CHAPTER 3. THE LOWER MOTOR NEURONS

OBJECTIVE:
    BE ABLE TO IDENTIFY IN SECTIONS AND GROSS SPECIMENS THE PRECISE LOCATION OF NUCLEI FORMED BY LOWER MOTOR NEURONS, THE COURSE AND RELATIONSHIPS OF THEIR INTRAMEDULLARY ROOTLETS, AND THEIR POINTS OF EMERGENCE FROM THE BRAIN OR SPINAL CORD.

Lower motor neurons are located in the anterior gray horn of the spinal cord and in the motor nuclei associated with certain cranial nerves. The morphological features of the alpha motoneurons can be observed in h & e (Sls. 67, 68) and silver (Sl. 63) preparations. Their axons terminate as motor end plates on skeletal muscle fibers (Sl. 99).

Lower motor neurons are classified as either general somatic efferent or special visceral efferent. The general somatic efferent innervate muscles derived from somites and include all the spinal lower motor neurons and those in the brainstem that comprise the oculomotor, trochlear, abducens, and hypoglossal nuclei. The special visceral efferent neurons innervate muscles derived from branchial arch mesoderm. These form the motor trigeminal and facial nuclei, and the nucleus ambiguus which contributes to the glossopharyngeal, vagus, and cranial accessory nerves.

Brainstem

Oculomotor Nucleus and CN III

Identify the oculomotor nucleus in the V-shaped ventral part of the periaqueductal gray at the level of the superior colliculus (Sls. 31, 32). The III CN rootlets which arch through the red nucleus, gather ventral to the red nucleus, and emerge in the wall of the interpeduncular fossa (Pls. 7, 10, 12, 13).

Trochlear Nucleus and CN IV

Identify the trochlear nucleus at the ventral border of the periaqueductal gray at the level of the inferior colliculus (Sls. 29, 30). Observe the IV CN rootlets as they arch dorsally and caudally in the outer part of the periaqueductal gray (Sls. 30, 29, 28, 27) to reach the superior medullary velum in which they decussate before emerging just caudal to the inferior colliculus (Pl. 9).

Motor Trigeminal Nucleus and Motor Root of CN V

Identify the motor trigeminal nucleus in the dorsolateral part of the tegmentum at the mid-pontine level (Sls. 23, 24). Lateral to it are motor trigeminal root fibers which eventually emerge as the motor root of the trigeminal nerve (Pl. 8).

Abducens Nucleus and CN VI

Identify the abducens nucleus beneath the facial colliculus in the floor of the fourth ventricle (Sls. 21, 22). Observe the VI CN rootlets as they pass ventrally and emerge at the pontomedullary junction next to the pyramid (Pl. 8).

Facial Nucleus and Motor Root of CN VII

Identify the facial nucleus in the lateral part of the tegmentum of the caudal pons (Sls. 21, 22).

The facial root fibers, upon emerging from the nucleus, stream in unobservable bundles dorsomedially to the floor of the fourth ventricle where they form a compact bundle, the ascending root of the facial nerve. Identify this bundle medial to the VIth nucleus. At the rostral pole of the VIth nucleus the fibers of the ascending root arch over it as the genu of the facial nerve (Sls. 21, 22). Observe that the fibers then course ventrolaterally between the facial and spinal trigeminal nuclei and emerge at the lateral aspect of the pontomedullary junction (Pl. 8).

Nucleus Ambiguus and CN IX, X, XI
Identify the nucleus ambiguus, an elusive column of alpha motoneurons in the ventrolateral part of the reticular formation about midway between the inferior olivary and spinal trigeminal nuclei. It can be more readily observed in slides 14 and 15. Its fibers emerge with the glossopharyngeal and vagus nerves and the cranial part of the accessory nerve (Pl. 8) which joins the vagus nerve after traveling briefly with the spinal part of the accessory nerve.

Hypoglossal Nucleus and CN XII

Identify the hypoglossal nucleus in the floor of the medullary part of the fourth ventricle near the midline (Sls. 11, 12, 13, 14, 15, 16). Observe the XII CN rootlets as they pass ventrally through the medulla and emerge at the preolivary sulcus (Pl. 8). Along their route they lie next to or in the lateral parts of the medial lemniscus and pyramid (Sls. 12, 14).

Spinal Cord

Identify in the various levels of the spinal cord (Sls. 1, 2, 3, 4, 5, 6, 7) the lateral and medial cell columns in lamina IX of the anterior horn. The medial column is uniform in size and, for the most part, extends through the length of the cord. The lateral column is relatively small in the thoracic segments but extremely large in the cervical and lumbar enlargements.