5. Somesthetic Sensation

The objectives of this chapter are to:

  1. Understand the organization of the anterolateral system.
  2. Understand the organization of the dorsal column-medial lemniscus system.

While accomplishing these objectives, it is important to think about the significance of these sensory systems in your daily experience. For example, what constraints do they place on you? If they are interrupted, what results? How would you be incapacitated? And, importantly, how would this appear to others?

The anterolateral system, also called the spinothalamic tract, is primarily involved with temperature and pain sensation. The dorsal column-medial lemniscus system functions mainly in discriminative touch (also called discrete touch) and position sense. The activity of both these somatic sensory systems begins in the fine-mesh screen of receptors in the body wall (#11841) that separates all of the "out-of-doors" from our subcutaneous selves. This receptor screen supplies the CNS with raw data about environmental temperature and about objects touching the body. Hence, we speak of contact sensation. Using these sensory data, the CNS constructs an impression of the immediate environment. With eyes closed and ears plugged while holding your breath, it is possible to tell whether the air is cool or warm, windy or calm, wet or dry. Think of even the banal things told us by our fingers. This contact and kinesthetic information is also important in developing awareness of our body. Thus, the somesthetic sensory systems are significant in constructing that fundamental, tridimensional body image we each have of ourselves.

The anterolateral and dorsal column-medial lemniscus systems have several structural features in common. These should become obvious. The following questions should be answered for each system: (1) Where are the primary, secondary, and tertiary sensory neurons located? (2) Where are the axons of the second-order cells (e.g., anterolateral system and medial lemniscus) located in each major brain stem level? (3) What are the spatial relations between these tracts at each level? (4) What are their positions relative to the cranial nerve motor nuclei?

The primary sensory neurons are associated with receptors, for example, a Pacinian corpuscle (#15160) or a Meissner corpuscle (#3999). They are examples of encapsulated receptors, which are associated with the primary sensory neurons of the dorsal column-medial lemniscus system. The peripheral axons of the primary sensory neurons of the anterolateral system end blindly in the skin, that is, they are naked or unencapsulated nerve endings. Their tips act as receptor organs. Whereas all the primary sensory axons of the dorsal column-medial lemniscus system are myelinated, the majority of the primary sensory axons of the anterolateral system are unmyelinated.

The cell bodies of the primary sensory neurons of both systems are located in dorsal root ganglia (#4295). The bluish structures are myelinated axons. The large cell bodies are surrounded by numerous densely staining satellite cells (#4363) (actually only their nuclei are stained). Dorsal root ganglion cells, which develop from neural crest cells, have a peripheral axonal process and a central axonal process. The peripheral process ends in the somatic body wall in relation to a receptor. The axons of one dorsal root ganglion innervate a particular stretch of skin called a dermatome. The central process enters the spinal cord through a dorsal root.

I. The Anterolateral System

In sections through the spinal cord, identify the dorsal horn (#6683) and tract of Lissauer (= dorsolateral fasciculus) (#4542).

(fig 5a) (fig 5b) (fig 5c) (fig 5d)

What forms this tract? What is its functional significance? The gray matter of the spinal cord is divided into a series of laminae. Each lamina has a characteristic set of cells and connections. The laminae are numbered I - X and are referred to as Rexed's laminae in honor of the Norwegian anatomist who first described them. The dorsal horn consists of laminae I - VI. Lamina I is adjacent to the tract of Lissauer and contains large cells (#52407 Golgi stain in a cat). Lamina II is deep to lamina I and corresponds to the substantia gelatinosa (#6686). (The large motor neurons of the ventral horn occur in clumps that are designated as lamina IX, and the central canal is surrounded by lamina X. (#6712)). The secondary sensory cell bodies of the anterolateral system occur mainly in laminae I and V. Their axons sweep across the midline, ventral to the central canal, and go through the opposite ventral horn into the white matter, where they collect to form the anterolateral system (#6717).

Next, locate the area of the anterolateral system fibers in the lateral and ventral funiculi. Although two separate anterolateral system tracts (lateral and ventral) used to be identified, they are now considered one large fasciculus. How are the secondary sensory axons arranged in the anterolateral system? How are the dermatomes represented in it?

