For a really good demonstration of how this works, see Eye simulator V2.0
- Optic Nerve (II) – this is the afferent nerve – it detects the light, and sends this signal to the brain. HOWEVER – a defect in the afferent nerve does not necessarily have to be in the optic nerve itself, it could be at the optic chiasm (likely to be bilateral), or in the optic tract. Anywhere along the afferent pathway.
- Occulomotor nerve (III) – this is the efferent nerve – when the brain receives a signal about strong light from EITHER EYE – it will send a signal down both occulomotor nerves, telling the pupil to constrict
Afferent pathway (red line)
- Action potential generated in optic nerve
- Action potential travels through right and left lateral geniculate bodies
- Axons synapse at right and left pre-tectal nuclei
Efferent pathway (green line)
- Action potential passes to right and left Edinger-westphal nuclei of CN III (oculomotor nerve) via interneuron’s (black lines with arrows)
- Then to the right and left ciliary ganglions via oculomotor nerve
- Pupil being illuminated (A) = direct reflex
- Contra lateral pupil (B) = consensual reflex
Marked – shining the light in the affected eye will result in no constriction of either pupil.
- E.g – a RAPD in the left eye – shining the light in the left eye will result in no constriction of either pupil. Shining the light in the right eye will result in constriction of both pupils.
- This is due to damage to the left afferent pathway
- Bilateral presentation – neither pupil will contract in response to light in either eye.
Mild – shining the light in the affected eye will result in a slow constriction of both pupils, but then a gradual dilation of both pupils.
- There is ptosis
- The pupil is naturally quite constricted. It will still respond relatively to light (ie. It will constrict further, and will dilate somewhat in the dark), but its generally ‘resting’ state is constricted to a greater degree than a normal pupil.
Third Nerve (Efferent) Palsy
- An eye that points down and out
- A dilated pupil that does not respond to light in either eye.
- The contralateral eye will respond to light in the affect eye. And the contralateral eye will respond normally to its own light.