5. Somatic Sensation
Revised July 14, 2010
The learning objectives of this chapter are to:
Describe the organization of the anterolateral system and the dorsal column-medial lemniscus system. In particular, be able to
- Describe the location of the primary, secondary, and tertiary sensory neurons for each system.
- Name the structures both systems have in common.
- For each system, describe the location of the axons of the second-order neurons at each major brain stem level.
- Describe the spatial relations between the two systems at each level.
- Describe the positions of these systems relative to the cranial nerve motor nuclei.
Somatic sensation is sensation that originates from receptors in the skin, muscles and joints. This chapter describes the two main systems that mediate conscious perception of somatic sensation from the body (somatic sensation from the head is covered in the next chapter):
2. The dorsal column-medial lemniscus (DCML) system mediates
- Two-point tactile discrimination: the ability to recognize that two blunt points closely applied to the skin are not a single point
- Stereognosis (recognition of object size, basic shape, and texture), tactile object recognition (identification of an object, e.g. as a key or a coin) and graphesthesia (identification of letters and numbers written on the skin)
- Proprioception (kinesthesia): Limb/joint position and direction of movement
- Light (simple, crude) touch
With disease of the DCML system, in addition to impairment of the above functions, the following three signs may occur:
Note that there will be little deficit in light touch if only one system is affected.
I. The Anterolateral System (ALS) (Spinothalamic Tract)
The anterolateral system is a three-neuron pathway.
The peripheral axons of the primary sensory neurons of the anterolateral system end as unencapsulated free nerve endings. These are the sensory receptors for the anterolateral system.
Primary sensory neurons
The primary sensory axons of the anterolateral system are mainly small-diameter myelinated (Aδ) and unmyelinated (C) cutaneous fibers (and the corresponding groups III and IV fibers in muscle nerves).
The cell bodies of the primary sensory neurons of both the anterolateral and DCML systems are located in dorsal root ganglia (DRG, spinal ganglia) (#4295). The bluish structures are myelinated axons. The large neuronal cell bodies are surrounded by numerous densely staining satellite cells (#4363) (actually only their nuclei are stained). The DRG neuronal cell bodies of the anterolateral system are smaller than those of the DCML system, corresponding to the diameters of their axons. Dorsal root ganglion neurons, which develop from neural crest cells, have a peripheral axonal process and a central axonal process. The peripheral 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.
In sections through the spinal cord, identify the dorsal horn (#6683) and Lissauer's tract (dorsolateral fasciculus) (#4542, fig 5a, fig 5b, fig 5c, fig 5d). What forms this tract? What is its functional significance?
Secondary sensory neurons
The cell bodies of the secondary sensory neurons of the anterolateral system are located in the dorsal horn.
The secondary sensory axons sweep across the midline, ventral to the central canal, in the anterior white commissure, and then ascend in the contralateral anterolateral white matter as a diffuse collection of fibers, the anterolateral system (#6717). The most important component of the anterolateral system is the spinothalamic tract, which is the major pathway in humans for pain and temperature sensation. How are the dermatomes represented in the anterolateral system?
In fig 5a, observe the central canal and notice its proximity to the anterior white commissure (#6713), in which the secondary sensory axons decussate (cross) to the other side of the cord. Should the canal expand, as in syringomyelia (#11730), what will happen to the commissure? What will be the result?
In the medulla, the anterolateral system axons (fig 6c, #6170) are lateral and dorsal to the inferior olivary nucleus. Where are they in respect to the nucleus ambiguus (fig 4e)? The anterolateral system and nucleus ambiguus are both in the dorsolateral quadrant of the medulla, nourished by branches of the vertebral artery, including small direct branches as well as the posterior inferior cerebellar artery (#8469). Hence, both the anterolateral system and the nucleus ambiguus are affected by infarction of this area (#9728) as part of the lateral medullary syndrome (Wallenberg's syndrome).
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).
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 nucleus (VPL) of the thalamus (#6734), the main relay nucleus of the thalamus for somatic sensation from the body. The axons of the anterolateral system that reach the thalamus are the spinothalamic tract.
(Spinothalamic axons also end in other thalamic nuclei besides the VPL. It should also be noted that many axons in the anterolateral system in the spinal cord do not reach the thalamus. Instead, they synapse in such areas as the reticular formation and the periaqueductal gray (#4772).)
Click for the Pathway Quiz
Click for the Movie: Anterolateral System 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. Charleston, South Carolina.
II. The Dorsal Column Medial Lemniscus (DCML) System
Like the anterolateral system, the dorsal column-medial lemniscus (DCML) system is a three-neuron pathway.
Receptors for the DCML system consist of cutaneous (skin) receptors and deep (muscle and joint) receptors. The four main types in glabrous (hairless) skin are Meissner corpuscles (#3999), Merkel cells, Ruffini endings, and Pacinian corpuscles (#15160). The main deep receptors are muscle spindles.
Primary sensory neurons
The primary sensory axons of the DCML system are mainly large-diameter myelinated fibers: Aα/β cutaneous fibers, and groups I and II fibers in muscle nerves.
