7. 2 Pupillary Reflexes and Visual Pathways Flashcards
Pupil constriction: decreases glare and increases depth of focus. What is it mediated by?
Mediated by parasympathetic fibres within CN III
Pupil dilation – by pupil dilator muscle
• Increases amount of light entering eye
What is it mediated by?
Mediated by sympathetic fibres – noradrenaline acting on α1 receptors
what is the direct light reflex?
Constriction of the pupil of the light-stimulated eye
what is the consensual light reflex?
Constriction of the pupil of the non-light-stimulated eye
Pupillary light Reflex
Afferent
• Nerve impulses generated by _______________
• Photoreceptors synapse on ___________ which in turn synapse on ganglion cells
• Afferent impulses travel along the optic nerve, optic chiasm and optic tract
• Pupil-specific ganglion cells exit at the posterior third of the optic tract before the _________________- to synapse on nerve cells in the pretectal nucleus
• Each pretectal nucleus then connects with both _________________ which are the parasympathetic nuclei of the oculomotor nerve
Efferent
• The efferent pathway begins at the Edinger-Westphal nucleus in the brainstem
• Parasympathetic pre-ganglionic fibers travel through the oculomotor nerve – reach ______________ in the orbit
• Post-ganglionic parasympathetic fibers travel through the _________________ to reach the eyeball where they innervate constrictor pupillae muscle of the iris
• Pupillary constriction
photoreceptors on the retina;
bipolar cells ;
lateral geniculate nucleus;
Edinger-Westphal nuclei ;
ciliary ganglion;
short ciliary nerves
Damage to left optic nerve (CN II)
- _______________ when light is shone in the left eye. In unilateral afferent pupillary defect, there is impaired direct light reflex in the affected eye and impaired consensual light reflex in the unaffected eye
- ____________ when the right eye is stimulate with light
Both pupils show weak pupillary constriction;
Normal pupillary constriction in both eyes
Relative afferent pupillary defect (RAPD) – may be due to optic neuropathy or extensive neural damage
- Damage to the afferent pathway is usually incomplete, hence there is often still some degree of pupillary response still present when the damaged side is stimulated
- Elicited by the ___________________
(alternating stimulation of right and left eye with light, demonstrating weakened response in damage to the afferent pathway)
- Both pupils ___________ when light swings to left undamaged side
- Both pupils ________________ when light swings to the right damaged side
- Due to a relatively reduced drive for pupillary constriction in both eyes – optic nerve damage reduces sensory (afferent) stimulus sent to the midbrain
- Pupil responds less vigorously – dilates from its prior constricted state when the light is moved away from the unaffected eye and towards the affected eye
swinging torch test;
constrict;
paradoxically dilate
What constitute the near reflex?
1) Pupillary constriction- contraction of the constrictor pupilae
2) convergence- contraction of both medial recti muscles
3) Accomodation- thickening of the lens (contraction of ciliary muscle)
Near reflex
- Afferent: Through the visual pathway to the visual cortex
- Visual cortex is connected to the eye field of the frontal cortex’
- Cortical fibers descend to the oculomotor nucleus, the oculomotor nerve travels through the ____________ for convergence
- Some cortical fibers synapse with the ______________ on both sides. The parasympathetic fibers travel to the _______________ and subsequently through the ____________ and subsequently reach the constrictor pupillae muscle of the iris to bring about pupillary constriction and also to ciliary muscle also resulting in accommodation
medial recti;
Edinger-Westphal nucleus;
ciliary ganglion;
short ciliary nerves
The visual pathway is the neurological pathway whereby visual stimuli are converted to neurological signals and transmitted from the eye to the visual cortex in the ________________.
- Eye: extensions of the brain, it is connected to the brain via the ______________.
- Optic nerve is made out of _______________ with cell bodies originating in the retina. the optic nerves from both eyes converge in the optic chiasm.
- 53% of ganglion fibres cross over to the optic tract at the contralateral side and the rest stay on the optic tract of the ipsilateral side at the optic chiasm
- Ganglion nerve fibers which had already originated from the retina then synapse at the next order neuron at the _______________, a relay station in the thalamus.
