Special%20Senses

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Special Senses
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Special Senses

• Receptors for the special senses smell, taste,
  vision, hearing, and equilibrium are
  anatomically distinct from one another.
• These receptors are concentrated in very specific
  locations in the head.
• They are usually embedded in the epithelial
  tissue within complex sensory organs such as the
  eyes and ears.
• The neural pathways for the special senses are
  more complex than those of the general senses.

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Smell Taste

• Smell and taste are chemical senses. They
  involve the interaction of molecules with the
  receptors.
• Impulses from these sense propagate to the limbic
  system and higher cortical areas. Consequently,
  they evoke emotional responses and memories.

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Chemosensor

• A chemosensor, also known as a chemoreceptor, is
  a sensory receptor that transduces a chemical
  signal into an action potential.
• A chemosensor detects certain chemicals in the
  environment.

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Olfactory Receptors

• There are between 10 100 million receptors for
  olfaction (sense of smell).
• They are contained in the olfactory epithelium
  (5cm2)

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Olfactory Receptors

• 3 Kinds of cells
• Olfactory receptors
• 1st order neurons bipolar
• Olfactory hairs cilia that project from the
  dendrite respond to chemicals called odorants
• Supporting cells
• Columnar epithelium of mucous membrane
• Support, nourish, detoxify chemicals
• Basal cells
• Stem cells produce new olfactory cells (live
  approx. 1 month)

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Olfactory Epithelium
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Olfactory Glands (Bowmans)

• Bowmans glands are in the connective tissue that
  supports the epithelium.
• They produce mucous which moistens the epithelial
  surface and dissolves the odorants.

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Innervation

• Branches of the facial nerve (CN VII) innervate
  the supporting cells and olfactory glands.
• They stimulate the olfactory glands and the
  lacrimal glands.
• The lacrimal glands produce tears from pepper and
  ammonia.

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Physiology of Olfaction

• There are hundreds of primary odors.
• Humans can recognize about 10,000 different
  odors.
• Different combinations of olfactory receptors
  stimulate different patterns of activity in the
  brain.

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Odor Thresholds and Adaptation

• All special senses have a low threshold including
  olfaction.
• Methyl mercaptan can be detected with as little
  as 1/25 billionth of a milligram/ mL air.
• Adaptation is a decrease in sensitivity. It
  occurs rapidly. 50 of the decrease occurs
  within the first second or so and then very
  slowly after that.

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Olfactory Pathway

• There are approximately 20 olfactory foramina on
  either side of the nose in the cribiform plate of
  the ethmoid bone.
• 40 or so bundles of axons form right and left
  olfactory nerves (CN I).
• They terminate in the olfactory bulbs below the
  frontal lobes of the cerebrum.

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Olfactory Pathway

• Axons of the olfactory bulbs form the olfactory
  tract which projects to the primary olfactory
  area of the cerebral cortex.
• Some project into the limbic system and
  hypothalamus (emotional and memory evoked
  responses.
• Olfactory sensations are the only sensations that
  reach the cerebral cortex without first synapsing
  in the thalamus.

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Olfactory Pathway

• The primary olfactory area has axons that extend
  to the orbitofrontal area (frontal lobe) region
  for odor identification.

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Hyposmia

• Hyposmia is a reduced ability to smell.
• Women have a keener sense of smell than men,
  especially at ovulation.
• Smoking impairs the sense of smell.
• Age deteriorates the olfactory receptors.
• Affects 50 over 65 and 75 over 80 years of age.

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Hyposmia

• Neurological changes impair the receptors.
• Head injury, Alzheimers, Parkinsons.
• Medications impair receptors.
• Antihistamines, analgesics, steroids.

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Aromatherapy

• Effects of smells on our psychology have been
  claimed.
• Lavender, Orange Blossom, Rose, and Sage are said
  to be calming.
• Sandlewood, Patachouli and Jasmine are said to
  alleviate mild depression.
• Association areas of the brain.

