-
Vision and Light
- •Vision - perception of light emitted or reflected from objects in the environment
- •Visible light
- •electromagnetic radiation with wavelengths from 400 to 750 nm
- •must cause a photochemical reaction to produce a nerve signal
- •radiation below 400 nm; energetic, kills cells
- •radiation above 750 nm; too little energy to cause photochemical reaction
-
Image Formation: Refraction, Accomodation, and Constriction
- •Refraction
- •Bending of light rays
- •Accommodation
- •Change in the shape of the lens
- •Constriction
- •Change in the size of the pupil
-
PRINCIPLE OF REFRACTION
- •Speed of light in a vacuum 3 X 10 5 km/s
- •Slows down in air, water, glass, other mediums
- •When light rays strike a surface at an angle the rays are bent
- •Refractive index – the ability of a medium to slow down light rays relative to air (1.0)
-
OPTICAL COMPONENTS
- •Structures refract light to focus on retina
- •cornea
- •transparent cover on anterior surface of eyeball
- •aqueous humor
- •serous fluid posterior to cornea, anterior to lens
- •lens
- • changes shape to help focus light
- •rounded with no tension
- •flattened due to pull of suspensory ligaments
- •vitreous humor
- •jelly fills space between lens and retina
-
REFRACTION
- Bending of light rays occurs when light passes through substance with different refractive index at any angle other than 90 degrees
- •refractive index of air is arbitrarily set to n = 1
- •refractive index
- •cornea is n = 1.38
- •lens is n = 1.40
- •Cornea refracts light more than lens does
- •due to shape of cornea
- •lens becomes rounder to increase refraction for near vision
-
EFFECTS OF CORRECTED LENSES
- •Hyperopia - farsighted (eyeball too short)
- •correct with convex lenses
- •Myopia - nearsighted (eyeball too long)
- •correct with concave lenses
-
Astigmatism
irregularities in the curvature of lens and cornea leading to blurred vision.
-
ACCOMMODATION OF LENS
- •Change in the shape of the lens to bring an object in focus
- •Ciliary muscles contract
- •Suspensory ligaments relax
- •Decreasing diameter of ciliary body
- •Allowing lens to become more rounded (convex)
- •Near point accommodation - closest point for an object in visual field while remaining in focus
-
Presbyopia
- "old vision"
- •Decreased ability to accommodate with age due to decreased flexibility of lens
- •Leads to difficulty reading and doing close handiwork
- •bifocals
-
CATARACTS
- clouding of lens
- •aging, diabetes, smoking, and UV light
-
Glaucoma
Aqueous Humor:
- Produced by ciliary body, flows to posterior chamber through pupil to anterior chamber - reabsorbed into canal of Schlemm
- •Glaucoma
- •death of retinal cells due to elevated pressure within the eye
- •obstruction of scleral venous sinus
- •colored halos and dimness of vision
-
DRUGS TO TREAT GLAUCOMA
- •Tonometry - to screen for glaucoma by measuring intraocular pressure and to permit evaluation of glaucoma
- • -pneumatic tonometry consists of the electronic measurement of pressure after a short burst of gas strikes the cornea
- •Cholinergic drugs - to produce miosis (constriction of the pupil) in the treatment of glaucoma. They reduce intraocular pressure by increasing the outflow of fluid from the anterior chamber of the eye.
- • Carbonic anhydrase inhibitors - lower intraoccular pressure by inhibiting the formation of fluid
- • Beta blocking agents - lower intraoccular pressure by reducing aqueous humor production; block the action of epinephrine and norepinephrine.
