Chapter 9 THE SENSES

           Chapter 9
          The Senses

 

CLASSIFICATIONOF SENSE ORGANS

 

 

˜General sense organs (Table 9-1)

ØOften exist as individual cells or receptor units

ØWidely distributed throughout the body

˜Special sense organs (Table 9-2)

ØLarge and complex organs

ØLocalized grouping of specialized receptors

 

˜Classification by presence or absence of covering capsule

 

ØEncapsulated

 

ØUnencapsulated (“free” or “naked”)

 

˜Classification by type of stimuli required to activate receptors

 

ØPhotoreceptors (light)

ØChemoreceptors (chemicals)

ØPain receptors (injury) 

ØThermoreceptors (temperature change)

ØMechanoreceptors (movement or deforming of capsule)

ØProprioceptors (position of body parts or changes
in muscle length or tension)

 

 

CONVERTING A

 

STIMULUS INTO A

 

SENSATION

 

˜All sense organs have common functional characteristics

ØAll are able to detect a particular stimulus

ØA stimulus is converted into a nerve impulse

ØA nerve impulse is perceived as a sensation
in the central nervous system

 

SPECIAL SENSE

 

ORGANS

 

˜The eye (Figure 9-2)

ØLayers of eyeball

•Sclera—tough outer coat; “white” of eye; cornea is transparent part of sclera over iris

•Choroid—pigmented vascular layer prevents scattering of light; front part of this layer made of ciliary muscle and iris, the colored part of the eye; the pupil is the hole in the center of the iris; contraction of iris muscle dilates or constricts pupil

•Retina (Figure 9-4)—innermost layer of the eye; contains rods (receptors for night vision) and cones (receptors for day vision and color vision)

ØConjunctiva—mucous membrane covering the front surface of the sclera and also lines the eyelid; kept moist by tears found in the lacrimal gland

ØLens—transparent body behind the pupil; focuses light rays on the retina

 

 

˜The eye (cont.)

ØEye fluids

•Aqueous humor—in the anterior chamber in front of the lens

•Vitreous humor—in the posterior chamber behind the lens

ØVisual pathway

•Innermost layer of retina contains rods and cones

•Impulse travels from the rods and cones through the bipolar and ganglionic layers of retina (Figure 9-4)

•Nerve impulse leaves the eye through the optic nerve; the point of exit is free of receptors and is therefore called a blind spot

•Visual interpretation occurs in the visual cortex of the cerebrum

•

˜The ear

ØThe ear functions in hearing and in equilibrium and balance—receptors called mechanoreceptors

ØDivisions of the ear (Figure 9-5)

•External ear

†Auricle (pinna)

†External auditory canal

–Curving canal 2.5 cm (1 inch) in length

–Contains ceruminous glands

–Ends at the tympanic membrane

 

 

˜The ear (cont.)

ØDivisions of the ear (cont.)

•Middle ear

†Houses ear

ossicles—malleus, incus, and

stapes

†Ends in the oval window

†The auditory (eustachian)

tube connects the middle ear

to the throat

†Inflammation called otitis media

•Inner ear (Figure 9-6)

†Bony labyrinth filled with

perilymph

†Subdivided into the

 vestibule, semicircular canals,

 and cochlea

†Membranous labyrinth filled

with endolymph

†The receptors for balance in

 the semicircular canals are

called cristae ampullaris

†Specialized hair cells on the

organ of Corti respond when

 bent by the movement of

 surrounding endolymph set in

 motion by sound waves

 (Figure 9-7)

 

 

 

 





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˜The taste receptors (Figure 9-8)

ØReceptors are chemoreceptors called taste buds

ØCranial nerves VII and IX carry gustatory impulses

ØMost pathologists list four kinds of “primary” taste sensations—sweet, sour, bitter, and salty

•Metallic and umami (meaty) tastes are also unique and may soon be added to the list of “primary” taste sensations

•Nasal congestion interferes with stimulation
of olfactory receptors and thereby dulls taste sensations

ØGustatory and olfactory senses work together
to permit creation of many other taste sensations

 

˜The smell receptors (Figure 9-9)

ØReceptors for fibers of olfactory or cranial nerves lie in olfactory mucosa of nasal cavity

ØOlfactory receptors are extremely sensitive but easily adapt (become fatigued)

ØOdor-causing chemicals initiate a nervous signal that is interpreted as a specific odor by the brain