- ______________ consists of 4th order neurons (anterior/lateral and posterior/medial bundles) relaying the signal from the lateral geniculate ganglion to the primary visual cortex within the occipital lobe of the brain for visual processing
- Primary visual cortex (striate cortex): within the _____________ of the occipital lobe then relays information to the Extrastriate cortex which is the region surrounding surrounding primary visual cortex within occipital lobe, involved in higher processing
occipital lobe of the cortex;
optic nerve (CN II);
myelinated ganglion nerve fibers;
lateral geniculate nucleus;
Optic Radiation;
calcarine sulcus
Retina – information from photoreceptors are passed along different order of neurones
= ________________ directly detects light and can be considered the first order neurones of the visual pathway
- Photoreceptors synapse upon the _________________, forming the second order neurons.
Third order retinal ganglion nerve fibers relay visual information out of the eye into the brain along the optic nerve. To improve signal transmission, retinal ganglion fibers become ______________ after entering the optic nerve. Half of the optic nerve fibers cross to the other side after entering the optic chiasm. Retinal ganglion fibers terminate at the______________ in the thalamus and synapse upon the 4th order neurons which relay visual information to the visual cortex
Photoreceptors;
retinal bipolar cells;
myelinated;
lateral geniculate nucleus
Optic chiasm – important landmark in the visual pathway
- Partial decussation of nerve fibers – 53% of ganglion fibers cross at the optic chiasm
- Lesions anterior to optic chiasm affect the visual field _____________ whereas lesions posterior to optic chiasm affect visual field ________________
Lesion posterior to optic chiasm affect visual field in both eyes because of the crossing of nerve fibers
- Partial decussation of nerve fibers
- 53% of ganglion fibers cross at the optic chiasm
- Crossed fibers originate from the _____________ retina of each eye and are responsible for _____________ visual field
- Uncrossed Fibers originate from the ___________ retina and are responsible for the ____________ visual field
in one eye only;
in both eyes;
nasal ;
temporal ;
temporal;
nasal
Retro-chiasmal visual pathway – due to partial decussation at the optic chiasm
- Optic tracts connect ____________ to lateral geniculate nuclei
- Each optic tract conveys nerve fibers from the ipsilateral temporal retina and contralateral nasal retina
- Lateral geniculate nucleus: relay station in the thalamus, receives information from both retinas
- Optic radiations : transverse the _____________ lobes, convey information from the lateral geniculate nucleus to the visual cortex in the occipital lobe
- Anterior/lateral bundle – pass though temporal lobe, transmits information of ________________ in lower retina, projects to __________________
- Posterior/medial bundle – at _____________, transmits information of inferior visual field in upper retina, projects to __________________-
optic chiasm;
parietal and temporal;
superior visual field;
lingual gyrus;
parietal lobe;
cuneus gyrus
Primary visual cortex – aka striate cortex,
- Situated along calcarine sulcus within the occipital lobe
- Characterized by a distinct stripe derived from the ____________________
- Role in neuronal processing of visual inputs including _______, ___________ and ___________
Representation
- Superior Visual Field projects to below the calcarine fissure
- Inferior Visual Field projects to above the calcarine fissure
- Right hemifield from both eyes projects to ________________
Left Hemifield from both eyes projects to the right primary visual cortex
- Disproportionately large area representing the _____________, located posteriorly. While most of the primary visual cortex is supplied by the posterior cerebral artery, the area represented macular function receives a dual blood supply where there is an anastomosis between the _______& _________occurring at the that site.
myelinated fiber of the optic radiation projecting into the cortex;
spatial frequency, orientation and direction;
left primary visual cortex;
macula;
posterior cerebral and middle cerebral arteries
Extrastriate cortex – area surrounding the primary visual cortex within the occipital lobe, converts basic visual information, orientation and position into complex information
Dorsal Pathway relays information from the primary visual cortex to the _________________
- Processes information about spatial orientation and movement
- Damage results in inability to _____________
Ventral Pathway relays information from the primary visual cortex to the _________________
- Processes information about recognition of objects, faces and colors
- damage results in cerebral ___________, an inability to perceive colour
posterior parietal lobe ;
perceive motion;
temporal visual cortex ;
acopsia
