AN61.1-3 | Midbrain — Glossary

Dorsal midbrain structure involved in visual reflexes and the visual startle response; lesion in Parinaud syndrome

Dorsal midbrain auditory relay centre connecting the lateral lemniscus to the medial geniculate body

Crus cerebri — large ventral midbrain structure containing descending fibres: corticospinal (middle 3/5), corticopontine, and corticobulbar

Supplies all extraocular muscles except lateral rectus and superior oblique; also carries parasympathetic fibres for pupil constriction and accommodation

Parasympathetic nucleus adjacent to CN III; sends preganglionic fibres to the ciliary ganglion for pupil constriction and accommodation

Large pigmented (iron) nucleus in the midbrain tegmentum; receives dentatorubral fibres, gives rise to the rubrospinal tract; lesion causes contralateral tremor

Structure between midbrain tegmentum and basis; pars compacta produces dopamine (loss causes Parkinson's), pars reticulata is GABAergic output

PAG — grey matter surrounding the cerebral aqueduct; involved in pain modulation and the descending opioid analgesic pathway

Ventral midbrain syndrome from basis pedunculi infarct: ipsilateral CN III palsy + contralateral hemiplegia

Midbrain tegmental syndrome: ipsilateral CN III palsy + contralateral involuntary movements (red nucleus) + contralateral sensory loss (medial lemniscus)

Dorsal midbrain syndrome from pineal tumour compressing the tectum: paralysis of upward gaze, light-near dissociation, convergence-retraction nystagmus

Only cranial nerve exiting from the dorsal brainstem; thinnest CN; supplies the superior oblique muscle (intorts, depresses, abducts the eye)

Pupils do not react to light but constrict on accommodation; seen in Parinaud syndrome and Argyll Robertson pupils

Berry aneurysm at the ICA-PCOM junction; classic cause of compressive CN III palsy with fixed dilated pupil — neurosurgical emergency

CN III palsy with normal pupil; suggests ischaemic cause (diabetes/hypertension) affecting the core fibres while sparing peripheral parasympathetics

Triangular depression between the cerebral peduncles containing the interpeduncular cistern and posterior perforated substance

Four bumps on the dorsal midbrain (two superior + two inferior colliculi); collectively form the tectum

Aqueduct of Sylvius — narrow channel through the midbrain connecting the 3rd and 4th ventricles; stenosis causes obstructive hydrocephalus

Descending motor tract from the red nucleus facilitating limb flexor movements; crosses immediately after origin

Germinoma — most common pineal region tumour in Indian adolescents; compresses the dorsal midbrain causing Parinaud syndrome