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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

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)

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

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


•      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


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

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:

Anterior Segment

•      Aqueous humor

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

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

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



Here are the videos for the Special Senses, none of them too long

Eyes    12:49  You may skip the section on muscles

Ears    12:24

Smell, Taste and Touch   8:58


Lab Images: