The midbrain or mesencephalon is a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, arousal, and temperature regulation.

It is comprised of the tectum, tegmentum, the cerebral aqueduct, and the cerebral peduncles, as well as several nuclei and fasciculi.

Caudally the midbrain adjoins the metencephalon the pons and cerebellum.

Rostrally it adjoins the diencephalon, the thalamus, and hypothalamus

Specifically, the midbrain consists of:


Inferior colliculi

Superior colliculi

The corpora quadrigemina are four solid lobes on the dorsal side of the cerebral aqueduct, where the superior posterior pair are called the superior colliculi and the inferior posterior pair are called the inferior colliculi.

The superior colliculus is involved with saccadic eye movements; while the inferior is a synapsing point for sound information. The trochlear nerve comes out of the posterior surface of the midbrain, below the inferior colliculus.

Its medial descending pathway comprises the vestibulospinal tracts, reticulospinal tract, and the tectospinal tract, running through the medial reticular formation, the lateral and medial vestibular nuclei.

The ventricular system comprises the choroid plexus. the lateral, third, and fourth ventricles responsible for circulating the CSF.

The choroid plexus produces cerebral spinal fluid (CSF).

The fourth ventricle is the most caudal aspect of the cerebral ventricular system, and is formed by the pons and medulla.

The third and fourth ventricles are connected via the cerebral aqueduct, the smallest ventricle in the ventricular system.

CSF is recycled in the fourth ventricle through spinal nerve sheaths through the epidural vein.

The substantia nigra is located in the midbrain.

The substantia nigra has a left and right region.

It has two primary regions; the substantia nigra pars compact and the substantia nigra pars reticular.

The substantia nigra contains dopaminergic tracts responsible for coordination of eye movement and voluntary motor movement.

The substantia nigra region undergoes extremely high metabolic synthesis of dopamine and norepinephrine through the metabolic conversion of tyrosine to L-DOPA

The loss of dopaminergic neurons in this region contributes to the progression of Parkinson’s disease.

The cerebral peduncles are paired structures, present on the ventral side of the cerebral aqueduct.

The cerebral peduncles carry tegmentum on the dorsal side and cresta or pes on the ventral side, and both of them accommodate the corticospinal tract fibers, from the internal capsule, the middle part of cerebral peduncles carry substantia nigra.

Between the cerebral peduncles is the interpeduncular fossa, from which the oculomotor nerve comes out, and the trochlear nerve is visible wrapping around the outside of the peduncles.

The cerebral aqueduct connects the third and fourth ventricle and the periaqueductal gray.

Obstruction of the cerebral aqueduct during development can lead to congenital hydrocephalus.


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