Refraction

1
Refraction

• Light rays are bent
• refractive index ratio of light in a vacuum to
  the velocity in that substance
• velocity of light in vacuum300,000 km/sec
• Light year 9.46 X 1012 km
• Refractive indices of various media
• air 1
• cornea 1.38
• aqueous humor 1.33
• lens 1.4
• vitrous humor 1.34

2
Refraction of light by the eye

• Refractive power of 59 D (cornea lens)
• Diopter 1 meter/ focal length
• Convex lens expressed as diopters
• Concave lens expressed as - diopters
• central point 17 mm in front of retina
• inverted image- brain makes the flip
• lens strength can vary from 20- 34 D (? 14)
• Ability to increase refractive power ? with age
• 14 (age 10) 8 (age 30) 2 (age 50)
• Parasympathetic increases lens strength
• Greater refractive power needed to read text

3
Accomodation

• Increasing lens strength from 20 -34 D
• Parasympathetic causes contraction of ciliary
  muscle allowing relaxation of suspensory
  ligaments attached radially around lens, which
  becomes more convex, increasing refractive power
  (illustration)
• Associated with close vision (e.g. reading)
• In addition, eyes roll in and pupils constrict
• Presbyopia- loss of elasticity of lens w/ age
• decreases accommodation

4
Errors of Refraction

• Emmetropia- normal vision ciliary muscle relaxed
  in distant vision
• Hyperopia-farsighted- focal pt behind retina
• globe short or lens weak convex lens to correct
• Myopia- nearsighted- focal pt in front of
  retina
• globe long or lens strong concave lens to
  correct
• Astigmatism- irregularly shaped
• cornea (more common)
• lens (less common)

5
Visual Acuity

• Snellen eye chart
• ratio of what that person can see compared to a
  person with normal vision
• 20/20 is normal
• 20/40 less visual acuity
• What the subject sees at 20 feet, the normal
  person could see at 40 feet.
• 20/10 better than normal visual acuity
• What the subject sees at 20 feet, the normal
  person could see at 10 feet

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Visual acuity

• The fovea centralis is the area of greatest
  visual acuity
• it is less than .5 mm in diameter (lt 2 deg of
  visual field)
• outside fovea visual acuity decreases to more
  than 10 fold near periphery
• acuity for point sources of light 25 sec of arc
  (angle of 25 seconds)
• point sources of light two ? apart on retina can
  be distinguished as two separate points

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Fovea and acute visual acuity

• Central fovea-area of greatest acuity
• composed almost entirely of long slender cones
• aids in detection of detail
• blood vessels, ganglion cells, inner nuclear
  plexiform layers are displaced laterally
• allows light to pass relatively unimpeded to
  receptors

8
Depth Perception

• Relative size
• the closer the object, the larger it appears
• learned from previous experience
• Moving parallax
• As the head moves, objects closer move across the
  visual field at a greater rate
• Stereopsis- binocular vision
• eyes separated by 2 inches- slight difference in
  position of visual image on both retinas, closer
  objects are more laterally placed

9
Formation of Aqueous Humor

• Secreted by ciliary body (epithelium)
• 2-3 ul/min
• flows into anterior chamber and drained by Canal
  of Schlemm (vein)
• intraocular pressure- 12-20 mmHg.
• Glaucoma- increased intraocular P.
• compression of optic N.-can lead to blindness
• treatment drugs surgery

10
Retina

• Peripheral extension of the CNS
• Processing of visual signal
• Photoreceptors
• Rods Cones
• Other Cells
• bipolar, ganglion, horizontal, amacrine
• Only retinal cells that generate action
  potentials are the ganglion cells

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Photoreceptors

• Rods Cones
• Light breaks down rhodopsin (rods) and cone
  pigments (cones)
• ? rhodopsin ? ? Na conductance
• photoreceptors hyperpolarize
• release less NT (glutamate) when stimulated by
  light

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Retinal responses

• Dark
• ?
• Rod/Cone
• depolarize
• ?
• ? NT
• Hyperpol Depolarize
• ON BC OFF BC

• Light
• ?
• Rod/Cone
• hyperpolarize
• ?
• NT
• Depolarize Hyperpol
• ON BC OFF BC

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Bipolar Cells

• Connect photoreceptors to either ganglion cells
  or amacrine cells
• passive spread of summated postsynaptic
  potentials (No AP)
• Two types
• ON- hyperpolarized by NT glutamate
• Invaginating bipolars
• OFF- depolarized by NT glutamate
• Flat bipolars

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Ganglion Cells

• Can be of the ON or OFF variety
• ON bipolar ON ganglion
• OFF bipolar OFF ganglion
• Generate AP carried by optic nerve
• Three subtypes
• X (P) cells
• Y (M) cells
• W cells

15
Ganglion cells
16
P (X) Ganglion Cells

• Most numerous (55) G cells
• Receive input mostly from bipolar c.
• Slower conduction velocity (14 m/sec)
• Sustained response-slow adaptation
• Small receptive field
• signals represent discrete retinal location
• Respond differently to different ?
• Responsible for color vision
• Project to Parvocellular layer of lateral
  geniculate nucleus (thalamic relay)

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M (Y) Ganglion Cells

• Receive input mostly from Amacrine
• Larger receptive field
• Transient-fast conduction velocity
• respond best to moving stimuli
• Not sensitive to different ?
• More sensitive to brightness
• Project to magnocellular LGN
• Black White images

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W Ganglion Cells

• smallest, slowest CV (8 m/sec)
• 40 of all ganglion cells
• many lack center-surround antagonistic fields
• they act as light intensity detectors
• some respond to large field motion
• detect directional movement
• Broad receptive fields
• Receive most of their input from rods
• Important for crude vision in dim light

19
Horizontal Cells

• Non spiking inhibitory interneurons
• Make complex synaptic connections with
  photorecetors
• Hyperpolarized when light stimulates input
  photoreceptors (just like receptor)
• When they depolarize they inhibit photoreceptors
• Maybe responsible for center-surround antagonism

20
Amacrine Cells

• Receive input from bipolar cells
• Project to ganglion cells
• Several types releasing different NT
• GABA, dopamine
• Transform sustained ON or OFF to transient
  depolarization AP in ganglion cells

21
Center-Surround Fields

• Receptive fields of bipolar gang. C.
• two concentric regions
• Center field
• mediated by all photoreceptors synapsing directly
  onto the bipolar cell
• Surround field
• mediated by photoreceptors which gain indirect
  access to bipolar cells via horizontal cells

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Center-Surround (cont)

• Photoreceptors contributing to center field of
  one bipolar cell contributes to surround field of
  other bipolar cells
• Because of center-surround antagonism, ganglion
  cells monitor differences in luminance between
  center surround fields

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Center-surround (cont)

• If center field is on, surround is off
• If center field is off, surround is on
• Simultaneous stimulation of light of both fields
  gives no net response
• antagonistic excitatory inhibitory inputs
  neutralize each other
• When surround is illuminated, the horizontal
  cells depolarize the cones in the center
  (opposite effect of light)

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Receptive field size

• In fovea- ratio can be as low as 1 cone to 1
  bipolar cell to 1 ganglion cell
• In peripheral retina- hundreds of rods can supply
  a single bipolar cell many bipolar cells
  connected to 1 ganglion cell

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Dark Adaptation

• In sustained darkness reformation of light
  sensitive pigments (Rhodopsin Cone Pigments)
• ? of retinal sensitivity 10,000 fold
• cone adaptationlt100 fold (1st 10 min.)
• rod adaptationgt100 fold (50 min.)
• dilation of pupil
• neural adaptation

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Cones

• 3 populations of cones with different
  pigments-each having a different peak absorption
  ?
• Blue sensitive (445 nm)
• Green sensitive (535 nm)
• Red sensitive (570 nm)

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Color Blindness

• Sex-linked trait carried on X chromosome
• Occurs almost exclusively in males but
  transmitted by the female
• Most common is red-green color blindness
• missing either red or green cones

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Loss of Cones

• Loss of Red Cones- Protanope
• decrease in overall visual spectrum
• Loss of Green Cones- Deuteranope
• normal overall visual spectrum
• problems distinguishing green, yellow, orange
  red (Ishihara Chart)
• Loss of Blue Cones- rare but may be
  under-represented Blue weakness

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Visual Pathway

• Optic N to Optic Chiasm
• Optic Chiasm to Optic Tract
• Optic Tract to Lateral Geniculate
• Lateral Geniculate to 10 Visual Cortex
• geniculocalcarine radiation

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Additional Visual Pathways

• From Optic Tracts to
• Suprachiasmatic Nucleus
• biologic clock function
• Pretectal Nuclei
• reflex movement of eyes-
• focus on objects of importance
• Superior Colliculus
• rapid directional movement of both eyes
• Orienting reactions

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Cells in visual pathway
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Primary Visual Cortex

• Brodman area 17 (V1)-2x neuronal density
• Simple Cells-responds to bar of light/dark
• above below layer IV
• Complex Cells-motion dependent but same
  orientation sensitivity as simple cells
• Color blobs-rich in cytochrome oxidase in center
  of each occular dominace band
• starting point of cortical color processing
• Vertical Columns-input into layer IV
• Hypercolumn-functional unit, block through all
  cortical layers about 1mm2

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Visual Association Cortex

• Visual signal is broken down sent over parallel
  pathways
• Visual analysis proceeds along many paths in
  parallel- at least 30 cortical areas processing
  vision
• Parvo-interblob
• High resolution static form perception (B W)
• Blob
• Color (V4)
• Achromatopsia
• Magno
• Movement (MT) Stereoscopic Depth

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Old vs. New visual system

• Old pathway projects to the superior colliculus
• Locating objects in visual field, so you can
  orient to it (rotate head eyes)
• Subconscious
• Blindsight
• New pathway projects to the cortex
• Consciously recognizing objects

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Blindsight

• Some patients who are effectively blind because
  of brain damage can carry out tasks which appear
  to be impossible unless they can see the objects.
• For instance they can reach out and grasp an
  object, accurately describe whether a stick is
  vertical or horizontal, or post a letter through
  a narrow slot.
• The explanation appears to be that visual
  information travels along two pathways in the
  brain. If the cortical pathway is damaged, a
  patient may lose the ability to consciously see
  an object but still be aware of its location and
  orientation via projections to the superior
  colliculus at a subconscious level.
• How the brain learns to see video

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Cortical fixation areas

• Voluntary fixation mechanism (anterior)
• Person moves eyes voluntarily to fix on an object
• Controlled by cortical field bilaterally in
  premotor cortex
• Involuntary fixation mechanism (posterior)
• Holds eyes firmly on object once it has be
  located
• Controlled by secondary visual areas in occipital
  cortex located just in front of primary visual
  cortex
• Works in conjunction with the superior colliculus
• Involuntary fixation is mostly lost when superior
  colliculus is destroyed.

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Control of Pupillary Diameter

• Para causes ? size of pupil (miosis)
• Symp causes ? size of pupil (mydriasis)
• Pupillary light reflex
• optic nerve to pretectal nuclei to
  Edinger-Westphal to ciliary ganglion to pupillary
  sphincter to cause constriction (Para)

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Horners Syndrome

• Interruption of SNS supply to an eye
• from cervical sympathetic chain
• constricted pupil compared to unaffected eye
• drooping of eyelid normally held open in part by
  SNS innervated smooth muscle
• dilated blood vessels
• lack of sweating on that side of face

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Function of extraoccular muscles

• Medial rectus of one eye works with the lateral
  rectus of the other eye as a yoked pair to
  produce lateral eye movements
• The superior inferior recti muscles elevate
  depress the eye respectively and are most
  effective when the eye is abducted
• The superior oblique muscles lower the eye when
  it is adducted
• The inferior oblique muscle elevates the eye when
  it is adducted

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Innervation of extraoccular muscles

• Extraoccular muscles controlled by CN III, IV,
  and VI
• CN VI controls the lateral rectus only
• CN IV controls the superior oblique only
• CN III controls the rest

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Summary of extraoccular ms.
Elevate Depress Intorsion Extors.
Adduct. Eye Inferior oblique Superior oblique Superior rectus Inferior rectus
Abduct. Eye Superior rectus Inferior rectus Superior oblique Inferior oblique
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Sound

• Units of Sound is the decibel (dB)
• I (measured sound)
• Decibel 1/10 log --------------------------
  
• I (standard
  sound)
• Reference Pressure for standard sound
• .02 X 10-2 dynes/cm2

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Sound

• Energy is proportional to the square of pressure
• A 10 fold increase in sound energy 1 bel
• One dB represents an actual increase in sound E
  of about 1.26 X
• Ears can barely detect a change of 1 dB

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Different Levels of Sound

• 20 dB- whisper
• 60 dB- normal conversation
• 100 dB- symphony
• 130 dB- threshold of discomfort
• 160 dB- threshold of pain

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Frequencies of Audible Sound

• In a young adult
• 20-20,000 Hz (decreases with age)
• Greatest acuity
• 1000-4000 Hz

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Tympanic Membrane Ossicles

• Impedance matching-between sound waves in air
  sound vibrations generated in the cochlear fluid
• 50-75 perfect for sound freq.300-3000 Hz
• Ossicular system
• reduces amplitude by 1/4
• increases pressure against oval window 22X
• increased force (1.3)
• decreased area from TM to oval window (17)

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Ossicular system (cont.)

• Non functional ossicles or ossicles absent
• decrease in loudness about 15-20 dB
• medium voice now sounds like a whisper
• attenuation of sound by contraction of
• Stapedius muscle-pulls stapes outward
• Tensor tympani-pull malleous inward

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Attenuation of sound

• CNS reflex causes contraction of stapedius and
  tensor tympani muscles
• activated by loud sound and also by speech
• latency of about 40-80 msec
• creation of rigid ossicular system which reduces
  ossicular conduction
• most effective at frequencies of lt 1000 Hz.
• Protects cochlea from very loud noises, masks low
  freq sounds in loud environment

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Cochlea

• System of 3 coiled tubes
• Scala vestibuli
• Scala media
• Scala tympani

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Scala Vestibuli

• Seperated from the scala media by Reissners
  membrane
• Associated with the oval window
• filled with perilymph (similar to CSF)

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Scala Media

• Separated from scala tympani by basilar membrane
• Filled with endolymph secreted by stria
  vascularis which actively transports K
• Top of hair cells bathed by endolymph

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Endocochlear potential

• Scala media filled with endolymph (K)
• baths the tops of hair cells
• Scala tympani filled with perilymph (CSF)
• baths the bottoms of hair cells
• electrical potential of 80 mv exists between
  endolymph and perilymph due to active transport
  of K into endolymph
• sensitizes hair cells
• inside of hair cells (-70 mv vs -150 mv)

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Scala Tympani

• Associated with the round window
• Filled with perilymph
• baths lower bodies of hair cells

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Function of Cochlea

• Change mechanical vibrations in fluid into action
  potentials in the VIII CN
• Sound vibrations created in the fluid cause
  movement of the basilar membrane
• Increased displacement
• increased neuronal firing resulting an increase
  in sound intensity
• some hair cells only activated at high intensity

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Place Principle

• Different sound frequencies displace different
  areas of the basilar membrane
• natural resonant frequency
• hair cells near oval window (base)
• short and thick
• respond best to higher frequencies (gt4500Hz)
• hair cells near helicotrema (apex)
• long and slender
• respond best to lower frequencies (lt200 Hz)

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Fourier analysis by the cochlea

• Any complex wave can be broken down into its
  component sine waves with differing phases,
  frequencies, amplitudes
• Fourier analysis
• Cochlea behaves like a Fourier analyser
• Acts a kind of auditory prism
• Sorting out vibrations of different frequencies
  into different positions along the membrane

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Central Auditory Pathway

• Organ of Corti to ventral dorsal cochlear
  nuclei in upper medulla
• Cochlear N to superior olivary N (most fibers
  pass contralateral, some stay ipsilateral)
• Superior olivary N to N of lateral lemniscus to
  inferior colliculus via lateral lemniscus
• Inferior colliculus to medial geniculate N
• Medial geniculate to primary auditory cortex

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Primary Auditory Cortex

• Located in superior gyrus of temporal lobe
• tonotopic organization
• high frequency sounds
• posterior
• low frequency sounds
• anterior
• S.Q.U.I.D
• changes in central sensitivities

