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.