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Survival Function

• Our sense of smell serves a survival function to
  help us select non-poisonous foods.
• There are very few naturally occurring toxic
  vapors that are odorless.
• Synthetic vapors often give false impressions to
  our senses. Our natural preferences can no
  longer be relied upon.

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Gustation (Taste)

• Chemical sense.
• There are only 5 primary tastes that can be
  distinguished.
• Sour, sweet, bitter, salty, umani (receptors
  stimulated by MSG)
• All other flavors are combinations of the 5
  primary tastes and smell.

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Taste Receptors
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Taste Buds
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Taste Receptor Cells

• Contrary to popular belief, there is no tongue
  map.
• Responsiveness to the five basic modalities
  bitter, sour, sweet, salty, and umami is
  present in all areas of the tongue.
• The taste receptor cells are tuned to detect each
  of the five basic tastes.
• A given gustatory receptor may respond more
  strongly to some tastants than others.

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Taste Buds and Papillae

• There are approximately 10,000 taste buds.
• Most are on the tongue.
• There are some on the soft palate, pharynx, and
  epiglottis.
• Each taste bud is an oval body with 3 kinds of
  epithelial cells.
• Supporting cells.
• Gustatory receptor cells (life span of approx. 10
  days).
• Basal cells.

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Epithelial Cells on Taste Buds

• The Supporting Cells surround approximately 50
  gustatory receptor cells in a taste bud.
• The Gustatory Receptor Cells synapse with 1st
  order neurons. ! 1st order neurons contacts many
  gustatory receptor cells.
• The Gustatory Hair (microvillus) projects through
  the taste pore.
• The Basal Cells are stem cells at the periphery
  of the taste bud. They produce supporting cells
  which will develop into gustatory cells.

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Taste Buds

• The taste buds are found in elevations on the
  tongue.
• Vallate (circumvallate) papillae
• Fungiform papillae
• Foliate papillae
• Filiform papillae

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Vallate (Circumvallate) Papillae

• About 12 very large circular vallate papillae
  form an inverted V-shaped row at the back of the
  tongue.
• Each of these papillae contains approximately
  100-300 taste buds.

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Fungiform Papillae

• The Fungiform (mushroom like) papillae are
  mushroom shaped elevations scattered over the
  entire surface of the tongue.
• They contain about 5 tastebuds each.

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Foliate Papillae

• The foliate (leaflike) papillae are located in
  small trenches on the lateral margins of the
  tongue, but most of their taste buds degenerate
  in early childhood.

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Filiform Papillae

• Filiform papillae cover the entire surface of the
  tongue.
• They are pointed, threadlike structures that
  contain tactile receptors but no taste buds.
• They increase friction between the tongue and the
  food, making it easier for the tongue to move
  food into the oral cavity.

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Papillae
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Tastants

• Tastants are chemicals that stimulate gustatory
  receptors.
• Tastants dissolve in saliva. They can then make
  contact with the plasma membrane of the gustatory
  hairs, which are the sites of taste transduction.
• This generates a receptor potential, which in
  turn triggers nerve impulses with first-order
  sensory neurons.

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Tastant Stimulation of Gustatory Receptors

• Different tastants stimulate the gustatory
  receptors in different ways to generate the
  receptor potential.
• The sodium ions in salty foods enter the
  gustatory receptor cells via Na channels in the
  membrane.
• The hydrogen ions in sour tastants flow in
  through H channels.

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Tastant Stimulation of Gustatory Receptors

• Other tastants (sweet, bitter, and umami) do not
  enter the gustatory receptor cells. They bind to
  receptors on the plasma membrane. They trigger
  second messengers in the cell.
• All tastants ultimately result in the release of
  neurotransmitters from the gustatory receptor
  cell.
• Different foods taste different because of the
  patterns of nerve impulses in groups of
  first-order neurons that synapse with the
  receptors.

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Taste Thresholds

• The threshold for taste varies for each of the
  primary tastes.
• The threshold for bitter substances (i.e.
  quinine) is the lowest.
• Poisonous substances are often bitter.
• The threshold for sour substances (i.e. lemon) is
  somewhat higher.
• The thresholds for salty substances and sweet
  substances are similar and higher than the others.

