Special Senses Sight Smell Taste Hearing and balance. Touch, not special, and not here (Ch 13)

Special Senses Sight Smell Taste Hearing and balance Touch, not special, and not here (Ch 13)

Eye, matey.

Eye and Associated Structures 70% of all body sensory receptors are in the eye About half of the cerebral cortex is involved in visual processing 5/6 ths of the eye is protected by a cushion of fat and the bony orbit

Eye and Associated Structures Accessory structures include eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles

Eyebrows Coarse hairs that overlie the supraorbital margins Functions include: Shading the eye Preventing perspiration from reaching the eye Associated muscles Orbicularis oculi muscle depresses the eyebrows Corrugator muscles move the eyebrows medially

Eyebrows Orbicularis oculi muscle depresses the eyebrows Corrugator muscles move the eyebrows medially

Palpebrae (Eyelids) Protect the eye anteriorly Palpebral fissure separates eyelids Canthi medial and lateral angles (commissures) Lacrimal caruncle contains glands that secrete a whitish, oily secretion: (Sandman s eye sand)

Conjunctiva Transparent membrane that: Lines the eyelids as the palpebral (or tarsal) conjunctiva Covers the whites of the eyes as the bulbar (or ocular) conjunctiva Lubricates and protects the eye

Palpebrae (Eyelids) Palpebral conjunctiva Bulbar conjunctiva

Lacrimal Apparatus Consists of the lacrimal gland and associated ducts Lacrimal glands secrete tears Tears Contain mucus, antibodies, and lysozyme Enter the eye via superolateral excretory ducts Exit the eye medially via the lacrimal punctum Drain into the nasolacrimal duct

Lacrimal Apparatus Lacrimal sac Lacrimal gland Excretory ducts of lacrimal glands Lacrimal punctum Lacrimal canaliculus Nasolacrimal duct

Extrinsic Eye Muscles Six straplike extrinsic eye muscles Enable the eye to follow moving objects Maintain the shape of the eyeball Four rectus muscles originate from the annular ring Two oblique muscles move the eye in the vertical plane

Extrinsic Eye Muscles Superior oblique muscle Superior oblique tendon Superior rectus muscle Lateral rectus muscle Inferior rectus Inferior oblique muscle muscle (a) Lateral view of the right eye

Extrinsic Eye Muscles Trochlea Superior oblique muscle Superior oblique tendon Superior rectus muscle Axis at center of eye Inferior rectus muscle Medial rectus muscle Lateral rectus muscle Common tendinous ring (b) Superior view of the right eye

Let s play, Pin the Lacrimal Gland on the Eye. Superior Or Inferior Lateral view of R eye

Let s play, Pin the Lacrimal Gland on the Eye. Medial Or Lateral Superior view of R eye

Let s play, Pin the Lacrimal Gland on the Eye. Ok, Superior and Lateral, right? So, now name this muscle. And, this one. Superior view of R eye

Summary of Cranial Nerves and Muscle Actions Names, actions, and cranial nerve innervation of the extrinsic eye muscles Muscle Lateral rectus Medial rectus Superior rectus Inferior rectus Inferior oblique Superior oblique Action Moves eye laterally Moves eye medially Elevates eye and turns it medially Depresses eye and turns it medially Elevates eye and turns it laterally Depresses eye and turns it laterally Controlling cranial nerve VI (abducens) III (oculomotor) III (oculomotor) III (oculomotor) III (oculomotor) IV (trochlear) (c) Summary of muscle actions and innervating cranial nerves

Structure of the Eyeball A slightly irregular hollow sphere with anterior and posterior poles The wall is composed of three tunics fibrous, vascular, and sensory The internal cavity is filled with fluids called humors The lens separates the internal cavity into anterior and posterior segments

Structure of the Eyeball Sclera Choroid Retina Fibrous Vascular Sensory Anterior segment (contains aqueous humor) Posterior segment (contains vitreous humor) (a) Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball. Hyaloid canal

