Organ
Midbrain
Eye movement and motor circuits above the pons
High-yield
Look for the aqueduct, colliculi, red nucleus, substantia nigra and nuclei associated with cranial nerves III and IV. The aqueduct connects the third and fourth ventricles and can become obstructed.
Overview
The midbrain lies between the pons and diencephalon. Its aqueduct separates the dorsal tectum from the tissue in front. The tegmentum includes the red nucleus and periaqueductal gray; the substantia nigra separates it from the more ventral crus cerebri. The tectum contains the paired superior and inferior colliculi.
Reference facts
- Location
- Between the pons below and diencephalon above, within the opening of the tentorium.
- Parts
- Ventral crus cerebri, substantia nigra, tegmentum, aqueduct and dorsal tectum; the tectum bears superior and inferior colliculi.
- Blood supply
- Small branches from the posterior cerebral, superior cerebellar and basilar arterial systems, with overlapping territories.
- Relations
- The tentorial margins lie nearby. CN III emerges ventrally between the cerebral peduncles; CN IV leaves dorsally. Transtentorial displacement can compress the midbrain and adjacent structures.
- Function
- Contributes to eye movements, pupil responses, orienting to visual and auditory signals, movement regulation and descending modulation of pain.
Clinical correlation
A lesion involving ventral midbrain CN III fibers and the corticospinal pathway can cause ipsilateral third-nerve palsy with contralateral weakness (Weber syndrome). Dorsal midbrain disease can impair upgaze and produce convergence-retraction movements and pupils that respond better to near viewing than to light.
Frequently asked questions
Where is the midbrain located?
Between the pons below and diencephalon above, within the opening of the tentorium.
What does the midbrain do?
Contributes to eye movements, pupil responses, orienting to visual and auditory signals, movement regulation and descending modulation of pain.
What are the parts of the midbrain?
Ventral crus cerebri, substantia nigra, tegmentum, aqueduct and dorsal tectum; the tectum bears superior and inferior colliculi.
What is the blood supply of the midbrain?
Small branches from the posterior cerebral, superior cerebellar and basilar arterial systems, with overlapping territories.