Flashcards
12 Visual Pathways and Reflexes
Cards in this deck (30)
- 1Describe the general pathway of the optic nerve, stating which cranial nerve this is alsoCNII
eye - optic chiasm - optic tract - 2Describe the LG-occipital cortex pathwayVisual Recognition
retinal ganglion cells supply axons that constitute CNII
most axons project to LGN (visual thalamic relay)
LGN send axons in optic radiation to primary visual cortex (area 17) in occipital lobe
damage to pathway - blindness - 3Describe the superior colliculus pathwayVisual Tracking and Fixation
some CNII fibres project to SC (aka optic tectum)
-paired structure, below thalamus, surrounds pineal glands, in midbrain
SC connects to MLF - links together and synchronises oculomotor nuclei
test - "follow my finger"
damage - PT has difficulty fixating on an object and tracking moving object - 4Some fibres from SC also project to...Edinger-Westphal nucleus
-Pupillary light reflex - 5Explain about the two divisions of optic radiationupper
-LGN to cuneus (upper part of calcarine sulcus)
lower
-loops from LGN anteriorly (Meyer's loop) then posteriorly to end in lower part of calcarine sulcus = lingual gyrus
-loops then runs alongside posterior horn of lateral ventricle - 6What makes up the left and right visual fields?LVF = everything to the left of a fixation point
RVF =everything to the right of a fixation point
both eyes contribute to both visual fields - 7How does the LVF project to the right visual cortex and vice versa?nasal hemiretina from left eye --> RVC
temporal hemiretina from right eye --> RVC
both hemiretina view objects in LVF
(and vice versa) - 8What unique condition may arise when a patient loses visual cortex?loss of visual cortex but patient still insists they can see
Anton-Babinski syndrome
-patient confabulates their visual surroundigns - 9Visual pathways to the brainstem mediate visual reflexes. These can persist although visual perception doesn't exist. What are the four key reflexes?pupillary light reflex
accommodation reflex
vestibule-ocular reflex
blink reflex - 10Explain the pupillary light reflex. What is capable of blocking the pupillary light reflex?In on II out on III
CNII axons to pretecal nucleus in SC
axons to PSN E-W nucleus
out on CNIII to ciliary ganglion
axons to pupillary constrictor muscle
constriction reduces pupil size
atropine - 11In what situations can an accommodation reflex occur? What is this reflex controlled by?conscious person
controlled by frontal eye fields - specialised parts of premotor area that are dedicated to motor control of extraocular eye muscles - 12What two things arise as a result of damage to frontal eye fields?inability to direct gaze from one object to another
loss of fast phase of nystagmus - 13How do we normally change our focal point?in a series of fast flick, simultaneous movements of both eyes between two points of fixation
=saccades
we are not aware of our own saccades and visual system switched off during saccade - 14Describe how ciliary muscle acts as a sphincterat rest lens under tension from ring of suspensory ligaments which stretch and flatten it away from its relaxed shape
ciliary muscle = sphincter
when it contracts it reduces diameter of ring
tension slackens in suspensory ligaments and lens bulges out to its relaxed shape - 15State the three essential parts of the accommodation reflexpupillary constriction
thickening of lens
convergence - 16Describe pupillary constrictionPC output pathway same as pupillary light reflex
input to cause constriction is from descending projections from FEF in premotor cortex
PC improves focusing on close objects - depth of field - 17Describe thickening of the lensthickening to focus on near objects
tension in SL reduced and lens thickens passively towards normal shape
output - CNIII (parasympathetic part) - 18Describe and explain the differences in the lens for distant and close visionlens at minimum strength for distant vision
lens at greater strength for close vision
distant
-light rays from distant objects nearly parallel so don't need as much refraction to bring them to focus
close
-light rays from close objects diverge and require more refraction for focusing - 19Describe the process of convergenceinvolves simultaneous activity in two medial rectus muscles (rotates eyes inwards during convergence movements)
mediated by CNIII
occurs as a results of visual cortex input to oculomotor nucleus
can't be triggered in unconscious patient
if occurs incorrectly or not at all --> diplopia - 20What happens if you turn your head to the side whilst looking at distant objects?eyes rotate in opposite direction to head (exactly opposite amounts) to keep direction of gaze constant and object stays in view
=vestibule-ocular reflex
aka
oculocephalic reflex/dolls eye reflex - 21State the input and output of the vestibulo-ocular reflexinput
-CNVIII (receives signals from semicircular canals)
output
-CNVI and CNIII (moving eye outwards and inwards respectively)
-cooperation and coactivation between oculomotor nerves III, IV and VI fundamental to all eye movements - 22Describe the basics of the vestibule-ocular reflexdirection of rotation
inhibition of extraocular muscles on one side
excitation of extraocular muscles on other side
compensating eye movement - 23How can the vestibulo-ocular reflex be tested?hold someone's eye open and gently rotate head from side to side
caloric stimulation test
-warm water irrigated into exteral auditory canal with syringe
(temp difference between body and water creates convective current in edolymph of nearby horizontal semicircular canal - 24Describe the outcome of the caloric stimulation testwarm water
-mimics head turn to ipsilateral side
-both eyes turn slowly away from irrigated ear toward contralateral ear followed by horizontal fast flick (nystagmus) towards irrigated ear
cold water
-mimics head turn to contralateral side
-both eyes turn slowly towards ipsilateral ear with nystagmus towards non-irrigated ear
COWS
-cold - flick to other ear
-warm - flick to same ear - 25Define the term nystagmus. How is this elicited?combination of initial slow rotation followed by fast flick back
-direction determined by direction of fast flick
elicited by spinning someone around for a few minutes
-fluid in semicircular canal has inertia and doesn't rotate at first
-after a while friction with walls starts rotation of fluid
-when rotation stops fluid continues to rotate inducing post-rotational N
-slow component of N follows movement of fluid so direction of fast phase of N is against direction of rotation - 26Describe the role of the semicircular canals in generating compensatory movementssemicircular canals detect movement in any axis, send signals to three oculomotor nuclei to generate compensatory movements
-signals from vestibular apparatus fed into MLF which directs activity in appropriate oculomotor nuclei - 27Describe the blink reflexprotects eye from foreign bodies e.g. grit
input
-sensory nerve endings in cornea or conjunctiva - branches of ophthalmic V3 branch of CNV
output
-motor fibres in CNVII to the orbicularis oculi muscle which pushes the eyelids together - 28What is meant by the term macula sparing?central parts of the eye (macula and fovea) have a bilateral projection to the visual cortex, so a lesion on one side will often not affect central vision
- 29Describe the different routes of fibres mediating vision from upper and lower quadrants (from LGN to primary visual cortex)vision from upper quadrants
-fibres loop laterally around inferior horn of lateral ventricle - Meyer's loop
vision from lower quadrants
-fibres travel straight back in posterior limb of internal capsule to the visual cortex - 30What is the result of the following lesions partial optic nerve complete optic nerve optic chiasm AT part of Meyer's loop optic radiation visual cortexipsilateral scotoma
blindness in that eye
bitemporal hemianopia
homonymous hemianopia
homonymous upper quadrant anopia (axons arranged with upper part of visual field anterior)
homonymous hemianopia
homonymous hemianopia (Macular sparing)
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- University Brain and Behaviour Flashcards on 12 Visual Pathways and Reflexes, created by Ifeoma Ezepue on 02/03/2016.
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