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					                      Refraction
• Light rays are bent
• refractive index = ratio of light in a vacuum to the
  velocity in that substance
• velocity of light in vacuum=300,000 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
   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 (Δ 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
                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
          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)
              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
               Visual acuity
• The fovea centralis is the area of greatest
  visual acuity
  – it is less than .5 mm in diameter (< 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
   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
            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
  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
                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
        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
           Retinal responses

     Dark                       Light
                                 
   Rod/Cone                   Rod/Cone
   depolarize                hyperpolarize
                                 
     ↑ NT                       NT

Hyperpol   Depolarize   Depolarize   Hyperpol
“ON” BC    “OFF BC      “ON” BC      “OFF” BC
            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
          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
            Ganglion cells
Cell type   P (X)          M (Y)
INPUT       Bipolar        Amacrine
Receptive Small        Large
Field
Conduc. Slow           Fast
Velocity
Response Slow adapting Fast adapting

Projection Parvocellular   Magno. of
           of LGN          LGN
        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)
       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
           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
          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
         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
   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
   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
    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)
      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
        Dark Adaptation
• In sustained darkness reformation of
  light sensitive pigments (Rhodopsin
  & Cone Pigments)
•  of retinal sensitivity 10,000 fold
• cone adaptation<100 fold (1st 10 min.)
• rod adaptation>100 fold (50 min.)
• dilation of pupil
• neural adaptation
               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)
        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
          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”
           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
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
Cells in visual pathway
 Receptor   Light      diffuse     orientation
                       light good important
                                   – no
 Ganglion small spot moderate no
            narrow bar
            over
            center
 Geniculate same as    poor        no
            ganglion
 Simple     narrow bar ineffective yes
            or edge
 Complex bar / edge ineffective yes
 Hypercom line / edge ineffective yes
 plex     that stops
        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
     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
    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
                     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
          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.
  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)
          Horner’s 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
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
    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
   Summary of extraoccular ms.
          Elevate    Depress   Intorsion Extors.


Adduct.   Inferior   Superior Superior Inferior
Eye       oblique    oblique rectus    rectus

Abduct.   Superior Inferior    Superior Inferior
Eye       rectus   rectus      oblique oblique
                      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
                  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
       Different Levels of Sound
•   20 dB- whisper
•   60 dB- normal conversation
•   100 dB- symphony
•   130 dB- threshold of discomfort
•   160 dB- threshold of pain
    Frequencies of Audible Sound
•   In a young adult
•   20-20,000 Hz (decreases with age)
•   Greatest acuity
•   1000-4000 Hz
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)
        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
        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 < 1000 Hz.
• Protects cochlea from very loud noises,
  masks low freq sounds in loud environment
                      Cochlea
• System of 3 coiled tubes
  – Scala vestibuli
  – Scala media
  – Scala tympani
            Scala Vestibuli
• Seperated from the scala media by
  Reissner’s membrane
• Associated with the oval window
• filled with perilymph (similar to CSF)
              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
       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)
             Scala Tympani
• Associated with the round window
• Filled with perilymph
  – baths lower bodies of hair cells
          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
              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 (>4500Hz)
• hair cells near helicotrema (apex)
  – long and slender
     • respond best to lower frequencies (<200 Hz)
 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
     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
     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
      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)
   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 > bone- sensory neural
  – bone conduction > air- air conduction loss
          Sound Localization
• Horizontal direction from which sound
  originates from determined by two principal
  mechanisms
  – Time lag between ears
     • functions best at frequencies < 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
    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)
              Taste receptors
• May have preference for stimuli
• influenced by past history
  – recent past
     • adaptation
  – long standing
     • memory
     • conditioning-association
     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
    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
                     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
                 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
    1000’s 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
     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
          Adaptation of taste
• Rapid-within minutes
• taste buds account for about 1/2 of
  adaptation
• the rest of adaptation occurs higher in CNS
           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)
     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
                    Olfaction
• Least understood
  – smell is subjective
  – hard to study in animals
  – rudimentary in humans
     • Humans are microsmatic
        – Poorly developed sense of smell
                         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
         Vomeronasal organ
• Aka Jacobson’s 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
           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
   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 Bowman’s glands
  – secrete mucus
• Sustentacular cells
  – supporting cells
    Characteristics of Odorants
• Volatile
• slightly water soluble-
  – for mucus
• slightly lipid soluble
  – for membrane of cilia
          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
  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
    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
          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
  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)
       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
             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 doesn’t relay via
    thalamus (from lateral pathway)
  – learned control/adversion
• Newer- passes through the thalamus to
  orbitofrontal cortex (from lateral pathway)
  – - conscious analysis of odor
   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

				
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