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Special senses

Olfactory (Smell), Gustatory (Taste), Vision, Auditory, Balance

Olfactory neurons in the olfactory epithelium (10 million)

Axons of bipolar neurons extend through cribriform plate to the olfactory bulb. Dendrites extend to the epithelial surface of the olfactory recess and enlarge into olfactory vesicles.

Olfactory vesicles

contain Cilia which lie in mucous film on the epithelium.


Odorants bind to chemoreceptor molecules, Depolarize and initiate action potentials in neurons, Low threshold for odor detection


Binds to a chemoreceptor on the olfactory hair membrane and causes depolarization of the membrane and initiates action potentials in the olfactory neurons.

Seven primary odors

1) camphoraceous, 2) musky, 3) floral, 4) pepperminty, 5) ethereal, 6) pungent, and 7)putrid. More than one type of odorant can stimulate a receptor.


the lack of the ability to smell.

Olfactory neurons

Most have very poor replication abilities, but are constantly being replaced. The entire epithelium, including the neurosensory cells, are replaced about every two months. Replaced by a proliferation of basal cells in the olfactory epithelium.

Olfactory pathway

Bundles of unmyelinated axons extend through approximately 20 olfactory foramina in the cribriform plate from each nasal cavity. Collectively these bundles are the olfactory nerves, (cranial nerve I). The olfactory neuron bundles synapse with mitral cells and/or tufted cells. These cells relay information via the olfactory tracts and synapse with association neurons in the olfactory bulb. The association neurons also receive information from nerve cell processes that enter the olfactory bulb from the brain. Thus association neurons can modify olfactory information before it leaves the olfactory bulb. The olfactory tract terminates in the olfactory cortex. It is the only major sensation that goes directly to the cerebral cortex without first passing through the thalamus.

Lateral olfactory area

located in the temporal lobe. More specifically within the lateral fissure of the cerebrum. Responsible for the conscious perception of smell

Medial olfactory area

responsible for the visceral and emotional reactions to odors, with connections to the limbic system, and subsequently to the hypothalamus

Intermediate olfactory area

located along the nerve tract to the olfactory bulb and is involved in the modulation of sensory information and accommodation

Taste buds

(about 10,000) mostly located on the papillae, but are also found on the lips, throat, and palate, and more so in children. Each consists of about 50 gustatory cells that are continuously replaced, each cell lasting about 10 days


have hairs (microvilli), can be vallate, fungiform, foliate, or filiform (no taste buds)


surounded by a wall


mushroom shaped


leaf shaped


filament shaped. no taste buds but most numerous on the surface of the tongue. provide rough surface for food to be manipulated more easily

Histology of taste buds

Sensory cells of each taste bud consist of about 50 taste (gustatory) cells. Nonsensory cells are basal cells and supporting cells

Function of taste buds

Receptors on hairs detect dissolved substances called tastants. These taste cells have no axons so cannot generate action potentials, but instead release neurotransmitters which cause associated sensory neurons to depolarize.

Taste types

Sour, salty, bitter, sweet, umami (savory)

Factors that Affect Taste

texture, temperature of food, combination, adaptation occurs within 1-5 min, variation of thresholds, olfactory sensations, appetite

The Chorda tympani

crosses tympanic membrane but it is really actually involved in TASTE!! Not hearing

Visual System

Eye, Accessory structures (Eyebrows, eyelids, eyelashes, tear glands that lubricate and protect eyes from sunlight and damaging particles), Optic nerve (II) (Tracts and Pathways), Eyes respond to light and initiate afferent action potentials

Palpebral fissure

the window through which we see.


are double or triple rows of hairs along the free edges of the eyelids (palpebrae) that help protect the eyes.

Ciliary glands

are modified sweat glands that lubricate the lashes. The plugging or inflammation of one of these glands = sty

Meibomian or tarsal glands

produce sebum which lubricates the palpebrae and restrains tears from overflowing the margins of the eyelids. A blockage or infection is referred to as a chalazion, or a meibomian cyst.


thin transparent mucous membranes that cover the inner surface of the eyelid and the anterior surface of the eye.


is an irritation or infection of the conjunctiva. Acute contagious conjunctivitis caused by bacteria is commonly called "pinkeye."

Lacrimal glands

are located in the lateral corners of the orbit and lubricate the eye. Most of the fluid evaporates, but excess fluid is collected in the lacrimal canaliculi which connects to the nasal cavity.

Lacrimal apparatus

Lacrimal Gland (produces tears to moisten, lubricate, wash)

Lacrimal Canaliculi

Collects excess tears


the opening of the lacrimal canaliculus

Lacrimal Sac

connects caniculi with nasolacrimal

Nasolacrimal duct

Opens into nasal cavity

Eye Anatomy

Three coats or tunics. Fibrous: Consists of sclera and cornea. Vascular: Consists of choroid, ciliary body, iris. Nervous: Consists of retina

Fibrous tunic

Outer later; sclera and cornea


White outer layer, maintains shape, protects internal structures, provides muscle attachment point, continuous with cornea.


Allows light to enter eye and bends and refracts it as part of focusing. Avascular and transparent due to proteoglycans small collagen fibers, and low water content.

Vascular tunic

Middle; iris, ciliary muscles, and choroid


Controls light entering pupil; smooth muscle

Ciliary muscles

Control lens shape; smooth muscle


the portion of the vascular tunic associated with the sclera.


Inner. Contains neurons sensitive to light. Contains 120 photoreceptor cells called rods and 6 or 7 million cones. Center of posterior retina has a macula, at the center of this is the fovea centralis where light is focused. Portion with greatest visual acuity because photoreceptor cells are more tightly packed. Optic disc is medial to macula.

