8 Special Senses

The Senses Special senses Smell Taste Sight Hearing Equilibrium

The Eye and Vision 70 percent of all sensory receptors are in the eyes Each eye has over a million nerve fibers Protection for the eye Most of the eye is enclosed in a bony orbit A cushion of fat surrounds most of the eye

Accessory Structures of the Eye Eyelids and eyelashes Conjunctiva Lacrimal apparatus Extrinsic eye muscles

Eyebrow Site where conjunctiva merges with cornea Eyelid Eyelashes Pupil Palpebral fissure Lacrimal caruncle Lateral commissure (canthus) Medial commissure (canthus) Sclera (covered by conjunctiva) Iris Eyelid Figure 8.1

Accessory Structures of the Eye Eyelids Meet at the medial and lateral commissure (canthus) Eyelashes Tarsal glands  produce an oily secretion that lubricates the eye Ciliary glands  are modified sweat glands located between the eyelashes

Accessory Structures of the Eye Conjunctiva Membrane that lines the eyelids Connects to the outer surface of the eye Secretes mucus to lubricate the eye and keep it moist

Homeostatic Imbalance Conjunctivitis  inflammation of the conjunctiva that results in reddened, irritated eyes Pinkeye  the infectious form of conjunctivitis cause by bacteria or viruses is highly contagious

Accessory Structures of the Eye Lacrimal apparatus = lacrimal gland + ducts Lacrimal gland  produces lacrimal fluid (tears); situated on lateral aspect of each eye Lacrimal canaliculi  drain lacrimal fluid from eyes medially Lacrimal sac  provides passage of lacrimal fluid towards nasal cavity Nasolacrimal duct  empties lacrimal fluid into the nasal cavity

Excretory duct of lacrimal gland Lacrimal gland Conjunctiva Anterior aspect Eyelid Eyelashes Tarsal glands (a) Eyelid Figure 8.2a

Lacrimal sac Lacrimal gland Excretory ducts of lacrimal gland Lacrimal canaliculus Nasolacrimal duct Inferior meatus of nasal cavity Nostril (b) Figure 8.2b

Accessory Structures of the Eye Function of the lacrimal apparatus Protects, moistens, and lubricates the eye Empties into the nasal cavity Lacrimal secretions (tears) contain: Dilute salt solution Mucus Antibodies Lysozyme (enzyme that destroys bacteria)

Accessory Structures of the Eye Extrinsic eye muscles Six muscles attach to the outer surface of the eye Produce eye movements

Superior oblique muscle Superior oblique tendon Superior rectus muscle Conjunctiva Lateral rectus muscle Optic nerve Inferior rectus muscle Inferior oblique muscle (a) Figure 8.3a

Superior oblique muscle Trochlea Superior oblique muscle Superior oblique tendon Axis at center of eye Superior rectus muscle Inferior rectus muscle Medial rectus muscle Lateral rectus muscle (b) Figure 8.3b

Structure of the Eye Layers forming the wall of the eyeball Fibrous layer Outside layer Vascular layer Middle layer Sensory layer Inside layer

humor (in anterior segment) Sclera Ciliary body Choroid Ciliary zonule Retina Cornea Fovea centralis Iris Pupil Optic nerve Aqueous humor (in anterior segment) Lens Scleral venous sinus (canal of Schlemm) Central artery and vein of the retina Vitreous humor (in posterior segment) Optic disc (blind spot) (a) Figure 8.4a

Ciliary body Vitreous humor in posterior segment Iris Retina Margin of pupil Choroid Sclera Aqueous humor (in anterior segment) Fovea centralis Optic disc Optic nerve Lens Cornea Ciliary zonule (b) Figure 8.4b

Structure of the Eye: The Fibrous Layer Sclera White connective tissue layer Seen anteriorly as the “white of the eye” Cornea Transparent, central anterior portion Allows for light to pass through Repairs itself easily The only human tissue that can be transplanted without fear of rejection

Structure of the Eye: Vascular Layer Choroid  is a blood-rich nutritive layer in the posterior of the eye Pigment prevents light from scattering Modified anteriorly into two structures Ciliary body  smooth muscle attached to lens Iris  regulates amount of light entering eye Pigmented layer that gives eye color Pupil  rounded opening in the iris

Structure of the Eye: Sensory Layer Retina contains two layers Outer pigmented layer Inner neural layer Contains receptor cells (photoreceptors) Rods Cones

Structure of the Eye: Sensory Layer Neurons of the retina and vision Rods Most are found towards the edges of the retina Allow dim light vision and peripheral vision All perception is in gray tones

Structure of the Eye: Sensory Layer Neurons of the retina and vision Cones Allow for detailed color vision Densest in the center of the retina Fovea centralis–lateral to blind spot Area of the retina with only cones Visual acuity (sharpest vision) is here No photoreceptor cells are at the optic disc, or blind spot

Pigmented layer of retina Rod Cone Bipolar cells Pathway of light Ganglion cells (a) Figure 8.5a

Structure of the Eye: Sensory Layer Cone sensitivity Three types of cones Different cones are sensitive to different wavelengths Color blindness  is the result of the lack of one cone type Night blindness  inhibited rod function that hinders the ability to see at night

Lens Biconvex crystal-like structure Held in place by a suspensory ligament attached to the ciliary body

Lens Cataracts  result when the lens becomes hard and opaque with age Vision becomes hazy and distorted Eventually causes blindness in affected eye Risk factors include: Diabetes mellitus Frequent exposure to intense sunlight Heavy smoking

Figure 8.7

Two Segments, or Chambers, of the Eye Anterior (aqueous) segment Anterior to the lens Contains aqueous humor Posterior (vitreous) segment Posterior to the lens Contains vitreous humor

humor (in anterior segment) Sclera Ciliary body Choroid Ciliary zonule Retina Cornea Fovea centralis Iris Pupil Optic nerve Aqueous humor (in anterior segment) Lens Scleral venous sinus (canal of Schlemm) Central artery and vein of the retina Vitreous humor (in posterior segment) Optic disc (blind spot) (a) Figure 8.4a

Anterior Segment Aqueous humor Watery fluid found between lens and cornea Similar to blood plasma Helps maintain intraocular pressure Provides nutrients for the lens and cornea Reabsorbed into venous blood through the scleral venous sinus, or canal of Schlemm

Homeostatic Imbalance Glaucoma If drainage of aqueous humor is blocked, fluid backs up like a clogged sink Pressure within the eye may increase to dangerous levels and compress the delicate retina and optic nerve Eventually causes pain and possibly blindness if not detected early

Posterior Segment Vitreous humor Gel-like substance posterior to the lens Prevents the eye from collapsing Helps maintain intraocular pressure

Ophthalmoscope Instrument used to illuminate the interior of the eyeball Can detect diabetes, arteriosclerosis, degeneration of the optic nerve and retina

Fovea centralis Macula Blood vessels Optic disc Retina Lateral Medial Figure 8.8

Pathway of Light Through the Eye Light must be focused to a point on the retina for optimal vision The eye is set for distance vision (over 20 feet away) Accommodation The lens must change shape to focus on closer objects (less than 20 feet away)

Light from distant source Focal point Retina Light from distant source Focal point (a) Light from near source Focal point Retina (b) Figure 8.9

Pathway of Light Through the Eye Image formed on the retina is a real image Real images are: Reversed from left to right Upside down Smaller than the object

Figure 8.10

Visual Fields and Visual Pathways Optic chiasma Location where the optic nerves cross Fibers from the medial side of each eye cross over to the opposite side of the brain Optic tracts Contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye

Occipital lobe (visual cortex) Fixation point Right eye Left eye Optic nerve Optic chiasma Optic tract Optic radiation Thalamus Occipital lobe (visual cortex) Figure 8.11

