SPECIAL SENSES

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SPECIAL SENSES
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The Special Senses

• Smell
• Taste
• Sight
• Hearing
• Equilibrium

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Innervation

• Special senses are served by various cranial
  nerves

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Smell (Olfaction)

• This special sense, along with taste, monitors
  chemicals
• Smell, and also taste, propagate to the limbic
  system as well as higher cortical areas.
• The limbic system can elicit strong emotional
  responses.

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Anatomy

• Receptors
• Bipolar neurons with knob shaped dendrites that
  have receptors on cilia site of transduction.
• Dendrites are located in the superior portion of
  the nasal cavity.
• These bipolar neurons are C.N. I
• Supporting cells
• insulators and detoxifiers
• Nourish and support receptors.
• Basal stem cells
• Produce new receptors
• Olfactory glands
• Produce mucous

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C.N. I

• Axons of C.N. I exit nasal cavity in 40 bundles
  through the olfactory foramina
• Synapse occurs in olfactory bulbs, just above
  cribiform plate
• Axons from olfactory bulb enter brain
• C.N. I is unusual in that new neurons are
  routinely produced

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Innervation of Supporting Cells and Olfactory
Glands

• C.N. VII

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Physiology of Olfaction

• There are 100s of primary scents.
• We can recognize about 10,000 scents that are
  created through combinations of the primary ones.
• An odorant molecule binds to a receptor in the
  cell membrane and through a second messenger
  causes a depolarizing generator potential that
  can lead to an impulse (action potential).
• Odorant molecules must be dissolved in water
  supplied by the mucosa, unless they are fat
  soluble.

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Thresholds and Adaptations

• Low threshold, requiring only a few molecules for
  odor to be perceived.
• Rapid adaptation with 50 of receptors adapting
  within 1 second.
• At least some adaptation occurs in CNS

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Olfactory Pathway

• About 40 bundles of axons enter the cranium
  through the cribiform plate as CN I.
• They terminate in the olfactory bulbs, which are
  bundles of gray matter below frontal lobes of
  cerebrum.
• Here the axon terminals of first order neurons
  synapse with dendrites and cell bodies of the
  second order neurons.

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Olfactory Pathway contd

• Axons of the second order neurons extend
  posteriorly as the olfactory tracts.
• They project to the olfactory area in the
  temporal lobe.
• This area is part of the limbic system.
• From here there are pathways to the frontal lobe,
  sometimes via the thalamus.
• Odor identification occurs in the frontal lobe
  the right is more active in odor processing.

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Cortical areas stimulated by olfaction
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Taste (Gustation)

• Chemical sense, also
• Stimuli
• Sour
• Salt
• Sweet
• Bitter Umami (meat)
• Food can also stimulate olfaction, which is much
  more sensitive to low chemical concentrations.
  That is why you cant taste well if you have a
  cold.

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Anatomy of Receptors

• Located in taste buds
• Buds (10,000 in youth, decreases with age) are
  found primarily in papillae on upper surface of
  the tongue of the adult
• Circumvallate
• Fungiform
• Filiform (rarely found in these)
• Foliate-degrade in early childhood

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Anatomical Relation of Taste Buds to Papillae

• Taste buds are located in papilla below the
  surface of the tongue.

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Taste Zones

• Individual taste buds respond to more than one
  primary taste, but may respond more strongly to
  one than another.

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Anatomy of Taste Bud
Basal cells produce supporting cells which mature
into receptor cells.
single microvillus
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Physiology of Taste

• Tastants, which are chemicals of food, must be
  dissolved in saliva.
• Different tastes arise from activation of
  different groups of taste neurons.
• Receptor potential stimulates exocytosis of
  neurotransmitter.
• Sour-opens H channels
• Salt-opens Na channels
• Sweet-second messenger system
• Bitter-second messenger system
• Umami (meat)-second messenger system
• Neurotransmitter stimulates first order sensory
  neurons that synapse with the receptor cells.

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Thresholds and Adaptation

• Threshold varies
• Bitter has lowest threshold
• Protective
• Harmful substances are often bitter
• Complete adaptation can occur in 3-5 minutes at
  any level
• Taste receptors
• Olfactory receptors
• Neurons

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Gustatory Pathway

• First order neurons of the anterior two-thirds of
  the tongue are part of CN VII.
• First order neurons from the posterior third are
  part of CN IX.
• A few buds are found on the epiglottis and in the
  throat. CN X serves these.

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Pathway contd

• Receptors to
• CN to
• Gustatory nucleus of medulla to
• Limbic and hypothalamus or to
• Thalamus to
• Primary gustatory area of insula, where we become
  conscious of the sensation

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VISION
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Acessory Structures
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Eyelid Anatomy

• Layers of the lower eye lid
• Epidermis
• Dermis
• Subcuaneous
• Orbicularis occuli
• Tarsal plate
• Tarsal glands
• Conjunctiva

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Common Eyelid Pathologies

• Chalazion
• Infection of a tarsal gland, which is a modified
  sebaceous gland.
• Sty
• Infection of a sebaceous gland associated with an
  eyelash.

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Accessory structures Lacrimal Apparatus
Tears contain lysozyme, an antibacterial enzyme
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Extrinsic Muscles of Eye

• Surrounded by periorbital fat
• Superior rectus
• Inferior rectus
• Medial rectus
• Lateral rectus
• Superior oblique
• Inferior obllique

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Tunic (layers) of the Eyeball

• Fibrous tunic
• Vascular tunic
• Nervous tunic (the retina)

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Components of Tunics

• Fibrous
• Sclera
• cornea
• Vascular
• Choroid
• Ciliary body
• Ciliary muscle
• Ciliary processes
• Zonular fibers
• Iris
• Nervous
• Retina

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Other Anatomical Components

• Lens
• Cavities
• Anterior
• Anterior chamber
• Posterior chamber
• Vitreous (posterior )
• Vitreous body

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Fibrous Tunic

• Cornea
• Sclera
• Scleral venus sinus (canal of Schlemm)
• Aqueous humor drainage

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Vascular Tunic

• Choroid
• Ciliary body
• Ciliary muscle
• Ciliary processses
• Zonular fibers (suspensory ligaments of lens)
• Iris
• pupil

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Iris, Pupil and ANS
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Nervous Tunic, the Retina

• Posterior three quarters of eyeball
• Is the beginning of the visual pathway
• Only place in the body where blood vessels can be
  viewed directly and examined for pathology
  (diabetes, hypertension)
• The ora serrata is the jagged anterior margin of
  the retina

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Retinal Anatomy

• Ora serrata
• Optic disc
• Macula
• Fovea

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Layers of Retina

• Pigmented epithelium
• Photoreceptors rods and cones
• Bipolar cells
• Ganglion cells
• Inner synaptic layer
• Outer synaptic layer
• Horizontal and amacrine cells

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Notable details

• Note the flow of information vs. light
• Lack of retinal cells, except for axons of
  ganglion cells, at optic disc
• Presence of only cones in fovea
• Greatest visual acuity

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Age-Related Macular Degeneration

• More likely to occur in smokers
• Dry-deterioration of pigmented layer loss of
  acute vision in fovea no treatment
• Wet-10 progress to wet degeneration
  proliferation of vessels in choroid with
  subsequent leaking of blood or plasma onto macula

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Loss of Vision in Macular Degeneration
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The Photoreceptors Rods and Cones

• Rods detect shades of gray and are active in low
  light
• Only one type
• Cones detect color and are active in bright
  lights
• Red
• Blue
• Green

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Horizontal and Amacrine Cells

• Horizontal and amacrine cells modify signals
  transmitted to bipolar and ganglion cells. They
  are absent in the central fovea, the area of
  greatest visual acuity.

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A Quick Physiological Experiment

• Check out your own blind spot, using cross and
  square in book.

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The Lens

• Made of layers of proteins called crystallins.
• Avascular and transparent.
• Biconvex
• Flexible rounds for near vision, flattens for
  far vision.

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Lens Proteins
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Interior of Eye

• Anterior cavity
• Vitreous cavity
• Central artery and vein
• Hyaloid canal

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Flow of Aqueous Humor

• Post. chamber
• Between iris and lens
• Through pupil
• Ant. Chamber
• Scleral venus sinus (canal of Schlemm)
• Blood

Intraoccular pressure is produced mainly by the
aqueous humor.
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Glaucoma

• Damage to retina because of increased pressure in
  anterior chamber.
• Due to build up of aqueous humor.
• Treatments include drugs to slow production of
  aqueous humor and surgeries to open canal of
  Schlemm.

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Types of Glaucoma

• Normal angle
• Acute angle

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Vision Loss of Glaucoma

• Tunnel vision

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Image Formation

• Light is focused onto the retina.
• The retina acts like film in a camera it must be
  exposed to the proper amount of light achieved
  through changing diameter of pupil.
• The retina responds neurologically and sends
  information to brain for processing.

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Processes leading to the formation of clear
images on the retina

• Refraction focuses light on retina
• Accommodation rounding of lens to focus on near
  objects
• Constriction of pupil reduces scattered light at
  edge of image

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Refraction

• Bending that occurs when light passes through the
  junction of 2 materials with different densities
• Cornea (75)
• Lens (10)
• Other structures and fluids of eye (150

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Image Formation on Retina as Result of Refraction

• The image on the retina is upside down right and
  left are reversed, due to the biconvex shape of
  the lens.
• The brain is responsible for correction of
  perception.

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Accommodation-Adaptations for Near Vision

• Involves the increase in curvature of lens for
  near vision.
• The lens of the eye is convex on both its
  anterior and posterior surfaces.
• The lens becomes more curved, resulting in
  increased refraction (bending) of light, when it
  is focusing on near objects.

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Accomodation and Aging

• Accommodation becomes more difficult in a
  persons 40s
• Condition is called presbyopia.
• Correction through glasses, contacts or surgery

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Refraction Abnormalities

• Emmertropic eye
• Normal eye
• Focus on retina
• Myopic eye
• Elongated eyeball or
• Thickened lens
• Focus in front of retina
• Hypermetropic eye
• Shortened eyeball or thin lens
• Focus behind retina

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Constriction of the Pupil in Accommodation for
Near Vision

• Constriction of the pupil by contracting the
  circular smooth muscle of the iris is an
  autonomic reflex.
• Constriction prevents peripheral light rays from
  entering and producing a blurred image (because
  the light rays could not be focused on the
  retina).

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Convergence

• Humans have binocular vision.
• Both eyes focus on the same object.
• The result is three dimensional vision.
• Convergence means the medial movement of the eyes
  so that they can both focus on the same object as
  we move closer to it.

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The Physiology of Vision

• Transduction of light into receptor potentials
  occurs in the outer segments of the rods and
  cones.
• The inner segments contain the usual cellular
  machinery.

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Photoreceptors Rods and Cones

• Outer segments are renewed at fast ratein rods
  1-3 discs per hour!
• Pigmented epithelium nourishes rods and cones

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The Photopigments

• Rods contain only one type of photopigment,
  rhodopsin, aka visual purple.
• There are three different photopigments for
  cones, one for each type.
• Color is perceived by the stimulation of
  combinations of these types of cones.
• Color blindness is the result of a lack of one of
  the three types it is the inability to
  distinguish certain colors from others.

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Composition of Photopigments

• Retinal the appropriate opsin photopigment
• Retinal is found in all 4 photopigments.
• It is a derivative of vit.A.
• A deficiency of vit. A can cause night blindness,
  also called nyctalopia.
• 4 different opsins exist, one for rods and one
  for each type of cone.
• The pigment of the rods is rhodopsin, aka visual
  purple

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Bleaching and Regeneration Transduction of Light

• Light is transduced by causing bent retinal, the
  cis-form, to straighten into trans-retinal.
• Results in cessation of neurotransmitter,
  glutamate, production by rods and cones.
• Retinal detaches from opsin and rebends into
  trans form.
• Colorless productbleaching
• The photopigment is regenerated by the
  reattachment of trans-retinal to opsin

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Dark Current of Photoreceptors

• Glutamate is produced in the dark, when sodium
  ion channels are open.
• Results in IPSP in bipolar cells they cannot
  generate an action potential.
• Consequently no action potentials are generated
  in the ganglion cells and no information goes to
  the thalamus and then to visual cortex.

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Photoreceptor Response to trans-Retinal

• Na channels close through cGMP second messenger
  system.
• Membrane hyperpolarizes
• The photoreceptor can no longer reach threshold.
• Photoreceptor ceases release of glutamate.

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Response of Bipolar Cells in Dark

• Bipolar cells have leakage channels for Na that
  are always open.
• When glutamate is present, negative ion channels
  also open.
• More negative ions enter than positive, resulting
  in IPSP and no action potential.

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Response of Bipolar Cells in Light

• When glutamate is no longer present, the negative
  ion channels close.
• The Na entering from the leakage channels leads
  to threshold and action potential.
• The bipolar cell releases a neurotransmitter that
  leads to EPSPs and threshold in the ganglion
  cells.
• Ganglion cells then have action potentials that
  carry information to thalamus.

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LIGHT AND DARK ADAPTATION

• In high light, more photopigment is bleached.
  Rhodopsin regeneration cannot keep up but
  pigments of the cones can. Therefore, rods
  contribute little to vision in bright light, but
  we perceive color.
• If light decreases quickly, sensitivity changes
  quickly at first and then slows.
• Cones regenerate in 8 minutes and at first dim
  light is perceived as colored
• Rods regenerate more slowly, but visual acuity
  increases until even a single photon can be
  detected. When the rods are functioning,
  however, only shades of gray are perceived.

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Processing in the Retina

• Some features are enhanced and some are
  discarded. Convergence plays a bigger role than
  divergence.
• Horizontal cells inhibit bipolar cells lateral to
  excited rods and cones to enhance contrast also
  help in differentiation of color.
• Bipolar cells excite amacrine cells, which
  synapse with ganglion cells and transmit
  information about changes in levels of
  illumination.

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The Visual Pathway

• Axons of the ganglion cells eventually run
  together and exit the eye through the optic
  foramen as the optic nerve (C.N.II).

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Brain pathways and visual fields

• Information from the right sides of both fields
  projects to the left side of the brain and vice
  versa. (The lateral halves cross, but the medial
  halves do not.)

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Pathways into Brain

• Optic chiasm
• Optic tract
• Superior colliculi
• Lateral geniculate nucleus of thalamus
• Optic radiations
• 10 visual area

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Hearing and Equilibrium

• The ear is divided into three parts
• Outer includes canal and tympanic membrane
• Collects sound waves
• Middle includes ossicles, muscles and ligaments
• Conveys sound vibrations to the oval window
• Inner includes vestibular cochlear apparatus
• Houses receptors for hearing and equilibrium

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Outer Ear

• Canal
• Ceruminous glands
• Tympanic membrane

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Middle Ear

• Malleus
• Incus
• Stapes
• Tensor tympani
• Stapedius
• Ligaments
• Oval window
• Round window
• Secondary tympanic membrane

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Stapedius and Tensor Tympani Protect the Inner Ear

• The stapedius (CN VII) is the smallest skeletal
  muscle. It dampens vibrations of the stapes due
  to loud sounds, protects the oval window, and
  decreases hearing sensitivity.
• The tensor tympani (CN V) limits movement of
  eardrum by increasing tension.

