The midbrain is the top section of the brainstem: a short stalk deep under the middle of the brain, just below the thalamus and above the pons, with the cerebellum behind and below it. [1][2]
The 3D brain needs JavaScript. The text below still says where the midbrain is and what it does.
Slice
Opens cut by a side (sagittal) slice through the midbrain. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
The uppermost of the three parts of the brainstem (midbrain, pons, medulla oblongata). It joins the diencephalon and the cerebrum above to the pons below. [1][2]
It lies mostly in the posterior cranial fossa, with its upper part reaching above the tentorial notch, the gap in the dura sheet that separates the cerebrum from the cerebellum. Above it is the diencephalon (thalamus and hypothalamus); below it the pons, medulla and cerebellum. [1]
Its principal regions are the tectum (the roof, behind the cerebral aqueduct, carrying the four colliculi), the tegmentum in front of the aqueduct, and the two cerebral peduncles at the front. [1]
A narrow channel, the cerebral aqueduct, runs through it and links the third ventricle above to the fourth ventricle below. [1][3]
What it does
It is functionally associated with vision, hearing, motor control, sleep and wakefulness, arousal (alertness) and temperature regulation. [1]
The inferior colliculi are part of the hearing pathway; the superior colliculi combine information about visual, auditory and body space, for example to turn the eyes and head towards something. [4]
Its substantia nigra, a basal ganglia structure, plays an important role in reward and movement; its compact part supplies the striatum with dopamine. [5]
The nuclei of two cranial nerves that move the eyes, the oculomotor (III) and trochlear (IV) nerves, are in the midbrain. [1][6]
Connections
The cerebral peduncles at its front carry the large descending tracts from the cortex, including the corticospinal tract on its way to the spinal cord. [1][7]
To the cerebellum through the superior cerebellar peduncles. [8]
The compact part of the substantia nigra sends dopamine fibres to the striatum (caudate nucleus and putamen); its reticular part carries basal ganglia signals on to other brain structures. [5]
The trochlear nerve (IV) is the only cranial nerve that leaves from the back of the brainstem, just below the inferior colliculi. [9]
Inside it
The substantia nigra, the red nucleus, the periaqueductal grey around the cerebral aqueduct, and part of the reticular formation. [1]
Other names
Mesencephalon, from the Greek mesos, “middle”, and enkephalos, “brain”. [1]
Size
The shortest segment of the brainstem: less than 2 cm long. [1]
On this model the midbrain measures about 29 mm front to back, 17 mm top to bottom and 29 mm side to side (the box it fits in, not its volume). [10]
Centre of the midbrain compared with the centre of the pons: 22 mm higher; 22 mm apart in a straight line. [10]
Centre of the midbrain compared with the centre of the left thalamus: 16 mm lower, 13 mm closer to the midline; 20 mm apart in a straight line. [10]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.