The Pons

The medulla oblongata and pons together form the floor of the fourth ventricle, the rhombic fossa. The floor? Well, basically it is a wall – at least in bipedal animals that stand upright. Furthermore, the medulla and pons are so similar that some anatomists encourage their students to refer to the rhombencephalon instead. Although this also includes the cerebellum, the classification still makes sense when considering the development of the embryo – all three develop from the same vesicle of the neural tube.

The conspicuous bulge consists mainly of a broad band of fibers. Earlier anatomists considered this to be a bridge connecting the two cerebellar hemispheres directly – hence the name “bridge” (pons Varolii), coined by the Italian anatomist Costanzo Varolio (1537–1575). However, this proved to be incorrect. Today we know that these are corticopontine fibers: pathways from the cerebral cortex are switched in the pons and then continue as pontocerebellar pathways to the opposite cerebellar hemisphere. 

But there is much more hidden in the pons. A fine groove, the sulcus basilaris, runs down the center of its front, through which the basilar artery, one of the large cerebral arteries, passes. The two elevations on the right and left give an idea of the course of the pyramidal tract inside. They connect the motor cortex with the spinal cord.

Surrounding these two elevations are numerous nuclei, the nuclei pontis. Their function can perhaps be compared to that of a bridge: by switching the corticopontine fibers mentioned above and sending them as pontocerebellar pathways to the opposite cerebellar hemisphere, they serve as a bridge for motor signals. Here, the voluntary motor signals from the cerebral cortex are forwarded as a kind of copy to the cerebellum, which then fine-tunes and refines the movement. In the front part of the pons, almost everything revolves around the fine-tuning of motor function.

As mentioned above, the pons is a continuation of the medulla oblongata. The two are closely linked in terms of function and are very similar in structure. The pons is where the reticular formation network, which performs essential vegetative tasks, continues. While the medulla controls basic vegetative functions such as breathing rhythm, heartbeat, and blood pressure, the pons contains centers that fine-tune these processes. For example, it regulates the duration of inhalation and exhalation, and connects the control of breathing with other brain activities such as sleep or attention.

The ascending reticular activating system also runs through the pons. It transmits signals to the thalamus and from there to the cerebral cortex, ensuring that we remain awake and alert. Without the reticular activating system, consciousness is not possible. If it is severely damaged, consciousness can be lost.

In addition to the countless nuclei of the pons, there are also several cranial nerve nuclei. Only the fifth cranial nerve (trigeminal nerve) emerges directly from the pons. The third and fourth cranial nerves, on the other hand, leave above the pons fiber bundle, while the sixth, seventh, and eighth cranial nerves leave at the transition between the pons and the medulla oblongata.

The importance of these nuclei is also evident in clinical practice: in unconscious patients, simple reflex tests provide information about the integrity of the pons. The masseter reflex in particular highlights the trigeminal nerve: when the chin is tapped lightly, the jaw muscle contracts briefly, demonstrating the function of this nerve.

In the corneal reflex, on the other hand, the trigeminal nerve works together with the facial nerve, the seventh cranial nerve: when the cornea is touched, the eye closes immediately as a reflex. If these reflexes fail, this may indicate damage to the pons.

Important sensory systems also converge in the pons. Here, auditory signals are processed and forwarded to higher centers. Taste impressions reach the pons via special nuclei. Large pathways for touch, pain, and temperature also pass through this section of the brain stem.

Damage in this area is often serious because both vital functions and sensory perceptions can be affected. However, unusual symptoms sometimes occur. Normally, during REM sleep, the muscles are blocked by inhibitory pathways in the pons. If this inhibition fails, for example in the case of a lesion of the reticular formation, those affected actually act out their dreams: they lash out, kick, or shout in their sleep – a disorder that can only be controlled with medication.

Even more serious is extensive damage to the anterior pons region. This is where the pyramidal tracts run, which transmit movement commands from the cerebrum to the spinal cord. If they are destroyed on both sides, for example due to an infarction of the basilar artery, the so-called locked-in syndrome occurs: Those affected are awake and conscious, can perceive their environment, but are almost completely paralyzed and can often only communicate through eye movements.

First published on August 28, 2011
Last updated on September 20, 2025