Visual Transduction

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Visual Transduction. Once light waves have been successfully focused on the retina, the information “stored” in that electromagnetic energy must be changed by photopigments in the photoreceptors into signals our brain can interpret - a process called visual transduction - PowerPoint PPT Presentation

Text of Visual Transduction

Chapter 17

Visual TransductionOnce light waves have been successfully focused on the retina, the information stored in that electromagnetic energy must be changed by photopigments in the photoreceptors into signals our brain can interpret - a process called visual transductionThe single type of photopigment in rods is rhodopsin, whereas there are 3 different cone photopigments Color vision results from different colors of light selectively activating the different cone photopigments

Visual TransductionThe first step in visual transduction is absorption of light by a photopigment, a colored protein that undergoes structural changes when it absorbs light in the outer segment of a photoreceptorLight absorption initiates a series of events that lead to the productionof a receptor potential (number 4 in the diagram)Visual TransductionAll photopigments associated with vision contain two parts: a glycoprotein known as opsin and a derivative of vitamin A called retinalAlthough there are 4 different opsins, retinal is the light-absorbing part of all visual photopigmentsTo simplify the process we can say that there is a cyclical bleaching and regeneration of photopigmentBleaching is a term describing a conformational change in the retinal molecule in response to lightVisual TransductionIn darkness, retinal has a bent shape called cis-retinalAbsorption of a photon of light causes it to straighten into the trans-retinal form in a process called isomerizationTrans-retinal completely separates from the opsin; since the final products look colorless, this part of the cycle is called bleaching of photopigmentAn enzyme converts trans-retinal cis-retinalThe cis-retinal regenerates the photopigmentIn darkness, the neurotransmitter glutamate is released keeping Na+ channels open and inhibiting the bipolar cell: This inflow of Na+is called the dark current. Photons cause Na+ channels to close, and the rod hyperpolarizes. Its strange that when your eyes are closed and you are asleep the rods are the most active.

4Bleaching and regeneration of photopigments are summarized here

Horizontal cells transmit inhibitory signals to bipolar cells in the areas lateral to excited rods and cones.5Visual TransductionIn daylight, regeneration of rhodopsin cannot keep up with the bleaching process, so rods contribute little to daylight vision. In contrast, cone photopigments regenerate rapidly enough that some of the cis form is always present, even in very bright lightAs a consequence, light adaptation (from dark conditions light conditions) happens in seconds; dark adaptation (from light dark) takes minutes to occur (up to 40 minutes to fully adapt)

After complete bleaching, regeneration of half of the rhodopsin takes 5 minutes; half of the cone photopigments regenerate in only 90 seconds. Full regeneration of bleached rhodopsin takes 30 to 40 minutes.6Visual TransductionMost forms of color blindness, an inherited inability to distinguish between certain colors, result from the absence or deficiency of one of the three types of conesMost common type is red-green color blindness in which red cones or green cones are missingProlonged vitamin A deficiency and the resulting below-normal amount of rhodopsin may cause night blindness or nyctalopia, an inability to see well at low light levels

The graded potentials generated by the photoreceptors undergo considerable processing at synapses among the various types of neurons in the retina (horizontal cells, bipolar cells, and amacrine cells)- certain features of visual input are enhanced while others are discardedOverall, convergence pre-dominates as 126 million photo-receptors impinge on only 1 million ganglion cellsThe Visual PathwayHorizontal cells transmit inhibitory signals to bipolar cells in the areas lateral to excited rods and cones. Horizontal cells also assist in the differentiation of various colors. Amacrine cells, which are excited by bipolar cells, synapse with ganglion cells and transmit information to them that signals a change in the level of illumination of the retina. When bipolar or amacrine cells transmit excitatory signals to ganglion cells, the ganglion cells become depolarized and initiate nerve impulses.8

The Visual PathwayThe axons of retinal ganglion cells provide output that travels back towards the light, exiting the eyeball as the optic nerve, which emerges from the vitreous surface of the retinaThe axons then pass through a crossover point called the optic chiasm

Some axons cross to the opposite side, while others remain uncrossed. Once through the optic chiasm the axons enter the brain matter as the optic tracts (most terminate in thalamus)Here they synapse with neurons that project to the 1o visual cortex in the occipital lobesThe Visual Pathway

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Visual field ofleft eyeTemporalhalfVisual field ofright eyeTemporalhalfNasalhalfMidbrainLeft eyeTemporalretinaOpticradiationsLeft eye and its pathwaysOptictractPrimary visual area of cerebralcortex (area 17) in occipital lobeLateral geniculate nucleusof the thalamusOpticradiationsMidbrainTemporalretinaNasalretinaRight eyeRight eye and its pathwaysNasalhalfNasal retina1122445533

Visual field ofleft eyeTemporalhalfVisual field ofright eyeTemporalhalfNasalhalfMidbrainLeft eyeTemporalretinaOpticradiationsLeft eye and its pathwaysOptictractPrimary visual area of cerebralcortex (area 17) in occipital lobeLateral geniculate nucleusof the thalamusOpticradiationsMidbrainTemporalretinaNasalretinaRight eyeRight eye and its pathwaysNasalhalfNasal retina112324345566

The EarAudition, the process of hearing, is accomplished by the organs of the ear. The ear is an engineering marvel because its sensory receptors can transduce sound vibrations with amplitudes as small as the diameter of an atom of gold into electrical signals 1000 times faster than the eye can respond to lightThe ear also contains receptors for equilibrium

The EarThe ear has 3 principle regionsThe external ear, which uses air to collect and channel sound wavesThe middle ear, which uses a bony system to amplify sound vibrationsThe internal ear, which generates action potentials to transmit sound and balance information to the brain13The pathways for sound transmission starts in the air, the to the solid bone of the middle ear, then to the endolymph of the inner ear.

The External EarThe anatomy of the external ear includesThe auricle (pinna), a flap of elastic cartilage covered by skin and containing ceruminous glandsA curved 1 long external auditory canal situated in the temporal bone leading from the meatus to the tympanic membrane (TM or ear drum) which separates the outer ear from the cavity of the middle ear

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The middle ear is an air-filled cavity in the temporal bone. It is lined with epithelium and contains 3 auditory ossicles (bones)The stapes (stirrup)The incus (anvil)The handle of the malleus (hammer) attaches to the TM

The Middle Ear

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The Middle EarTwo small skeletal muscles (the tensor tympani and stapedius) attach to the ossicle and dampen vibrations to prevent damage from sudden, loud sounds16The Middle Ear

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The Middle EarThe Eustachian (auditory) tube connects the middle ear with the nasopharynx (upper portion of the throat)It consists of bone and hyaline cartilage and is normallypassively collapsed. It opens to equalize pressures on each sideof the TM(allowing itto vibrate freely)The chamber of the middle ear is continuous through the eustachian tube with the nasopharynx, but also with the mastoid antrum and mastoid air cells. Infection of the mucosa lining the middle ear will extend to the mastoid air cells in the bone behind the ear.18The Inner EarThe internal ear (inner ear) is also called the labyrinth because of its complicated series of canalsStructurally, it consists of two main divisions: an outer bony labyrinth that encloses an inner membranous labyrinththe bony labyrinth is sculpted out of the petrous part of the temporal bone, and divided into three areas: (1) the semicircular canals, (2) the vestibule,and (3) the cochlea

19The oval window starts the inner ear. As the stapes rocks back and forth the oval window and round window oscillates (like pushing on the end of a waterbed).

The Inner EarThe vestibule is the middle part of the bony labyrinth The membranous labyrinth in the vest