-
CONSTRICTION OF THE PUPIL
- •Light enters the eyes through the pupils
- •Pupillary constrictor - smooth muscle encircling the pupil
- •parasympathetic stimulation narrows pupil
- •Pupillary dilator - spokelike myoepithelial cells
- •sympathetic stimulation widens pupil
- •Active when light intensity changes or gaze shifts from distant object to nearby object
- •photopupillary reflex -- both pupils constrict if one eye is illuminated (type of consensual reflex)
-
NEAR RESPONSE
- Allows eyes to focus on nearby object (that sends oblique light waves to eyes)
- 1.convergence of eyes
- •eyes orient their visual axis towards object
- 2.constriction of pupil
- •blocks peripheral light rays and reduces spherical aberration (blurry edges)
- 3.accomodation of lens
- •ciliary muscle contracts, lens takes convex shape
- •light refracted more strongly and focused onto retina
-
TUNICS OF THE EYEBALL
- Fibrous layer - sclera and cornea
- •Vascular layer - choroid, ciliary body and iris
- •Internal layer - retina and optic nerve
-
NONRECEPTOR RETINAL CELLS
- •Bipolar cells (1st order neurons)
- •synapse on ganglion cells
- •large amount of convergence
- •Ganglion cells (2nd order neurons)
- •axons of these form optic nerve
- •more convergence occurs (114 receptors to one optic nerve fiber)
- •Horizontal and amacrine cells form connections between other cells
- •enhance perception of contrast, edges of objects and changes in light intensity
-
TEST FOR BLIND SPOT
- •Optic disk = blind spot
- •optic nerve exits posterior surface of eyeball
- •no receptor cells
- •Blind spot - use test illustration above
- •close eye, stare at X and red dot disappears
- •Visual filling - brain fills in green bar across blind spot area
-
PHOTORECEPTOR CELLS
- •Posterior layer of retina - pigment epithelium
- •purpose is to absorb stray light and prevent reflections
- •Photoreceptors
- •rod cells (night - scotopic vision)
- •outer segment - stack of coinlike membranous discs studded with rhodopsin pigment molecules
- •cone cells (color - photopic vision)
- •outer segment tapers to a point
-
DETACHED RETINA
- •Blow to head or lack of vitreous
- •Blurry areas in field of vision
- •Disrupts blood supply, leads to blindness
- •How does a detached retina disrupt the blood supply to neural retina?
-
VISUAL PIGMENTS
- •Rod cells have rhodopsin
- •has absorption peak at wavelength of 500 nm
- •2 major parts of molecule
- •opsin - protein portion
- •retinal - a vitamin A derivative
- •Cones contain photopsin
- •opsin moieties contain different amino acids that determine wavelengths of light absorbed
- •3 kinds of cones absorbing different wavelengths of light produce color vision
-
RHODOPSIN BLEACHING/REGENERATION
- •Rhodopsin absorbs light, converted from bent shape (cis-retinal) to straight (trans-retinal)
- •retinal dissociates from opsin (bleaching)
- •5 minutes to regenerate 50% of bleached rhodopsin
-
GENERATING VISUAL SIGNALS
- •Opsin is an enzyme
- •Inactive when associated with cis-retinal
- •In light trans-retinal disassociates from opsin making it active
- •Activated opsin interacts with transducin to facilitate the breakdown of cGMP by phosphodiesterase
- •Breakdown of cGMP will lead to closure of ligand gated Na clannels
-
GENERATING VISUAL SIGNALS 2
- In the light ligand gated Na channels are closed by the breakdown of cGMP
- •Na influx ceases
- •Decreasing glutamate release at distal synapse
- •In the light there is a decrease in glutamate signaling on bipolar cells
-
NIGHT VISION
- •Rods sensitive – react even in dim light
- •extensive neuronal convergence
- •600 rods converge on 1 bipolar cell
- •many bipolar converge on each ganglion cell
- •results in high degree of spatial summation
- •one ganglion cells receives information from 1 mm2 of retina producing only a coarse image
-
NIGHT VISION
- •Edges of retina have widely-spaced rod cells, act as motion detectors
- •Covers a large area in visual field
- •Small depolarization to very low levels of light in several rods can activate bipolar cell
- •Images generated are not clear because these photoreceptors cover a large area and converge
-
DAY VISION
- Fovea contains only 4000 tiny cone cells (no rods)
- •no neuronal convergence
- •each foveal cone cell has "private line to brain"
- •High-resolution color vision
- •little spatial summation so less sensitivity to dim light
- •Small depolarization in cones cannot activate bipolar cells because there is no additive effect
-
-
- •Primates have well developed color vision
- •nocturnal vertebrates
- have only rods
- •Cones named for absorption
- peaks of photopsins
- •blue cones peak sensitivity at 420 nm
- •green cones peak at 531 nm
- •red cones peak at 558 nm (orange-yellow)
- •Color perception based on mixture of nerve signals
-
COLOR BLINDNESS
- •Hereditary lack of one photopsin
- •red-green is common (lack either red or green cones)
- •incapable of distinguishing red from green
- •sex-linked recessive (8% of males)
- • Usually Ishihara plates are used to test color vision.
