Nuclei of the brainstem

The word "bulb" (i.e., bulbar) is descriptive and refers to the brainstem. The cortico-bulbar fibers do not form a single pathway. The fibers end in a wide variety of nuclei of the brainstem; those fibers ending in the pontine nuclei are considered separately (see Figure 48).

Wide areas of the cortex send fibers to the brainstem as projection fibers (see Figure 16). These axons course via the internal capsule and continue into the cerebral peduncles of the midbrain (see Figure 26). The fibers involved with motor control occupy the middle third of the cerebral peduncle along with the cortico-spinal tract (described with the previous illustration; see Figure 48), supplying the motor cranial nerve nuclei of the brainstem (see Figure 8A and Figure 48), the reticular formation and other motor-associated nuclei of the brainstem.

• Cranial Nerve Nuclei: The motor neurons of the cranial nerves of the brainstem are lower motor neurons (see Figure 8A and Figure 48); the cortical motor cells are the upper motor neurons. These motor nuclei are generally innervated by fibers from both sides, i.e., each nucleus receives input from both hemispheres.

There are two exceptions to this rule, which are very important in the clinical setting:

• The major exception is the cortical input to the facial nucleus. The portion of the facial nucleus supplying the upper facial muscles is supplied from both hemispheres, whereas the part of the nucleus supplying the lower facial muscles is innervated only by the opposite hemisphere (crossed).

• The cortical innervation to the hypoglossal nucleus is not always bilateral. In some individuals, there is a predominantly crossed innervation.

• Brainstem motor control nuclei: Cortical fibers influence all the brainstem motor nuclei, particularly the reticular formation, including the red nucleus and the substantia nigra, but not the lateral vestibular nucleus (see Figure 49A, Figure 49B, and Figure 50). The cortico-retic-

ular fibers are extremely important for voluntary movements of the proximal joints (indirect voluntary pathway) and for the regulation of muscle tone.

• Other brainstem nuclei: The cortical input to the sensory nuclei of the brainstem is consistent with cortical input to all relay nuclei; this includes the somatosensory nuclei, the nuclei cuneatus and gracilis (see Figure 33). There is also cortical input to the periaqueductal gray, as part of the pain modulation system (see Figure 43).

Clinical Aspect

Loss of cortical innervation to the cranial nerve motor nuclei is usually associated with a weakness, not paralysis, of the muscles supplied. For example, a lesion on one side may result in difficulty in swallowing or phonation, and often these problems dissipate in time.

Facial movements: A lesion of the facial area of the cortex or of the cortico-bulbar fibers affects the muscles of the face differentially. A patient with such a lesion will be able to wrinkle his or her forehead normally on both sides when asked to look up, but will not be able to show the teeth or smile symmetrically on the side opposite the lesion. Because of the marked weakness of the muscles of the lower face, there will be a drooping of the lower face on the side opposite the lesion. This will also affect the muscle of the cheek (the buccinator muscle) and cause some difficulties with drinking and chewing (the food gets stuck in the cheek and oftentimes has to be manually removed); sometimes there is also drooling.

This clinical situation must be distinguished from a lesion of the facial nerve itself, a lower motor neuron lesion, most often seen with Bell's palsy (a lesion of the facial nerve as it emerges from the skull); in this case, the movements of the muscles of both the upper and lower face are lost on one (affected) side.

Tongue movements: The fact that the hypoglossal nucleus may or my not receive innervation from the cortex of both sides or only from the opposite side makes interpretation of tongue deviation not a reliable sign in the clinical setting. A lesion affecting the hypoglossal nucleus or nerve is a lower motor lesion of one-half of the tongue (on the same side) and will lead to paralysis and atrophy of the side affected.

FIGURE 46: Cortico-Bulbar Tracts — Nuclei of the Brainstem

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