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Taste Adaptation

• Complete adaptation of a taste can occur in 1-5
  minutes if continuous stimulation.

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Gustatory Pathway

• Three cranial nerves contain axons for the
  gustatory pathways.
• Facial Nerve (CN VII) serves taste buds in
  anterior 2/3 of the tongue.
• Glossopharyngeal Nerve (CN IX) serves taste
  buds in the posterior 1/3 of the tongue.
• Vagus Nerve (CN X) serves taste buds in the
  throat and epiglottis.
• Impulses propagate to the gustatory nucleus in
  the medulla oblongata.

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Gustatory Pathway

• Some axons carrying taste signals project into
  the limbic system and the hypothalamus.
• Others project to the thalamus and from there to
  the primary gustatory area in the parietal lobe
  of the cerebral cortex.
• This allows us to perceive taste.

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Taste Aversion

• The taste projections to the hypothalamus and
  limbic system account for the strong association
  between taste and emotions.
• Sweet foods evoke reactions of pleasure, while
  bitter foods can evoke reactions of disgust.
  This is true even in newborn babies.
• Animals learn to avoid foods that upset the
  digestive system. This is known as taste
  aversion.
• Certain medications cause upset stomach and can
  cause taste aversion to all foods.

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Vision

• Sight is extremely important for human survival.
• More than half of the sensory receptors in the
  human body are located in the eyes.

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Electromagnetic Radiation

• Electromagnetic radiation is energy in the form
  of waves that radiate from the sun.
• Many types
• Gamma rays, X-rays, UV rays
• Visible light
• Infrared radiation, Microwaves, Radio waves
• The range of electromagnetic radiation is known
  as the electromagnetic spectrum.

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Electromagnetic Spectrum
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Wavelength

• The distance between two consecutive peaks of an
  electromagnetic wave is the wavelength.
• Wavelengths range from short to long.
• Gamma rays short than a nanometer.
• Radio waves greater than a meter
• The eyes are responsible for the detection of
  visible light.

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Wavelength

• The color of visible light depends upon its
  wavelength.
• Wavelength of 400 nm is violet
• Wavelength of 700 nm is red
• An object will absorb certain wavelengths of
  light and reflect others. It will appear the
  color of the wavelengths it reflects.
• White reflects all wavelengths of visible
  light.
• Black absorbs all wavelengths of visible light.

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Anatomy of the Eye
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Accessory Structures of the Eye

• Eyelids
• Eyelashes
• Eyebrows
• Lacrimal apparatus
• Extrinsic eye muscles

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Eyelids

• The palpebrae or eyelids (palprbra singlular)
  shade the eyes during sleep, protect the eyes
  from excessive light and foreign objects, and
  spread lubricating secretions over the eyeballs.
• The upper eyelid contains the levator palpebrae
  superioris muscle and is more moveable than the
  lower.
• The lacrimal caruncle is a small, reddish
  elevation on the medial border and contains
  sebaceous (oil) and sudoriferous (sweat) glands.

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Eyelids

• The Meibomian glands are embedded in the eyelids
  and secrete fluid that prevents the eyelids from
  adhering to each other. Infection of the glands
  produces a cyst known as a chalazion.
• The bulbar conjunctiva passes from the eyelids to
  the surface of the eyeball and covers the sclera
  (white of the eye). The conjunctiva is
  vascular. Irritation or infection cause
  bloodshot eyes.

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Eyelashes and Eyebrows

• The eyelashes and eyebrows help protect the
  eyeballs from foreign objects, perspiration, and
  the direct rays of the sun.
• Sebaceous ciliary glands are located at the base
  of the hair follicles of the eyelashes. The
  release a lubricating fluid into the follicles.
• Infection of these glands results in a sty.

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Lacrimal Apparatus

• The lacrimal apparatus is a group of structures
  that produces and drains lacrimal fluid or tears.
• The lacrimal ducts empty tears onto the surface
  of the conjunctiva of the upper lid.

