brain stem

Midbrain

by Sharadsaini on

The midbrain can be divided into three main parts: the tectum (quadrigeminal plate); the tegmentum, which is a continuation of the pons tegmentum; and the very large crus cerebri, which contains  the corticofugal fibers. The midbrain contains two cranial nerve nuclei, the oculomotor and trochlear nuclei. The most   prominent nuclear mass in the midbrain is the substantia nigra, a huge area darkly  pigmented with melanin, a metabolic byproduct of dopamine breakdown. The substantia nigra, which sends dopaminergic  projections to the basal ganglia, is very important clinically since its degeneration produces a loss of dopamine terminations in the basal ganglia, resulting in the extrapyramidal  disorder Parkinson’s disease. The structure of the midbrain is most usually  demonstrated using transverse sections at the level of the inferior and superior colliculi.

                                 Transection at the level of the inferior  colliculus reveals that the pontine tectum or covering, i.e., the superior medullary velum, is now replaced by the inferior and  superior colliculi, swellings caused by the  masses of nuclei serving as relay stations for transmission of auditory and other signals to the brain. At this level the cerebral   aqueduct replaces the fourth ventricle and decussation of the fibers of the superior cerebellar peduncles is visible.

                             Several tegmental nuclear groups surround the cerebral aqueduct in the periaqueductal gray matter. These include the locus ceruleus, a pigmented cell mass which sends many norepinephrine-containing projections to the cerebellum and cerebral cortex. The locus ceruleus appears to be involved in modulation of cortical sensory and association areas, and in sleep activation. (Parts of several nuclei, including the nucleus ceruleus, are also seen in rostral sections of pontine areas; it is wrong to compartmentalize brain stemnuclei as strictly pontine or midbrain etc.) Also in this region is the mesencephalic nucleus of the trigeminal nerve, a collection of unipolar sensory neurons, and the dorsal nucleus of the raphe. The trochlear nucleus lies ventrally in the periaqueductal gray matter and sends efferents to the superior oblique muscle of the eye.

                               Several tracts can be seen in transverse section. The most prominent is the decussation of the cerebellar peduncles. The lateral lemniscus is seen where it enters the inferior colliculus and the medial lemniscus en route to the thalamus. Just medial is the ventral trigeminothalamic tract. Clustered medially are the dorsal trigeminothalamic tract, central tegmental tract, the medial longitudinal fasciculus, and the tectospinal tract. The ventrally placed crus cerebri contains the  massive descending corticospinal and corticobulbar tracts, and temperopontine fibers.

                    Transection at the level of the superior colliculi shows the prominent bilateral  red nucleus, so called because it appears pinkish red in freshly cut sections. The red nucleus runs continuous with the crossed superior cerebellar peduncle, and it is the origin of descending motor tracts, which decussate in the ventral tegmentum to become the rubrospinal tract.

                      The superior colliculi communicate through the posterior commissure and integrate auditory, cortical, spinal, and retinal afferents in the control of eye movements and reflex reflexes. The superior brachium carries the retinal inputs. The oculomotor nucleus lies ventrally in the periaqueductal gray matter, and its efferent projections cross the red nucleus, emerge in the interpeduncular fossa and  run to optic and extra-optic muscle.

Midbrain

 

Brain Stem

by Sharadsaini on

Brain Stem

The brain stem consists of the medulla oblongata (or medulla), the pons and the midbrain. The three brain areas each contain cranial nerve nuclei, and the fourth ventricle lies partly in the pons and partly in the medulla. The brain stem may occasionally be referred to as the ‘bulb’ in such terms as the ‘corticobulbar’ tract.

The medulla is around 3 cm long in adult humans and widens rostrally. It is continuous with the spinal cord from just belowthe foramen magnum, at the level of the upper rootlet of the first cranial nerve, and extends through to the lower  (caudal) border of the pons. The medulla lies on the basilar part of the occipital bone, and is obscured from view by the cerebellum. Externally, the spinal cord and medulla appear to merge imperceptibly, but internal examination reveals extensive reorganization of white and gray matter at the junction. In the medulla the central canal widens into the fourth ventricle.

From the ventral aspect, the central median fissure appears as a central groove, which is a continuation of that of the spinal cord. The progress of the fissure is interrupted by the decussation (crossing over) of the fiber tracts of the corticospinal tract, where they cross over at the pyramid of the medulla to form the lateral corticospinal tract. Lateral to the pyramids on each side is the olive, made up of a convoluted mass of gray matter called the inferior olivary nucleus. The olive is separated from the pyramids by the rootlets of the hypoglossal nerve (XII). Rootlets of the vagus (X) and the cranial accessory (XI) nerves arise lateral to the olive, the latter two being united with the spinal accessory nerve (XI). The facial (VII) and vestibulocochlear (VIII) nerves arise at the border between the lateral medulla and the pons.

The pons is about 2.5 cm in length. Its name is Latin for ‘bridge’, since it appears to connect the cerebellar emispheres though this is not actually the case. Ventrally, the pons is a sort of relay station, where cerebral cortex fibers terminate ipsilaterally on pontine nuclei, whose axons become the contralateral middle cerebellar peduncles. Thus the ventral (or basal) pons is a sort of massive synaptic junction that connects each cerebral hemisphere with the contralateral cerebellar hemisphere. Functionally, this system maximizes efficiency of voluntary movement.

The ventral surface of the midbrain extends rostrally from the pons to the mamillary bodies, which mark the caudal  border of the diencephalon. On either side are prominent swellings called the crus cerebri (basis pedunculi). These are made up of the fiber tracts of the descending pyramidal motor system, and fibers from the cortex to the pons (corticopontine fibers). Although not shown here, the midbrain is penetrated by several small blood vessels in the floor of the interpeduncular  fossa, and the area has been named the posterior perforated substance because of these blood vessels. The oculomotor nerve (III) to the eye leaves the brain through the cavernous venous sinus from each side of the interpeduncular fossa. The optic chiasm and optic nerves, together with the diencephalic tuber cinereum are exposed on the ventral surface. brain stem