In (fig 5c) observe the central canal and notice its proximity to the anterior white commissure (#6713), in which the secondary sensory axons decussate (i.e., cross to the other side of the cord). Should the canal expand, as it sometimes does in syringomyelia (#11730), what will happen to the commissure? What will be the result?

In the medulla (fig 5e), the anterolateral system axons are lateral and dorsal to the inferior olivary nucleus (#6170). Where are they in respect to the nucleus ambiguus? The anterolateral system and nucleus ambiguus are both nourished by branches of the posterior inferior cerebellar artery (#8469); hence, they face ischemia together. Only the dorsolateral quadrant of the medulla is vascularized by this artery. The midline structures, extending from the hypoglossal nucleus to the pyramids, are nourished by branches of the anterior spinal artery (#4194). Consequently, occlusion of this artery will produce symptoms different from those caused by the occlusion of the posterior inferior cerebellar artery. Predict what cranial nerve would be involved with occlusion of the anterior spinal artery?

In the pons (fig 5f, fig 5g), the anterolateral system is in the ventrolateral tegmentum, similar to its location in the medulla. It maintains this relative position throughout its course in the brain stem (fig 5h). It is near the pial surface at the level of the inferior colliculus (#6606). Neurosurgeons have taken advantage of this superficial location to transect the tract (a mesencephalic tractotomy operation). Assuming that such an operation is successful, what will the sensory examination show? What dermatomes will be affected and on which side of the body?

At the level of the junction between the midbrain and thalamus, the anterolateral system fibers join with those of the medial lemniscus (to be discussed next) (fig 5i). They can be followed together into the thalamus (fig 5j). Both the anterolateral system and medial lemniscus terminate in the ventral posterolateral thalamic nucleus (#6734), the specific sensory nucleus for somatic sensation.

In addition, some anterolateral system axons end in intralaminar thalamic nuclei (#8296). (It should be noted here that a large portion of the axons in the anterolateral system in the spinal cord do not reach the thalamus. Instead, they synapse in the reticular formation of the medulla (#10050), pons (#10152) and in the midbrain tegmentum (#10241) as well as the central gray matter of the midbrain (#4772).

Click for the Pathway Quiz
Click for the Movie From: "Parallel Pathways, A Tour Through the CNS". Edward Allen Neilson, W. Curtis Wise, Henry F. Martin. Department of Physiology, The Medical University of South Carolina. Charlston, South Carolina.

II. The Dorsal Column Medial Lemniscus System (DCML)

The central axonal processes of the primary sensory neurons of the DCML form the dorsal columns of the cord (#11684).

(fig 5a) (fig 5b) (fig 5c) (fig 5d)

In cervical segments, each dorsal column consists of two fasciculi or tracts, the gracile fasciculus (#6718) and cuneate fasciculus (#6719). How are the axons arranged in these fasciculi? What dermatomes are represented in the gracile fasciculus? In the cuneate fasciculus? Is there a cuneate fasciculus in sacral segments? Where are the cell bodies for the axons in these fasciculi? Is the cuneate fasciculus on the left side ipsilateral or contralateral to the dermatomes represented in it?

The dorsal column axons (#6691) synapse in the dorsal column nuclei, also known as the gracile and cuneate nuclei. They contain the cell bodies of the secondary sensory neurons of the DCML system. On the dorsal surface of the medulla, on either side of midline, are protuberances known as the gracile tubercles (#5274) and cuneate tubercles (#5275). Located in them are the gracile nuclei and cuneate nuclei (fig 5k). What axons synapse in these nuclei? What dermatomes are represented in the gracile nuclei? In the cuneate nuclei? Is the cuneate nucleus on the left ipsilateral or contralateral to the dermatomes that are represented in it? The gracile and cuneate nuclei (= dorsal column nuclei) are seen in sections through caudal medulla. What fiber system originates in them?