Fibrous Tunic Forms the outermost coat of the eye and is composed of: Opaque sclera (posteriorly) Clear cornea (anteriorly) The sclera protects the eye and anchors extrinsic muscles The cornea lets light enter the eye

Structure of the Eyeball: Fibrous Tunic Cornea Sclera Choroid Retina Vascular Sensory Anterior pole Anterior segment (contains aqueous humor) Posterior segment (contains vitreous humor)

Vascular Tunic (Uvea): Has three regions: choroid, ciliary body, and iris Choroid region A dark brown membrane that forms the posterior portion of the uvea Supplies blood to all eye tunics

Vascular Tunic (Uvea): Choroid Region Cornea Sclera Choroid Retina Sensory Anterior segment (contains aqueous humor) Posterior segment (contains vitreous humor)

Vascular Tunic (Uvea): Ciliary Body A thickened ring of tissue surrounding the lens Composed of smooth muscle bundles (ciliary muscles) Anchors the suspensory ligament that holds the lens in place

Vascular Tunic (Uvea): Ciliary Body Ciliary body Ciliary zonule (suspensory ligament) Cornea Sclera Choroid Retina Sensory Anterior segment (contains aqueous humor) Posterior segment (contains vitreous humor)

Vascular Tunic (Uvea): Iris The colored part of the eye Pupil central opening of the iris Regulates the amount of light entering the eye during: Close vision and bright light pupils constrict Distant vision and dim light pupils dilate Changes in emotional state pupils dilate when the subject matter is appealing or requires problem-solving skills

Vascular Tunic (Uvea): Iris Ciliary body Ciliary zonule (suspensory ligament) Iris Sclera Choroid Retina Sensory Anterior segment (contains aqueous humor) Posterior segment (contains vitreous humor)

Pupil Dilation and Constriction Parasympathetic + Sympathetic + Sphincter pupillae muscle contraction decreases pupil size. Iris (two muscles) Sphincter pupillae Dilator pupillae Dilator pupillae muscle contraction increases pupil size.

Sensory Tunic: Retina A delicate two-layered membrane Pigmented layer the outer layer that absorbs light and prevents its scattering Neural layer, which contains: Photoreceptors that transduce light energy Bipolar cells and ganglion cells Amacrine and horizontal cells

Structure of the Eyeball: Sensory Tunic Ciliary body Ciliary zonule (suspensory ligament) Sclera Choroid Retina Iris Anterior segment (contains aqueous humor) Posterior segment (contains vitreous humor)

Sensory Tunic: Retina Pathway of light Neural layer of retina Pigmented layer of retina Choroid Sclera (a) Posterior aspect of the eyeball

Ganglion cells Bipolar cells Photoreceptors Rod Cone Pathway of light Amacrine cell Horizontal cell Pathway of signal output Pigmented Pathway of light layer of retina (b) Cells of the neural layer of the retina

Nuclei of ganglion cells Outer segments of rods and cones Choroid Axons of Nuclei Nuclei of ganglion of bipolar rods and cells cells cones (c) Photomicrograph of retina Pigmented layer of retina

The Retina: Ganglion Cells and the Optic Disc Ganglion cell axons: Run along the inner surface of the retina Leave the eye as the optic nerve The optic disc: Is the site where the optic nerve leaves the eye Lacks photoreceptors (the blind spot)

Sensory Tunic: Retina Pathway of light Optic disc Neural layer of retina Pigmented layer of retina Choroid Sclera Optic nerve (a) Posterior aspect of the eyeball

The Retina: Optic Disc Optic disc Blind Spot Retina

The Retina: Photoreceptors Rods: Respond to dim light Are used for peripheral vision Cones: Respond to bright light Have high-acuity color vision Are found in the macula lutea Are concentrated in the fovea centralis

The Retina: Optic Disc Macula lutea Optic disc Blind Spot Retina

Sensory Tunic: Retina Bipolar cells and ganglion cells Carry the action potential to the optic nerve Amacrine and horizontal cells Assist with visual processing Modify output of bipolar cells

