Pons

Named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio (pons Varolii), the pons is a structure located on the brain stem. It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it is above the medulla, below the midbrain, and anterior to the cerebellum. This white matter includes tracts that conduct signals from the cerebrum down to the cerebellum and medulla, and tracts that carry the sensory signals up into the thalamus.

The pons measures about 2.5 cm in length. Most of it appears as a broad anterior bulge rostral to the medulla. Posteriorly, it consists mainly of two pairs of thick stalks called cerebellar peduncles. They connect the cerebellum to the pons and midbrain.

The pons contains nuclei that relay signals from the forebrain to the cerebellum, along with nuclei that deal primarily with sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture.

Within the pons is the pneumotaxic center, a nucleus in the pons that regulates the change from inspiration to expiration.

The pons also contains the sleep paralysis center of the brain and also plays a role in generating dreams.

Embryonic development
During embryonic development the embryonic metencephalon develops from the rhombencephalon and gives rise to two structures: the pons and the cerebellum. The alar plate produce 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 abducent 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 of the facial nerve.

Cranial nerve nuclei
A number of cranial nerve nuclei are present in the pons:
 * mid-pons: The chief or pontine nucleus of the trigeminal nerve sensory nucleus (V)
 * mid-pons: the motor nucleus for the trigeminal nerve (V)
 * lower down in the pons: abducens nucleus (VI)
 * lower down in the pons: facial nerve nucleus (VII)
 * lower down in the pons: vestibulocochlear nuclei (vestibular nuclei and cochlear nuclei) (VIII)

The functions of these four nerves include sensory roles in hearing, equilibrium, and taste, and in facial sensations such as touch and pain; as well as motor roles in eye movement, facial expressions, chewing, swallowing, urination, and the secretion of saliva and tears.

Related diseases

 * Central pontine myelinosis, a demyelination disease that causes difficulty with sense of balance, walking, sense of touch, swallowing and speaking. In a clinical setting it is often associated with transplant. Undiagnosed it can lead to death or locked-in syndrome.

Evolution
The pons first evolved as an offshoot of the medullary reticular formation. Since lampreys possess a pons, it has been argued that it must have evolved as a region distinct from the medulla by the time the first agnathans appeared, 505 million years ago.