Sensory nuclei and ascending tracts

Dorn Spinal Therapy

Spine Healing Therapy

Get Instant Access

This diagrammatic presentation of the internal structures of the brainstem is shown from the dorsal perspective (as in Figure 10 and Figure 36). The information concerning the various structures will be presented in an abbreviated manner, as most of the major points have been reviewed previously. The orientation of the cervical spinal cord representation should be noted.

The major sensory systems include:

• Dorsal column-medial lemniscus (discriminative touch, joint position, and vibration) and its nuclei

• Anterolateral system (pain and temperature)

• Trigeminal system and its nuclei (discriminative touch, pain, and temperature)

• Lateral lemniscus (audition), with its nuclei

The Dorsal Column-Medial Lemniscus

The dorsal columns (gracile and cuneate tracts) of the spinal cord terminate (synapse) in the nuclei gracilis and cuneatus in the lowermost medulla (see Figure 9B). Axons from these nuclei then cross the midline (decussate) as the internal arcuate fibers (see Figure 67C), forming a new bundle called the medial lemniscus. These fibers ascend through the medulla, change orientation in the pons, and move laterally, occupying a lateral position in the mid-brain.

The Anterolateral System

This tract, having already crossed in the spinal cord, ascends and continues through the brainstem. In the medulla it is situated posterior to the inferior olive. At the upper pontine level, this tract becomes associated with the medial lemniscus, and the two lie adjacent to each other in the midbrain region.

The Trigeminal Pathway

The sensory afferents for discriminative touch synapse in the principal nucleus of V; the fibers then cross at the level of the mid-pons and form a tract that joins the medial lemniscus. The pain and temperature fibers descend and form the descending trigeminal tract through the medulla with the nucleus adjacent to it. These fibers synapse and cross, over a wide area of the medulla, eventually joining the other trigeminal tract. The two tracts form the trigem-inal pathway, which joins with the medial lemniscus in the uppermost pons (see Figure 36).

The Lateral Lemniscus

The auditory fibers (of CN VIII) enter the brainstem at the uppermost portion of the medulla. After the initial synapse in the cochlear nuclei, many of the fibers cross the midline, forming the trapezoid body. Some of the fibers synapse in the superior olivary complex. From this point, the tract known as the lateral lemniscus is formed. The fibers relay in the inferior colliculus.

Clinical Aspect

This diagram allows the visualization of all the pathways together, which assists in understanding lesions of the brainstem. The cranial nerve nuclei affected help locate the level of the lesion.

One of the classic lesions of the brainstem is an infarct of the lateral medulla (see Figure 67B), known as the Wallenberg syndrome. (The blood supply of the brainstem is reviewed with Figure 58.) This lesion affects the pathways and cranial nerve nuclei located in the lateral area of the medulla, including the anterolateral tract and the lateral lemniscus, but not the medial lemniscus; the descending trigeminal system is also involved, as are the nuclei of CN IX and X. Additional deficits may include vestibular or cerebellar signs, as the vestibular nuclei are nearby and afferents to the cerebellum may be interrupted. Notwithstanding the fact that the lateral lemniscus is most likely involved in this lesion, auditory deficits are not commonly associated with this clinical syndrome, probably due to the fact that this is a bilateral pathway. The lateral meduallary syndrome is discussed with Figure 67B.

Additional Detail

The superior cerebellar peduncles are shown in this diagram, although not part of the sensory systems. These will be described with the cerebellum (see Figure 57). This fiber pathway from the cerebellum to the thalamus decussates in the lower midbrain at the inferior collicular level (shown in cross-section, see Figure 65B).

The red nucleus is one of the prominent structures of the midbrain (see Figure 65A); its contribution to motor function in humans is not yet clear (discussed with Figure 47).

Decussation of superior cerebellar peduncles

Inferior colliculus

Superior cerebellar peduncle

Trigeminal nerve (CN V)

Principal n. of V

Decussation of superior cerebellar peduncles

Inferior colliculus

Superior cerebellar peduncle

Spinal Nucleus Trigeminal

Anterolateral system Trigeminal pathway

Medial lemniscus

Medial lemniscus Internal arcuate fibers Cuneatus n. Gracilis n.

Cuneatus tract Gracilis tract

Dorsal root of spinal nerve

Trigeminal nerve (CN V)

Principal n. of V

Descending (spinal) tract of V

Descending (spinal) n. of V

Anterolateral system

Cervical spinal cord

Descending (spinal) tract of V

Descending (spinal) n. of V

Anterolateral system

Cervical spinal cord

Anterolateral system Trigeminal pathway

Medial lemniscus

Lateral lemniscus

Trapezoid body

Superior olivary complex

Cochlear nn.

Vestibulocochlear nerve (CN VIII)

Medial lemniscus Internal arcuate fibers Cuneatus n. Gracilis n.

Cuneatus tract Gracilis tract

Dorsal root of spinal nerve

FIGURE 40: Sensory Systems — Sensory Nuclei and Ascending Tracts

Was this article helpful?

0 0
Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Post a comment