Chapter 10: Brainstem I: The Medulla

Intro

Objectives

In this chapter, the student is expected to:

  1. Develop a general understanding of the gross anatomical arrangement of the medulla.
  2. Know the internal organization of the major fiber pathways that ascend and descend through the medulla.
  3. Be able to identify the major cell groups at different levels of the medulla.
  4. Develop the basis for an understanding of why syndromes associated with damage to the lateral aspect of the lower brainstem differ from those associated with the medial aspect of the lower brainstem.

Gross Anatomical View and Internal Organization

Gross Anatomical View

The purpose of this chapter is to begin to develop an understanding of the organization of the brainstem by considering the neuroanatomy of the medulla. Knowledge of the anatomy of the principal neural cell groups and pathways of the medulla is essential in acquiring an appreciation of their functions and insight into the clinical disorders resulting from damage to particular areas of this region of the brainstem. One may view the medulla as an extension of the spinal cord, in that it contains a number of the fiber tracts present in the spinal cord and, like the spinal cord, also includes sensory, motor, and autonomic neurons.

Thus, the principal pathways include long ascending and descending fiber systems that begin or terminate in the spinal cord and that pass through the medulla. Several other pathways mediating sensory, motor, and autonomic functions arise from the medulla and, to a considerable extent, are linked to cranial nerves. In addition, one of the three cerebellar peduncles, which connect the cerebellum to the brainstem, includes the inferior cerebellar peduncle. This massive fiber bundle represents an important conduit for transmission of information from the spinal cord and medulla to the cerebellum and is attached to the cerebellum from the dorsolateral aspect of the rostral medulla.

The medulla, or myelencephalon, is located between the pons and spinal cord. At its caudal end, it is continuous with the spinal cord. At its rostral end, the medulla is continuous with the pons (Fig. 10-1). Within the caudal half of the medulla, one can observe the positions of the fasciculus gracilis and fasciculus cuneatus, which ascend from the spinal cord (Fig. 10-1B). These tracts mediate conscious proprioception, vibratory sensation, and some tactile sensation from the body to the brain. As these structures pass in a rostral direction, they form two swellings (seen on the dorsal surface) referred to as the tuberculum gracilis and tuberculum cuneatus (or gracile and cuneate tubercles [Figs. 10-1, 10-2 and 10-3]).

They reflect the underlying nucleus gracilis and nucleus cuneatus (the latter lies immediately lateral to the nucleus gracilis); these nuclei receive inputs from the fasciculus gracilis and cuneatus, respectively, and thus constitute a relay system for the transmission of conscious proprioception, vibratory sensation, and tactile impulses to higher regions of the brain. The gracile and cuneate tubercles extend rostrally to a level where the fourth ventricle begins.

The rostral third of the medulla, which contains a portion of the fourth ventricle, is known as the “open” part of the medulla (described later in the “Levels of the Medulla” section). The part of the medulla where the central canal is present is referred to as the “closed” part of the medulla (described later). At the rostral end of the medulla, the dorsolateral surface is expanded to form the inferior cerebellar peduncle, which contains proprioceptive and vestibular fibers which project to the cerebellum (see Chapter 1, Fig. 1-8 for orientation).

Figure 10-1. Different views of brainstem.

Different views of brainstem.

(A) Anterior view of the medulla. (B) Posterior view of the medulla. Note the position at which the fourth ventricle begins (the “open” part of the medulla) at the level of the area postrema. In this illustration, the cerebellum has been removed in order to see the structures situated on the dorsal surface of the medulla. The midbrain and pons are included for purposes of orientation. Cranial nerves (CN) VII–X and XII are presented to illustrate their medial or lateral positions along the neuraxis of the medulla.

Figure 10-2. Ascending and descending pathways of fiber tracts and associated nuclei.

Ascending and descending pathways of fiber tracts and associated nuclei.

Note the most prominent ascending pathways (the dorsal column-medial lemniscus) and descending pathways (the medial longitudinal fasciculus, corticospinal and corticobulbar tracts, and descending tract of cranial nerve [CN] V) that traverse the medulla. In the lower half of the medulla, the nucleus gracilis and nucleus cuneatus are shown. At more rostral levels, the relative positions of the nucleus ambiguus (CN IX and X); spinal nucleus of CN V and its rostral extension in the pons; the main sensory nucleus of CN V; hypoglossal nucleus (CN XII); inferior olivary nucleus; inferior (I), medial (M), lateral (L), and superior (S) vestibular nuclei; and cochlear nuclei (CN VIII) are depicted.

Figure 10-3. Cross-sectional diagrams of the lower medulla.

Cross-sectional diagrams of the lower medulla.

(A) Level of the decussation of the pyramids. (B) Level of the decussation of the medial lemniscus. CN = cranial nerve.

