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Chapter 16

The Special Senses


Chemical Senses

•      Chemical senses – gustation (taste) and olfaction (smell)

•      Their chemoreceptors respond to chemicals in aqueous solution

•    Taste – to substances dissolved in saliva

•    Smell – to substances dissolved in fluids of the nasal membranes

Taste Buds

•      The 10,000 or so taste buds are mostly found on the tongue

•      Found in papillae of the tongue mucosa

•      Papillae come in three types: filiform, fungiform, and circumvallate

•      Fungiform and circumvallate papillae contain taste buds

Anatomy 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

•    Gustatory cells – taste cells

Taste Sensations

•      There are four basic taste sensations

•    Sweet – sugars, saccharin, alcohol, and some amino acids

•    Salt – metal ions

•    Sour – hydrogen ions

•    Bitter – alkaloids such as quinine and nicotine

Physiology of Taste

•      In order to be tasted, a chemical:

•    Must be dissolved in saliva

•    Contact gustatory hairs

•      Binding of the food chemical:

•    Depolarizes the taste cell membrane, releasing neurotransmitter

•    Initiates a generator potential that elicits an action potential

Taste Transduction

•      The stimulus energy of taste is converted into a nerve impulse by:

•    Na+ influx in salty tastes

•    H+ and blockage of K+ channels in sour tastes

•    Gustducin in sweet and bitter tastes

Gustatory Pathways

•      Cranial Nerves VII  and IX  carry impulses from taste buds to the solitary nucleus of the medulla

•      These impulses then travel to the thalamus, and from there fibers branch to the:

•    Gustatory cortex (taste)

•    Hypothalamus and limbic system (appreciation of taste)

Influence of Other Sensations on Taste

•      Taste is 80% smell

•      Thermoreceptors, mechanoreceptors, nociceptors also influence tastes

•      Temperature and texture enhance or detract from taste

Sense of Smell

•      The organ of smell is the olfactory epithelium, which covers the superior nasal concha

•      Olfactory receptor cells are bipolar neurons with radiating olfactory cilia

•      They are surrounded and cushioned by supporting cells

•      Basal cells lie at the base of the epithelium

Physiology of Smell

•      Olfactory receptors respond to several different odor causing chemicals

•      When bound to ligand these proteins initiate a G protein mechanism, which uses cAMP as a second messenger

•      cAMP opens sodium channels, causing depolarization of the receptor membrane that then triggers an action potential

Olfactory Pathway

•      Olfactory receptor cells synapse with mitral cells

•      Glomerular mitral cells process odor signals

•      Mitral cells send impulses to:

•    The olfactory cortex

•    The hypothalamus, amygdala, and limbic system

Eye and Associated Structures

•      70% of all sensory receptors are in the eye

•      Photoreceptors – sense and encode light patterns

•      The brain fashions images from visual input

•      Accessory structures include:

•    Eyebrows, eyelids, conjunctiva

•    Lacrimal apparatus and extrinsic eye muscles


•      Coarse hairs the overlie the supraorbital margins

•      Functions include:

•    Shading the eye

•    Preventing perspiration from reaching the eye

•      Orbicularis 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”)

•      Tarsal plates of connective tissue support the eyelids internally

•      Levator palpebrae superioris – gives the upper eyelid mobility

Accessory Structures of the Eye

•      Eyelashes

•    Project from the free margin of each eyelid

•    Initiate reflex blinking

•      Lubricating glands associated with the eyelids

•    Meibomian glands and sebaceous glands

•    Ciliary glands


•      Transparent membrane that:

•    Lines the eyelids as the palpebral conjunctiva

•    Covers the whites of the eyes as the ocular conjunctiva

•    Lubricates and protects the eye

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

Extrinsic Eye Muscles

•      Six straplike extrinsic eye muscles

•    Enable the eye to follow moving objects

•    Maintain the shape of the eyeball

•      The two basic types of eye movements are:

•    Saccades – small, jerky movements

•    Scanning movements – tracking an object through the visual field

Summary of Cranial Nerves and Muscle Actions

•      Names, actions, and cranial nerve innervation of the extrinsic eye muscles

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 fluid filled with humors – aqueous and vitreous

•      The lens separates the internal cavity into anterior and posterior segments

Fibrous Tunic

•      Forms the outermost coat of the eye and is composed of:

•    Opaque sclera (posterior)

•    Clear cornea (anterior)

•      Sclera – protects the eye and anchors extrinsic muscles

•      Cornea – lets light enter the eye

Vascular Tunic (Uvea): Choroid Region

•      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: 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: 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

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

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)

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

Blood Supply to the Retina

•      The neural retina receives it blood supply from two sources

•    The outer third receives its blood from the choroid

•    The inner two-thirds are served by the central artery and vein

•      Small vessels radiate out from the optic disc and can be seen with an ophthalmoscope

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

Anterior Segment

•      Composed of two chambers

•    Anterior – between the cornea and the iris

•    Posterior – between the iris and the lens

•      Aqueous humor

•    A plasmalike fluid that fills the anterior segment

•    Drains via the canal of Schlemm

•      Supports, nourishes, and removes wastes

The Lens

•      A biconvex, transparent, flexible, avascular structure that:

•    Allows precise focusing of light onto the retina

•    Is composed of epithelium and lens fibers

•      Lens epithelium – anterior cells that differentiate into lens fibers

•      Lens fibers – cells filled with the clear protein crystalline

•      With age, the lens becomes more compact and dense and loses its elasticity


•      Electromagnetic radiation – all energy waves from short gamma rays to long radio waves

•      Our eyes respond to a small portion of this spectrum called the visible spectrum

•      Different cones in the retina respond to different wavelengths of the visible spectrum

Refraction and Lenses

•      When light passes from one transparent medium to another its speed changes and it refracts (bends)

•      Light passing through a convex lens (as is in the eye) is bent so that the rays converge to a focal point

•      When a convex lens forms an image, the image is upside down and reversed right to left

Focusing Light on the Retina

•      Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, and the 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 (20ft)

Focusing for Close Vision

•      Close vision requires:

•    Accommodation – changing the lens shape by ciliary muscles 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

Problems of Refraction

•      Emmetropic eye – normal eye with light focused properly

•      Myopic eye (nearsighted) – the focal point is in front of the retina

•    Corrected with a concave lens

•      Hyperopic eye (farsighted) – the focal point is behind the retina

•    Corrected with a convex lens

Photoreception: Functional Anatomy of Photoreceptors

•      Photoreception – process by which the eye detects light energy

•      Rods and cones contain visual pigments (photopigments)

•    Arranged in a stack of
disklike infoldings of the plasma membrane that change shape as they absorb light


•      Functional characteristics

•    Sensitive to dim light and best suited for night vision

•    Absorb all wavelengths of visible light

•    Perceived input is in gray tones only

•    Sum visual input from many rods feed into a single ganglion cell

•    Results in fuzzy and indistinct images


•      Functional characteristics

•    Need bright light for activation (have low sensitivity)

•    Pigments that furnish a vividly colored view

•    Each cone synapses with a single ganglion cell

•    Vision is detailed and has high resolution

Chemistry of Visual Pigments

•      Retinal – a light-absorbing molecule

•    Combines with opsins to form visual pigments

•    Similar to and is synthesized from vitamin A

•    Two isomers: 11-cis and all-trans

•      Isomerization of retinal initiates electrical impulses in the optic nerve

Excitation of Rods

•      The visual pigment of rods is rhodopsin (opsin + 11-cis retinal)

•      Light phase

•    Rhodopsin breaks down into all-trans retinal + opsin (bleaching of the pigment)

•      Dark phase

•    All-trans retinal converts to 11-cis form

•    11-cis retinal is also formed from vitamin A

•    11-cis retinal + opsin regenerate rhodopsin

Excitation of Cones

•      Visual pigments in cones are similar to rods (retinal + opsins)

•      There are three types of cones: blue, green, and red

•      Intermediate colors are perceived by activation of more than one type of cone

•      Method of excitation is similar to rods


•      Light energy splits rhodopsin into all-trans retinal, releasing activated opsin

•      The freed opsin activates the G protein transducin

•      Transducin catalyzes activation of phosphodiesterase (PDE)

•      PDE hydrolyzes cGMP to GMP and releases it from sodium channels

•      Without bound cGMP, sodium channels close, the membrane hyperpolarizes, and neurotransmitter cannot be released


•      Adaptation to bright light (going from dark to light) involves:

•    Dramatic decreases in retinal sensitivity – rod function is lost

•    Switching from the rod to the cone system – visual acuity is gained

•      Adaptation to dark is the reverse

•    Cones stop functioning in low light

•    Rhodopsin accumulates in the dark and retinal sensitivity is restored

Visual Pathways

•      Axons of retinal ganglion cells form the optic nerve

•      Medial fibers of the optic nerve decussate at the optic chiasm

•      Most fibers of the optic tracts continue to the lateral geniculate body of the thalamus

•      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

Depth Perception

•      Achieved by both eyes viewing the same image from slightly different angles

•      Three-dimensional vision results from cortical fusion of the slightly different images

•      If only one eye is used, depth perception is lost and the observer must rely on learned clues to determine depth

Retinal Processing: Receptive Fields of Ganglion Cells

•      On-center fields

•    Stimulated by light hitting the center of the field

•    Inhibited by light hitting the periphery of the field

•      Off-center fields have the opposite effects

•      These responses are due to receptor types in the “on” and “off” fields

Thalamic Processing

•      The lateral geniculate nuclei of the thalamus:

•    Relay information on movement

•    Segregate the retinal axons in preparation for depth perception

•    Emphasize visual inputs from regions of high cone density

•    Sharpen the contrast information received by the retina

Cortical Processing

•      Striate cortex processes

•    Basic dark/bright and contrast information

•      Prestriate cortices (association areas) processes

•    Form, color, and movement

•      Visual information then  proceeds anteriorly to the:

•    Temporal lobe – processes identification of objects

•    Parietal cortex and postcentral gyrus – processes spatial location

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

•      Receptors for hearing and balance:

•    Respond to separate stimuli

•    Are activated independently

Outer Ear

•      The auricle (pinna) is composed of:

•    Helix (rim)

•    The lobule (earlobe)

•      External auditory canal

•    Short, curved tube filled with ceruminous glands

•      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

Middle Ear (Tympanic Cavity)

•      A small, air-filled, mucosa-lined cavity

•    Flanked laterally by the eardrum

•    Flanked medially by the oval and round windows

•      Epitympanic recess – superior portion of the middle ear

•      Pharyngotympanic tube – connects the middle ear to the nasopharynx

•    Equalizes pressure in the middle ear cavity with the external air pressure

Ear Ossicles

•      The tympanic cavity contains three small bones: the malleus, incus, and stapes

•    Transmit vibratory motion of the eardrum to the oval window

•    Dampened by the tensor tympani and stapedius muscles

•Loudness is perceived by:

•Varying thresholds of cochlear cells

•The number of cells stimulated

The Organ of Corti

•      Is composed of supporting cells and outer and inner hair cells

•      Afferent fibers of the cochlear nerve attach to the base of hair cells

•      The stereocilia (hairs):

•    Protrude into the endolymph

•    Touch the tectorial membrane

Excitation of Hair Cells in the Organ of Corti

•      Bending cilia:

•    Opens mechanically-gated ion channels

•    Causes a graded potential and the release of a neurotransmitter (probably glutamate)

•      The neurotransmitter causes cochlear fibers to transmit impulses to the brain, where sound is perceived

Auditory Pathway to the Brain

•      Impulses from the cochlea pass via the spiral ganglion to the cochlear nuclei

•      From there, impulses are sent to the:

•    Superior olivary nucleus

•    Inferior colliculus (auditory reflex center)

•      From there, impulses pass to the auditory cortex

•      Auditory pathways decussate so that both cortices receive input from both ears

Auditory Processing

•      Pitch is perceived by:

•    The primary auditory cortex

•    Cochlear nuclei

•      Loudness is perceived by:

•    Varying thresholds of cochlear cells

•    The number of cells stimulated

•      Localization is perceived by superior olivary nuclei that determine sound


•      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

•      Tinnitus – ringing or clicking sound in the ears in the absence of auditory stimuli

•      Meniere’s syndrome – labyrinth disorder that affects the cochlea and the semicircular canals, causing vertigo, nausea, and vomiting

Mechanisms of Equilibrium and Orientation

•      Vestibular apparatus – equilibrium receptors in the semicircular canals and vestibule

•    Maintain our orientation and balance in space

•    Vestibular receptors monitor static equilibrium

•    Semicircular canal receptors monitor dynamic equilibrium

Anatomy of Maculae

•      Maculae – the sensory receptors for static equilibrium

•    Contain supporting cells and hair cells

•    Each hair cell has stereocilia and kinocilium embedded in the otolithic membrane

Anatomy of Maculae

•      Otolithic membrane – jellylike mass studded with tiny CaCO3 stones called otoliths

•      Uticular hairs respond to horizontal movement

•      Saccular hair respond to vertical movement

Effect of Gravity on Utricular Receptor Cells

•      Otolithic movement in the direction of the kinocilia:

•    Depolarizes vestibular nerve fibers

•    Increases the number of action potentials generated

•      Movement in the opposite direction :

•    Hyperpolarizes vestibular nerve fibers

•    Reduces the rate of impulse propagation

•      From this information, the brain is informed of the changing position of the head

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

•      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

Transduction of Rotational Stimuli

•      Cristae respond to changes in velocity of rotatory movements of the head

•      Directional bending of hair cells in the cristae causes either:

•    Depolarizations and rapid impulses reach the brain at a faster rate

•    Hyperpolarizations and fewer impulses reach the brain

•      The result is that the brain is informed of rotational movements of the head

Balance and Orientation Pathways

•      There are three modes of input for balance and orientation

•    Vestibular receptors

•    Visual receptors

•    Somatic receptors

•      These receptors allow our body to respond reflexively

Developmental Aspects

•      All special senses are functional at birth

•      Chemical senses – few problems occur until the fourth decade, when these senses begin to decline

•      Vision – optic vesicles protrude from the diencephalon during the 4th week of development

•    These vesicles indent to form optic cups and their stalks form optic nerves

•    Later, the lens forms from ectoderm

•      Vision is not fully functional at birth

•      Babies are hyperopic, see only gray tones, and eye movements are uncoordinated

•      Depth perception and color vision is well developed by age five and emmetropic eyes are developed by year six

•      With age the lens loses clarity, dilator muscles are less efficient, and visual acuity is drastically decreased by age 70

•      Ear development begins in the 3rd week

•      Inner ears develop from otic placodes, which invaginate into the otic pit and otic vesicle

•      The otic vesicle becomes the membranous labyrinth, and the surrounding mesenchyme becomes the bony labyrinth

•      Middle ear structures develop from the pharyngeal pouches

•      The branchial groove develops into outer ear structures