The Special Senses Taste, Smell, Touch (later), Vision, and Hearing.

The Special Senses

Taste, Smell, Touch (later), Vision, and Hearing

Sense of Taste Gustatory cell (taste) receptors detect

chemicals dissolved in saliva Taste buds: sensory receptor cells for taste.

located on and between papillae (pimplelike protuberances on the tongue)

Also located on palate (roof of mouth), throat, and epiglottis

A child has approx 10,000 taste buds With age the number of taste buds will drop to

fewer than 5,000

Taste Buds Each taste bud is

structured like an orange whose “segments” consist of approx 25 gustatory receptor cells.

The receptor cells have hairlike tips that project into a hole (the taste pore) in the tongue’s surface.

Primary Sensations Tip of tongue: sweet

and salty Sides of tongue: sour Back of tongue: bitter 80% of taste is actually

smell Other influences:

thermoreceptors (temperature), mechanoreceptors (texture), nocioreceptors (pain)

Sense of Smell Specialized neurons with olfactory cilia in

upper nasal cavity Sniffing draws air forcefully into the nose Neurons detect molecules or tiny particles

known as odorants floating on the air Smell is more sensitive than taste and is

able to distinguish between more than 10,000 odors.

Sensory cells are affected by a variety of factors: age, nutrition, hormones, drugs, therapeutic radiation

Smell is long-lasting and stimulation by similar smells can trigger memory of events that occurred long ago

Olfactory receptors easily fatigue- adaptation occurs Process of conforming to the environment after

continuous stimulation of constant intensity These changes in awareness of odors allows us

to continue to function at an optimum level

Homeostatic Imbalances: Anosmias Some genetic causes Head injuries that tear olfactory nerves After effects of nasal cavity inflammation

Cold, allergy, smoking Physical destruction of nasal cavity due to

polyps Aging

Sense of Touch Sensory receptors make it possible for the

body to respond to environmental stimuli Specialized endings of nerve cells in the skin or

in deeper tissues Receptors respond to a stimulus and

convert the stimulus to a nerve impulse Nerve impulses travel by afferent sensory

neurons to the brain for interpretation

Touch Mechanoreceptors/extroreceptors; located

on the body surfaces; respond to touch, stretch, and pressure Meissner’s corpuscles: in fingertips, lips, and

hairless parts for fine touch Pacinian corpuscles: in skin, joints and genitals

for deep pressure and stretch Krause’s end bulbs: in eyelids, lips, and

genitals for light touch Ruffini’s corpuscles: found in skin for

continuous touch

Heat/Cold: thermoreceptors

Pain: nocioreceptors; free nerve endings for pain, tickle, itch noci/o (pain, injury)

Sense of Vision- Anatomy Eyebrows: physical protection of eyes; short,

coarse hairs Eyelids (palpebrae): physically protect the eye

and prevent the cornea from drying via blink reflex

Eyelashes: hairs with glands at the base for lubrication; inflammation=sty

Meibomian glands: secrete a lipid tear film spread by blinking; reduces evaporation of tear film, prevents tear film from running down face, gives even spread over eyeball

Lacrimal glands: secrete aqueous tear film containing globulins and lysozyme; supplies nourishment to the cornea and provides antimicrobial activity; nasolacrimal duct empties into nasal cavity; secretions decrease with age

Conjunctiva: membranes that lines the eyelid; secretes a mucous tear component that helps reduce surface tension; accumulates in the medial canthus as “sleep”; inflammation= pinkeye

Extrinsic eye muscles Sclera: outermost white covering of the eyeball;

anchor site for muscles Cornea: transparent front of the sclera; no blood

vessels but richly supplied with sensory nerves; depends on tear film for nutrition, O2, and removal of waste; window for light to enter; extraordinary capacity for regeneration

Choroid: highly vascular middle layer of the eye; pigment absorbs light to prevent scatter and reflection internally

Ciliary body: encircles the lens Iris: visible colored part of the eye; regulates amount

of light entering the lens Pupil: round central opening of the iris; allows light to

enter Lens: transparent disk of tissue that changes shape

for near or far vision Retina: thin layer of light sensitive rod and cone cells;

absorbs light Fovea: composed of cone cells; visual acuity Optic disc: the “blind spot” where neurons exit the

eyeball as the optic nerve

Sequence of Vision Rays of light enter the eye through the clear,

domed front of the eyeball, the cornea, where they are partly bent (refracted).

