Up to this point, we have discussed the ascending and descending pathways of the spinal cord and the positions of the cranial nerve nuclei in the brainstem, along with their emerging root fibers and their central connections. This section deals with the topography of the pathways that traverse the brainstem, as well as the site and function of other nuclei besides those that have already been described. Knowledge of the topographical anatomy of the brainstem is essential for a proper understanding of the clinical syndromes produced by lesions affecting the medulla, pons, and midbrain.
Internal Structure of the Brainstem
The brainstem contains important nuclei, including the reticular formation, the olives, the red nucleus, the substantia nigra, and others, each of which will be described in the subsection dealing with the part of the brainstem in which it is located. The connections that these nuclei make with each other and with the cerebrum, cerebellum, and spinal cord will also be discussed.
▶Fig. 4.52 and ▶Fig. 4.53 contain longitudinal and cross-sectional diagrams of the brainstem, showing the individual nuclei, the ascending and descending pathways, and their spatial relationships.
▶Fig. 4.54 and ▶Fig. 4.55 depict the spatial relationships of the individual fiber pathways in lateral and dorsal views of the brainstem.
Medulla
The spatial arrangement of the gray and white matter in the medulla already differs from that in the spinal cord at the lowest medullary level, i.e., at the level of the pyramidal decussation (▶Fig. 4.52). The anterior horns can still be seen: they contain the motor nuclei for the first cervical nerve and for the roots of the accessory nerve. The descending fibers of the corticospinal tracts are located in the pyramids; most of these fibers cross the midline at this level and then descend in the contralateral lateral funiculus of the spinal cord. In the region of the posterior columns, two nuclei are found, i.e., the cuneate nucleus and the gracile nucleus. These are the relay nuclei for the ascending posterior column fibers of the spinal cord. They, in turn, project impulses by way of the medial lemniscus to the contralateral thalamus. These two nuclei possess a somatotopic arrangement (point-to-point projection), in which the cuneate nucleus contains fibers for the upper limbs, while the gracile nucleus contains fibers for the lower limbs. This somatotopy is preserved in the medial lemniscus, in the thalamus, and all the way up to the primary sensory cortex. ▶Fig. 4.55 shows the twisting course of the medial lemniscus: the fibers carrying impulses for the lower limb are more lateral, and those carrying impulses for the upper limb are more medial.
The lateral spinothalamic tract (pain, temperature), anterior spinothalamic tract (touch, pressure), and spinotectal tract (to the quadrigeminal region) have essentially the same position in the caudal medulla as in the cervical spinal cord.
An extensive network of cells, the lateral reticular nucleus, receives incoming fibers from the reticular formation of the spinal cord. This nucleus lies dorsal to the inferior olivary nucleus. The spinoreticular fibers carry sensory impulses from the skin and internal organs. These fibers run more diffusely in the spinal cord, some of them in association with the spinothalamic tract.
The posterior spinocerebellar tract, which originates in Clarke’s column (the thoracic nucleus) and ascends ipsilaterally in the spinal cord, at first keeps its position in the caudal medulla, and then takes a progressively more dorsal position and finally accompanies the olivocerebellar tract as it travels, via the inferior cerebellar peduncle, to the cerebellum (▶Fig. 4.54b and ▶Fig. 4.55b). The anterior spinocerebellar tract, part of which is crossed, traverses the medulla and pons and finally enters the cerebellum by way of the superior cerebellar peduncle and the superior medullary velum (▶Fig. 4.54b and ▶Fig. 4.55b).
