Refraction

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# Refraction - PowerPoint PPT Presentation

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 10 12 km Refractive indices of various media air = 1 cornea = 1.38 aqueous humor = 1.33 lens = 1.4

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## PowerPoint Slideshow about 'Refraction' - zelda

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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
Dark

Rod/Cone

depolarize

↑ NT

Hyperpol Depolarize

“ON” BC “OFF BC

Light

Rod/Cone

hyperpolarize

NT

Depolarize Hyperpol

“ON” BC “OFF” BC

Retinal responses
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
P (X) Ganglion Cells
• Most numerous (55%) G cells
• Receive input mostly from bipolar c.
• Slower conduction velocity (14 m/sec)
• 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
• 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
• 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
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
• 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
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
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
• 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
• 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
• Rapid-within minutes
• 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
• 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)
• 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)