5. Somatosensory Processes
Part 5 of 10
Patrick Dougherty, Ph.D.
(Content provided by Chiyeko Tsuchitani, Ph.D.).
Clinical Examples:
Central Nervous System: The Spinal Cord
Spinal Cord Damage: Although there are numerous tracts in the spinal cord, the tracts considered to be of major clinical importance are limited. There are three major ascending tracts in the spinal cord, the posterior funiculus (which includes the gracilis and cuneatus fasciculi, aka posterior columns); the spinothalamic tract (in the anterior and lateral funiculi); and the posterior spinocerebellar tract (in the lateral funiculus).
- Clinical symptoms of spinal cord damage are obvious when the posterior columns and/or spinothalamic tract are affected. The posterior spinocerebellar tract carries unconscious proprioceptive information and damage to it does not result in sensory losses.
- When a somatosensory tract is sectioned in the spinal cord, the sensory losses start at the level of the lesion and extend down to encompass lower parts of the body. Such a loss occurs because the flow of ascending afferent information from the lesion site and lower body is interrupted and cannot reach the thalamus and cerebral cortex.
- The spinothalamic tracts are crossed in the spinal cord. The decussation in the medial lemniscal pathway occurs in the lower medulla.
- The cranial nerves, which are associated with the brain stem, supply the motor and sensory innervation of the face. Consequently, spinal cord lesions do not produce sensory or motor losses involving the face. When somatosensory losses are isolated to the body, spinal cord lesions should be suspected.
- Unilateral lesion of one descending tract, the corticospinal tract
in the lateral funiculus, produces the most obvious motor deficits (i.e., deficits in
voluntary motor control below the level of the lesion).
The patient suffers from loss of discriminative touch and proprioception (i.e., vibration and position sensations) from the right half of the body starting just below the right nipple and extending down to and including his right foot.
Figure 5.15 |
Symptoms: The patient complains of problems with walking, especially at night when there is little light. He also reports a loss of sensation in his right foot. Physical examination determines that there are decreases in vibration and position sensations and poor localization of tactile stimuli involving the right half of his body starting just below the right nipple and extending down to include his right foot (Figure 5.15). Pain sensation is normal in the right torso, leg and foot. Touch, vibration, position and pain sensations are normal for the rest of the body and face. The Romberg test is positive. (i.e., The patient has difficulty standing upright with his feet together and his eyes closed.) You conclude that the somatosensory losses in his right lower body involve
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| Neurons in the medial lemniscal pathway (MLP) process discriminative touch and proprioceptive information from the body. The MLP 1° afferents ascend uncrossed in the spinal cord within the posterior funiculus. In contrast, the 2° afferents of the neospinothalamic pathway (NSTP), which carry pain and temperature information, decussate in the spinal cord and ascend the cord in the lateral funiculus. Consequently, within the spinal cord, discriminative touch and proprioception of the right side of the body is represented in the ipsilateral (right) posterior funiculus and pain and temperature from the right side of the body is represented in the contralateral (left) lateral funiculi. | Figure 5.16 |
Side & Level of Damage: The sensory losses (Figure 5.17)
- do not involve the face
- involve only the medial lemniscal pathway
- start at the nipple and extend to the foot
- are limited to the right side of the body
Figure 5.17 |
The results for testing somatosensory sensations for example 3. Applying a vibrating tuning fork on the right foot and manipulating right foot produce no vibration or proprioceptive sensations. However, a pin prick to the right foot produces a well-localized sensation of sharp pain. Press Vibration and pain sensations are normal in the rest of the body. Press |
to view the course of action potentials generated in response to the tuning fork on the right foot and pin prick to the right foot.
to view the course of action potentials generated in response to application of a vibrating tuning fork to the right and left hands. - damage is limited to the posterior column of the spinal cord as pain sensation is not affected (Figure 5.18)
- the fifth thoracic segment of the spinal cord may be involved (Figure 5.11)
- symptoms are ipsilesional (i.e., involve damage to the right half of the spinal cord, Figure 5.18)
Figure 5.18 |
The posterior column has been damaged at upper thoracic level (T5) on the right side. The afferents pain and temperature sensations from the right and left side of the body were spared as the lateral and anterior columns were not damaged. |
When only the posterior column of the spinal cord is damaged, there are losses involving discriminative touch and proprioception, but no loss of pain, temperature or crude touch sensitivity. The deficit is ipsilesional and extends down the body from the level of the lesion. There is an inability to appreciate vibrating stimuli and the position and movement in the ipsilesional lower body. The remaining tactile sense in the ipsilesional lower body is poorly localized as the spinothalamic tracts are undamaged. The Romberg test is positive as the patient has lost proprioception in a leg and cannot maintain normal posture with eyes closed.
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