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Lacrimal Apparatus

• The fluid passes into through the lacrimal
  puncta, into lacrimal canals, to the lacrimal
  sac, and then into the nasolacrimal duct.
• The lacrimal glands are supplied by
  parasympathetic fibers of the facial nerves
  (VII).
• Tears are cleared away by either evaporation or
  by passing into the lacrimal ducts.

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Flow of Tears
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Lacrimal Apparatus
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Extrinsic Eye Muscles

• Six extrinsic eye muscles move each eye
• Superior rectus, inferior rectus, medial rectus,
  inferior oblique (CN III - Oculomotor).
• Superior oblique (CN IV Trochlear).
• Lateral rectus (CN VI Abducens).
• They are supplied by cranial nerves III, IV, VI.
  SO4LR6.
• Motor units in these muscles tend to be small
  with each motor neuron serving only 2 or 3 muscle
  fibers. This permits smooth, precise, and rapid
  movements.

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Accessory Structures of the Eye
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Anatomy of the Eyeball

• The adult eyeball is about 2.5 cm (1 inch) in
  diameter.
• Only 1/6 of the surface area is exposed. The
  remainder is protected by the orbit.
• The wall of the eyeball consists of three layers
• Fibrous tunic
• Vascular tunic
• Retina

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Fibrous Tunic

• The fibrous tunic is the superficial layer of the
  eyeball and consists of the anterior cornea and
  posterior sclera.

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Cornea

• The cornea is a transparent coat that covers the
  colored iris.
• It is curved and helps to focus light.
• The central part of the cornea receives oxygen
  from the outside air.

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Sclera

• The sclera (white of the eye) covers the entire
  eyeball except the cornea.
• The sclera gives shape to the eyeball, makes it
  more rigid, protects its inner parts, and serves
  as a site of attachment for the extrinsic eye
  muscles.

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Canal of Schlemm

• At the junction of the sclera and cornea is an
  opening known as the scleral venous sinus (canal
  of Schlemm).
• The aqueous humor drains into this sinus.

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Vascular Tunic

• The vascular tunic or uvea is the middle layer of
  the eyeball.
• It is composed of three parts
• Choroid
• Ciliary body
• Iris

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Choroid

• The choroid is highly vascularized.
• It provides nutrients to the posterior surface of
  the retina.
• It contains melanocytes which produce the pigment
  melanin.

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Choroid

• The melanin absorbs stray light rays, which
  prevents reflection and scattering of light
  within the eyeball.
• Consequently, the image cast on the retina by the
  cornea and the lens remains sharp and clear.
• Albinos lack melanin in all parts of the body,
  therefore bright light is perceived as a bright
  glare due to scattering.

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Ciliary Body

• In the anterior portion of the vascular tunic,
  the choroid becomes the ciliary body.
• It contains melanin producing melanocytes.
• The ciliary processes produce aqueous humor.
• Zonular fibers extend from the ciliary processes
  and attach to the lens.

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Ciliary Body

• The ciliary muscle is a circular band of smooth
  muscle that controls the tightness of the zonular
  fibers.
• Contraction or relaxation of the ciliary muscle
  changes the tightness on the zonular fibers,
  which alters the shape of the lens, adapting it
  for near or far vision.

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Iris

• The iris ( rainbow) is the colored portion of
  the eyeball.
• It is shaped like a flattened doughnut.
• It is suspended between the cornea and the lens.
• It contains melanocytes. The amount of melanin
  produced determines eye color.
• It contains circular and radial smooth muscle
  fibers.

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Iris

• A principle function is to regulate the amount of
  light entering the eyeball through the pupil, the
  hole in the center of the iris.
• When bright light stimulates the eye,
  parasympathetic fibers of the oculomotor nerve
  (CN III) stimulate the circular muscles
  (sphincter pupillae) to contract causing a
  decrease in pupil size (constriction).
• In dim light, sympathetic neurons stimulate the
  radial muscles (dilator pupillae) to contract,
  causing an increase in the pupils size
  (dilation).