The central axonal processes of the primary sensory neurons of the DCML system ascend in the ipsilateral dorsal column of the cord (#11684, fig 5a). In the lower cord, the dorsal column on each side consists of a single fasciculus, the gracile fasciculus (fig 5a sacral cord, fig 5b lumbar cord). In the upper cord, each dorsal column consists of two fasciculi, the gracile fasciculus and the cuneate fasciculus (fig 5c upper thoracic cord, fig 5d cervical cord).
How are the axons arranged in these fasciculi? What dermatomes are represented in the gracile fasciculus? In the cuneate fasciculus? 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?
Secondary sensory neurons
The cell bodies of the DCML secondary sensory neurons are within the dorsal column nuclei and their axons form the medial lemniscus.
Dorsal column nuclei
The dorsal column axons (#6691) ascend to the caudal medulla, where they synapse in the gracile nuclei and cuneate nuclei (fig 5k), which together are called the dorsal column 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 tubercle (#5274) and the cuneate tubercle (#5275). Located in them are the gracile and cuneate nuclei.
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 axons from the gracile and cuneate nuclei sweep ventrally and medially through the tegmentum. Along this portion of their course, they are called the internal arcuate fibers (fig 5k, #4063). They cross the midline (#4064) and turn rostrally. Along the rest of their course, these axons are known as the medial lemniscus (fig 6c). (Note: only the ventral two-thirds of the axons along the midline of the medulla belong to the medial lemniscus (fig 8b).) 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 (fig 6c); and the hypoglossal nerve rootlets are adjacent to the medial lemniscus on their way to exiting the medulla. All these structures are vascularized by branches of the anterior spinal artery in the caudal medulla, and by penetrating branches of the vertebral artery and vertebral-basilar junction in the rostral medulla. Medial medullary infarction (which usually occurs in the rostral medulla) (#41976), thus involves hypoglossal function, the medial lemniscus, and the pyramids, and will produce symptoms, the medial medullary syndrome, different from those of the lateral medullary syndrome.
In the caudal pons at the level of the abducens nucleus the medial lemniscus rounds up (#6175). In the mid pons at the level of the motor nucleus of V, it stretches out like a fibrous lid over the basilar pons (basis pontis, pons proper). It is at this level in the mid pons that the medial lemniscus (#6178) and anterolateral system (#6731) finally meet. The anterolateral system is located at the lateral edge of the medial lemniscus.
Sensory dissociation is a pattern of somatosensory loss in which only one of the two primary somatosensory systems (pain/temperature vs. vibration/proprioception/tactile discrimination) is affected in a specific body region.
Notice that in fig 5d and fig 6c, the DCML and anterolateral systems are separated and can be separately involved in neurologic problems. For example, a lesion in the dorsolateral medulla (e.g., lateral medullary syndrome) can 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, vibration and position sense, however, will be spared.
Another example is a lesion of the medial medulla (e.g., medial medullary syndrome) 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?
III. Thalamocortical Connections of the Anterolateral and DCML Systems
Tertiary sensory neurons
The cell bodies of the tertiary sensory neurons of both systems are within the thalamus, and their axons project to the primary somatosensory cortex.
The medial 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 nucleus (VPL) (#6750) of the thalamus. VPL contains the cell bodies of the tertiary sensory neurons of the anterolateral and DCML systems. It is organized somatotopically with the lower limb laterally and the upper limb medially. Does the left VPL represent the ipsilateral or contralateral body?
VPL is vascularized by thalamic 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?
Primary somatosensory cortex
The thalamus is the foyer to the cerebral cortex. 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 posterior limb of the internal capsule (fig 5l, #6390) to the primary somatosensory cortex (SI; Brodmann's areas 3a, 3b, 1, and 2), which occupies the postcentral gyrus (#4208, #74246) and the adjacent posterior part of the paracentral lobule (#74248, #74247). These axons are called the somatosensory radiation. They terminate in a somatotopic manner in the primary somatosensory cortex. The foot and leg are represented medially in the posterior part of the paracentral lobule; and the trunk, arm, and hand, are represented laterally in the upper half of the postcentral gyrus. 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 (#51167, fig 2e), and the lateral surface and internal capsule by branches of the middle cerebral artery (fig 2d). What will be the sensory loss due to interruption of the middle cerebral artery (MCA) near its origin? Where will the sensory loss occur with a more distal MCA lesion if the internal capsule is spared?
Other cortical processing
Affective response to pain. The pain pathway activates other cortical areas in addition to the primary somatosensory cortex. In particular, activation of the anterior cingulate gyrus is believed to be involved with the processing of the affective (emotional) / motivational responses to pain.
Graphesthesia and tactile object recognition. Perception of pain, temperature, light touch, two-point tactile discrimination, stereognosis, vibration, and proprioception all occur within the primary somatosensory cortex (SI). However, graphesthesia and tactile object recognition require synthesis of SI perceptions in other areas of the parietal cortex, and then association with previous knowledge to identify the figure drawn on the skin or the object being manually examined. In unusual cases, a lesion of the extra-SI parietal cortex may result in contralateral agraphesthesia or tactile agnosia while stereognosis and the other SI functions are little affected.
Click for the Syllabus Quiz
Three Critical Pathways
- Three Critical Pathways (2 sensory, 1 motor) Ignore trigeminothalamic pathways until Chapter 6.
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