Sensory Tunic: Retina Ganglion cells Bipolar cells Photoreceptors Rod Cone Pathway of light Amacrine cell Horizontal cell Pathway of signal output Pigmented Pathway of light layer of retina (b) Cells of the neural layer of the retina

Blood Supply to the Retina The neural retina receives its blood supply from two sources The outer third receives its blood from the choroid The inner two-thirds is served by the central artery and vein Small vessels radiate out from the optic disc and can be seen with an ophthalmoscope

The Retina: Central artery and vein Central artery and vein emerging from the optic disc Optic disc Retina

Inner Chambers and Fluids The lens separates the internal eye into anterior and posterior segments The posterior segment is filled with a clear gel called vitreous humor that: Transmits light Supports the posterior surface of the lens Holds the neural retina firmly against the pigmented layer Contributes to intraocular pressure

Inner Chambers and Fluids Posterior segment (contains vitreous humor)

Anterior Segment Composed of two chambers Anterior Ch. between the cornea and the iris Posterior Ch. between the iris and the lens Aqueous humor A plasmalike fluid that fills the anterior segment Produced by the ciliary processes Drains via the scleral venous sinus (Canal of Schlemm) Supports, nourishes, and removes wastes

Anterior Segment Cornea Lens Anterior segment (contains aqueous humor) Anterior chamber Posterior chamber Scleral venous sinus 3 1 Ciliary processes Ciliary muscle Ciliary body

Lens A biconvex, transparent, flexible, avascular structure that: Allows precise focusing of light onto the retina Is composed of epithelium and lens fibers

Anterior Segment Cornea Lens Lens epithelium Lens 2 Ciliary processes Ciliary muscle Ciliary body

Lens Is composed of epithelium and lens fibers Lens epithelium anterior, cuboidal cells that differentiate into lens fibers Lens fibers cells filled with the transparent protein crystallin

Anterior Segment Figure 15.8

Lens With age, the lens becomes more compact and dense and loses its elasticity, and you need

Focusing Light on the Retina Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, neural layer of the retina to the photoreceptors Light is refracted: At the cornea Entering the lens Leaving the lens The lens curvature and shape allow for fine focusing of an image

Focusing for Distant Vision Light from a distance needs little adjustment for proper focusing Far Point of Vision the distance beyond which the lens does not need to change shape to focus (20 ft.)

Focusing for Distant Vision Nearly parallel rays from distant object Lens Sympathetic activation Ciliary zonule Ciliary muscle (a) Lens is flattened for distant vision. Sympathetic input relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens. Inverted image

Focusing for Close Vision Close vision requires: Accommodation changing the lens shape by ciliary muscle contraction and lens ligament relaxation to increase refractory power Constriction the pupillary reflex constricts the pupils to prevent divergent light rays from entering the eye Convergence medial rotation of the eyeballs toward the object being viewed

Focusing for Close Vision Divergent rays from close object Parasympathetic activation Ciliary muscle Inverted image Lens ligaments (b) Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge.

Visual Pathways Axons of retinal ganglion cells form the optic nerve Medial fibers of the optic nerve decussate at the optic chiasm Optic chiasm

Visual Pathways

Visual Pathways Most fibers of the optic tracts continue to the lateral geniculate body of the thalamus (gateway to the cortex, conscious senses all pass through the thalamus)

Visual Pathways

Visual Pathways Other optic tract fibers end in superior colliculi (initiating visual reflexes) and pretectal nuclei (involved with pupillary reflexes) Optic radiations travel from the thalamus to the visual cortex

Visual Pathways

End, matey.

New senses. Taste and Smell.