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Pupil Response to Light
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Retina

• The retina is the inner layer of the eyeball.
• It is the beginning of the visual pathway.
• We can view the anatomy of the retina through an
  ophthalmoscope.
• Landmarks visible through the ophthalmoscope
• Optic disc the site where the optic nerve (CN
  II) exits the eyeball.
• Central retinal artery and central retinal vein.
• Macula lutea.
• Fovea centralis.

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Retina
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Photoreceptors

• Photoreceptors are specialized cells that begin
  the process by which light rays are converted to
  nerve impulses.
• Two types
• Rods (approximately 120 million per retina)
• Allow us to see in dim light.
• Do not provide color vision.
• Cones (approximately 6 million per retina)
• Stimulated in brighter light.
• Produce color vision.

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Three Types of Cones

• There are three types of cones in the retina
• Blue cones sensitive to blue light.
• Green cones sensitive to green light.
• Red cones sensitive to red light.
• Color vision results from the stimulation of
  various combinations of these three types of
  cones.

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Rods and Cones

• Most of our experiences are mediated by the cone
  system, the loss of which produces legal
  blindness.
• A person who loses rod vision mainly has
  difficulty seeing in dim light.

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Blind Spot

• The optic disc is the site where the optic nerve
  exits the eyeball.
• The optic disc is also called the blind spot.
• There are no rods or cones where the blind spot
  is.
• We are typically not aware of having a blind spot
  because the two eyes compensate for one another.

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Microscopic Structure Retina
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Detached Retina

• A detached retina may occur due to trauma, such
  as a blow to the head, in various eye disorders,
  or as a result of age-related degeneration.
• Detachment occurs between the neural portion of
  the retina and the pigment epithelium.
• Fluid accumulates between these layers and forces
  the retina outward.
• This results in distorted vision and blindness in
  the corresponding fields.
• Laser surgery or cryosurgery can correct this.

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Macula Lutea
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Maculae Receptors
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Age-related Macular Degeneration (AMD)

• Age-related macular disease (AMD), also known as
  macular degeneration, is a degenerative disorder
  of the retina in persons 50 years of age or
  older.
• Abnormalities occur in the region of the macula
  lutea, which is ordinarily the most acute area of
  vision.
• Victims of AMD retain their vision but lose the
  ability to look straight ahead.
• They cannot see facial features to identify a
  person in front of them.

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Age-related Macular Degeneration (AMD)

• AMD is the leading cause of blindness in those
  over age 75, afflicting 13 million Americans.
• AMD is 2.5 times more common in 1 pack / day
  smokers.

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Lens

• The lens is behind the pupil and the iris, within
  the cavity of the eyeball.
• Proteins called crystallins, arranged like layers
  of an onion, make up the refractive media of the
  lens.
• The lens is normally perfectly transparent and
  lacks blood vessels.
• The lens helps focus images on the retina to
  facilitate clear vision.

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Interior of the Eyeball

• The lens divides the interior of the eyeball into
  two cavities the anterior cavity and vitreous
  chamber.
• The anterior cavity the space anterior to the
  lens consists of two chambers.
• Anterior chamber lies between the cornea and
  iris.
• Posterior chamber lies behind the iris and in
  front of the lens.
• Both chambers of the anterior cavity are filled
  with aqueous humor, a transparent watery fluid
  that nourishes the lens and the cornea.

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Iris Chambers
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Interior of the Eyeball

• The posterior cavity of the eyeball is the
  vitreous chamber, which lies between the lens and
  the retina.
• The vitreous body lies within the vitreous
  chamber. The vitreous body is a transparent
  jellylike substance that holds the retina flush
  against the choroid, giving the retina an even
  surface for the reception of clear images.

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Interior of the Eyeball

• Occasionally, collections of debris may cast a
  shadow on the retina and create the appearance of
  specks that dart in and out of the field of
  vision. These are known as vitreous floaters.

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Intraocular Pressure

• The pressure of the eye is referred to as
  intraocular pressure.
• It is produced mainly by the aqueous humor and
  partly by the vitreous body.
• It is normally about 16 mmHg.
• It helps to maintain the shape of the eyeball and
  prevent it from collapsing.
• Punctures of the eyeball can cause a loss of
  aqueous humor and the vitreous body, thereby
  decreasing intraocular pressure, a detached
  retina, and sometimes blindness.

