Chapter
9
The Senses
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)
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
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
of olfactory receptors and thereby dulls taste sensations
ØGustatory
and olfactory senses work together
to permit creation of many other taste sensations
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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