Chemical Senses Chemical senses = olfaction (smell) and gustation (taste) Their chemoreceptors respond to chemicals in aqueous solution Smell to substances dissolved in fluids of the nasal membranes Taste to substances dissolved in saliva

Sense of Smell (this one s easy) The organ of smell is the olfactory epithelium, which covers the superior nasal concha

Olfactory Receptors Olfactory epithelium Olfactory tract Olfactory bulb (a)

Sense of Smell (this one s easy) Olfactory receptor cells are bipolar neurons with radiating olfactory cilia Olfactory receptors are surrounded and cushioned by supporting cells Basal cells lie at the base of the epithelium

Olfactory Receptors Olfactory epithelium Olfactory tract Olfactory tract Olfactory bulb Cribriform plate of ethmoid bone Filaments of olfactory nerve (a) Route of inhaled air Olfactory gland Olfactory epithelium Mucus Axon Basal cell Olfactory receptor cell Supporting cell Dendrite Olfactory cilia (b) Route of inhaled air containing odor molecules

Olfactory Pathway Olfactory receptor cells synapse with mitral cells at glomeruli Mitral cells process odor signals Mitral cells send impulses to: The olfactory cortex (via the thalamus for conscious perception of the sense of smell) The hypothalamus, amygdala, and limbic system (for unconscious perceptions and emotional response)

Olfactory Receptors Olfactory epithelium Olfactory tract Mitral cell (output cell) Glomeruli Olfactory tract Olfactory bulb Nasal conchae Olfactory gland (a) Route of inhaled air Olfactory epithelium Mucus (b) Route of inhaled air containing odor molecules

Olfactory Receptors, end. (told U it was easy)

Taste

Taste Buds Most of the 10,000 or so taste buds are found on the tongue Some are other places in the mouth. Taste buds are found in papillae of the tongue mucosa Papillae come in three types: filiform (foliate), fungiform, and circumvallate (vallate)

Taste Buds Epiglottis Palatine tonsil Lingual tonsil Foliate papillae Circumvallate (vallate) Fungiform papillae (a) Taste buds are associated with fungiform, foliate, and circumvallate (vallate) papillae.

Taste Buds Circumvallate papilla Taste bud (b) Enlarged section of a circumvallate papilla.

Structure of a Taste Bud Gourd-shaped

Taste Buds Connective tissue Taste fibers of cranial nerve Gustatory hair Basal cells Gustatory (taste) cells Taste pore Stratified squamous epithelium of tongue (c) Enlarged view of a taste bud.

Structure of a Taste Bud Each gourd-shaped taste bud consists of three major cell types Supporting cells insulate the receptor Basal cells dynamic stem cells, replace gustatory cells every 7-10 days. Gustatory cells taste cells, transmit signals to sensory dendrites of Cranial Nerves VII and IX (facial and?, to thalamus )

Taste Buds Connective tissue Taste fibers of cranial nerve Gustatory hair Basal cells Gustatory (taste) cells Taste pore Stratified squamous epithelium of tongue

Taste Sensations There are five basic taste sensations Sweet sugars, saccharin, alcohol, and some amino acids Salt metal ions (NaCl = Na+ and Cl-) Sour hydrogen ions, acids Bitter alkaloids such as quinine and nicotine Umami elicited by the amino acid glutamate (MSG) And, maybe now a 6 th, calcium. Most tastes are combinations

Physiology of Taste In order to be tasted, a chemical: Must be dissolved in saliva Must contact gustatory hairs Binding of the food chemical: Depolarizes the taste cell membrane, releasing neurotransmitter to the sensory dendrite

Gustatory Pathway Cranial Nerves VII (Facial) and IX (? *Hint, well named for tongue stuff) carry impulses from taste buds to the thalamus and then to the gustatory cortex (Conscious perception of taste)

Gustatory cortex (in insula) Thalamus Pons Solitary nucleus in medulla oblongata Facial nerve (VII) Glossopharyngeal nerve (IX)

Influence of Other Sensations on Taste Taste is 80% smell, food tastes bland when you have a cold. Thermoreceptors, mechanoreceptors, nociceptors also influence tastes Temperature and texture enhance or detract from taste

End of Taste and Smell.