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Image Formation

• In many ways the eye operates like a camera.
• It has optical elements which focus as image on
  the retina.
• Three processes help to focus the image
• The refraction or bending of light by the lens
  and cornea.
• The change in shape of the lens (accommodation).
• The constriction or narrowing of the pupil.

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Refraction of Light Rays

• When light rays pass from one substance (air) to
  another substance with a different density
  (water), they bend at the junction between the
  two substances.
• This bending is known as refraction.
• Light is refracted at both the cornea and the
  lens so that it comes into exact focus on the
  retina.

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Refraction of Light Rays

• Images focused on the retina are inverted (upside
  down). They also undergo light to left reversal.
• 75 of the refraction occurs at the cornea.
• 25 occurs at the lens, which also changes the
  focus to view either distant or near objects.

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Refraction
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Accomodation and the Near Point of Vision

• Convex surface that curves outward.
• When a lens is convex, it will refract incoming
  light rays towards one another.
• Concave surface that curves inward.
• When a lens is concave, it refract incoming light
  rays away from each other.
• The lens of the eye is convex on both the
  anterior and posterior surfaces.

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Accomodation and the Near Point of Vision

• As the curvature becomes greater, its focusing
  power increases.
• When the eye is focusing on a close object, the
  lens becomes more curved, causing greater
  refraction of the light rays.
• This increase in the curvature of the lens is
  called accommodation.
• The near point of vision is the minimum distance
  from the eye that an object can be clearly
  focused with the maximum accommodation.

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Accomodation and the Near Point of Vision

• When viewing distant objects, the ciliary muscle
  is relaxed and the lens is flatter because the
  taught zonular fibers are stretching it in all
  directions.
• When viewing close objects, the ciliary muscle
  contracts, which pulls the ciliary processes
  towards the lens. This releases tension on the
  lens and zonular fibers. The lens is elastic and
  then becomes more spherical.
• Parasympathetic fibers of CN III (oculomotor)
  innervate the ciliary muscle.

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Refraction Abnormalities

• Emmetropic eye normal eye can sufficiently
  refract light rays from objects 6 m (20 ft) away
  so that a clear image is focused on the retina.
• Myopia nearsightedness can see close objects
  clearly, but not distant objects.
• Hyperopia (hypermetropia) farsightedness can
  see distant objects clearly, but not close ones.
• Astigmatism either the cornea or the lens has
  an irregular curvature. Parts of the image are
  out of focus.

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Refraction Abnormalities Corrections
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Constriction of the Pupil

• Constriction of the pupil is a narrowing of the
  diameter of the hole through which light enters
  the eye due to contraction of the circular
  muscles of the iris.
• This occurs automatically during accommodation to
  prevent light from entering at the periphery of
  the lens.
• The pupil also constricts in bright light.

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Pupil Response To Light
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Convergence

• Binocular vision is focusing on one set of
  objects with both eyes.
• This allows us to perceive depth and the three
  dimensional nature of objects.
• When we look ahead at an object, light is
  refracted to comparable spots on the retinas of
  both eyes.
• As we move closer to an object, the eyes must
  rotate medially towards the object being viewed.
• Convergence is the medial movement of the two
  eyeballs so that both are pointed towards the
  object.

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Physiology of Vision
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Photoreceptors and Photopigments

• Rods and cones were named for the different
  appearance of the outer segment the distal end
  next to the pigmented layer.
• The outer segment of rods are cylindrical or
  rod-shaped those of cones are tapered or
  cone-shaped.
• Transduction of light energy into a receptor
  potential occurs in the outer segment.
• Photopigments are integral proteins in the plasma
  membrane.