Along the medial aspect of the ventral surface of the medulla, there is a swelling referred to as the pyramid, which is also present throughout the rostro-caudal extent of the medulla. The pyramid is composed of numerous nerve fibers that arise from the precentral, postcentral, and premotor regions of the cerebral cortex. These nerve fibers pass to the spinal cord as the corticospinal tracts or terminate within the medulla as corticobulbar fibers. As described in Chapter 19, the corticospinal tract mediates voluntary control over movements of the body. Similarly, the corticobulbar tract mediates voluntary control over movements of the head region.

At the caudal end of the medulla immediately above the rostral end of the spinal cord, the pyramid is considerably smaller because, at this level, 90% of the fibers are crossed in a dorsolateral direction to the opposite side in a series of bundles called the decussation of the pyramids. The decussated axons then descend into the spinal cord as the lateral corticospinal tract (Fig. 10-3). The remaining (uncrossed) component of the corticospinal tract passes ipsilaterally into the ventral white matter of the spinal cord as the ventral corticospinal tract (see Chapter 9).

At the level of the rostral half of the medulla, another swelling located just lateral to the pyramids is referred to as the inferior olivary nucleus.

Internal Organization

Major Fiber Tracts and Associated Nuclei

Figures 10-2 and 10-3 depict major fiber tracts and their associated nuclei.

Pyramidal (Corticospinal) Tract

The pyramidal tract is situated in a ventromedial position throughout most of the medulla but changes position at its caudal end. Here, approximately 90% of the fibers, en route to the spinal cord, pass in a dorsolateral direction while crossing the midline to reach the dorsolateral aspect of the caudal medulla. The fibers, which cross to the contralateral side of the brainstem, form the decussation of the pyramid and descend into the dorsolateral aspect of the lateral funiculus of the spinal cord as the lateral corticospinal tract. As described in Chapter 9, the corticospinal tract mediates voluntary control over motor responses, and damage to this pathway results in an upper motor neuron paralysis.

Medial Lemniscus

The medial lemniscus originates from the dorsal column nuclei (described in Chapter 9). These fibers collectively pass ventrally for a short distance in an arc-like trajectory and are referred to as internal arcuate fibers. These second-order somatosensory fibers then cross to the opposite side of the medulla as the decussation of the medial lemniscus. The medial lemniscal fibers then pass rostrally in a medial position throughout the remainder of their trajectory through the medulla and continue into the pons. The function of the medial lemniscus is to transmit information associated mainly with conscious proprioception and vibratory stimuli to the thalamus (Fig. 10-2).

The Medial Longitudinal Fasciculus

The medial longitudinal fasciculus (MLF) tract is located in a dorsomedial position within the medulla and consists of both ascending and descending axons (Fig. 10-2 and Fig. 10-3B). Ascending axons arising from the lateral, medial, and superior vestibular nuclei project to the pons and midbrain; they provide information about the position of the head in space to cranial nerve nuclei that mediate control over the position and movements of the eyes. The descending fibers, which arise from the medial vestibular nucleus and pass caudally to cervical levels of the cord, are often called the medial vestibulospinal tract. This tract serves to adjust changes in the position of the head in response to changes in vestibular inputs.

Descending Tract of Nerve V

The descending fibers of the trigeminal nerve extend caudally as far as the second cervical segment and occupy a far lateral position within the medulla immediately lateral to the spinal nucleus of the trigeminal nerve (cranial nerve [CN] V [Figs. 10-2 and 10-3]). The spinal nucleus of CN V is present throughout the entire length of the medulla, extending from the level of the lower pons caudally to the spinal cord–medulla border, where it becomes continuous with the substantia gelatinosa. In the middle of the pons, the spinal nucleus is replaced by the main sensory nucleus of CN V.

The descending fibers of CN V constitute first-order axons that mediate somatosensory inputs from the head region to the brain; they synapse at different levels along the rostro-caudal axis of the spinal nucleus. Therefore, the spinal nucleus constitutes the second-order neuron, which transmits somatosensory information from the head region to the thalamus (for subsequent transmission to the cerebral cortex). Further details concerning functions of the trigeminal nuclei are presented in Chapter 14.

Other Fiber Tracts

Several other tracts, shown in part in Figure 10-3, reflect pathways that mediate sensory and motor functions. Sensory tracts include the spinothalamic and spinocerebellar pathways. The spinothalamic and spinocerebellar fibers are situated laterally within the medulla and, thus, retain the same general positions that they occupied within the lateral funiculus of the spinal cord. The lateral spinothalamic tract mediates pain and temperature sensation from the contralateral side of the body to the thalamus, and the spinocerebellar tracts mediate unconscious proprioception (i.e., from muscle spindles and Golgi tendon organs) to the cerebellum.

