CHAPTER 5. THE BRAINSTEM MOTOR CENTERS

OBJECTIVE:
    BE ABLE TO IDENTIFY THE VESTIBULAR NUCLEI, RETICULAR FORMATION, AND RED NUCLEUS.

Vestibular

Vestibular Nuclei

Identify the vestibular nuclear complex consisting of four nuclei beneath the vestibular area in the floor and wall of the 4th ventricle (Sls. 14, 15, 17, 18, 19, 20, 21, 22).

Vestibulospinal Tracts

The lateral vestibulospinal tract arises from the lateral vestibular nucleus and extends throughout the length of the spinal cord. 

This tract descends ipsilaterally.  Rostrally it is in the ventral part of the lateral funiculus but it spreads medially into the anterior funiculus at caudal levels.

The medial vestibulospinal tract arises from the medial and inferior vestibular nuclei.

This tract descends bilaterally in the medullary part of the medial longitudinal fasciculus and in the spinal cord is located in the anterior funiculus. It influences muscles of the neck and upper limbs and descends only to about mid-thoracic levels.

Reticular

Two areas of the reticular formation give rise to reticulospinal fibers, one in the pons and the other in the medulla. The pontine or medial reticulospinal tract arises from groups of large neurons in the pontine reticular formation.  

Fibers of the medial reticulospinal tract descend ipsilaterally in close relation to the medial longitudinal fasciculus; in fact, at medullary levels it is usually considered a part of the MLF.  Within the spinal cord this bundle is found in the medial part of the anterior funiculus.

The medullary or lateral reticulospinal fibers arise chiefly from neurons located in the medial part of the medullary reticular formation.

These fibers descend bilaterally and are initially found in the ventral part of the lateral funiculus.  They gradually move medially and eventually intermingle with the pontine reticulospinal fibers in the anterior funiculus.

Rubral

The red nucleus, slightly reddish in the fresh brain due to its vascularity, is a huge oval structure in the ventral part of the midbrain tegmentum mainly at the levels of the superior colliculus (Pls. 16, 34; Sls. 31, 32) and pretectum (Pl. 28; Sl. 33), although it does extend rostrally to overlap with diencephalic structures (Pl. 27; Sls. 34, 35, 36).

Upon emerging from the medial aspect of the red nucleus, rubrofugal fibers immediately cross the midline in the ventral tegmental decussation.  This small tract is limited chiefly to the cervical spinal cord, where it is located anterior to and partially intermingled with the fibers of the lateral corticospinal tract.

Other Centers

Two other brainstem motor control centers consist of neurons in the superior colliculus and the interstitial nucleus (of Cajal). Both influence head movements through projections to the spinal cord.

Anatomical Basis for Spinal Motor Organization

The lower motor neurons that innervate the axial muscles are in the medial motor nuclei, while those innervating the proximal and distal limb muscles are in the lateral motor nuclei (Sls. 1, 2, 3, 4, 6, 7).  The medial and lateral motor nuclei are most strongly influenced by the nearest descending paths. 

The ventromedial paths, the medial vestibulo- and reticulospinal in the MLF at cervical and upper thoracic levels and the more medial parts of the lateral vestibulo- and reticulospinal tracts at lumbosacral levels, most strongly influence the axial and proximal limb muscles. Distal limb muscles are more strongly influenced by the more laterally descending fibers located in the rubrospinal tract at cervical and upper thoracic levels and the more lateral parts of the lateral vestibulo- and reticulospinal tracts at lumbosacral levels.

The lateral corticospinal tract, forming the dorsal half of the lateral funiculus (Sls. 1, 2, 3, 4, 5, 6, 7, 8), exerts its influence on all lower motor neurons, but not uniformly. Its influence on the medial motor nuclei innervating axial muscles is weak and indirect, that is, via interneurons and propriospinal neurons. In contrast, its influence on the lateral motor nuclei, especially those dorsolaterally at C.8 and T.1 which innervate the intrinsic finger muscles, is strong, is monosynaptic, and is their sole supraspinal innervation.

Bordering the spinal gray matter at all levels is the fasciculus proprius system of intraspinal axons. The anterior fasciculus proprius contains the long propriospinal axons which ascend and descend bilaterally throughout the length of the cord. These propriospinal neurons receive input chiefly from the ventromedial paths and integrate movements of the entire vertebral column.

The lateral fasciculus proprius contains the axons of intermediate propriospinal neurons most ventrally and short propriospinal more dorsally. These propriospinal neurons receive input chiefly from the lateral descending paths. The intermediate integrate an entire cervical or lumbosacral enlargement, while the short integrate only the segments that control the most distal muscles of the limbs.