Transcript 8) Special Senses

BIOLOGY OF HUMAN AGING
CHAPTER 8
The Special Senses
Eye and Associated Structures
• 70% of all sensory receptors are in the eye
• Most of the eye is protected by a cushion of
fat and the bony orbit
• Accessory structures include eyebrows,
eyelids, conjunctiva, lacrimal apparatus, and
extrinsic eye muscles
Conjunctiva
• Transparent membrane that:
– Covers the whites of the eyes as the ocular
conjunctiva
– Lubricates and protects the eye
Lacrimal Apparatus
• Consists of the lacrimal gland and
associated ducts
• Lacrimal glands secrete tears
• Tears
– Contain mucus & antibodies
– Drain into the nasolacrimal duct
Lacrimal Apparatus
Figure 15.6
Extrinsic Eye Muscles
Figure 15.7a, b
Structure of the Eyeball
• A slightly irregular hollow sphere with
anterior and posterior poles
• The wall is composed of three tunics –
fibrous, vascular, and sensory
• The internal cavity is filled with fluids
called humors
Structure of the Eyeball
Figure 15.8a
Fibrous Tunic
• Forms the outermost coat of the eye and is
composed of:
– Opaque sclera
– Clear cornea
– The sclera protects the eye and anchors
extrinsic muscles
• The cornea lets light enter the eye
Vascular Tunic: Choroid Region
• Has three regions: choroid, ciliary body, and
iris
• Choroid region
– A dark brown membrane
– Supplies blood to all eye tunics
Vascular Tunic: Ciliary Body
• A thickened ring of tissue surrounding
the lens
• Composed of smooth muscle bundles
(ciliary muscles)
• Anchors the suspensory ligament that
holds the lens in place
Vascular Tunic: Iris
• The colored part of the eye
• Pupil – central opening of the iris
– Regulates the amount of light entering the eye
during:
• Close vision and bright light – pupils constrict
• Distant vision and dim light – pupils dilate
Sensory Tunic: Retina
• A delicate two-layered membrane
• Pigmented layer – the outer layer that
absorbs light and prevents its scattering
• Neural layer, which contains:
– Photoreceptors that transduce light energy
Sensory Tunic: Retina
Figure 15.10a
The Retina: the Optic Disc
The optic disc:
– Is the site where the optic nerve leaves the eye
– Lacks photoreceptors (the blind spot)
The Retina: Optic Disc
Figure 15.10b
The Retina: Photoreceptors
• Rods:
– Respond to dim light
– Are used for peripheral vision
• Cones:
– Respond to bright light
– Have high-acuity color vision
– Are concentrated in the fovea centralis
• Aqueous humor
– A plasma like fluid that fills the anterior
segment
• Supports, nourishes, and removes wastes
Anterior Segment
Figure 15.12
Lens
• A biconvex, transparent, flexible, avascular
structure that:
– Allows precise focusing of light onto the retina
• With age, the lens becomes more compact
and dense and loses its elasticity
Refraction and Lenses
• When light passes from one transparent
medium to another its speed changes and it
refracts (bends)
• Light passing through a convex lens (as in
the eye) is bent so that the rays converge to
a focal point
Problems of Refraction
• Emmetropic eye – normal eye with light
focused properly
• (nearsighted) – the focal point is in front of
the retina
– Corrected with a concave lens
• (farsighted) – the focal point is behind the
retina
– Corrected with a convex lens
Problems of Refraction
Figure 15.18
Adaptation
• Adaptation to bright light (going from
dark to light) involves:
– Dramatic decreases in retinal sensitivity –
rod function is lost
– Switching from the rod to the cone system
– visual acuity is gained
• Adaptation to dark is the reverse
– Cones stop functioning in low light
Age-related changes
Visual problems
Slight shrinkage and degeneration of some cells. Increase in the amount of
connective tissues. Reduced blood supply.
Loss of fat and elastic tissues thinning of the skin in the eyelids (bags).
Dry cornea irritation and inflammation
Production of tears by the lacrimal gland is reduced with aging.
