Pons
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Pons

The pons (from Latin pons, "bridge") is part of the brainstem that in humans and other mammals, lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.

The pons is also called the pons Varolii ("bridge of Varolius"), after the Italian anatomist and surgeon Costanzo Varolio (1543–75). This region of the brainstem includes neural pathways and tracts that conduct signals from the brain down to the cerebellum and medulla, and tracts that carry the sensory signals up into the thalamus.

The pons in humans measures about 2.5 centimetres (0.98 in) in length. It is the part of the brainstem situated between the midbrain and the medulla oblongata. The horizontal medullopontine sulcus demarcates the boundary between the pons and medulla oblongata on the ventral aspect of the brainstem, and the roots of cranial nerves VI/VII/VIII emerge from the brainstem along this groove. The junction of pons, medulla oblongata, and cerebellum forms the cerebellopontine angle. The superior pontine sulcus separates the pons from the midbrain. Posteriorly, the pons curves on either side into a middle cerebellar peduncle.

A cross-section of the pons divides it into a ventral and a dorsal area. The ventral pons is known as the basilar part, and the dorsal pons is known as the pontine tegmentum.

The ventral aspect of the pons faces the clivus, with the pontine cistern intervening between the two structures. The ventral surface of the pons features a midline basilar sulcus along which the basilar artery may or may not course. There is a bulge to either side of the basilar sulcus, created by the pontine nuclei that are interweaved amid the descending fibres within the substance of the pons. The superior cerebellar artery winds around the upper margin of the pons.

Most of the pons is supplied by the pontine arteries, which arise from the basilar artery. A smaller portion of the pons is supplied by the anterior and posterior inferior cerebellar arteries.

During embryonic development, the metencephalon develops from the rhombencephalon and gives rise to two structures: the pons and the cerebellum. The alar plate produces sensory neuroblasts, which will give rise to the solitary nucleus and its special visceral afferent (SVA) column; the cochlear and vestibular nuclei, which form the special somatic afferent (SSA) fibers of the vestibulocochlear nerve, the spinal and principal trigeminal nerve nuclei, which form the general somatic afferent column (GSA) of the trigeminal nerve, and the pontine nuclei which relays to the cerebellum.

Basal plate neuroblasts give rise to the abducens nucleus, which forms the general somatic efferent fibers (GSE); the facial and motor trigeminal nuclei, which form the special visceral efferent (SVE) column, and the superior salivatory nucleus, which forms the general visceral efferent fibers (GVE) of the facial nerve.

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