midbrain function

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

 

Transverse Section of Medulla Oblongata

by Sharadsaini on

Higher transection of the medulla oblongata at the level of the middle of the olivary nuclei clearly shows the fourth ventricle, the roof of which is formed by the choroid plexus in the inferior medullary velum at the base of the cerebellum. The floor of the ventricles is pushed up by the hypoglossal and dorsal vagal nuclei. The reticular formation, a network of nerve cells in the brain stem, is nowclearly visible, as are the major fiber tracts.

                                      The pyramids, medial lemnisci, and tectospinal tract lie medially in section. The tectospinal tract carries descending fibers from the tectum, which is the roof of the midbrain, consisting of superior and inferior colliculi. Also prominent is the inferior vestibular nucleus, which lies just medial to the inferior cerebellar peduncle.

                                             The most prominent feature of the transverse section at this level is the convoluted inferior olivary nucleus, which has a massive input to the cerebellum through the olivocerebellar tract which constitutes most of the inferior cerebellar peduncle. If it could be dissected entirely, the inferior olive would resemble a collapsed purse or bag. Axons of olivary cells leave the nucleus and decussate to the other side of the medulla and sweep up into the peduncle. The fibers radiate to virtually all parts of the cerebellum and many have an excitable effect on cerebellar Purkinje cells. The inferior olivary complex has been divided into the principal, medial accessory and dorsal accessory olivary nuclei, based mainly on their cerebellar connections. For example, the fibers arising from the medial portion of the principal nucleus and those from the accessory nuclei terminate mainly in the vermis of the cerebellum.

                                            The olive receives descending corticoolivary fibers from the occipital, parietal, and temporal cortex, which terminate bilaterallymainly in the principal olivary nucleus. The principal olive also receives rubro-olivary fibers from the red nucleus, and fibers in the central tegmental tract from the periaqueductal gray matter in the midbrain, some of which also terminate in the medial accessory nuclei. The dorsal and medial accessory olives receive ascending fibers in the spino-olivary tract, which runs up the cord in the anterior (ventral) funiculus of the white matter.

                                           There are other nuclei at this level. The nucleus ambiguus is a longitudinal column of nerve cells within the reticular formation, extending through the medulla from the medial lemniscus to the midrostral portion of the inferior olive. The cells are multipolar motoneurons, and the efferents from this nucleus arch upward to join efferents from the dorsal vagal nucleus and from the nucleus of the tractus solitarius. Efferents from the rostral part of the nucleus ambiguus become visceral efferents of the lossopharyngeal nerve,which innervate the stylopharyngeus muscle. The more caudal portion of the nucleus gives rise to fibers of the spinal accessory nerve.The nucleus of the tractus solitarius gives rise to fibers, which, among other destinations, target the hypothalamic nuclei which release the peptide vasopressin. The reticular formation contains several important raphe nuclei which extend in the pons, and which project 5-HT neuronal processes to the midbrain, diencephalon and cerebral cortex. These central gray projections appear to mediate rhythmic processes such as arousal.

Medulla Oblongata

Medulla Oblongata

by Sharadsaini on

Medulla Oblongata

The spinal cord becomes the medulla oblongata, which also contains white and gray matter, but the arrangement hanges, due to the embryonic expansion of the central canal to form the hindbrain vesicle, which will become the fourth ventricle. Development of the ventricle pushes dorsally situated structures more dorsolaterally. The transition is clearly seen in transverse section. The spinal cord becomes the medulla, which initially resembles the upper cervical segments. The substantia gelatinosa is now much larger in size and has become the spinal nucleus of the trigeminal nerve. In transverse section, descending fibers of the spinal trigeminal tract can be seen immediately dorsolateral to the nucleus. There is an increase in the amount of gray matter surrounding the central canal.

                                       At low medullary level, the most prominent sign of transition to medulla is the appearance of the decussation at the pyramids. This is where the descending corticospinal motor tracts cross over. These fibers cross ventral (anterior) to the central gray matter and project dorsolaterally across the base of the ventral horn of the medulla. The pyramidal decussation almost eliminates the spinal anterior median fissure. (In the human, approximately 90% of the descending corticospinal fibers decussate and descend the cord in the lateral corticospinal tract, while about 10% do not cross, and descend in the uncrossed lateral and ventral corticospinal tracts.) The decussation explains the contralateral control of body movements by the motor cortex. At this level can also be seen the tracts of the gracile and cuneate fasciculi, which are the CNS projections of the cells of the spinal ganglia, and the lower ends of the gracile and cuneate nuclei where they terminate. At this level are also the cut fibers of the ascending ventral (anteriorand lateral spinocerebellar tracts, which carry information from the sense organs in tendons and muscle spindles, the inferior olivary nucleus, and the spinal root of the accessory nerve.

                                       Transaction at a higher level of the medulla (B) reveals another prominent decussation, that of the medial emniscus. This is where fiber tracts from the ascending gracile and cuneate nuclei cross the midline of the medulla on their way up to higher centers. The nuclei are complex and arranged to correspond topographically with the body areas from which the ascending 0fibers come. Ascending fibers from the nuclei curve round the central gray matter and decussate to form the medial lemniscus. At this level, the spinal nucleus of the trigeminal nerve, which innervates the head region, is prominent, and immediately dorsolateral to it are the fibers of the descending trigeminal nerve. At both levels, the ascending spinocerebellar and spinothalamic tracts are both visible, and in B the medial accessory olivary nucleus lies medial to these tracts.

                                              In summary, the transition from spinal cord to medulla is marked by (i) the expansion of the central canal; (ii) decussation at the pyramids; (iii) formation of the medial lemniscus through the decussation of ascending fibers arising from the cuneate and gracile nuclei; (iv) dorsolateral displacement of the dorsal horn of gray matter; (v) appearance of cranial nerve nuclei and various relay nuclei projecting to the cerebellum.

Medulla Oblongata