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Photoreceptors and Photopigments

• In cones, the plasma membrane is folded back and
  forth in a pleated fashion.
• In rods, the outer segment contains a stack of
  about 1000 discs, piled up like coins in a
  wrapper.
• The inner segment contains the cell nucleus,
  Golgi complex, and many mitochondria.
• The proximal end expands into synaptic terminals
  filled with synaptic vesicles.

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Photoreceptors and Photopigments

• The photopigment undergoes a structural change
  when it absorbs light, which leads to a receptor
  potential.
• Rods contain the pigment rhodopsin.
• Cones contain three different photopigments, one
  for each of the three types of cones.
• Different colors of light activate different cone
  pigments.
• All photopigments contain the glycoprotein opsin
  and a derivative of vitamin A called retinal.

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Rod and Cone Structure
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Rods and Cones
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Light and Dark Adaptation

• When you emerge from dark surroundings into the
  light, light adaptation occurs. Your visual
  system adjusts within seconds to the brighter
  surroundings.
• When you enter a darkened room, dark adaptation
  occurs. Your sensitivity increases slowly over
  several minutes.

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Rods Do Not See Red

• The light response of the rods peaks sharply in
  blue light. They respond little to red light.
• In bright light, the color sensitive cones
  predominate. At twilight, the less-sensitive
  cones begin to shut down and most of the vision
  comes from the rods.
• The attainment of optimum night vision can take
  up to a half hour.
• You can view things with red light at night
  without activating the cones and therefore, you
  will not lose your night vision.

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Color Blindness and Night Blindness
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Release of Neurotransmitter by Photoreceptors

• A ligand known as cyclic GMP (guanosine
  monophosphate) or cGMP allows the inflow of Na
  ions to depolarize the photoreceptor.
• Light causes a hyperpolarizing receptor potential
  in photoreceptors, which decreases release of an
  inhibitory neurotransmitter (glutamate).
• The cGMP channels close.
• The photoreceptor cells become excited and
  stimulate the ganglion cells to form action
  potentials.

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Visual Pathway

• Visual signals from the retina exit the eyeball
  as the optic nerve (CN II) and proceed to the
  brain.

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Processing of Visual Input in the Retina

• Visual input is processed in the retina before
  proceeding to the optic nerve.
• There are 126 million photoreceptors in the human
  eye, but only 1 million ganglion cells.
• Some features of visual input are enhanced, while
  others are discarded.

110
Processing of Visual Input in the Retina

• Between 6 and 600 rods synapse with a single
  bipolar cell a cone more often synapses with a
  single bipolar cell.
• Convergence of many rods onto a single bipolar
  cell increases the light sensitivity, but may
  slightly blur the image.
• Cone vision is less sensitive, but sharper due to
  the one to one synapse with the bipolar cell.

111
Brain Pathway and Visual Fields

• 1. Axons of all retinal ganglion cells in one
  eye exit the eyeball at the optic disc and form
  the optic nerve on that side.
• 2. At the optic chiasm, axons from the temporal
  half of each retina do not cross but continue
  directly to the lateral geniculate nucleus of the
  thalamus on the same side.
• 3. Axons from the nasal half of each retina
  cross the optic chiasm and continue to the
  opposite hypothalamus.

112
Brain Pathway and Visual Fields

• 4. Each optic tract consists of crossed and
  uncrossed axons that project from the optic
  chiasm to the thalamus on one side.
• 5. Axon collateral extend to the midbrain to
  govern pupil constriction and to the hypothalamus
  to govern patterns of sleep and other circadian
  rhythms relevant to light and darkness.
• 6. Axons of thalamic neurons form the optic
  radiations and project to the primary visual area
  of the cortex on the same side.

113
Visual Pathway
114
Hearing and Equilibrium

• The ear can transduce sound vibrations with
  amplitudes as small as the diameter of an atom of
  gold (0.3 nm) into electrical signals 1000 times
  faster than photoreceptors can respond to light.
• The ear also contains receptors for equilibrium.

115
Anatomy of the Ear

• The ear is divided into three main regions
• External ear collects sound waves and channels
  them inward.
• Middle ear conveys sound vibrations to the oval
  window.
• Internal ear houses the receptors for hearing
  and equilibrium.