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Air vs. Bone conduction

• Air conduction pathway involves external ear
  canal, middle ear, and inner ear
• Bone conduction pathway involves direct
  stimulation of cochlea through the vibration of
  the skull as the cochlea is imbedded in the
  petrous portion of the temporal bone
• reduced hearing may involve
• ossicles (air conduction loss)
• cochlea or associated neural pathway (sensory
  neural loss)

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Differentiating a hearing loss

• If there is a known bad ear
• Weber test (512 Hz) tunning fork placed on
  midline of the skull
• If sounds louder in bad ear- conduction loss in
  bad ear. (external canal or ossicles involved)
• If sounds louder in good ear- sensory neural loss
  in bad ear
• Rinne test- confirms results of Weber
• air conduction gt bone- sensory neural
• bone conduction gt air- air conduction loss

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Sound Localization

• Horizontal direction from which sound originates
  from determined by two principal mechanisms
• Time lag between ears
• functions best at frequencies lt 3000 Hz.
• Involves medial superior olivary nucleus
• neurons that are time lag specific
• Difference in intensities of sounds in both ears
• involves lateral superior olivary nucleus

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Exteroceptive chemosenses

• Taste
• Works together with smell
• Categories (Primary tastes)
• sweet
• salt
• sour
• bitter (lowest threshold-protective mechanism)
• Umami (savory/pungent)
• Olfaction (Smell)
• Primary odors (100-1000)

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Taste receptors

• May have preference for stimuli
• influenced by past history
• recent past
• adaptation
• long standing
• memory
• conditioning-association

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Primary sensations of taste

• Sour taste-
• caused by acids (hydrogen ion concentration)
• Salty taste-
• caused by ionized salts (primarily the Na)
• Sweet taste-
• most are organic chemicals (e.g. sugars, esters
  glycols, alcohols, aldehydes, ketones, amides,
  amino acids) inorganic salts of Pb Be

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Primary sensations of taste

• Bitter- no one class of compounds but
• long chain organic compounds with N
• alkaloids (quinine, strychnine, caffeine,
  nicotine)
• Umami/Savory
• Flavor associated with MSG
• Receptor responds to amino acids

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Taste

• Taste sensations are generated by
• complex transactions among chemical and receptors
  in taste buds
• subsequent activities occuring along the taste
  pathways
• There is much sensory processing, centrifugal
  control, convergence, global integration among
  related systems contributing to gustatory
  experiences

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Taste Buds

• Taste neuroepithelium consists of taste buds
  distributed over tongue, pharynx, larynx.
• Aggregated in relation to 3 kinds of papillae
• fungiform-blunt pegs 1-5 buds /top
• foliate-submerged pegs in serous fluid with
  1000s of taste buds on side
• circumvallate-stout central stalks in serous
  filled moats with taste buds on sides in fluid
• 40-50 modified epithelial cells grouped in barrel
  shaped aggregate beneath a small pore which opens
  onto epithelial surface

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Innervation of Taste Buds

• each taste nerve arborizes innervates several
  buds (convergence in 1st order)
• receptor cells activate nerve endings which
  synapse to base of receptor cell
• Individual cells in each bud differentiate,
  function degenerate on a weekly basis
• taste nerves
• continually remodel synapses on newly generated
  receptor cells
• provides trophic influences essential for
  regeneration of receptors buds

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Adaptation of taste

• Rapid-within minutes
• taste buds account for about 1/2 of adaptation
• the rest of adaptation occurs higher in CNS

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CNS pathway-taste

• Anterior 2/3 of tongue
• lingual N. to chorda tympani to facial (VII CN)
• Posterior 1/3 of tongue
• IX CN (Petrosal ganglion)
• base of tongue and palate
• X CN
• All of the above terminate in nucleus tractus
  solitarius (NTS)

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CNS pathway (taste cont)

• From the NTS to VPM of thalamus via central
  tegmental tract (ipsilateral) which is just
  behind the medial lemniscus.
• From the thalmus to lower tip of the post-central
  gyrus in parietal cortex adajacent opercular
  insular area in sylvian fissure

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Olfaction

• Least understood
• smell is subjective
• hard to study in animals
• rudimentary in humans
• Humans are microsmatic
• Poorly developed sense of smell

