Side by side
| Compared | Midbrain | Pons |
|---|---|---|
| Where it is | The top section of the brainstem, between the diencephalon (thalamus and hypothalamus) above and the pons below; the narrow cerebral aqueduct runs through it. [1][5] | The middle section of the brainstem, below the midbrain and above the medulla oblongata, directly in front of the cerebellum; its back surface forms the upper part of the floor of the fourth ventricle. [2][6] |
| What it does | Its superior colliculi turn the eyes and head towards what is seen and its inferior colliculi relay hearing; the substantia nigra and ventral tegmental area hold dopamine-making cells used in movement and reward. [1][7][8][9] | Relays signals from the cerebral cortex to the cerebellum; its pontine respiratory group helps time the switch from breathing in to breathing out, and it is involved in sleep. [2][10] |
| Connects with | The cerebral peduncles on its front carry fibres from the cortex down towards the pons and spinal cord; the superior cerebellar peduncles join it to the cerebellum; the nuclei of cranial nerves III and IV lie in it. [1][11][12] | Pontine nuclei pass signals from the cortex to the opposite half of the cerebellum through the middle cerebellar peduncles; the nuclei of cranial nerves V to VIII lie in it. [2][13] |
| Size measured from the model | 29 mm front to back, 17 mm top to bottom, 29 mm side to side [4] | 34 mm front to back, 29 mm top to bottom, 42 mm side to side [4] |
Size is the box each part fits in on this one model (midbrain; pons), not its volume. Real brains vary.
How far apart are they?
Measured from the model [4], from the midbrain to the pons:
- Centre to centre: 22 mm in a straight line.
- The centre of the pons is 2 mm further forward, 22 mm lower and the same distance from the midline than the centre of the midbrain.
- Nearest points of the two shapes: under 1 mm apart.
A “centre” here is the average position of the points on the part’s 3D surface, so for a curved part such as a C shape it can sit outside the part itself. The nearest-points figure is measured between the corners of the simplified meshes, so a gap of 2 or 3 mm or less is within the model’s coarseness: read it as “touching or nearly so”.
How to tell them apart
- From the front: the midbrain shows two thick stalks, the cerebral peduncles, with the oculomotor nerves (III) coming out in the hollow between them; the pons is the broad, rounded bulge with transverse fibres just below. [1][2][14]
- Inside, the midbrain carries only a thin canal, the cerebral aqueduct; at the level of the pons that canal opens out into the much wider fourth ventricle, between the pons and the cerebellum. [5][6]
- They come from different parts of the embryo: the midbrain is the whole mesencephalon, while the pons, with the cerebellum, comes from the metencephalon, part of the hindbrain. [1][15][16]