains glands producing thick secretions and contributes to gritty deposits that appear after good night’s sleep Conjunctiva: Epithelium covering inner surfaces of eyelids (palpebral conjunctiva) and outer surface of eye (ocular conjunctiva)

Figure 17-4a External Features and Accessory Structures of the Eye.
Eyelashes Lateral canthus Pupil Palpebra Palpebral fissure Sclera Medial canthus Lacrimal caruncle Corneal limbus a Gross and superficial anatomy of the accessory structures

Lacrimal Apparatus: Produces, distributes, and removes tears Fornix: Pocket where palpebral conjunctiva joins ocular conjunctiva Lacrimal gland (tear gland) Secretions contain lysozyme, an antibacterial enzyme

Tears Collect in the lacrimal lake Pass through: Lacrimal puncta Lacrimal canaliculi Lacrimal sac Nasolacrimal duct To reach inferior meatus of nose

Figure 17-4b External Features and Accessory Structures of the Eye.
Superior rectus muscle Tendon of superior oblique muscle Lacrimal gland ducts Lacrimal punctum Lacrimal gland Lacrimal caruncle Ocular conjunctiva Superior lacrimal canaliculus Lateral canthus Medial canthus Lower eyelid Inferior lacrimal canaliculus Orbital fat Lacrimal sac Inferior rectus muscle Nasolacrimal duct Inferior nasal concha Inferior oblique muscle Opening of nasolacrimal duct b The organization of the lacrimal apparatus

17-3 The Eye Three Layers of the Eye Outer fibrous layer
Intermediate vascular layer Deep inner layer

17-3 The Eye Eyeball Is hollow and divided into two cavities
Large posterior cavity Smaller anterior cavity The Fibrous Layer Sclera (white of the eye) Cornea Corneal limbus (border between cornea and sclera)

Figure 17-5a The Sectional Anatomy of the Eye.
Fornix Palpebral conjunctiva Eyelash Optic nerve Ocular conjunctiva Ora serrata Cornea Lens Pupil Iris Corneal limbus Fovea Retina Choroid Sclera a Sagittal section of left eye

Figure 17-5b The Sectional Anatomy of the Eye.
Fibrous layer Vascular layer (uvea) Cornea Anterior cavity Iris Sclera Ciliary body Choroid Posterior cavity Inner layer (retina) Neural part Pigmented part b Horizontal section of right eye

Figure 17-5c The Sectional Anatomy of the Eye.
Visual axis Anterior cavity Cornea Posterior chamber Anterior chamber Edge of pupil Iris Ciliary zonule Nose Corneal limbus Lacrimal punctum Conjunctiva Lacrimal caruncle Lower eyelid Medial canthus Lateral canthus Ciliary processes Lens Ciliary body Ora serrata Sclera Choroid Retina Posterior cavity Ethmoidal labyrinth Lateral rectus muscle Medial rectus muscle Optic disc Fovea Optic nerve Orbital fat Central artery and vein c Horizontal dissection of right eye

17-3 The Eye Vascular Layer (Uvea) Functions
Provides route for blood vessels and lymphatics that supply tissues of eye Regulates amount of light entering eye Secretes and reabsorbs aqueous humor that circulates within chambers of eye Controls shape of lens, which is essential to focusing

Figure 17-5c The Sectional Anatomy of the Eye.
Visual axis Anterior cavity Cornea Posterior chamber Anterior chamber Edge of pupil Iris Ciliary zonule Nose Corneal limbus Lacrimal punctum Conjunctiva Lacrimal caruncle Lower eyelid Medial canthus Lateral canthus Ciliary processes Lens Ciliary body Ora serrata Sclera Choroid Retina Posterior cavity Ethmoidal labyrinth Lateral rectus muscle Medial rectus muscle Optic disc Fovea Optic nerve Orbital fat Central artery and vein c Horizontal dissection of right eye

Figure 17-6 The Pupillary Muscles.
Pupillary constrictor (sphincter) Pupil Iris contains papillary muscles and change diameter of pupil Pupillary dilator (radial) The pupillary dilator muscles extend radially away from the edge of the pupil. Contraction of these muscles enlarges the pupil. The pupillary constrictor muscles form a series of concentric circles around the pupil. When these sphincter muscles contract, the diameter of the pupil decreases. Decreased light intensity Increased sympathetic stimulation Increased light intensity Increased parasympathetic stimulation

17-3 The Eye The Inner Layer Outer layer called pigmented part
Inner called neural part (retina) Contains visual receptors and associated neurons Rods and cones are types of photoreceptors Rods: Do not discriminate light colors are highly sensitive to light Cones: Provide color vision

Figure 17-7a The Organization of the Retina (Part 1 of 2).
Horizontal cell Cone Rod Pigmented part of retina Rods do not discriminate light colors. Highly sensitive to light Cones Provide color vision. Densely clustered in fovea, at center of macula Rods and cones Amacrine cell Bipolar cells Ganglion cells LIGHT a The cellular organization of the retina. The photoreceptors are closest to the choroid, rather than near the posterior cavity (vitreous chamber).

Figure 17-7a The Organization of the Retina (Part 2 of 2).
Choroid Pigmented part of retina Rods and cones Bipolar cells Ganglion cells Retina LM × 350 Nuclei of ganglion cells Nuclei of rods and cones Nuclei of bipolar cells a The cellular organization of the retina. The photoreceptors are closest to the choroid, rather than near the posterior cavity (vitreous chamber).

Figure 17-7b The Organization of the Retina.
Pigmented part of retina Neural part of retina Central retinal vein Optic disc Central retinal artery Sclera Choroid Optic nerve b The optic disc in diagrammatic sagittal section.

Figure 17-7c The Organization of the Retina.
Optic disc (blind spot) Fovea Optic Disc: Circular region just medial to fovea. Origin of optic nerve Blind spot Macula Central retinal artery and vein emerging from center of optic disc c A photograph of the retina as seen through the pupil.

17-3 The Eye Inner Neural Part
Bipolar cells :Neurons of rods and cones synapse with ganglion cells Horizontal cells: Extend across outer portion of retina Amacrine cells: Comparable to horizontal cell layer where bipolar cells synapse with ganglion cells

17-3 The Eye The Chambers of the Eye
Ciliary body and lens divide eye into: Large posterior cavity (vitreous chamber) Posterior chamber is between iris, ciliary body, and lens Smaller anterior cavity Anterior chamber extends from cornea to iris

Figure 17-9 The Circulation of Aqueous Humor.
Anterior chamber extends from cornea to iris Cornea Pupil Anterior cavity Anterior chamber Scleral venous sinus Posterior chamber Posterior chamber is between iris, ciliary body, and lens Body of iris Ciliary process Conjunctiva Lens Ciliary body Ciliary zonule Sclera Pigmented epithelium Posterior cavity (vitreous chamber) Choroid Retina

17-3 The Eye The Lens Lens fibers Cells in interior of lens
No nuclei or organelles Filled with crystallins, which provide clarity and focusing power to lens

Figure 17-10 Factors Affecting Focal Distance.
Light Refraction: Bending of light by cornea and lens Focal point: Specific point of intersection on retina Focal distance: Distance between center of lens and focal point Focal distance Focal distance Focal distance Close source Focal point Light from distant source (object) Lens The closer the light source, the longer the focal distance The rounder the lens, the shorter the focal distance a b c

17-3 The Eye Light Refraction of Lens
Accommodate the shape of lens changes to focus image on retina Astigmatism Condition where light passing through cornea and lens is not refracted properly Visual image is distorted

17-3 The Eye Light Refraction of Lens Image reversal Visual acuity
Clarity of vision “Normal” rating is 20/20

Figure 17-12d Image Formation.
Optic nerve Light rays projected from a horizontal object show why the image arrives with a left and right reversal. The image also arrives upside down. (As noted in the text, these representations are not drawn to scale.) d

Figure 17-13 Refractive Problems (Part 1 of 5).
The eye has a fixed focal distance and focuses by varying the shape of the lens. A camera lens has a fixed size and shape and focuses by varying the distance to the film.