Visual problems
Conjuctiva become thinner and more fragile
Astigmatism
Deposition of calcium and cholesterol salts senile ring
Ciliary body produces less aqueous humor affecting nourishment and
cleansing of the lens  atrophy of muscle cells
General decrease of photoreceptors
Photopigement (rhodopsin) decreases with aging
Lipofuscins accumulates in the retina
Inclusion bodies floaters may increase with aging
Iris become hardened with aging and atrophy of the
dilator muscles
Poor drainage of aqueous humor
Lens become yellow and less transparent with age
Lens change shape
Atherosclerosis of choroid and blood vessels of the retina
Adaptation takes longer
Less light reaching the retina sensitivity to light decreases with aging
Slower scanning process in the brain  takes longer for older persons
to identify objects
Age-related Dysfunction
Presbyopia
Blindness
Glaucoma
Diabetic retinopathy
Cataracts
Macular degeneration
Detached retina
The Ear
The Ear: Hearing and Balance
• The three parts of the ear are the inner,
outer, and middle ear
• The outer and middle ear are involved with
hearing
• The inner ear functions in both hearing and
equilibrium
The Ear: Hearing and Balance
Figure 15.25a
Outer Ear
• The auricle (pinna)
• External auditory meatus
– Short, curved tube
Tympanic membrane (eardrum)
– Thin connective tissue membrane that vibrates in
response to sound
– Transfers sound energy to the middle ear ossicles
– Boundary between outer and middle ears
Ear Ossicles
• The tympanic cavity contains three small
bones
– Transmit vibratory motion of the eardrum to the
oval window
Inner Ear
• Bony labyrinth
– Contains the vestibule, the cochlea, and the
semicircular canals
– Filled with perilymph
Inner Ear
Figure 15.27
The Vestibule
• The central egg-shaped cavity of the bony
labyrinth
• Suspended in its perilymph are two sacs:
the saccule and utricle
• These sacs:
– House equilibrium receptors called maculae
– Respond to gravity and changes in the position
of the head
The Semicircular Canals
• It houses equilibrium receptors
• These receptors respond to angular
movements of the head
The Cochlea
• A spiral, conical, bony chamber that:
– Contains the cochlear duct
– Contains the organ of Corti (hearing receptor)
Sound and Mechanisms of Hearing
• Sound vibrations beat against the eardrum
• The eardrum pushes against the ossicles,
which presses fluid in the inner ear against
the oval and round windows
– This movement sets up shearing forces that
pull on hair cells
– Moving hair cells stimulates the cochlear nerve
that sends impulses to the brain via
vestibulocochlear nerve
Resonance of the Basilar Membrane
Figure 15.32
Age-related changes
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Auricle become long, wide, and lose flexibility
Auditory meatus becomes wider and stiffer
Excessive earwax
Joints between ossicles may ossify less freely
movable problem in vibration
• Degeneration of spiral organ cells
• Thickening of the walls of capillaries supplying the
cochleadecrease in nutrients
• Decrease in the number of nerve fibers
problems in balance and coordination
Age-related Dysfunction
Presbycusis
Tinnitus
Deafness
Dizziness and Verdigo
Taste and Smell
The Special Senses
Chemical Senses
• Chemical senses – gustation (taste) and
olfaction (smell)
• Their chemoreceptors respond to chemicals
in aqueous solution
– Taste – to substances dissolved in saliva
– Smell – to substances dissolved in fluids of the
nasal membranes
Taste Buds
• Most of the 10,000 or so taste buds are
found on the tongue
• Microvilli
Taste Buds
Figure 15.1
Taste Sensations
• There are five basic taste sensations
– Sweet – sugars, saccharin, alcohol, and some
amino acids
– Salt – metal ions
– Sour – hydrogen ions
– Bitter – alkaloids such as quinine and nicotine
Age-related changes
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General decrease in taste perception
Decrease in the number of taste buds
Decrease in volume of saliva secreted
Increase in the viscosity of saliva
Atrophy of the salivary glands
Cigarette smoking may contribute to the
decrease in taste perception
Influence of Other Sensations on
Taste
• Taste is 80% smell
• Thermoreceptors, mechanoreceptors,
nociceptors also influence tastes
Sense of Smell
Figure 15.3
Age-related changes
• There may be a decrease in the
olfactory sensory cells with aging
• Loss of neurons in olfactory bulbs
• Sense of smell gradually decline
gradually