Anterior & Posterior compartments

Aqueous humor circulated via the canal of Schlemm. Helps maintain intraocular pressure

Vitreous or Postremal compartments

Vitreous humor, thicker and not as rapidly turned over and exchanged. Helps maintain eyeball shape and intraocular pressure. Holds lens and retina in place


Held by suspensory ligaments attached to ciliary muscles, transparent, biconvex

Functions of the eye

like a camera in some respects, iris allows light into eye, then cornea, lens, humors focus light onto retina, light striking retina is converted into action potentials relayed to brain

Visible light

Portion of electromagnetic spectrum detected by human eye: 400-700 nm. Each color is a different wavelength


Bending of light


Light striking a concave surface


Light striking a convex surface. As they converge they cross and a focal point occurs (focusing)

Focal point

Point where light rays converge and cross


Normal resting condition of lens

Far point of vision

The distance at which the lens does not have to thicken for focusing to occur - 20 feet or more from the eye.

Near point of vision

The closest distance at which the eye can focus. Things closer start to blur. This point is usually 2-3 inches for children and up to 60 inches for 80 year olds. (Accommodation, Pupil constriction, Convergence)

Pigmented retina

provides black backdrop and increases visual acuity

Sensory retina

contains three neuron layers; photoreceptor, bipolar, and ganglionic.


120 million rods in the retina. Photoreceptor portion of each rod contains 700 double layered membranous disks containing rhodopsin.

Rhodopsin reduction

Light adaptation

Rhodopsin production

Dark adaptation


6-7 million in the retina. Vital for color vision & visual acuity. Use iodopsin visual pigment. As light decreases, fewer cone cells respond to dim light.


Nearsightedness. Focal point too near lens, image focused in front of retina


Farsightedness. Image focused behind retina


Degeneration of accommodation, corrected by reading glasses; regular sight problems with age


Cornea or lens not uniformly curved


Lack of parallelism of light paths through eyes (eyes may turn in or out)

Retinal detachment

Can result in complete blindness; a hole or tear in retina allowing fluid to accumulate between neural and pigmented layers


Increased intraocular pressure by inhibition of aqueous humor circulation (causing build up)


Scarring of the cornea because of Chlamydia trachomatis infection.


Clouding of lens caused by buildup of proteins

Macular degeneration

Common in older people, loss in acute vision


dysfunction of peripheral circulation causes degeneration or detachment of the retina.

External ear

Hearing; terminates at eardrum or tympanic membrane. Includes auricle or pinna (elastic cartilage covered by skin) and the external acoustic meatus.

Middle ear

Hearing; air filled space that contains auditory ossicles. Auditory, pharyngotympanic, or eustachian tube, and oval window.

Inner ear

Hearing and balance; interconnecting fluid-filled tunnels and chambers

External Acoustic Meatus

Part of external ear. Lined with hairs and ceruminous glands that produce cerumen (earwax)

Tympanic membrane

Ear drum. Separates external ear from middle ear. Vibrated by sound waves

Auditory or pharyngotympanic (eustachian) tube

Part of middle ear. Passage way that opens into pharynx, equalizes pressure.


Part of inner ear that is bony and contains membranous labyrinth (outer surface covered with perilymphatic cells)

Bony Cochlea


Bony Vestibule


Bony Semicircular canals



Part of inner ear. Endolymph and Perilymph


Clear fluid that fills the membranous labyrinth. High in K+, low in Na+.


Fills the space between the membranous and bony labyrinths. Similar to cerebrospinal fluid, high concentrations of Na+ & low concentrations of K+.


produce sound waves

Volume or loudness

Function of wave amplitude


Function of wave frequency


Resonance quality or overtones of sound

Static Labyrinth

Of the vestibule. Evaluates position of head relative to gravity, Detects linear acceleration and deceleration. Utricle and saccule.


Patch of epithelium on utricle and saccule. Consists of support cells & hair cells which contain many microvilli called stereocilia and one kinocilium embedded in gelatinous mass containing otoliths

Kinetic Labyrinth

Consists of three semicircular canals. Evaluates movements of head.


The base of each semicircular canal. Contains crista ampullaris, cupula, which is displaced by fluid movements within the semicircular canals which bend the hairs and initiates action potentials.

Crista ampullaris

crest of epithelia in the ampulla covered by a curved gelatinous mass


the gelatinous mass


Ringing, clicking, whistling in ear due to disorders in middle or inner ear

Motion sickness

Dysfunctions caused by excessive stimulation of semicircular canals during motion

Space sickness

Dysfunction caused by lack of normal stimulation from the vestibule and semicircular canals

Otitis Media

infections in the middle ear

Meniere's syndrome

Characterized by tinnitus, vertigo, and hearing loss


Results from otitis media, dental abscesses, TMJ pain

Aging effects on special senses

Slight loss in ability to detect odors, Decreased sense of taste, Lenses of eyes lose flexibility, Development of cataracts, macular degeneration, glaucoma, diabetic retinopathy, Decline in visual acuity and color perception, Decrease in hearing and balance

Macula lutea

Area in the retina, at the center of which is the fovea centralis: Area of greatest visual acuity.

Optic disc

Blind spot


Important in low light conditions. Do not see color.

Fovea Centralis

At the center of the macula in the retina. Contains 35,000 cones and no rods so it contributes to visual acuity.

Auditory ossicles

Part of middle ear. Consists of Malleus, incus, and stapes. Transmit vibrations from tympanic membrane to oval window

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