Myopia (nearsightedness) If the eyeball is too deep or the resting curvature of the lens is too great, the image of a distant object is projected in front of the retina. Myopic people see distant objects as blurry and out of focus. Vision at close range will be normal because the lens is able to round as needed to focus the image on the retina. Myopic corrected with a diverging, concave lens Diverging lens

Hyperopia (farsightedness) If the eyeball is too shallow or the lens is too flat, hyperopia results. The ciliary muscle must contract to focus even a distant object on the retina. And at close range the lens cannot provide enough refraction to focus an image on the retina. Older people become farsighted as their lenses lose elasticity, a form of hyperopia called presbyopia (presbys, old man). Hyperopia corrected with a converging, convex lens Converging lens

17-4 Visual Physiology Color Vision: Integration of information from red, green, and blue cones Color blindness: Inability to detect certain colors

Figure 17-15 Cone Types and Sensitivity to Color.
100 Rods Red cones Blue cones Green cones 75 Light absorption (percent of maximum) 50 25 WAVELENGTH (nm) Violet Blue Green Yellow Orange Red

17-4 Visual Physiology Recovery after Stimulation
Bleaching :Rhodopsin molecule breaks down into retinal and opsin Night blindness: Results from deficiency of vitamin A

17-4 Visual Physiology Light and Dark Adaptation
Dark: Most visual pigments are fully receptive to stimulation Light: Pupil constricts and bleaching of visual pigments occurs

17-4 Visual Physiology The Visual Pathways begin at photoreceptors and end at visual cortex of cerebral hemispheres. Message crosses two synapses before it heads toward brain Photoreceptor to bipolar cell Bipolar cell to ganglion cell Ganglion Cells: M cells and P cells M Cells: Are ganglion cells that monitor rods are relatively large and provide information about: General form of object Motion Shadows in dim lighting

17-4 Visual Physiology Ganglion Cells P cells
Are ganglion cells that monitor cones Are smaller, more numerous Provide information about edges, fine detail, and color

17-4 Visual Physiology Ganglion Cells On-center neurons
Are excited by light arriving in center of their sensory field Are inhibited when light strikes edges of their receptive field Off-center neurons Inhibited by light in central zone Stimulated by illumination at edges

17-4 Visual Physiology Central Processing of Visual Information
Axons from ganglion cells converge on optic disc Penetrate wall of eye Proceed toward diencephalon as optic nerve (II) Two optic nerves (one for each eye) reach diencephalon at optic chiasm

Figure 17-20 The Visual Pathways.
Combined field of vision arrive at visual cortex of opposite occipital lobe Left half arrive at right occipital lobe Right half arrive at left occipital lobe Combined Visual Field Left side Right side Left eye only Right eye only Binocular vision The Visual Pathway Photoreceptors in retina Retina Optic disc Optic nerve (II) Optic chiasm Optic tract Suprachiasmatic nucleus Lateral geniculate nucleus Diencephalon and brain stem Projection fibers (optic radiation) Visual cortex of cerebral hemispheres Superior colliculus Left cerebral hemisphere Right cerebral hemisphere

17-5 The Ear The External Ear
Auricle: Surrounds entrance to external acoustic meatus protects opening of canal and provides directional sensitivity External acoustic meatus: Ends at tympanic membrane (eardrum) Tympanic membrane: Is a thin, semitransparent sheet and separates external ear from middle ear Ceruminous glands: Secrete waxy material (cerumen) Keeps foreign objects out of tympanic membrane Slows growth of microorganisms in external acoustic meatus

Figure 17-21 The Anatomy of the Ear.
External Ear Middle Ear Internal Ear Elastic cartilages Auditory ossicles Oval window Semicircular canals Petrous part of temporal bone Auricle Facial nerve (VII) Vestibulocochlear nerve (VIII) Bony labyrinth of internal ear Cochlea Tympanic cavity Auditory tube To nasopharynx External acoustic meatus Tympanic membrane Round window Vestibule

17-5 The Ear The Middle Ear (tympanic cavity) : Communicates with nasopharynx via auditory tube and permits equalization of pressures on either side of tympanic membrane Encloses and protects three auditory ossicles Malleus (hammer) Incus (anvil) Stapes (stirrup)