The Ear: Hearing and Balance

The Ear: Hearing and Balance Wake up, sit up, pay attention, this one is complicated. Sitzender Junge ("Sitting youngster") by Werner Stötzer, 1956

The Ear: Hearing and Balance The three parts of the ear are the inner, outer, and middle ear The outer and middle ear are involved with hearing The inner ear functions in both hearing and equilibrium

Three regions of the ear External ear Middle ear Internal ear (labyrinth)

Outer Ear The auricle (pinna) contains: The helix (rim) The lobule (earlobe) External auditory canal Short, curved tube filled with ceruminous glands

The Ear: Hearing and Balance External ear Middle ear Internal ear (labyrinth) Auricle (pinna) Helix Lobule External acoustic meatus

Outer Ear Tympanic membrane (eardrum) Thin connective tissue membrane that vibrates in response to sound Transfers sound energy to the middle ear ossicles Boundary between outer and middle ears

The Ear: Hearing and Balance External ear Middle ear Internal ear (labyrinth) Tympanic membrane

Middle Ear (Tympanic Cavity) A small, air-filled, mucosa-lined cavity Flanked laterally by the eardrum (tympanic membrane) Flanked medially by the oval and round windows

Middle Ear Oval window (deep to stapes) Tympanic membrane Round window

Middle Ear: Ear Ossicles The tympanic cavity contains three small bones: malleus, incus, and stapes Transmit vibratory motion of the eardrum to the oval window Dampened by the tensor tympani (tube to malleus) and stapedius muscles (cavity to the stapes)

Middle Ear: Ear Ossicles Auditory ossicles Malleus (hammer) Incu (anvil) Stapes (stirrup)

Middle Ear (Tympanic Cavity) Pharyngotympanic tube connects the middle ear to the nasopharynx Equalizes pressure in the middle ear cavity with the external air pressure

Middle Ear Pharyngotympanic (auditory) tube

The Inner Ear: Hearing and Balance Receptors for hearing and balance: Respond to separate stimuli Are activated independently

Inner Ear: Hearing and Balance

Inner Ear: Hearing and Balance Semicircular ducts in semicircular canals Anterior Posterior Lateral Utricle in vestibule Saccule in vestibule Cochlear duct in cochlea

Inner Ear: Hearing and Balance Bony labyrinth Tortuous channels worming their way through the temporal bone Contains the vestibule, the cochlea, and the semicircular canals Filled with perilymph, similar to CSF, supports the Membranous labyrinth Series of membranous sacs within the bony labyrinth Filled with endolymph, similar to intracellular fluid

Inner Ear: Hearing and Balance Membranous labyrinth inside bony labyrinth inside which bone? Semicircular ducts in semicircular canals Temporal bone Anterior Posterior Lateral Balance Cristae ampullares in the membranous ampullae Utricle in vestibule Saccule in vestibule Stapes in oval window Hearing Maculae Spiral organ (of Corti) Cochlear duct in cochlea Round window

Mechanisms of Equilibrium and Orientation

Mechanisms of Equilibrium and Orientation Vestibular apparatus equilibrium receptors in the semicircular canals and vestibule Maintains our orientation and balance in space Vestibular receptors monitor static equilibrium Semicircular canal receptors monitor dynamic equilibrium

Inner Ear: Balance: The Vestibule

Inner Ear: Balance: The Vestibule The Vestibule The central cavity of the bony labyrinth Suspended in its perilymph are two sacs: the utricle and saccule The utricle extends into the semicircular canals (balance) (U is a semi circle) The saccule extends into the cochlea (cochlea is for hearing)

Inner Ear: Balance: The Vestibule Utricle in vestibule Saccule in vestibule

Inner Ear: Balance: The Vestibule The utricle and saccule Have equilibrium receptors called maculae Respond to gravity and changes in the position of the head

Inner Ear: Balance: The Vestibule Maculae Utricle in vestibule Saccule in vestibule

