The bridge (the pons) is a ventrally protruding section of the brainstem. It lies between the midbrain and medulla.
What is the bridge?
The bridge (from the Latin “pons”) is a section in the human brain. Together with the cerebellum, the pons is part of the hindbrain (metencephalon). Even a cursory examination of the brain reveals the bridge as a rather distinct raised transverse bulge. It is located between the midbrain (mesencephalon) and the medulla (myelencephalon), and together with both in the central nervous system it forms the brainstem of the brain.
Anatomy and structure
The bridge is divided into an anterior part – the base (lat. : Pars basilaris pontis) – and a posterior part – the bridge cap (lat. : Pars dorsalis pontis). At the base there are two longitudinal bulges. Through both passes the so-called pyramidal tract (main part of the system of movement control, tractus pyramidalis). In the groove in the middle (lat. : sulcus basilaris) runs the arteria basilaris, which is an important inflow for the supply of blood to the brain. In the cross section of the brain there is a clearly visible connecting line of both halves (raphe), which is crossed by a multitude of nerve fibers. The trapezoid body (lat. : corpus trapezoideum) is located behind the transverse fibers of the pontine base. It forms a station of the auditory pathway (central nervous part of the auditory system). Dorsally, cranial nerves VII (facial nerve) and VIII (lat. : vestibulocochlear nerve, vestibular nerve) enter the surface of the brain at the caudal edge of the bridge at the angle of the cerebellum. The VI cranial nerve (lat. : abducens nerve, responsible with other nerves for the movement of the eyeball) exits the bridge in the sulcus bulbopontinus at the downward bridge base. The trigeminal nerve (very powerful V cranial nerve, “triplet nerve”, mediating tactile sensations in the face and scent perception) exits or enters laterally at the bridge. A part of the floor of the rhomboid fossa (lat. : Fossa rhomboidea) forms the dorsal closure of the bridge cap and thus of the 4th ventricle (cavity filled with cerebral fluid). Through the middle cerebellar peduncle (lat. : pedunculus cerebelli medius), the connection to the cerebellum is established on both sides.
Function and tasks
The pons forms the passageway for all the tracts that connect the areas of the central nervous system located in front of and behind it, both between brain areas and with the spinal cord. The white matter of the pons contains, in addition to these longitudinal strands of fibers (Latin: fibrae pontis longitudinales), strong tracts of transverse fibers (Latin: fibrae pontis transversae). These connect the bridge with the cerebellum. The tracts connecting the two parts of the metencephalon originate from so-called bridge nuclei (lat. : nuclei pontis), which are considered switching stations. Through these, mainly the cortical areas in the cerebrum cortex are connected to those of the cerebellum (usually crossed). The nuclei of the bridge (mediators of projections from the cerebral cortex and the contralateral cerebellar cortex) unfold strongly. Embedded in the pontine formation reticularis (extensive, diffuse neuronal network in the brainstem), the motor origin nuclei of several nerves of the brain (e.g., nucleus motorius nervi trigemini, nucleus nervi abducentis, and nucleus motorius nervi facialis), among others, lie in the bridge cap. The pons is considered the regulatory center for circulation and respiration. It also provides for the function of hearing and also taste.
Diseases
Typical diseases of the bridge include central pontine myelinolysis (ZPM), Millard-Gubler syndrome (so-called bridge syndrome), and tumors. Central pontine myelinolysis is a neurological disease. It involves damage to the sheathing of nerve fibers in the pons. This disease is caused when an abnormally low sodium level (hyponatremia) in the organism is corrected too quickly. Extrapontine myelinolysis is a particular form of ZPM in which demyelination occurs in the cerebellum, near the ventricles, in the basal ganglia, in the bars, and in the internal capsule. Both forms of ZPM are considered osmotic demyelinating diseases, which can also occur simultaneously. Low-salt diets combined with high levels of drinking (e.g.e.g., in malnutrition and anorexia), side effects of medications such as diuretics or carbamazepine), hormonal disorders (e.g., Schwartz-Bartter syndrome, central salt wasting syndrome), so-called “water intoxication” (e.g., in the case of faulty infusion therapy or drowning accidents), and alcoholism can trigger hyponatremia and thus ZPM. Millard-Gubler syndrome is a so-called bridge syndrome in which circulatory disturbances occur in the area of the bridge foot (pars basilaris pontis) (e.g., due to a stroke). Furthermore, so-called lateral and paramedian bridge syndromes can occur. There are also bridge canopy syndromes. Lateral bridge syndromes usually result from occlusion of the arteriae circumferentes breves and damage the lateral pedunculus cerebellaris medius (bridge arm) on one side. Symptoms of lateral bridge syndromes include impaired movement and sensation. Paramedian bridge syndromes – also known as bridge foot syndromes – result from occlusion of the branches of the basilar artery and may present with symptoms such as spastic hemiplegia. In bridge cap syndromes, hearing impairment, gaze paralysis, sensory paralysis, or cerebellar ataxia (disturbance of movement patterns) occur due to cranial nerve deficits. A tumor in the area of the pons can lead to damage of the brain stem. Signs of such a tumor may include squinting of the eyes, paralysis of the facial nerve (drooping of one half of the face), gaze direction disorders, irregular breathing, loss of voluntary motor activity (except for eye and eyelid movements), or paralysis of both arms and both legs (complete paraplegia). The sense of hearing may also be impaired; further, there may be disturbances of consciousness.