Figure 17-22a The Middle Ear.
Auditory Ossicles Malleus Incus Stapes Temporal bone (petrous part) Stabilizing ligaments Oval window Muscles of the Middle Ear Branch of facial nerve VII (cut) Tensor tympani muscle External acoustic meatus Stapedius muscle Tympanic cavity (middle ear) Round window Auditory tube Tympanic membrane a The structures of the middle ear

Figure 17-22b The Middle Ear.
Malleus attached to tympanic membrane Tendon of tensor tympani muscle Malleus Incus Base of stapes at oval window Stapes Stapedius muscle Inner surface of tympanic membrane b The tympanic membrane and auditory ossicles

Figure 17-22c The Middle Ear.
Incus Malleus Points of attachment to tympanic membrane Stapes Base of stapes c The isolated auditory ossicles

17-5 The Ear Vibration of Tympanic Membrane converts arriving sound waves into mechanical movements Auditory ossicles conduct vibrations to inner ear Tensor tympani muscle: Stiffens tympanic membrane Stapedius muscle: Reduces movement of stapes at oval window

17-5 The Ear The Internal Ear contains fluid called endolymph and Bony labyrinth surrounds and protects membranous labyrinth Subdivided into: Vestibule: provide sensations of gravity and linear acceleration Semicircular canals: Receptors stimulated by rotation of head Cochlea: Receptors provide sense of hearing

Figure 17-23b The Internal Ear.
KEY Membranous labyrinth Semicircular ducts Anterior Bony labyrinth Lateral Posterior Vestibule Cristae within ampullae Maculae Endolymphatic sac Semicircular canal Cochlea Utricle Saccule Vestibular duct Cochlear duct Tympanic duct Spiral organ b The bony and membranous labyrinths. Areas of the membranous labyrinth containing sensory receptors (cristae, maculae, and spiral organ) are shown in purple.

Figure 17-23a The Internal Ear.
KEY Membranous labyrinth Bony labyrinth Perilymph Bony labyrinth Endolymph Membranous labyrinth a A section through one of the semicircular canals, showing the relationship between the bony and membranous labyrinths, and the boundaries of perilymph and endolymph.

17-5 The Ear The Internal Ear Vestibule Semicircular canals Cochlea
Encloses saccule and utricle Receptors provide sensations of gravity and linear acceleration Semicircular canals Contain semicircular ducts Receptors stimulated by rotation of head Cochlea Contains cochlear duct (elongated portion of membranous labyrinth) Receptors provide sense of hearing

17-5 The Ear The Internal Ear Stimuli and Location
Round window :Thin, membranous partition. Separates perilymph from air spaces of middle ear Oval window: Formed of collagen fibers. Connected to base of stapes Stimuli and Location Sense of gravity and acceleration :From hair cells in vestibule Sense of rotation: From semicircular canals Sense of sound: From cochlea

17-5 The Ear Equilibrium Sensations provided by receptors of vestibular complex Hair cells Basic receptors of inner ear Provide information about direction and strength of mechanical stimuli

Figure 17-24a The Semicircular Ducts.
The Semicircular Ducts are continuous with utricle and each duct contains: Ampulla with gelatinous cupula associated sensory receptors Vestibular branch (N VIII) Semicircular ducts Anterior Cochlea Ampulla Posterior Endolymphatic sac Lateral Endolymphatic duct Utricle Maculae Saccule a An anterior view of the right semicircular ducts, the utricle, and the saccule, showing the locations of sensory receptors.

Figure 17-24b The Semicircular Ducts.
Ampulla filled with endolymph Cupula Hair cells Crista ampullaris Supporting cells Sensory nerve b A cross section through the ampulla of a semicircular duct.

Figure 17-24d The Semicircular Ducts.
Displacement in this direction stimulates hair cell Displacement in this direction inhibits hair cell Gelatinous material Kinocilium Stereocilia Kinocilium – single large cilium Stereocilia – resemble long microvilli are on surface of hair cell Hair cell Sensory nerve ending Supporting cell d A representative hair cell (receptor) from the vestibular complex. Bending the sterocilia toward the kinocilium depolarizes the cell and stimulates the sensory neuron. Displacement in the opposite direction inhibits the sensory neuron.

17-5 The Ear The Utricle and Saccule provide equilibrium sensations
Are connected with the endolymphatic duct, which ends in endolymphatic sac Maculae: Oval structures where hair cells cluster Statoconia: Densely packed calcium carbonate crystals on surface of gelatinous mass Otolith (ear stone)  gelatinous matrix and statoconia

Figure 17-25ab The Saccule and Utricle.
Endolymphatic sac Endolymphatic duct Utricle Saccule a The location of the maculae Otoliths Gelatinous layer forming otolithic membrane Hair cells Nerve fibers b The structure of an individual macula

17-5 The Ear Pathways for Equilibrium Sensations
Vestibular receptors activate sensory neurons of vestibular ganglia Axons form vestibular branch of vestibulocochlear nerve (VIII) Synapse within vestibular nuclei

17-5 The Ear Four Functions of Vestibular Nuclei
Integrate sensory information about balance and equilibrium from both sides of head Relay information from vestibular complex to cerebellum Relay information from vestibular complex to cerebral cortex Provide conscious sense of head position and movement Send commands to motor nuclei in brain stem and spinal cord

Figure 17-26 Pathways for Equilibrium Sensations.
To superior colliculus and relay to cerebral cortex Red nucleus N III Semicircular canals Vestibular ganglion N IV Vestibular branch Vestibular nucleus N VI To cerebellum Vestibule Cochlear branch N XI Vestibulocochlear nerve (VIII) Vestibulospinal tracts

17-5 The Ear Eye, Head, and Neck Movements are reflexive motor commands From vestibular nuclei Distributed to motor nuclei for cranial nerves Peripheral Muscle Tone, Head, and Neck Movements instructions descend in vestibulospinal tracts of spinal cord

Diagrammatic and sectional views of the cochlear spiral
Figure 17-27b The Cochlea. Temporal bone (petrous part) Vestibular membrane Scala vestibuli (contains perilymph) Tectorial membrane Cochlear duct (contains endolymph) Basilar membrane Spiral organ From oval window Spiral ganglion Scala tympani (contains perilymph) To round window Cochlear nerve Vestibulocochlear nerve (VIII) b Diagrammatic and sectional views of the cochlear spiral

Diagrammatic and sectional views of the cochlear spiral
Figure 17-27b The Cochlea. Vestibular membrane Basilar membrane Temporal bone (petrous part) Scala vestibuli (contains perilymph) Cochlear duct (contains endolymph) Spiral organ Spiral ganglion Scala tympani (contains perilymph) Cochlear nerve Cochlear spiral section LM × 60 b Diagrammatic and sectional views of the cochlear spiral

17-5 The Ear Hearing Cochlear duct receptors provide sense of hearing
Auditory ossicles convert pressure fluctuation in air into much greater pressure fluctuations in perilymph of cochlea Frequency of sound Determined by which part of cochlear duct is stimulated Intensity (volume) Determined by number of hair cells stimulated

17-5 The Ear Hearing Cochlear duct receptors Basilar membrane
Separates cochlear duct from tympanic duct Hair cells lack kinocilia Stereocilia in contact with overlying tectorial membrane

Figure 17-28b The Spiral Organ (Part 1 of 2).
Tectorial membrane Outer hair cell Basilar membrane Inner hair cell Nerve fibers b Diagrammatic and sectional views of the receptor hair cell complex of the spiral organ

Figure 17-28b The Spiral Organ (Part 2 of 2).
Cochlear duct Vestibular membrane Tectorial membrane Scala tympani Basilar membrane Hair cells of spiral organ Spiral ganglion cells of cochlear nerve Spiral organ LM × 125 b Diagrammatic and sectional views of the receptor hair cell complex of the spiral organ

17-5 The Ear An Introduction to Sound Pressure waves
Consist of regions where air molecules are crowded together Adjacent zone where molecules are farther apart Sine waves S-shaped curves

17-5 The Ear Pressure Wave Wavelength: Distance between two adjacent wave troughs Frequency Number of waves that pass fixed reference point at given time Physicists use term cycles instead of waves Hertz (Hz) number of cycles per second (cps)

Figure 17-29a The Nature of Sound.
Wavelength Tympanic membrane Tuning fork Air molecules Sound waves (here, generated by a tuning fork) travel through the air as pressure waves. a

Figure 17-29b The Nature of Sound.
1 wavelength Sound energy arriving at tympanic membrane Amplitude Time (sec) b A graph showing the sound energy arriving at the tympanic membrane. The distance between wave peaks is the wavelength. The number of waves arriving each second is the frequency, which we perceive as pitch. Frequencies are reported in cycles per second (cps), or hertz (Hz). The amount of energy carried by the wave is its amplitude. The greater the amplitude, the louder the sound.

