Flashcards
NS 38 + 39 - Optical Pathways and Lesion + Retinal Function and Visual Perception
Cards in this deck (58)
- 1Visual system parts (2)sensory organ (eye)
contains optical elements and photoreceptors
corresponding neural system in the brain
optic tract, nuclei and visual cortex - 2Function of visual systemdetect and interpret photic stimuli
EM waves between 400-750nm
(purple = 400, red = 750) - 3Structures of the eye (8)Sclera - outer coat, connective tissue
Choroid - nutritional layer + light absorbing pigments
Retina - photoreceptors and neurons
Cornea - most anterior surface, transparent window, main light refraction point
Aqueous humour - between cornea/lens, produced by ciliary
Vitreous humour - behind lens, produced by retinal cells
Iris - pupil diameter (contraction of radial + longitudinal smooth muscle fibres)
Lens - focal length (ciliary muscle contraction)
Macula - region containing fovea (highest visual activity) - 4Convex surfaces converging light rays to a focal point (2)cornea (2/3) convex shape but can't be altered
lens (1/3) biconvex, refractive power can be adjusted - 5Accommodationprocess by which lens alters focal length
- 6Control of refractionCiliary muscles
distant vision (light less divergent)
= need flattened lens = ciliary muscle relaxes = suspensory ligaments pull outwards
near vision (light more divergent)
= more convex lens = ciliary muscle contracts = suspensory ligament tension reduced (mediated by parasympathetic) - 7Image inversionimage on retina is inverted vertically and laterally -> corrected by brain
- 8Iris?rings of circular (middle) and radial (outer) smooth muscle
gives eye colour - 9Pupil?round opening in middle of iris
- 10Effects of pupil on vision (2)level of light
constriction reduces light entering eye
ensures photoreceptor stimulation is optimal
depth of focus
constriction increases depth of focus (cuts out peripheral light, distant image) - 11Retina structureseveral cell layers
1st = photoreceptors (rods and cones) located to external side - light passes through all layers of retina before reaching photoreceptors
2nd = bipolar cells
3rd = ganglion cells - 12Fovearegion with highest visual acuity
area where most photoreceptors are - 13Retina - how many: - photoreceptors - bipolar cells - ganglion cells130 million
6 million
1 million - 14Photoreceptors Rods Cones what do they detect?photons
light intensity
colour + provide sharper image - 15Location of rods and conesrods - peripheral areas
cones - highest density in macula (fovea contains only cones) - 16internal structure of conesouter segment = series of membranous plates (discs) - specialised receptor region
Inner segment = mitochondria + other organelles - 17signal route from photoreceptor to optic nervephotoreceptor -> bipolar cell -> ganglion cell -> optic nerve
- 18Rod characteristicshigh sensitivity to light
low acuity - ability to distinguish between 2 nearby points - many rods converge onto same ganglion cell
do not contribute to colour vision - 19Neurotransmitter in RodsGlutamate - normally excitatory but inhibitory here
- 20Photoreceptors in light conditionscGMP levels low = cGMP gated Na+ channels closed = less Na+/Ca2+ influx = less dark current hyperpolarised to -60mV = no glutamate release = excitation of neurons
- 21Photoreceptor in darkcGMP levels high = cGMP gated Na+ channels open = steady inward current (dark current) = cell kept depolarised at -40mV = glutamate release = inhibit excitation of neurons
- 22Rhodopsin what is it structurephotosensitive pigment on membrane of out segment of photoreceptor cell
retinal (vit A derivative) + opsin + chromophore - 23Effect of light on rhodopsinretinal undergoes conformational change (cis -> trans) = doesn't fit with opsin so opsin becomes metarhodopsin
- 24Macular degenerationloss of photoreceptor cells - blindness
- 25photoreceptor synapse effect of lighton-centre + off-centre
light = membrane hyperpolarises = on-centre bipolar cell fires because reduction of glutamate release from photoreceptor = release of NT from bipolar cell increases = generates AP in ganglion cell - 26phototransductioncGMP maintains Na+ leak into rods + NT release
absorption of light changes cis-retinal to trans-retinal = reduce cGMP = Na+ leak stops
membrane hyperpolarises, rate of glutamate release lowered = signal transmitted - 27Cones characteristics classesLow sensitivity - needs hundreds of photos for response
High acuity - less convergence in 'wiring patterns'
3 classes blue (S) green (M) red (L)
population = 16% blue (430nm), 10% green (530nm), 74% red (600nm) - 28Colour blindness cause inheritance pattern symptomInherited absence of 1 or more class of cone pigment
L and M cone pigment found on X chromosome
X-linked recessive - more common in men
Inability to distinguish between red and green - 29Dark adaptationlight = insufficient to activate cones = must wait for regeneration of rod rhodopsin (20-30mins)
