5. Somatosensory Processes
Part 9 of 10
Patrick Dougherty, Ph.D.
(Content provided by Chiyeko Tsuchitani, Ph.D.).
Clinical Examples:
Central Nervous System: The Brain
Brain Stem. Trauma, stroke, multiple sclerosis (a disease of myelin), and brain tumors are the major causes of brain stem lesions. The location of the lesion site can often be deduced by the loss in cranial nerve function.
- Eleven of the thirteen cranial nerves are associated with the brain stem and may be damaged with brain stem lesions.
- Most of the 1° somatosensory afferents carrying crude touch, pain and temperature information from the face enter the brain stem at mid pontine levels and descend down to second cervical segment of the spinal cord.
- Axons in the spinothalamic pathway decussate in the spinal cord. The decussation in the medial lemniscal pathway occurs in the lower medulla. Consequently, both the spinothalamic tract and medial lemniscus represent the contralateral side of the body at and above the upper (open) medulla.
- The spinal trigeminal 2° afferents decussate upon leaving the spinal trigeminal nucleus in the medulla and lower pons, whereas the main sensory trigeminal nucleus axons decussate at mid pons levels immediately upon leaving the nucleus.
- Above the level of the pons, all of the somatosensory pathways have decussated and are traveling in close proximity.
- The initial higher order processing the discriminative touch and proprioception occurs exclusively within the primary somatosensory cortex. In contrast, pain information is processed in multiple cortical areas.
The patient suffers from a decrease in pain and temperature sensations involving the left side his body and the right side of his face.
Figure 5.32 |
Symptoms: The patient exhibits decrease in pain and temperature sensations that involve the left side of his body and right side of his face (Figure 5.32). Discriminative touch, vibration and position senses are normal in these areas. Touch, vibration, position, temperature, and pain sensations are normal for the rest of the body and face. You conclude that the somatosensory losses involve
Pathway(s) Affected: You conclude that structures in the following somatosensory pathways (Figure 5.33) have been affected
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| Neurons of the spinothalamic pathways (NSTP, neospinothalamic and PSTP, paleospinothalamic) process pain, temperature and crude touch information from the body. Whereas neurons of the spinal trigeminal pathway (STP) process pain, temperature and crude touch information from the face. | Figure 5.33 |
Side & Level of Damage: The sensory losses (Figure 5.34)
- involve pain sensation in the body and face
- involve both the spinothalamic and spinal trigeminal pathways
- are bilateral (analgesia in the left body and the right face)
Figure 5.34 |
Results of testing somatosensory sensation for Example 7. Pin pricks into the right side of the face and the left hand do not produce well-localized, sharp pain sensations. Press The vibration of a tuning fork applied to the right jaw and left hand, as well as manipulation of the jaw and fingers of the left hand produce normal vibration and proprioceptive sensations. Press |
to view the course of action potentials generated in response to pin pricks into the right side of the face and the left hand.
to view the course of action potentials generated in response to a vibrating tuning fork applied to the right side of the face and the left hand. - the damage must be in the brain stem (Figure 5.35)
- the spinothalamic and spinal trigeminal tracts are involved
- the symptoms are bilateral but the damage is unilateral and involves the right half of the medulla. That is, the symptoms are ipsilesional (right) for the face and contralesional (left) for the body
Figure 5.35 |
Wallenberg's Syndrome. In the medulla, both the spinothalamic tracts and the spinal trigeminal tracts are located posteriorly in the area that normally receives blood via branches of the posterior inferior cerebella artery (PICA) (Figure 5.36). Consequently, an obstruction of the PICA blood supply to the medulla will result in analgesia and thermo-anesthesia of the contralesional body (spinothalamic tracts) and of the ipsilesional face (spinal trigeminal tract). Branches of the anterior spinal and vertebral arteries supply more anterior areas of the upper medulla. Therefore, an infarct involving the PICA blood supply will not affect the medial lemniscus or ventral trigeminal lemniscus. Consequently, discriminative touch and proprioception from the body and pain, temperature and crude touch in the contralesional half of the face will not be affected with an infarct involving PICA.
Figure 5.36 |
Obstruction of the posterior inferior cerebellar artery (PICA) will result in damage to the posterior quadrant of the medulla. The descending spinal trigeminal tract and nucleus and the ascending spinothalamic tract would be damaged, whereas the medial lemniscus and ventral trigeminal lemniscus would be spared. |
Above the level of the pons (Figure 5.1), all of the major somatosensory tracts are crossed and located in close proximity. Consequently, if the brain stem were hemisected above the pons, there would be anesthesia of the contralesional body (section of the spinothalamic tracts which decussate in the spinal cord, and the medial lemniscus, which decussates in the medulla) and contralesional face (section of the ventral trigeminal lemniscus which consists of 2° afferents of the spinal and main sensory trigeminal nuclei that decussate in the medulla and pons).
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