Figure 17-31a Frequency Discrimination.
Stapes at oval window Cochlea 16,000 Hz 6000 Hz 1000 Hz Round window Basilar membrane a The flexibility of the basilar membrane varies along its length, so pressure waves of different frequencies affect different parts of the membrane.

Figure 17-31b Frequency Discrimination.
Stapes moves inward Basilar membrane distorts toward round window Round window pushed outward b The effects of a vibration of the stapes at a frequency of 6000 Hz. When the stapes moves inward, as shown here, the basilar membrane distorts toward the round window, which bulges into the cavity of the middle ear.

Figure 17-31c Frequency Discrimination.
Stapes moves outward Basilar membrane distorts toward oval window Round window pulled inward c When the stapes moves outward, as shown here, the basilar membrane rebounds and distorts toward the oval window.

Figure 17-30 Sound and Hearing (Part 1 of 2).
External acoustic meatus Malleus Incus Stapes Oval window 3 Movement of sound waves 2 1 Tympanic membrane Round window 1 2 3 Sound waves arrive at tympanic membrane. Movement of the tympanic membrane causes displacement of the auditory ossicles Movement of the stapes at the oval window establishes pressure waves in the perilymph of the scala vestibuli.

Figure 17-30 Sound and Hearing (Part 2 of 2).
Cochlear branch of cranial nerve VIII 6 Scala vestibuli (contains perilymph) Vestibular membrane Cochlear duct (contains endolymph) Basilar membrane 4 Scala tympani (contains perilymph) 5 4 5 6 The pressure waves distort the basilar membrane on their way to the round window of the scala tympani. Vibration of the basilar membrane causes hair cells to vibrate against the tectorial membrane. Information about the region and the intensity of stimulation is relayed to the CNS over the cochlear branch of cranial nerve VIII.

17-5 The Ear Auditory Pathways Cochlear branch
Formed by afferent fibers of spiral ganglion neurons Enters medulla oblongata Synapses at dorsal and ventral cochlear nuclei Information crosses to opposite side of brain Ascends to inferior colliculus of midbrain

17-5 The Ear Auditory Pathways Ascending auditory sensations
Synapse in medial geniculate nucleus of thalamus Projection fibers deliver information to auditory cortex of temporal lobe

Figure 17-32 Pathways for Auditory Sensations (Part 1 of 2).
Stimulation of hair cells at a specific location along the basilar membrane activates sensory neurons. Cochlea Low-frequency sounds High-frequency sounds Vestibular branch 2 Sensory neurons carry the sound information in the cochlear branch of the vestibulocochlear nerve (VIII) to the cochlear nucleus on that side. KEY Primary pathway Vestibulocochlear nerve (VIII) Secondary pathway Motor output

Figure 17-32 Pathways for Auditory Sensations (Part 2 of 2).
6 Projection fibers then deliver the information to specific locations within the auditory cortex of the temporal lobe. High- frequency sounds Low-frequency sounds To ipsilateral auditory cortex Thalamus 5 Ascending acoustic information goes to the medial geniculate nucleus. 4 The inferior colliculi direct a variety of unconscious motor responses to sounds. To reticular formation and motor nuclei of cranial nerves Superior olivary nucleus 3 Information ascends from each cochlear nucleus to the superior olivary nucleus of the pons and the inferior colliculi of the midbrain. KEY Primary pathway Secondary pathway Motor output Motor output to spinal cord through the tectospinal tracts

17-5 The Ear Hearing Range From softest to loudest represents trillionfold increase in power Never use full potential Young children have greatest range

Table 17-1 Intensity of Representative Sounds.

17-5 The Ear Effects of Aging on the Ear With age, damage accumulates
Tympanic membrane gets less flexible Articulations between ossicles stiffen Round window may begin to ossify

An Introduction to the Special Senses

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