Tracts mediating motor functions in addition to the corticospinal tract include the tectospinal tract, which follows the MLF to cervical levels and mediates postural movements from the superior colliculus; the rubrospinal tract, which passes in a ventrolateral position in the medulla from the red nucleus to all levels of the spinal cord and facilitates spinal cord flexor motor neuron activity; and the lateral and medial reticulospinal tracts, which arise from the medulla and pons, respectively, pass to different levels of the spinal cord, and modulate muscle tone. The functions of these tracts are considered in greater detail in Chapters 9, 19, 23, and 26.

Internal Nuclei of the Brainstem

Reticular Formation

The internal core of the entire brainstem contains mainly a complex set of neuronal groups (coupled with related fiber bundles) that are collectively referred to as the reticular formation. The nature of the structural and functional organization of this complex region of the brainstem is considered in detail in Chapter 23. In brief, it appears to be involved in such processes as modulation of sensory transmission to the cortex, regulation of motor activity, autonomic regulation, sleep and wakefulness cycles, and modulation of emotional behavior.

Major Nuclei of the Medulla

In the lower levels of the medulla, the principal nuclei of the caudal (or lower) medulla include the nucleus gracilis and nucleus cuneatus, which receive first-order somatosensory fibers from the fasciculus gracilis and fasciculus cuneatus, respectively, and the spinal nucleus of CN V, which receives first-order pain and temperature signals from the region of the head (Fig. 10-2).

In the upper levels of the medulla, numerous important nuclei are present. The spinal nucleus is present at this level, as it is at lower levels of the medulla. Other sensory neurons include the cochlear nuclei, which receive first-order auditory fibers (CN VIII), and vestibular nuclei, which also receive first-order vestibular inputs (CN VIII). In addition, a number of different cranial nerve nuclei appear. These include somatomotor nuclei (hypoglossal nucleus [CN XII] and nucleus ambiguus of CN X and CN IX) and sensory and motor autonomic nuclei (the solitary nucleus [CN IX and X] and dorsal motor nucleus of CN X)

Levels of the Medulla

In describing the anatomy of the medulla, we can distinguish at least two levels. The caudal half, which includes the “closed” medulla, contains two decussations: (1) the pyramidal decussation and (2) a second level that contains the decussation of the medial lemniscus. The upper half of the medulla, which includes the “open” medulla, contains the inferior olivary nucleus. The following sections consider the neuroanatomical organization of the medulla on the basis of the structures contained at each of these levels.

Level of the Pyramidal Decussation

At the level of the pyramidal decussation, the general organization of the medulla is very similar to that of the cervical spinal cord. For example, the dorsal aspect of this level of the medulla contains the fasciculus gracilis and fasciculus cuneatus (Figs. 10-3 and 10-4). The spinocerebellar and spinothalamic tracts also lie in the same general lateral position they occupy within the spinal cord. Some anterior horn cells can generally be seen at this level as well.

Figure 10-4. Photograph of a cross section of the human medulla at the level of the decussation of the pyramids (Weigert myelin stain).

Photograph of a cross section of the human medulla at the level of the decussation of the pyramids (Weigert myelin stain).

Note that the fasciculus gracilis and fasciculus cuneatus are still clearly present at this level, and note the positions occupied by the ascending sensory axons. (Reproduced with permission from Parent A: Carpenter’s Human Neuroanatomy, 9th ed. Baltimore: Williams & Wilkins, 1996, P. 424.)

There are major new features found at this level of medulla that are not present in the spinal cord. The most obvious change is the presence of the pyramidal decussation. As indicated earlier, about 90% of the fibers of each pyramid cross over to the contralateral side of the brain in their descent into the spinal cord as the lateral corticospinal tract. Approximately 8% of the fibers continue to descend uncrossed into the spinal cord as the anterior corticospinal tract and are situated in a ventromedial position; ultimately, these fibers cross over to the opposite side of the spinal cord. The remaining 2% of the corticospinal tract supplies the ipsilateral spinal cord grey matter.

A second change is the presence of the spinal nucleus of the trigeminal nerve. It replaces the substantia gelatinosa, in that it is situated in the dorsolateral aspect of the medulla, which is the approximate position occupied by the substantia gelatinosa in the dorsal horn of the spinal cord. The anatomical similarities between the substantia gelatinosa and the spinal nucleus of CN V are matched by their functional similarities. Both structures mediate pain and temperature modalities of sensation. The difference between the two relates to the fact that the substantia gelatinosa mediates these sensations from the body, whereas the spinal nucleus mediates sensations associated with the region of the anterior two thirds of the head, oral and nasal cavities, and the cutaneous surfaces of the ear and external auditory meatus.