A Closer Look Emmetropia Eye focuses images correctly on the retina Myopia (nearsighted) Distant objects appear blurry Light from those objects fails to reach the retina and are focused in front of it Results from an eyeball that is too long

A Closer Look Hyperopia (farsighted) Near objects are blurry while distant objects are clear Distant objects are focused behind the retina Results from an eyeball that is too short or from a “lazy lens”

A Closer Look Astigmatism Images are blurry Results from light focusing as lines, not points, on the retina due to unequal curvatures of the cornea or lens

The Ear Houses two senses Hearing Equilibrium (balance) Receptors are mechanoreceptors Different organs house receptors for each sense

Anatomy of the Ear The ear is divided into three areas External (outer) ear Middle ear (tympanic cavity) Inner ear (bony labyrinth)

External (outer) ear Middle ear Internal (inner) ear Vestibulocochlear nerve Auricle (pinna) Semicircular canals Oval window Cochlea Vestibule Round window Pharyngotympanic (auditory) tube Tympanic membrane (eardrum) Hammer (malleus) Anvil (incus) Stirrup (stapes) External acoustic meatus (auditory canal) Auditory ossicles Figure 8.12

The External Ear Involved in hearing only Structures of the external ear Auricle (pinna) External acoustic meatus (auditory canal) Narrow chamber in the temporal bone Lined with skin and ceruminous (wax) glands Ends at the tympanic membrane

The Middle Ear (Tympanic Cavity) Air-filled cavity within the temporal bone Only involved in the sense of hearing

The Middle Ear (Tympanic Cavity) Two tubes are associated with the inner ear The opening from the auditory canal is covered by the tympanic membrane The auditory tube connecting the middle ear with the throat Allows for equalizing pressure during yawning or swallowing This tube is otherwise collapsed

Bones of the Middle Ear (Tympanic Cavity) Three bones (ossicles) span the cavity Malleus (hammer) Incus (anvil) Stapes (stirrup) Function Vibrations from eardrum move the hammer  anvil  stirrup  inner ear

External (outer) ear Middle ear Internal (inner) ear Vestibulocochlear nerve Auricle (pinna) Semicircular canals Oval window Cochlea Vestibule Round window Pharyngotympanic (auditory) tube Tympanic membrane (eardrum) Hammer (malleus) Anvil (incus) Stirrup (stapes) External acoustic meatus (auditory canal) Auditory ossicles Figure 8.12

Inner Ear or Bony Labyrinth Includes sense organs for hearing and balance Filled with perilymph (plasma like fluid) Contains a maze of bony chambers within the temporal bone Cochlea Vestibule Semicircular canals

External (outer) ear Middle ear Internal (inner) ear Vestibulocochlear nerve Auricle (pinna) Semicircular canals Oval window Cochlea Vestibule Round window Pharyngotympanic (auditory) tube Tympanic membrane (eardrum) Hammer (malleus) Anvil (incus) Stirrup (stapes) External acoustic meatus (auditory canal) Auditory ossicles Figure 8.12

Organs of Equilibrium Equilibrium receptors of the inner ear are called the vestibular apparatus Vestibular apparatus has two functional parts Static equilibrium Dynamic equilibrium

Semicircular canals Ampulla Vestibular nerve Vestibule (a) Figure 8.14a

Static Equilibrium Changes in position of the head in space with respect to the pull of gravity when the body is not moving Maculae  receptors in the vestibule Report on the position of the head Send information via the vestibular nerve

Membranes in vestibule Otoliths Otolithic membrane Hair tuft Hair cell Supporting cell Nerve fibers of vestibular division of cranial nerve VIII (a) Figure 8.13a

Force of gravity Otolithic Otoliths membrane Hair cell Head upright Head tilted (b) Figure 8.13b

