The pons is also called the pons Varolii ("bridge of Varolius"), after the Italian anatomist and surgeon Costanzo Varolio (1543–75).[1] 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.[2]
Structure
The pons in humans measures about 2.5 centimetres (0.98 in) in length.[2] It is the part of the brainstem situated between the midbrain and the medulla oblongata,[3] and in front of the cerebellum.[citation needed] 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.[4] The junction of pons, medulla oblongata, and cerebellum forms an angle - the cerebellopontine angle.[5] The superior pontine sulcus separates the pons from the midbrain.[6] Posteriorly, the pons curves on either side into a middle cerebellar peduncle.[3]
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.[3]
Functions of these four cranial nerves (V-VIII) include regulation of respiration, control of involuntary actions, 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, and the secretion of saliva and tears.[2]
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.[2]
The pons is implicated in sleep paralysis, and may also play a role in generating dreams.[9]
Clinical significance
Central pontine myelinolysis is a demyelinating 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 or rapid correction of blood sodium. Undiagnosed, it can lead to death or locked-in syndrome.
Other animals
Evolution
The pons first evolved as an offshoot of the medullary reticular formation.[10] 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, 525 million years ago.[11]
