BIOLOGY OF HUMAN AGING

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BIOLOGY OF HUMAN AGING

• CHAPTER 8
• The Special Senses

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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

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Conjunctiva

• Transparent membrane that
• Covers the whites of the eyes as the ocular
  conjunctiva
• Lubricates and protects the eye

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Lacrimal Apparatus

• Consists of the lacrimal gland and associated
  ducts
• Lacrimal glands secrete tears
• Tears
• Contain mucus antibodies
• Drain into the nasolacrimal duct

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Lacrimal Apparatus
Figure 15.6
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Extrinsic Eye Muscles
Figure 15.7a, b
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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

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Structure of the Eyeball
Figure 15.8a
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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

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Vascular Tunic Choroid Region

• Has three regions choroid, ciliary body, and
  iris
• Choroid region
• A dark brown membrane
• Supplies blood to all eye tunics

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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

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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

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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

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Sensory Tunic Retina
Figure 15.10a
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The Retina the Optic Disc

• The optic disc
• Is the site where the optic nerve leaves the eye
• Lacks photoreceptors (the blind spot)

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The Retina Optic Disc
Figure 15.10b
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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

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• Aqueous humor
• A plasma like fluid that fills the anterior
  segment
• Supports, nourishes, and removes wastes

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Anterior Segment
Figure 15.12
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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

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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

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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

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Problems of Refraction
Figure 15.18
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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

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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
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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
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Age-related Dysfunction

• Presbyopia
• Blindness
• Glaucoma
• Diabetic retinopathy
• Cataracts
• Macular degeneration
• Detached retina

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The Ear
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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

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The Ear Hearing and Balance
Figure 15.25a
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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

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Ear Ossicles

• The tympanic cavity contains three small bones
• Transmit vibratory motion of the eardrum to the
  oval window

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Inner Ear

• Bony labyrinth
• Contains the vestibule, the cochlea, and the
  semicircular canals
• Filled with perilymph

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Inner Ear
Figure 15.27
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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

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The Semicircular Canals

• It houses equilibrium receptors
• These receptors respond to angular movements of
  the head

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The Cochlea

• A spiral, conical, bony chamber that
• Contains the cochlear duct
• Contains the organ of Corti (hearing receptor)

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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

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Resonance of the Basilar Membrane
Figure 15.32
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Age-related changes

• 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

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Age-related Dysfunction

• Presbycusis
• Tinnitus
• Deafness
• Dizziness and Verdigo

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Taste and Smell

• The Special Senses

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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

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Taste Buds

• Most of the 10,000 or so taste buds are found on
  the tongue
• Microvilli

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Taste Buds
Figure 15.1
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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

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Age-related changes

• 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

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Influence of Other Sensations on Taste

• Taste is 80 smell
• Thermoreceptors, mechanoreceptors, nociceptors
  also influence tastes

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Sense of Smell
Figure 15.3
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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