The brainstem is the stalk at the base of the brain: it runs down from under the middle of the cerebrum, in front of the cerebellum, and continues as the spinal cord where it leaves the skull. [1][2]
The 3D brain needs JavaScript. The text below still says where the brainstem is and what it does.
Slice
Opens cut by a side (sagittal) slice through the brainstem. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
Three parts, top to bottom: the midbrain, the pons and the medulla oblongata. [1][2]
It reaches from just above the tentorial notch (the gap in the sheet of dura between the cerebrum and the cerebellum) down to the level of the first cervical vertebra, where the medulla carries on as the spinal cord. [1]
At the top the midbrain is continuous with the thalamus; some definitions count the diencephalon (thalamus and hypothalamus) as part of the brainstem; this page uses the usual three-part definition above. [1]
The cerebellum sits behind it, joined to it by three pairs of cerebellar peduncles; the fourth ventricle fills the gap between the pons and medulla in front and the cerebellum behind. [3][4]
On this model the brainstem is the atlas’s midbrain, pons and medulla oblongata plus the four colliculi on the back of the midbrain. The model stops at the bottom of the medulla, so the join with the spinal cord is not shown. [5]
What it does
All information passing between the body and the cerebrum and cerebellum goes through it: the ascending sensory tracts and the descending motor tracts run its whole length. [1]
Ten of the twelve pairs of cranial nerves come from the brainstem; they give the face and neck most of their motor and sensory nerve supply. [1][6]
The pons and the medulla regulate the cardiovascular and respiratory systems: heart rate and breathing rate. [2][1]
The reticular formation, a diffuse region of grey matter running through it, is related to sleep and wakefulness, general brain activity and attention. [2]
The four colliculi on the back of the midbrain are relays: the inferior pair is part of the hearing pathway, and the superior pair combines information about visual, auditory and body space. [2]
Connections
To the cerebellum through three pairs of peduncles: the superior from the midbrain, the middle from the pons and the inferior from the medulla. [3]
The corticospinal tract, carrying voluntary movement commands from the motor cortex, passes down through it and mostly crosses to the opposite side at the pyramids of the lower medulla (the pyramidal decussation). [7][8]
Above, the midbrain is continuous with the thalamus; below, the medulla is continuous with the spinal cord. [1]
Inside it
The nuclei of the cranial nerves, the reticular formation, and in the midbrain the substantia nigra and the red nucleus. [1][9]
Other names
Also written brain stem. Latin: truncus encephali. [1]
Size
Small for what it does: around 2.6 percent of the brain’s total weight. [1]
On this model the brainstem measures about 46 mm front to back, 81 mm top to bottom and 42 mm side to side (the box it fits in, not its volume). [5]
Centre of the brainstem compared with the centre of the cerebellum: 33 mm further forward, 5 mm higher; 33 mm apart in a straight line. [5]
Centre of the brainstem compared with the centre of the left thalamus: 37 mm lower, 12 mm closer to the midline; 40 mm apart in a straight line. [5]
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.