Module 10: Special Senses Flashcards Preview

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Flashcards in Module 10: Special Senses Deck (313)
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271

• lack of accommodation with age which requires + glass to increase

Presbyopia

272

• too great a curvature of the cornea in one plane of the eye
• Multiple focal points

Astigmatism

273

• Rods and cones
• Photosensitive cells that convert chemical energy into electrical energy

Receptor cells

274

• horizontal direction
• Photoreceptors to bipolar cell
• Always inhibitory
• Mechanism for lateral inhibition
• High visual accuracy in transmitting contrast borders in the visual image

Horizontal cells

275

• Vertical direction
• From rods, cones and horizontal cells to the ganglion and amacrine cells

Bipolar cells

276

• interneurons that analyze visual signals before they leave retina
• 2 directions
- Vertically : Bipolar to ganglion cells
- Horizontally: within the inner plexiform layer

Amacrine cells

277

• Axons generate to the optic nerve
• Send repetitive action potentials to the CNS

Ganglion Cells

278

Rods and Cones

Rods
• Generally narrower and longer
• scoptic vision
• Low threshold for detecting light

Cones
• photopic vision
• High threshold for light
• For high VA and color vision

279

Peripheral vs. Central Retina

• Higher visual acuity in the central retina
- Longer and fewer rods and cones in the center and as the fovea is approached rods disappear
- Number of optic nerve fibers leaving this part of the retina is almost equal to the number of cones

• Greater sensitivity of peripheral retina to weak light
- Rods are more sensitive to light than cones
- More rods in the periphery
- As many as 200 rods converge on a single optic nerve fiber

280

• Transduction of signal requires hyperpolarization of rods and cones
• Light energy is absorbed to be detected by the retina (Visual pigment: rhodopsin)
• Rhodopsin = 11-cis retinal + opsin
• After absorption of light
- 11-cis retinal -> 11-trans retinal
- Retinal -> retinol

Visual Transduction

281

Photosensitive chemical: rhodopsin
Location: Outer segment
Opsin: scotopsin
Chromophore: 11 cis retinal

Rods

282

Photosensitive chemical: Color pigments
Location: Outer segment
Opsin: photopsin
Chromophore: 11 cis retinal

Cones

283

Pathway 1 (light state)

Precursor: All trans Retinal
Enzyme: Retinal isomerase
Final Product: 11 cis retinal

284

Pathway 2 (dark state)

Precursor: Retinol
Enzyme: Isomerase
Intermediate Product: 11 cis retinol
Final Product: 11 cis retinal

285

• Poor night vision due to deficiency of Retinol
• Retinol (Vitamin A) is derived from carotenoids
- cannot be synthesized
• Rhodopsin contains retinal (an aldehyde of retinol)

NIGHT BLINDEDNESS

286

Visual Transduction: DARKNESS

• Slightly depolarized
• cGMP-gated Na+ channels are open
• Continuous current of Na+ influx = Dark Current
• Glutamate is tonically released in the synapses

287

Visual Transduction: BRIGHT

• Light is absorbed
• photoisomerization of rhodopsin -> activates G protein transluscent -> activates cyclic guanosine monophosphate phosphodiesterase -> hydrolyzes cGMP to 5'-GMP
• lowers the cGMP concentration
• closing of the cGMP-gated Na+ channels
• hyperpolarization of the photoreceptor membrane

288

• Visual area seen by an eye at a given instant
• Nasal side – temporal field of vision
• Temporal side – nasal field of vision

Visual Field

289

• Charting of the field of vision
• Blind spot
- Lack of rods and cones in the retina over optic disc
- 15 degrees lateral to the central point of vision

Perimertry

290

• Damage to the optic nerve
• Glaucomas, immune reactions in the retina or toxic conditions
• Blind spot in portions other than the optic disc

SCOTOMA

291

• Degeneration of parts of retina
• Excessive melanin deposits
• Blindness in the peripheral field of vision first and then gradually on the central area

RETINITS PIGMENTOSA

292

Visual Pathway

Visual Field -> Retina ganglion -> Optic Nerve
-> Optic Chiasm -> Optic Tract -> Lateral Geniculate Nucleus -> Optic Radiations -> Primary Visual cortex (BA 17/Striate cortex)

293

(Visual Pathway and Visual Defects)

Target is reversed in the retina by the lens system

• Left Visual Field: L nasal and R temporal
• Right Visual Field: R nasal and L temporal

294

(Visual Pathway and Visual Defects)

Projection may be crossed or uncrossed

• Axons at temporal: uncrossed
• Axons at nasal: crossed at optic chiasm

295

(Visual Pathway and Visual Defects)

Some fibers go to temporal lobe as Meyer’s Loop

• Lower retinal quadrant: superior visual field

296

(Visual Pathway and Visual Defects)

Fibers that pass through parietal lobe

• Contralateral lower visual field

297

(Visual Pathway and Visual Defects)

Optic radiation ends in the Stiate Cortex

Cuneus
- Dorsal of fissure
- upper part of hemiretina

Lingual gyrus
- Ventral of fissure
- lower part of hemiretina

298

• Lies in the calcarine fissure extending towards the occipital pole
• Medial to each occipital cortex
• Brodmann’s area 17

Primary Visual Cortex

299

• Visual association areas
• Where analysis and meaning to visualized objects occur
• Lateral, anterior, superior and inferior to the primary visual cortex
• Brodmann’s area 18

Secondary Visual Cortex

300

• Helps the eye adapt to extremely rapidly changing light conditions
• Impulses from retina are sent to Edinger-Westphal nucleus causing constriction of iris.
• Darkness: inhibition of reflex causing mydriasis

Pupillary Light Reflex