= scotopic vision (colours not distinguished) - 30Light adaptationrod cells 'burn out' = pigment bleaches and brain switches to cone-only information processing
- 31Visual pathwayoptic nerve - formed by ganglion cell axons
optic chiasm - where axons cross midline - medial (nasal) decussate, temporal do not
optical tract - signals of left eye go to right side of brain etc
lateral geniculate nucleus (in thalamus) - tracts synapse here
optic radiation - project to visual cortex
primary visual cortex - occipital lobe - 32Cortical processingfrom lateral geniculate nucleus, optic radiation projects different visual info to different regions of visual cortex
cortical neurons fire when they receive particular pattern of illumination
re-inversion of image - 33Further visual processingparietal lobe = perception of movement
temporal lobe = perception of shape/colour - 34Visual system components (4)Eye
Optic nerve (CN II)
Lateral geniculate nucleus
visual cortex - 35Visual pathway (v long but vague af kms)cornea + lens refract light into a small image and reflect it onto retina
retina transduces image into electrical signals using rods/cones
optic nerve carries signals through optic canal and into cranium
at optic chiasm, medial fibres from both eyes decussate to opposite sides, lateral fibres from each side remain on same side
R+L fibres continue down optic tracts = wrap around cerebral peduncles and divide into lateral and medial roots into respective sides of brain:
lateral = LGN, medial = pretectal area + superior colliculus
these go to visual cortex for processing - 36size of optic canals: infraorbital intracanalicular intracranial25mm
9mm
16mm - 37Role of lateral geniculate nucleusconscious visual sensation
- 38Role of pretectal areabehavioural responses to changes in ambient light (pupillary reflex) + temporary changes to circadian rhythm
- 39Role of superior colliculusmajor part of midbrain
directs eye movements:
- visual grasp reflex
- automatic scanning of images
visual association pathways - 40Role of visual cortexprocessing image - making sense of what you see
- 41Optic nerve damagemonocular blindness
R optic nerve damage = completely blind in R eye - 42Optic chiasm damageBitemporal hemianopsia
partial blindness - vision missing from lateral half of L + R visual fields - 43Optic tract damageHomonymous hemianopsia
partial blindness - vision missing from lateral half of 1 eye and medial half of other eye
R optic tract = L lateral visual field gone, R medial visual field gone - 44Normal limits of visual fieldsuperior = 60deg
Inferior = 75deg
temporal (lat) = 100deg
nasal (med) = 60deg - 45Striabmusone eye looks at object you are viewing, other eye is misaligned in/out/up/down
- 46Causes of striabmus (3)poor vision in 1 eye (most common)
eye physically stuck (post traumatic)
muscle weakness (CN palsy) - 47Inward striabmusesotropia
- 48Outward striabmusexotropia
- 49Upward striabmushypertropia
- 50Downward striabmushypotropia
- 51Corneal reflexblink reflex - involuntary blinking of eyelids by stimulation of cornea
- 52Corneal reflex afferent + efferentafferent - opthalmic (Va)
efferent - facial (VII)
- stim orbicularis oris - 53Pupillary light reflexinvoluntary contraction of sphincter pupillae muscle of iris to control amout of light entering pupil
consensual = light in one eye causes other pupil to also contract - 54Pupillary light reflex afferent + efferentafferent - optic nerve -> pretectal nucleus (above superior colliculus) -> neurons project bilaterally to Edinger Westphal nuclei (in midbrain)
efferent - oculomotor nerve -> cilliary ganglion -> sphincter pupillae muscle - 55Ciliary ganglion position sympathetics parasympatheticsjust lateral to optic nerve
from superior cervical ganglionm hitches ride with V1
sympathetic -> dilator pupillae
= good long-distance vision in fight/flight
parasympathetic -> spinchter pupillae (constrictor)
-> ciliary muscle (relax ligament = lens thicken -> for accommodation reflex) - 56Accommodation reflexallows us to focus on near objects as result of changes in pupil (near = constrict, far = dilate) + lens (near = more convex, far = less convex)
- 57Argyll-Robertson pupilcan accommodate, but no pupillary light reflex
sign of neurosyphilis - lesion in pretectal nucleus - 58Summary of visual pathwayRetina
Optic nerve
Optic chiasma
Optic tract
Lateral geniculate body
Optic radiation
Visual cortex (association fibres)
Frontal lobes (descend via anterior limb internal capsule)
Superior colliculus of midbrain
EW of midbrain (bilateral)
Oculomotor nerve
Ciliary ganglion
Muscles of iris + ciliary body
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- JC3 (Neuroscience ) Flashcards on NS 38 + 39 - Optical Pathways and Lesion + Retinal Function and Visual Perception, created by Erica Lai on 01/12/2018.
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