Wednesday, April 10, 2013

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)
 
 
 

 



 
˜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
 

 
 
 


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