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Inner Ear

• Vestibular apparatus
• Semicircular canals
• ampullae
• Vestibule
• Utricle
• saccule
• Cochlea
• Helicotrema
• Bony labyrinth
• Perilymph
• Membranous labyrinth
• Endolymph
• Endolymphatic duct and sac (return of endolymph
  to dura mater sinus)

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Characteristics of Perilymph and Endolymph

• Perilymph is similar to CSF
• Endolymph has unusually high potassium ion levels

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Cochlea

• Modiolus
• Scala vestibuli
• Tympanic duct
• Scala tympani
• Vestibular membrane
• Helicotrema
• Basilar membrane
• Spiral organ

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C.N. VIII

• Vestibular part
• Cochlear part

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Detail of One Turn of Cochlea

• Scala vestibuli
• Scala tympani
• Spiral organ
• Vestibular membrane
• Basilar membrane
• Spiral ganglion

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The Spiral Organ

• Inner hair cells
• Outer hair cells
• Synapse with C.N. VIII
• Tectorial membrane

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Pathway of Pressure Waves
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Transduction of Sound

• Hair cells bend against the techtorial K enters
• This produces receptor potentials which are
  carried by the Coclear portion of CN VIII.

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Pressure waves created by the movement of the
perilymph push the vestibular membrane back and
forth, causing pressure waves in the endolymph of
the cochlear duct. This causes the basilar
membrane to vibrate. This in turn moves the hair
cells against the tectorial membrane, which
produces a receptor potential by
opening mechanically gated potassium ion channels.
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Sound

• Pressure waves in air or water
• Air molecules crowd together with adjacent areas
  that have fewer molecules
• Graph as sine waves
• Wavelength distance between adjacent peaks (or
  troughs) in graph
• Frequency number of waves (cycles) past a fixed
  point/unit time inversely proportional to
  wavelength measured in Hertz
• Different parts of Organ of Corti respond to
  different frequencies

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Pitch Sensory Response to Frequency

• High frequency (short wavelength)
• 15,000Hz or more
• Low frequency (long wavelength)
• 100 Hz or less

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Frequency Discrimination
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Amplitude intensity

• Reflects amount of energy used to produce sound
• Perceived as loudness
• Measured in decibels
• 0 lowest audible sound
• 60 conversation
• 80 alarm clock limit of safety
• 120 rock concert immediate danger for damage
• 140 gunshot immediate danger for damage
• 160 rocket launch hearing loss certain

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The Auditory Pathway

• First order neurons are the cochlear branch of
  VIII
• They end in the ipsilateral cochlear nuclei of
  the medulla
• Signals go to the superior olivary nucleus
• Signals arrive at slightly different times from
  the two ears allowing us to locate the source of
  the sound
• Some fibers decussate in the medulla
• Fibers from the medulla then go to the inferior
  colliculus of the midbrain
• Then to the thalamus
• Then to primary auditory area
• Because some fibers decussate, information from
  both ears goes to each primary auditory center

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Equilibrium

• Static equilibrium
• Dynamic equilibrium

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The Vestibular Apparatus
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Saccule and Utricle Otolithic Organs

• Both contain a thickened region called the
  macula.
• The maculae are perpendicular to each other
• They function in static equilibrium.
• Perceive head position
• They also function in dynamic equilibrium.
• Detect linear acceleration and deceleration

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Maculae and Transduction of Movement

• Hair cells
• Otolithic membrane
• Calcium carbonate crystals
• When head moves the membrane lags a little and
  pulls on the hair cells.
• The tips bend, K enters and a depolarizing
  potential results.
• The hair cells synapse with first order neurons
  on the vestibular portion of VIII.

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The Semicircular Canals

• Function in dynamic equilbrium together with the
  saccule and utricle.
• The semicircular canals lie at right angles to
  one another.
• They detect rotational acceleration or
  deceleration.

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The Ampullae

• Each duct has a dilated portion called the
  ampulla.
• The elevation in the ampulla is called the crista
  and consists of
• Hair cells
• Supporting cells
• The cupula covers the cells.

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Ampullae and Transduction of Movement

• Hair cells bend against the cupula which lags
  behind.
• K enters
• This produces receptor potentials which are
  carried by the vestibular portion
• Of CN VIII.

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The Vestibular Pathway

• CN VIII TO
• SEVERAL NUCLEI IN THE MEDULLA AND PONS.
• SOME FIBERS ENTER THE CEREBELLUM.
• INFORMATION ULTIMATETLY REACHES THE CEREBRUM
  WHERE APPROPRIATE MOTOR RESPONSES ARE GENERATED.

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END!