-
VISUAL ACUITY
Snellen eye chart-visual acuity 20/20, 20/40, 20/15
-
OPHTHALMOSCOPIC EXAM OF EYE
- •Direct evaluation of blood vessels
- •Ophthalmoscopy – used to detect retinal changes associated with hypertension, diabetes mellitus, atherosclerosis, and increased intracranial pressure
- •-an ophthalmoscope is used to examine the interior of the eyeball, with or without the use of dilating drops
-
STEREOSCOPIC VISION (STEREOPSIS)
- Depth perception - ability to judge distance to objects
- •requires 2 eyes with
- overlapping visual fields
- •panoramic vision has eyes
- on sides of head (horse)
- •Fixation point
- •farther away requires image focus medial to fovea
- •closer results in image focus lateral to fovea
-
VISUAL PROJECTION PATHWAY
- Bipolar and ganglion cells in retina - 1st and 2nd order neurons (ganglion cell axons of form CN II)
- •Hemidecussation in optic chiasm
- •1/2 of fibers cross over so that images of all objects in left visual field fall on right half of each retina
- •each side of brain sees what is on side where it has motor control over limbs
- •3rd order neurons in lateral geniculate nucleus of thalamus form optic radiation to 1 visual cortex where conscious visual sensation occurs
- •Few fibers project to superior colliculi and midbrain for visual reflexes (photopupillary and accommodation)
-
PITCH AND LOUDNESS
- Pitch - frequency vibrates specific parts of ear
- •hearing range is 20 (low pitch) - 20,000 Hz (cycles/sec)
- •speech is 1500-4000 where hearing is most sensitive
- •Loudness – amplitude; intensity of sound energy
-
THE EAR
- •The subdivisions of the ear are:
- •Outer ear - Collects sound waves and passes them to tympanic membrane via external auditory canal
- •Middle ear - Contains the tympanic membrane and the ossicles; conducts sound waves into inner ear
- •Inner ear - Composed of the bony labyrinth and the membranous labyrinth; Contains the receptors for hearing and equilibrium
- THE EAR
-
SPIRAL ORGAN
- Cochlear duct separated from
- •scala vestibuli by vestibular membrane
- •scala tympani by basilar membrane
- Spiral organ (organ of corti)
- •Stereocilia of hair cells attach to gelatinous tectorial membrane
- •Inner hair cells
- •hearing
- •Outer hair cells
- •adjust cochlear responses to different frequencies
- •increase precision
-
COCHLEAR HAIR CELLS
- •Stereocilia of IHCs
- •bathed in high K+
- •creating electrochemical gradient
- •tips embedded in tectorial membrane
- •bend in response to movement of basilar membrane
- •pulls on tip links and opens ion channels
- •K+ flows in – depolarization causes release of neurotransmitter
- •stimulates sensory dendrites at base
-
SENSORY CODING
- Vigorous vibrations excite more inner hair cells over a larger area
- •triggers higher frequency of action potentials
- •brain interprets this as louder sound
- •Pitch depends on which part of basilar membrane vibrates
- •at basal end, membrane narrow and stiff
- •brain interprets signals as high-pitched
- •at distal end, 5 times wider and more flexible
- •brain interprets signals as low-pitched
-
BASILAR MEMBRANE FREQUENCY RESPONSE
Notice high and low frequency ends
-
COCHLEAR TUNING
- •Increases ability of cochlea to receive some sound frequencies
- •Outer hair cells contract reducing basilar membranes freedom to vibrate
- •fewer signals from that area allows brain to distinguish between more and less active areas of cochlea
-
TYMPANIC REFLEX
- •Tensor tympani muscle
- •Tenses eardrum
- •Stapeduis muscle
- •Reduces movement of stapes
- •Make up tympanic reflex
- •Muffles transfer of sound waves to inner ear
-
TYMPANIC REFLEX 2
- •Works for loud sounds that gradually build up
- •Thunder
- •Dampens our speech allowing us to hear others when we speak
- •Not effective for sustained loud noises
- •Breaks sterocilia
- •Irreversible hearing loss
-
AUDITORY PROJECTION PATHWAY
- •Spiral ganglion formed by cell bodies of sensory neurons
- •Axons form cochlear nerve portion of CN VIII (vestibulocochlear)
- •Synapse in cochlear nuclei (near pons)
- •Second order neurons pass superior olivary nucleus (pons)
- •compares sounds from both sides to identify direction Binaural hearing
- •Cochlear tuning
- •Tympanic reflex
-
AUDITORY PROJECTION PATHWAY 2
- •Inferior colliculus helps
- •locate origin of sound
- •process fluctuations in pitch during speech
- •produce startle response; head turning to loud sound
- •Third order neurons from inferior colliculus go to thalamus
- •Fourth order neurons to cerebral cortex
-
DISORDERS ASSOCIATED WITH HEARING