The axons from the gracile and cuneate nuclei sweep ventrally and medially through the tegmentum (fig 5k). Along this portion of their course they are called the internal arcuate fibers (#4063). They cross the midline (#4064) and turn rostrally (fig 5e). Along the rest of their course, these axons are known as the medial lemniscus. Thus, the medial lemniscus is composed of the secondary sensory axons of the DCML system. In the medulla, the medial lemniscus is a column of axons ventral to the hypoglossal nucleus and dorsal to the pyramid. All three structures are vascularized by branches of the anterior spinal artery. (Note: only the ventral two-thirds of the axons along the midline of the medulla belong to the medial lemniscus.)

In the pons at the level of the abducens nucleus the lemniscus rounds up (#6175). But, at the level of the motor nucleus of V, it stretches out like a fibrous lid over pons proper. It is at this level that the medial lemniscus (#6178) and anterolateral system (#6731) finally meet. The anterolateral system is located at the lateral edge of the lemniscus. In the midbrain, as the lemniscus becomes displaced laterally (#6180), the anterolateral system (#6606) shifts dorsolaterally, but it is still joined to the lemniscus.

Notice that in (fig 5d) and (fig 5e) the DCML and anterolateral systems are separated and can be separately involved in neurologic problems. For example, a lesion in the dorsolateral medulla will interrupt the function of the anterolateral system but not the DCML. Thus, there will be a loss of temperature and pain sensation on the opposite side of the body. Discrete tactile and position sense, however, will be spared. The loss of some cutaneous sensations and the sparing of others in the same region of the body is called sensory dissociation. Another example of this is a midline lesion of the medulla that disrupts the medial lemniscus and not the anterolateral system. In this case, discrete touch and position sense are lost on the opposite side of the body while temperature sensation and pain remain intact. What sensory dissociation occurs with a lesion involving all the left half of the cord?

The lemniscus and anterolateral system are dorsal and lateral to the red nucleus, producing a "Cleopatra eye-like" effect (fig 5l). They travel forward into the thalamus and terminate in the ventral posterolateral (VPL) (#6750) nucleus of thalamus. This thalamic nucleus contains the cell bodies of the tertiary sensory neurons of the anterolateral and DCML systems. The medial lemniscus and anterolateral system end in the nucleus in a somatotopic sequence: caudal dermatomes are represented laterally and rostral dermatomes, medially. Is the left VPL ipsilateral or contralateral to the dermatomes represented in it? The somatotopic representation of the body in VPL is not a perfect miniature of the individual. Certain regions of the body are exaggerated. What regions are they? What is the basis of this exaggeration? VPL is vascularized by the thalamogeniculate branches of the posterior cerebral artery. Disease of these vessels can produce "pure sensory strokes." What sensations will be lost? Where will the sensory loss occur?

The thalamus is the portal or foyer to the cerebral cortex (fig 5l). The sensory systems project to specific thalamic nuclei that, in turn, project to restricted areas of the cerebral cortex. Axons of VPL neurons project through the internal capsule (#6390) to the primary somatosensory cortex (somesthetic cortex) of the postcentral gyrus (#4208). These axons are called the somatosensory radiation. They terminate in a somatotopic manner in the postcentral gyrus. The foot and leg are represented on the medial surface of the postcentral gyrus and the trunk, arm, and hand, on the lateral surface. Here, too, body geometry is distorted. What body regions are exaggerated in relation to the rest of the body? What is the basis for this? Recall that the medial surface of the hemisphere is supplied by branches of the anterior cerebral artery (#6061) and the lateral surface by branches of the middle cerebral artery (#8508, #9812). What will be the sensory loss due to interruption of the middle cerebral artery? Where will the sensory loss occur?

The longitudinal extent of the medial lemniscus can be appreciated in a stained parasagittal section through a monkey brain (fig 5m). The medial lemniscus is the compact bundle that extends from the pons to the thalamus. The ventral posterolateral nucleus of the thalamus can also be seen (#6192).

Click for the Syllabus Quiz
Click for the Pathway Quiz
Click for the Movie From: "Parallel Pathways, A Tour Through the CNS". Edward Allen Neilson, W. Curtis Wise, Henry F. Martin. Department of Physiology, The Medical University of South Carolina. Charlston, South Carolina.

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