Inner Ear: Balance: The Vestibule Utricle Macula Saccule

Anatomy of Maculae Maculae are the sensory receptors for static equilibrium Contain supporting cells and hair cells Each hair cell has stereocilia and kinocilium embedded in an otolithic membrane Otolithic membrane jellylike mass studded with tiny CaCO 3 stones called otoliths Utricular hairs respond to horizontal movement Saccular hairs respond to vertical movement

Inner Ear: Balance: The Macula Stereocilia Kinocilium Otoliths Otolithic membrane Hair bundle Macula of utricle Macula of saccule Vestibular nerve fibers Hair cells Supporting cells

Inner Ear: Balance: The Macula Otolithic membrane Kinocilium Stereocilia Receptor potential Nerve impulses generated in vestibular fiber Depolarization When hairs bend toward the kinocilium, the hair cell depolarizes, exciting the nerve fiber, which generates more frequent action potentials. Hyperpolarization When hairs bend away from the kinocilium, the hair cell hyperpolarizes, inhibiting the nerve fiber, and decreasing the action potential frequency.

Inner Ear: Balance: The Semicircular Canals

Inner Ear: Balance: The Semicircular Canals Three canals that each define two-thirds of a circle and lie in the three planes of space Membranous semicircular ducts line each canal and communicate with the utricle

Inner Ear: Balance: The Semicircular Canals Semicircular ducts in semicircular canals Anterior Posterior Lateral

Inner Ear: Balance: The Ampulla The ampulla is the swollen end of each canal and it houses equilibrium receptors in a region called the crista ampullaris These receptors respond to angular movements of the head

Inner Ear: Balance: The Ampulla Semicircular ducts in semicircular canals Anterior Posterior Lateral Cristae ampullares in the membranous ampullae

Crista Ampullaris and Dynamic Equilibrium The crista ampullaris (or crista): Is the receptor for dynamic equilibrium Is located in the ampulla of each semicircular canal Responds to angular movements

Inner Ear: Balance: The Ampulla Crystea ampullares in the ampullea

Crista Ampullaris and Dynamic Equilibrium Each crista has support cells and hair cells that extend into a gel-like mass called the cupula Dendrites of vestibular nerve fibers encircle the base of the hair cells

Inner Ear: Balance: The Ampulla Cupula Endolymph Membranous Crista labyrinth ampullaris Fibers of vestibular nerve Hair bundle (kinocilium plus stereocilia) Hair cell Supporting cell

Inner Ear: Balance: The Ampulla Cupula (b) Scanning electron micrograph of a crista ampullaris (200x)

Inner Ear: Balance: The Ampulla Section of ampulla, filled with endolymph Cupula Fibers of vestibular nerve Flow of endolymph At rest, the cupula stands upright. (c) Movement of the cupula during rotational acceleration and deceleration During rotational acceleration, endolymph moves inside the semicircular canals in the direction opposite the rotation (it lags behind due to inertia). Endolymph flow bends the cupula and excites the hair cells. As rotational movement slows, endolymph keeps moving in the direction of the rotation, bending the cupula in the opposite direction from acceleration and inhibiting the hair cells.

Inner Ear: Balance Input: Information about the body s position in space comes from three main sources and is fed into two major processing areas in the central nervous system. Vestibular receptors Visual receptors Somatic receptors (from skin, muscle and joints) Cerebellum Central nervous system processing Vestibular nuclei (in brain stem) Oculomotor control (cranial nerve nuclei III, IV, VI) (eye movements) Spinal motor control (cranial nerve XI nuclei and vestibulospinal tracts) (neck movements) Output: Fast reflexive control of the muscles serving the eye and neck, limb, and trunk are provided by the outputs of the central nervous system.