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The Nose

• 3 conchae bilaterally
• Highly vascularized organs covered with erectile
  tissue
• Fxns to moisten and warm incoming air
• Limit loss of heat H2O in expired air
• Engorged with blood when you have a cold
• Block air from reaching olfactory receptors
• Partial loss of smell
• Olfactory cleft at the top
• Olfactory epithelium
• Associated with the olfactory receptors
• Normally only a small portion of air reaches here
• Sniffing ? the by creating turbulence around
  conchae

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Vomeronasal organ

• Aka Jacobsons organ
• Located medially on septum in lower part of nasal
  cavity
• Appears to contribute to olfaction
• Probably more receptive than olfactory epithelium
  to phermones which have profound effects on
  behavior

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Olfactory Membrane

• Superior part of nostril
• Olfactory cells
• bipolar nerve cells
• 100 million in olfactory epithelium
• 6-12 olfactory hairs/cell project in mucus
• react to odors and stimulate cells

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Cells in Olfactory Membrane

• Olfactory cells-
• bipolar nerve cells which project hairs in mucus
  in nasal cavity
• stimulated by odorants
• connect to olfactory bulb via cribiform plate
• Cells which make up Bowmans glands
• secrete mucus
• Sustentacular cells
• supporting cells

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Characteristics of Odorants

• Volatile
• slightly water soluble-
• for mucus
• slightly lipid soluble
• for membrane of cilia

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Threshold for smell

• Very low
• methyl mercaptan
• 1/25 billion of mg/ml of air can be detected
• mixed with natural gas so gas leaks can be
  detected

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Stimulation of Olfactory Cells

• Odorant binds to receptor protein
• Inside of protein is coupled to a G-protein
• 3 subunits
• G-protein activates adenyl cyclase
• Adenyl cyclase converts ATP ? cAMP
• cAMP causes protein gated Na channels to open
• Ca enters as well which opens choride channels
• High Cl- concentraction inside olfactory
  receptors (unusual)
• Efflux of Cl- prolongs depolarization
• At every step the effect is amplified

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Primary sensations of smell

• Anywhere from 100 to 1000 based on different
  receptor proteins
• odor blindness has been described for at least 50
  different substances
• may involve lack of a specific receptor protein

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Olfactory Receptor

• Resting membrane potential when not activated
  -55 mv
• 1 impulse/ 20 sec to 2-3 impulses/ sec
• When activated membrane pot. -30 mv
• 20 impulses/ sec
• Prolongation of response in response to
• Na and Ca influx during depolarization
• Ca influx binds to and opens Chloride channel
  protein
• High Chloride content intracellularly (atypical),
  therefore when stimulated, Cl- efflux will
  prolong depolarization

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Glomerulus in Olfactory Bulb

• several thousand/bulb
• Connections between olfactory cells and cells of
  the olfactory tract
• receive axons from olfactory cells (25,000)
• receive dendrites from
• large mitral cells (25)
• smaller tufted cells (60)

84
Cells in Olfactory bulb

• Mitral Cells- (continually active)
• send axons into CNS via olfactory tract
• Tufted Cells- (continually active)
• send axons into CNS via olfactory tract
• Granule Cells
• inhibitory cell which can decrease neural traffic
  in olfactory tracts
• receive input from centrifugal nerve fibers
• Periglomerular Cells
• Inhibitory cells between glomerulus

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CNS pathways

• Very old- medial olfactory area
• feeds into hypothalamus primitive areas of
  limbic system (from medial pathway)
• basic olfactory reflexes
• Less old- lateral olfactory area
• prepyriform pyriform cortex -only sensory
  pathway to cortex that doesnt relay via thalamus
  (from lateral pathway)
• learned control/adversion
• Newer- passes through the thalamus to
  orbitofrontal cortex (from lateral pathway)
• - conscious analysis of odor

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Medial and Lateral pathways

• 2nd order neurons form the olfactory tract
  project to the following 1o olfactory
  paleocortical areas
• Anterior olfactory nucleus
• Modulates information processing in olfactory
  bulbs
• Amygdala and olfactory tubercle
• Important in emotional, endocrine, and visceral
  responses of odors
• Pyriform and periamygdaloid cortex
• Olfactory perception
• Rostral entorhinal cortex
• Olfactory memories