Dynamic Equilibrium These receptors respond to angular or rotary movements Crista ampullaris (in the ampulla of each semicircular canal)—dynamic equilibrium receptors are located in the semicircular canals Tuft of hair cells covered with cupula (gelatinous cap) If the head moves, the cupula drags against the endolymph

Cupula of crista ampullaris Endolymph Flow of endolymph Cupula of crista ampullaris Cupula Nerve fibers Direction of body movement (b) (c) Figure 8.14b-c

Organs of Hearing Organ of Corti Located within the cochlea Receptors = hair cells on the basilar membrane Gel-like tectorial membrane is capable of bending hair cells Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe

Mechanism of Hearing Vibrations from sound waves move tectorial membrane Hair cells are bent by the membrane An action potential starts in the cochlear nerve Impulse travels to the temporal lobe Continued stimulation can lead to adaptation

Figure 8.16 EXTERNAL EAR MIDDLE EAR INTERNAL EAR Auditory canal Ear- drum Hammer, anvil, stirrup Oval window Fluids in cochlear canals Pinna Upper and middle lower Pressure Spiral organ of Corti stimulated Time One vibration Amplitude Amplification in middle ear Figure 8.16

Mechanism of Hearing High-pitched sounds disturb the short, stiff fibers of the basilar membrane Receptor cells close to the oval window are stimulated Low-pitched sounds disturb the long, floppy fibers of the basilar membrane Specific hair cells further along the cochlea are affected

20,000 (High notes) 2,000 200 20 (Low notes) Stapes Fibers of sensory neurons Scala vestibuli Oval window Perilymph Round window Scala tympani Basilar membrane Cochlear duct (a) Fibers of basilar membrane Apex (long, floppy fibers) Base (short, stiff fibers) 20,000 (High notes) 2,000 200 20 (Low notes) Frequency (Hz) (b) Figure 8.17

Homeostatic Imbalance Deafness Hearing loss of any degree (slight loss to total inability to hear sound) Conduction Deafness Results when something interferes with the conduction of sound vibrations (earwax) Sensorineural Deafness Degeneration or damage to the receptor cells in the spiral organ of Corti, cochlear nerve, or neurons of auditory cortex (extended listening to excessively loud sounds)

Olfaction  The Sense of Smell Olfactory receptors are in the roof of the nasal cavity Neurons with long cilia Chemicals must be dissolved in mucus for detection Impulses are transmitted via the olfactory nerve Interpretation of smells is made in the cortex

Olfactory receptor cell Olfactory bulb Cribriform plate of ethmoid bone Olfactory tract Olfactory filaments of the olfactory nerve Supporting cell Olfactory mucosa Olfactory receptor cell Olfactory hairs (cilia) Mucus layer (a) Route of inhaled air containing odor molecules (b) Figure 8.18

The Sense of Taste Taste buds house the receptor organs Location of taste buds  specific receptors for the sense of taste Most are on the tongue Soft palate Cheeks

Epiglottis Palatine tonsil Lingual tonsil Fungiform papillae (a) Figure 8.19a

Circumvallate papilla Taste buds (b) Figure 8.19b

The Tongue and Taste The tongue is covered with projections called papillae Filiform papillae  sharp with no taste buds Fungiform papillae  rounded with taste buds Circumvallate papillae  large papillae with taste buds Taste buds are found on the sides of papillae

Structure of Taste Buds Gustatory cells are the receptors Have gustatory hairs (long microvilli) Hairs are stimulated by chemicals dissolved in saliva

Structure of Taste Buds Impulses are carried to the gustatory complex by several cranial nerves because taste buds are found in different areas Facial nerve Glossopharyngeal nerve Vagus nerve

Taste Sensations Sweet receptors (sugars) Saccharine Some amino acids Sour receptors Acids Bitter receptors Alkaloids Salty receptors Metal ions

Chemical Senses: Taste and Smell Both senses use chemoreceptors Stimulated by chemicals in solution Taste has four types of receptors Smell can differentiate a large range of chemicals Both senses complement each other and respond to many of the same stimuli