Inner Ear: Hearing: The Cochlea

Inner Ear: Hearing: The Cochlea The Cochlea A spiral, conical, bony chamber that: Extends from the anterior vestibule (saccule area) Coils around a bony pillar called the modiolus Contains the cochlear duct, which ends at the cochlear apex (helicotrema) Contains the organ of Corti (hearing receptor)

Modiolus The Cochlea Helicotrema

Inner Ear: Hearing: The Cochlea The cochlea is divided into three chambers: Scala vestibuli Scala media Scala tympani

Inner Ear: Hearing: The Cochlea Cochlear duct (scala media; contains endolymph) Scala vestibuli (contains perilymph) Spiral organ (of Corti) Scala tympani (contains perilymph)

Inner Ear: Hearing: The Cochlea The cochlea is divided into three chambers: Scala vestibuli Continuous with vestibule, abuts the oval window Scala media The cochlear duct, the hearing part Scala tympani Abuts round window, meets the scala vestibuli at the helicotrema.

The Cochlea and Hearing

The Cochlea The floor of the scala media (aka cochlear duct) is composed of: The osseous/bony spiral lamina The basilar membrane, which supports the organ of Corti The cochlear branch of nerve VIII runs from the organ of Corti to the brain (CN VIII is.?) Vestibulocochlear

The Cochlea Osseous spiral lamina Cochlear duct (scala media; contains endolymph) Spiral organ (of Corti) Basilar membrane Scala vestibuli (contains perilymph) Scala tympani (contains perilymph) Spiral ganglion Cochlear nerve, division of the vestibulocochlear nerve (VIII)

Organ of Corti Tectorial membrane Hairs (stereocilia) Outer hair cells Inner hair cell Afferent nerve fibers Supporting cells Fibers of cochlear nerve Basilar membrane

Air pressure A little physio: Sound and Mechanisms of Hearing Wavelength Area of high pressure (compressed molecules) Area of low pressure (rarefaction) Crest Trough Distance Amplitude (a) A struck tuning fork alternately compresses and rarefies the air molecules around it, creating alternate zones of high and low pressure. (b) Sound waves radiate outward in all directions.

A little physio: Sound and Mechanisms of Hearing Sound vibrations beat against the eardrum The eardrum pushes against the ossicles, which press fluid in the inner ear against the oval and round windows This movement sets up shearing forces that pull on hair cells Moving hair cells stimulates the cochlear nerve that sends impulses to the brain

Transmission of Sound to the Inner Ear Auditory ossicles Malleus Incus Stapes Cochlear nerve Oval window Scala vestibuli Helicotrema Scala tympani 2 3 Cochlear duct Basilar membrane 1 Tympanic Round membrane window (a) Route of sound waves through the ear 1 Sound waves vibrate the tympanic membrane. 2 Auditory ossicles vibrate. Pressure is amplified. 3 Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli. Sounds with frequencies below hearing travel through the helicotrema and do not excite hair cells. Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells.

Transmission of Sound to the Inner Ear Basilar membrane High-frequency sounds displace the basilar membrane near the base. Medium-frequency sounds displace the basilar membrane near the middle. Base (short, stiff fibers) Fibers of basilar membrane Apex (long, floppy fibers) Low-frequency sounds displace the basilar membrane near the apex. (b) Different sound frequencies cross the basilar membrane at different locations. Frequency (Hz)

Transmission of Sound to the Inner Ear The route of sound to the inner ear follows this pathway: Outer ear pinna, auditory canal, eardrum Middle ear malleus, incus, and stapes to the oval window Inner ear scalas vestibuli and tympani to the cochlear duct/scala media Stimulation of the organ of Corti Generation of impulses in the cochlear nerve

Inner Ear: Hearing: The Cochlea Medial geniculate nucleus of thalamus Hearing Primary auditory cortex in temporal lobe Inferior colliculus Lateral lemniscus Superior olivary nucleus (pons-medulla junction) Midbrain Cochlear nuclei Vibrations Vibrations Medulla Vestibulocochlear nerve Spiral ganglion of cochlear nerve Bipolar cell Spiral organ (of Corti)

Deafness Conduction deafness something hampers sound conduction to the fluids of the inner ear (e.g., impacted earwax, perforated eardrum, osteosclerosis of the ossicles) Sensorineural deafness results from damage to the neural structures at any point from the cochlear hair cells to the auditory cortical cells

End