Pretectal (olivary pretectal) nucleus
Relay nuclei · Light-near dissociation
Midbrain nucleus at the level of the superior colliculus that receives pupillomotor fibres from both optic tracts through the brachium of the superior colliculus. Each pretectal nucleus projects to both Edinger-Westphal nuclei, the crossing fibres running in the posterior commissure, which is why the light reflex is consensual.
Traced from the start
- Temporal hemiretinaNasal hemiretinaMacula and papillomacular bundle
- Optic nerve (CN II)
- Optic tract
- Pretectal (olivary pretectal) nucleus
Detail
- Input
- Both optic tracts, so light in either eye drives both pretecta
- Output
- Bilateral, to both Edinger-Westphal nuclei via the posterior commissure
- Bypasses
- The LGN and cortex, so the reflex is intact in cortical blindness
- Near response
- Driven by a separate cortical pathway to Edinger-Westphal, not through the pretectum
When it goes wrong
Afferent lesion anywhere from retina to optic tract (e.g. optic neuritis)
Relative afferent pupillary defect: on the swinging-torch test both pupils dilate when light moves to the affected eye because pretectal drive falls; pupils remain equal in size because the output is bilateral
Dorsal midbrain (Parinaud) syndrome from a pineal tumour or hydrocephalus compressing the pretectum and posterior commissure
Mid-dilated pupils with light-near dissociation, upgaze palsy, convergence-retraction nystagmus and lid retraction
Neurosyphilis damaging the pretectal and periaqueductal region bilaterally
Argyll Robertson pupils: small irregular pupils that constrict to near but not to light
Practise this structure
1 question in the bank tagged Reflexes.