- •Deafness
- •Sensorineural-death of hair cells
- •Continued exposure to lound noise
- •Conduction-disruption of sound wave transmission to inner ear
- •Damaged ear drum
- •Otosclerosis- fusion of ossicles to each other or fusion of stapes to oval window
- •Ottis media- middle ear infection common in children due to short horizontal auditory tubes
- •Fluid accumulates in tympanic cavity
- •Myringotomy- cutting eardrum and inserting small drainage tube
- •Audiometry
- •test to evaluate an individual’s hearing acuity
- While in a soundproof room an individual uses earphones to listen for sounds produced by an audiometer
-
EQUILIBRIUM
- •Control of coordination and balance
- •Receptors in vestibular apparatus
- •semicircular ducts contain crista
- •saccule and utricle contain macula
- •Static equilibrium – perceived by macula
- •perception of head orientation
- •Dynamic equilibrium
- •perception of motion or acceleration
- •linear acceleration perceived by macula
- •angular acceleration perceived by crista
-
STATIC EQUILIBRIUM
- •Saccule and Utricle
- •Contain macula
- •hair cells with stereocilia and one kinocilium buried in a gelatinous otolithic membrane
- •otoliths add to the density and inertia and enhance the sense of gravity and motion
-
DYNAMIC EQUILIBRIUM
- Semicircular canals
- •Contains crista ampullaris
- •Consists of hair cells buried in a mound of gelatinous membrane (one in each duct)
- •Orientation causes ducts to be stimulated by rotation in different planes
- As head turns, endolymph lags behind, pushes cupula, stimulates hair cells
- •After 30 sec endolymph catches up and hair cells no longer stimulated
-
EQUILIBRIUM PROJECTION PATHWAYS
- •Hair cells of macula and semicircular ducts synapse on vestibular nerve
- •Fibers end in vestibular nucleus in pons and medulla
- •Then to thalamus
- •Finally relayed to cerebral cortex for awareness of position and movement
-
DISORDERS ASSOCIATED WITH EQUILIBRIUM
- •Meniere’s Syndrome
- •May lead to deafness and a loss of equilibrium
- •Accumulation of excessive endolymph in the inner ear
- •Vertigo
- •Sensing motion when at rest
- •Unsual movement of endolymph in inner ear
- •Motion Sickness
- •Characterized by nausea and vomiting
- •Brought about by repetitive angular, linear or vertical movement
-
Chemical Senses-Taste
- Gustation-Sensation of taste
- -results from action of chemicals on taste buds
- Lingual Papillae
- -filiform (no taste buds)
- important for texture
- -foliate (no taste buds)
- -funigorm
- at tip and sides of tongue
- -Vallate (circumvallate)
- at rear of tongue
- contains half of taste buds
-
Physiology of Taste
- Molecules must dissolve in saliva
- 5 Primary Sensations-throughout tongue
- 1. Sweet-tip
- 2. Salty-Lateral margins
- 3. Sour-Lateral margins
- 4. Bitter-posterior
- 5. Umami-taste of amino acids (MSG)
- Influenced by food texture, aroma, tempurature, and appearance
- -mouthfeel -dectected by lingual nerve in papillae
- Hot pepper stimulates free nerve endings (pain)
-
Physiology of Taste (Mechanisms of Action)
- activate 2nd messenger systems
- -sugars, alkaloids, and glutamates bind to receptors depolarize cells directly
- -sodium and acids penetrate cells
-
Projection Pathways for Taste
- Innervation of taste buds
- -facial nerve (VII) -anterior 2/3 of tongue
- -glossopharyngeal nerve (IX)-posterior 1/3
- -vagus nerve (X)- palate, pharynx, epiglottis
- To medulla
- To hypothalamus and amygdala
- -activate autonomic reflexes eg salivation, gagging, vomiting
- To thalamus, then postcentral gyrus of cerebrum
- -conscious sense of taste
-
Chemical Sense-Smell
- Olfactory mucosa
- -contains receptor cells for olfaction
- -highly sensitive up to 10,000 odors
- -on 5cm2 of superior concha & nasal septum
-
Olfactory Epithelial Cells
- Olfactory Cells
- -olfactory hairs neurons with 20 cilia: bind molecules in thin layer of mucus
- -axons pass through cribrifrom plate, survive 60 days
- -Supporting cells
- -Basal Cells
-
Physiology of Smell
- Molecules bind to receptor on olfactory hair
- -hydrophilic -diffuse through mucus
- -hyrdophobic-transport by odorant-binding protein
- Activate G protein and cAMP system
- Opens ion channels for Na or Ca ions (creates receptor potential)
- Action potential travels to brain
- Receptor adapt quickly -due to synaptic inhibition in olfactory bulbs
-
Olfactory Pathway
- Olfactory cells synapse in olfactory bulb
- -on mitral and tufted cell dendrites
- -in spherical clusters called glomeruli (each glomeruli dedicated to single odor)
|
|
