Mlf and associated tracts

This diagram shows the brainstem from the posterior perspective (as in Figure 10 and Figure 40). Note the orientation of the spinal cord (with the ventral horn away from the viewer).

The MLF is a tract within the brainstem and upper spinal cord that links the visual world and vestibular events with the movements of the eyes and the neck, as well as linking up the nuclei that are responsible for eye movements. The tract runs from the midbrain level to the upper thoracic level of the spinal cord. It has a rather constant location near the midline, dorsally, just anterior to the aqueduct of the midbrain and the fourth ventricle (see brainstem cross-sections, e.g., Figure 65A, Figure 66A, and Figure 67A).

The MLF is, in fact, composed of several tracts running together:

• Vestibular fibers: Of the four vestibular nuclei (see previous illustration), descending fibers originate from the medial vestibular nuclei and become part of the MLF; this can be named separately the medial vestibulo-spinal tract. There are also ascending fibers that come from the medial, inferior, and superior vestibular nuclei that also are carried in the MLF. Therefore, the MLF carries both ascending and descending vestibular fibers.

• Visuomotor fibers: The interconnections between the various nuclei concerned with eye movements are carried in the MLF (as described in the previous illustration).

• Vision-related fibers: Visual information is received by various brainstem nuclei.

• The superior colliculus is a nucleus for the coordination of visual-related reflexes, including eye movements (see Figure 9A). The superior colliculus coordinates the movements of the eyes and the turning of the neck in response to visual information. It also receives input from the visual association cortical areas, areas 18 and 19 (see Figure 17 and Figure 41B). The descending fibers from the superior colliculus, called the tecto-spinal tract, are closely associated with the MLF and can be considered part of this system (although in most books it is discussed separately). As shown in the upper inset, these fibers cross in the midbrain. (Note that the superior colliculus [SC] of only one side is shown in order not to obscure the crossing fiber systems at that level.)

• The small interstitial nucleus and its contribution have already been noted and discussed with the previous illustration.

The lower inset shows the MLF in the ventral funic-ulus (white matter) of the spinal cord, at the cervical level (see Figure 68 and Figure 69). The three components of the tract are identified, those coming from the medial vestibular nucleus, the fibers from the interstitial nucleus, and the tecto-spinal tract. These fibers are mingled together in the MLF.

In summary, the MLF is a complex fiber bundle that is necessary for the proper functioning of the visual apparatus. The MLF interconnects the three cranial nerve nuclei responsible for movements of the eyes, with the motor nuclei controlling the movements of the head and neck. It allows the visual movements to be influenced by vestibular, visual, and other information, and carries fibers (upward and downward) that coordinate the eye movements with the turning of the neck.

The diagram also shows the posterior commissure (not labeled). This small commissure carries fibers connecting the superior colliculi. In addition, it carries the important fibers for the consensual pupillary light reflex coordinated in the pretectal "nucleus"' (discussed with Figure 41C).

Clinical Aspect

A lesion of the MLF interferes with the normal conjugate movements of the eyes. When a person is asked to follow an object (e.g., the tip of a pencil moving to the right) with the head steady, the two eyes move together in the horizontal plane. With a lesion of the MLF (such as demy-elination in multiple sclerosis), the abducting eye (the right eye) moves normally but the adducting eye (the left eye) fails to follow; yet, adduction is preserved on convergence. Clearly the nuclei and the nerves are intact; the lesion, then, is in the fibers coordinating the movement. This condition is known as internuclear ophthalmople-gia. Sometimes there is also monocular horizontal nystagmus (rapid side-to-side movements) of the abducting eye.

Interstitial n. Pretectal area

Interstitial n. Red n

Interstitial n. Pretectal area

Vestibulocochlear nerve (CN VIII)

Medial vestibulo-spinal tract Lateral vestibulo-spinal tract

Medial Longitudinal Fasciculus

Superior colliculus

Oculomotor n.

Trochlear n.

Medial longitudinal fasciculus (MLF)

Abducens n.

Tecto-spinal tract Interstitio-spinal tract

Interstitio-spinal fibers Tecto-spinal fibers Superior colliculus MLF

Vestibulocochlear nerve (CN VIII)

Medial vestibulo-spinal tract Lateral vestibulo-spinal tract

Tecto-spinal tract Interstitio-spinal tract

Medial vestibulo-spinal tract Lateral vestibulo-spinal tract sV = Superior vestibular n. lV = Lateral vestibular n. mV = Medial vestibular n. iV = Inferior vestibular n.

Superior colliculus

Oculomotor n.

Trochlear n.

Medial longitudinal fasciculus (MLF)

Abducens n.

Cervical spinal cord

FIGURE 51B: Medial Longitudinal Fasciculus (MLF)

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Responses

  • Kieran
    What are associated tracts?
    8 years ago

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