The olivary nuclear complex is located in the rostral portion of the medulla. The inferior olive (▶Fig. 4.54 and ▶Fig. 4.55), which resembles a sheet of gray matter that has been folded up to form a bag, receives most of its afferent input from the red nucleus of the midbrain, by way of the central tegmental tract. It receives further afferent input from the striatum, the periaqueductal gray matter, the reticular formation, and the cerebral cortex, by way of the cortico-olivary tract, which runs together with the corticospinal tract. Efferent fibers from the inferior olive cross the midline and form the olivocerebellar tract, which enters the cerebellum through the inferior cerebellar peduncle (▶Fig. 4.54b and ▶Fig. 4.55b) and conveys impulses to the entire neocerebellar cortex. This olivocerebellar projection belongs to the system for coordination of voluntary movement; it will be discussed further in the chapters concerning the cerebellum (▶Chapter 5) and basal ganglia (▶Chapter 8).
The accessory olive is phylogenetically older than the inferior olive. It is connected to the archicerebellum and plays a role in the maintenance of balance.
Lesions of the inferior olive or of the central tegmental tract produce rhythmic twitching of the soft palate, the pharynx, and sometimes the diaphragm (myorhythmia, myoclonus, singultus). Ischemia is the usual cause.
The courses of the corticospinal and corticonuclear tracts are depicted in the cross-sectional diagrams of the brainstem and in ▶Fig. 4.54a and ▶Fig. 4.55a.
The rubrospinal tract also passes through the medulla. This tract originates in the red nucleus of the midbrain and crosses the midline a short distance below it in the ventral tegmental decussation (of Forel). It accompanies the lateral corticospinal tract as it descends in the lateral funiculus of the spinal cord (▶Fig. 4.55).
The tectospinal tract originates in the midbrain tectum and immediately crosses the midline, swinging around the periaqueductal gray in the dorsal tegmental decussation (of Meynert). The tectospinal tract at first descends near the midline and then gradually takes a more ventral and lateral position, coming to lie in the ventrolateral portion of the medulla, near the rubrospinal tract. Along its way into the medulla, the tectospinal tract gives off collaterals to the nuclei innervating the extraocular muscles, as well as to the nucleus of the facial nerve and the cerebellum. It ends in the cervical spinal cord. Function: The superior colliculi receive visual input from the retina and auditory input from the inferior colliculi. Intense visual and auditory stimuli evoke reflex closure of the eyes, turning of the head away from the stimulus, and sometimes also raising of the arms (defense position); these reflexes are mediated by the tectonuclear and tectospinal pathways. The functional interaction of the occipital lobe and the superior collicular plate was mentioned in an earlier section. These two structures work together with the tectospinal pathways to enable automatic pursuit movements of the eyes and head when the individual looks at a moving object.
On the various cross-sectional images of the medulla, pons, and midbrain, one can see, in the spaces between the larger nuclei and the ascending and descending pathways, a number of diffusely distributed nuclei of varying size that occasionally cluster into nuclear groups, with an extensive network of fibers connecting them. These interconnected groups of neurons are known collectively as the reticular formation, a structure whose great importance was first recognized by Moruzzi and Magoun (1949). The reticular formation extends from the spinal cord (where it lies between the lateral and posterior funiculi) upward (▶Fig. 2.21), through the medulla and pons, to the oral part of the midbrain (▶Fig. 4.52 and ▶Fig. 4.53). We will discuss its function later.
One of the important nuclei in the medulla is the dorsal nucleus of the vagus nerve, which lies beneath the floor of the fourth ventricle (▶Fig. 4.1b). It contains autonomic motor (i.e., parasympathetic) neurons, which are analogous to the (sympathetic) neurons of the lateral horns of the spinal cord from T1 to L2. The more laterally lying nucleus of the tractus solitarius is a somatosensory and special sensory nucleus. Its rostral portion receives gustatory input from CNs VII, IX, and X. Its caudal portion, which receives afferent fibers from the thoracic and abdominal viscera, is interconnected with the dorsal nucleus of the vagus nerve, with visceral centers in the reticular formation, and with neurons projecting to the autonomic nuclei in the lateral horns of the spinal cord. All of these nuclei can thus participate in reflex arcs that regulate and control cardiovascular, respiratory, and alimentary function, and other vegetative processes (see ▶Fig. 4.56).
