Pupillary reflexes (light reflex, accommodation, RAPD) Flashcards
Describe the pupillary light reflex pathway
Afferent pupillary fibres start at the retinal ganglion cell layer and travel through optic nerve, optic chiasm, optic tract and then to the pretectal area of the midbrain
The pretectal area sends fibres BILATERALLY to the efferent Edinger- Westphal nuclei of the oculomotor complex
From the EW nuclei efferent pupillary parasympathetic preganglionic fibres travel on the oculomotor nerve to synapse in the ciliary ganglion
The ciliary ganglion sends parasympathetic postganglionic fibres to innervate the sphincter pupillae via M3 muscarinic receptors
Due to bilateral innervation of EW nuclei, a direct and consensual pupillary response is produced
All postganglionic parasympathetic fibres hitchhike on branches of what cranial nerve?
Trigeminal
The accommodation reflex is required for…
Near vision
What are the 3 C’s in relation to the accommodation reflex?
Convergence
Pupillary constriction
Convexity of the lens to increase refractive power
Cerebral cortex must be involved because it is relating to image analysis
What muscle causes convergence?
Medial rectus
What muscle is responsible for pupil constriction?
Sphincter pupillae
What muscle is responsible for changing shape of lens?
Ciliary muscle
Does the light reflex involve any cortical areas?
No it is a simple reflex
Describe the pathway of the accommodation reflex
Afferent arm: retina and fibres in optic nerve, synapse in LGN which projects to visual cortex - patient processes image getting closer to face
Projections in visual cortex go to EW nucleus and CN III
EW nucleus stimulates ciliary ganglion and therefore sphincter pupillae contraction and ciliary muscle contraction thickening the lens
Activation of CNIII nucleus that controls medial recti to contract leading to convergence response
Stimulation of the right afferent pupillary fibres will cause bilateral or unilateral pupillary constriction?
Bilateral
If the right optic nerve is severed what would happen in regards to pupillary response?
No afferent conduction from right eye - no right direct or left consensual response from shining light into right eye
A consensual response would remain in right eye when shining light into left (this is because the efferent system for right eye is still intact and can be innervated by afferent fibres from left eye)
What would happen to pupillary response if right optic nerve was damaged incompletely?
A sluggish pupillary response when light shined into right eye (reduced afferent innervation)
Normal right pupillary response when light shone into left eye (the right eye efferent system is intact)
This may be difficult to determine, so can exaggerate by swinging light from left to right eye while observing pupil we are shining light into
If both afferent arms of the pathway are intact what would you expect to happen to pupils when swinging the light between two eyes?
The constriction response to be equal and maintained
If the right optic nerve is damaged incompletely and the light is presented from left to right, what would happen to the right eye?
It would dilate! This is right relative afferent pupillary defect
The afferent information from left eye which no longer has light shone into it is demanding dilation rather than constriction. The damaged afferent system of right eye is overpowered by the intact afferent system of left, so instead of constricting it dilates
What can cause afferent defects?
Optic neuritis
Optic atrophy
Retinal disease