Rays then pass through the transparent lens, which changes shape to fine-focus the image, mechanism known as accommodation.

Light continues through the fluid within the eyeball and shines and upside-down image onto the retina lining.

The retina contains over 120 million cone cells and about 7 million rod cells Rods are scattered through the retina and

respond to low levels of light, do not differentiate colors

Cones are concentrated in the fovea and take bright light to distinguish colors and fine detail

Nerve fibers from the rods and cones connect via intermediate retinal cells to the fibers that form the optic nerve.

The image is then transmitted to the visual cortex in the brain, where it is turned upright.

20/20? Visual Acuity Clearness/sharpness of visual perception

recorded as 2 numbers: 1st # represents the distance in feet between the

subject and the test chart 2nd # represents the number of feet a person with

normal acuity would stand to see clearly 20/20 is considered normal acuity 20/100 a person can see objects at 20 ft. that

a person with normal can see at 100. Worse than 20/200 after correction is

considered legally blind

Homeostatic Imbalances Myopia: nearsighted; focus falls short of

the retina; far objects are blurred Hyperopia: farsightedness; focus falls

behind the retina; close objects are blurred

Astigmatism: cornea is not spherical, focused image is distorted

Color blindness: congenial lack of one or more types of cones (red, green, blue); sex-linked

Pathological Conditions Glaucoma: abnormally high pressure

inside the eye due to buildup of fluid. The pressure may permanently damage nerve fibers in the retina or the optic nerve.

Cataracts: excess glucose draws water into the lens causing opaque changes.

Diabetic retinopathy: small retinal hemorrhages occur due to excess glucose in blood- disrupts O2 to the rods and cones- blindness

Sense of Hearing- External Ear Anatomy Auricle (pinna): the flap that funnels sound

waves; helix=rim; lobule=earlobe External auditory meatus: opening to the

auditory canal, lined with cerumen/wax glands External auditory canal: short, narrow

chamber extends from auricle to tympanic membrane

Tympanic membrane: the eardrum that stretches across the canal and vibrates in response to sound waves

Middle Ear Anatomy:tiny cavity in the temporal bone Auditory ossicles: 3 bones that vibrate to

transmit sound waves to the inner ear Malleus: hammer shaped, handle attached to

tympanic membrane Incus: anvil shaped Stapes: stirrup shaped

Oval/vestibular window: opens to internal ear

Round/cochlear window: covered by membrane, opens to internal ear

Pharyngotympanic/auditory/Eustachian tube: connects middle ear to pharynx; helps to equalize pressure so eardrum will vibrate Myringotomy= lancing of eardrum to relieve

pressure- insertion of tubes for drainage of fluid/pus

Mastoid sinuses: air spaces in the temporal bone that drain into middle ear

Inner Ear Anatomy Labyrinth, located in the hollowed out

portion of the temporal bone Vestibule and semicircular canals:

involved in equilibrium Cochlea: snail like part of the inner ear for

hearing

Sequence of Hearing Sound waves caught by auricle, channeled

through the auditory canal and strike against the tympanic membrane causing it to vibrate

Vibrations move the malleus, incus, and stapes against the oval window

Pressure is exerted inward to the perilymph of the scala vestibuli (within the cochlea)

Vibrations continue to move to the organ of Corti- receptor organ for hearing

Homeostatic Imbalances Conduction deafness: something interferes

with the conduction of sound vibrations to the fluids of the inner ear Impacted earwax, perforated/ruptured eardrum,

otitis media, otosclerosis of ossicles Sensorineural deafness: damage to neural

structures at any point from cochlear hair cells to auditory cortical cells Gradual loss of receptor cells, exposure to single

loud noise, degeneration of cochlear nerve, cerebral infarcts, tumors

Tinnitus: ringing or clicking sound in the ears in the absence of auditory stimuli; can be first symptom of cochlear nerve degeneration or inflammation of middle/inner ear or side effect of medications

Vertigo: may result from disturbance affecting the organs of balance in the inner ear, the nerve that connects the inner ear to the brain, or areas of the brain concerned with balance