116
External Ear

• The external (outer) ear consists of the auricle,
  external auditory canal, and eardrum.
• The tympanic membrane (eardrum) is a thin,
  semitransparent partition between the external
  auditory canal and the middle ear.
• Tearing of the tympanic membrane is called a
  perforated eardrum.
• Pressure from a cotton swab, trauma, or a middle
  ear infection can cause perforation. It usually
  heals within 1 month.

117
External Ear

• The membrane can be examined using an otoscope.
• Ceruminous glands secrete cerumen (earwax).
• Cerumen and hairs help prevent dust and foreign
  particles from collecting in the ear.
• Impacted cerumen can impair hearing.

118
Middle Ear

• The middle ear contains the auditory ossicles.
• The malleus attaches to the internal surface of
  the tympanic membrane.
• The head of the malleus articulates with the
  incus.
• The incus articulates with the head of the
  stapes.
• The stapes fits into the oval window which is
  enclosed by a secondary tympanic membrane.
• The stapedius muscles (supplied CN VII) dampens
  vibrations of the stapes due to loud noises.

119
Middle Ear (Eustachean Tube)

• The auditory (pharyngotympanic) tube, also known
  as the eustachian tube connects the middle ear to
  the nasopharynx.
• It helps to equalize pressure in the middle ear.
• If pressure is not equalized, intense pain,
  hearing impairment, ringing in the ears, and
  vertigo can develop.
• It is a route for pathogens to enter and cause
  otitis media.

120
Internal Ear

• The internal ear is also called the labyrinth
  because of its complicated series of canals
  including the semicircular canals and cochlea.
• The spiral organ or Corti contains supporting
  cells including approximately 16,000 hair cells,
  which are the receptors for hearing.

121
Anatomy of the Ear
122
Nature of Sound Waves

• Sound waves are alternating high and low pressure
  regions traveling in the same direction through
  some medium (such as air).
• The frequency of a sound wave is the pitch.
• The human ear most acutely detects sounds waves
  between 500 and 5000 hertz (Hz).

123
Nature of Sound Waves

• The audible range extends between 20 and 20,000
  Hz.
• Sounds of speech are between 100 and 3000 Hz.
• The larger the intensity (size or amplitude) of
  the vibration, the louder the sound.
• Sound intensity is measured in units called
  decibels (dB).

124
Physiology of Hearing

• 1. The auricle directs sound waves into the
  external auditory canal.
• 2. Sound waves strike the tympanic membrane
  causes it to vibrate back and forth. The
  distance it moves depends upon the intensity and
  frequency of the waves.
• 3. The central eardrum connects to the malleus,
  which also starts to vibrate. This vibration is
  then transmitted to the incus and stapes.
• . As the stapes moves back and forth, it pushes
  the membrane of the oval window in and out.

125
Physiology of Hearing

• 5. The movement of the oval window sets up fluid
  pressure waves in the perilymph of the cochlea.
• 6. Pressure waves are transmitted to the round
  window, causing it to bulge outward.
• 7. These waves in turn create pressure waves in
  the endolymph of the cochlear duct.
• 8. This causes the basilar membrane to vibrate,
  which moves the hair cells leading to receptor
  potentials and ultimately nerve impulses.

126
Stimulation of Auditory Receptors
127
Auditory Pathway

• Bending of the stereocilia of the hair cells of
  the spiral organ causes the release of a
  neurotransmitter, which causes nerve impulses in
  the sensory neurons.
• These nerve impulses pass along the axons to form
  the cochlear branch of the vestibulocochlear
  nerve (CN VIII).
• They synapse in the cochlear nuclei in the
  medulla oblongata on the same side.

128
Auditory Pathway

• Some axons decussate in the medulla and terminate
  in the midbrain on the opposite side.
• Other axons continue to the pons on the same
  side.
• Slight differences in the timing of the impulses
  allow us to locate the source of the sound.
• The axons are then conveyed to the thalamus and
  ultimately to the primary auditory area of the
  cerebral cortex in the temporal lobe.

129
Auditory Pathway