5. Somatosensory Processes
Part 10 of 10
Patrick Dougherty, Ph.D.
(Content provided by Chiyeko Tsuchitani, Ph.D.).
Somatosensory Cortex. The sensory loss from head trauma or stroke that damages the somatosensory cortex will
- be contralesional, as decussations in all somatosensory pathways occur below the thalamus
- be more complex, as somatosensory information is integrated at cortical levels
- not result in analgesia, as pain is represented by multiple pathways (e.g. the neospinothalamic and paleospinothalamic pathways) and in multiple cortical areas
- often be accompanied by motor deficits, as the somatosensory cortex provides input to the motor cortex and motor control signals to the brain stem and spinal cord.
The patient suffers from deficits in discriminative touch and proprioceptive sensations involving the right side of his body and face. Tactile and pain sensations are also poorly localized on his right side. He has difficulty walking and controlling his right arm and hand and the right side of his face.
Figure 5.37 |
Symptoms: The patient exhibits deficits in fine motor control and in discriminative touch and proprioception on the right side of his body and face (Figure 5.37). He has problems manipulating and identifying objects placed in his right hand (stereognosis). He is unable to identify letters or numbers written on the skin of the right face and the palm of his right hand (graphesthesia). He also has difficulty in judging weight differences (baragnosis) and cannot appreciate textures with his right hand. He is unable to detect the passive movement of his right foot and the fingers of his right hand. Compared with the left side of his body pain sensations are not as sharp, well defined or easily localized on the right side of his body. Touch, vibration, position, thermal, and pain sensations are normal for the rest of the body and face. The patient has difficulty walking and the Romberg test is positive. You conclude that the somatosensory losses in his body involve
Pathway(s) Affected: You conclude that structures in the following somatosensory pathways (Figure 5.38) may have been affected
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| Neurons of the medial lemniscal pathway (MLP) process discriminative touch and proprioception information from the body, whereas those of the main sensory trigeminal pathway (MSTP) process discriminative touch and proprioception information from the face. The neurons of the neospinothalamic pathway (NSTP) process sharp, cutting pain and cool/cold information from the body, whereas those of the archispinothalamic and paleospinothalamic pathways (PSTP) processes dull and aching pain, warm/hot and crude touch from the body. The neurons of the spinal trigeminal pathway (STP) process all pain, temperature and crude touch information from the face. Notice that thalamic neurons in the paleospinothalamic (PSTP) and spinal trigeminal pathways (STP) send axons to multiple cortical areas. | Figure 5.38 |
Side & Level of Damage: The sensory losses (Figure 5.39)
- involve the body and face
- are unilateral and complex in nature
- involve higher order cortical functions, e.g., graphesthesia
- do not include complete loss of pain information
- are accompanied by motor deficits
Figure 5.39 |
The results of testing somatosensory sensation in Example 8. The vibration of a tuning fork applied to the right jaw or right hand, as well as manipulation of the right foot, produce no vibration or proprioceptive sensations. Press Pinching the right cheek or right hand produce pain sensations. Press |
to view the course of action potentials generated in response to a vibrating tuning fork applied to the right jaw and the right hand.
to view the course of action potentials generated in response to pinching the right side of the face and the right hand. So, you conclude that the damage
- involves the left somatosensory parietal cortical area
- may involve a branch of the middle cerebral artery (Figure 5.40)
Figure 5.40 |
Subdural hemorrhage involving a parietal branch of the middle cerebral artery injured somatosensory areas of the parietal lobe. |
Somatosensory Cortex. Hemorrhage limited to somatosensory parietal areas produces contralesional astereognosis, baragnosis, and losses in the ability to discriminate object size and texture. Also decreased or lost on the contralesional side of the body are the ability to discriminate position and movement of body parts and the control of fine movements. The hemorrhage would not produce a total loss of pain sensation as other cortical areas are also involved in the perception of painful stimuli. For example, the cingulate gyrus in the frontal lobe and part of the insular cortex appear to be involved in the perception of, and emotional reaction to, painful stimuli (Figure 5.41).
Figure 5.41 |
Cortical areas involved in pain sensation. The thalamic neurons of the spinothalamic pathways and spinal trigeminal pathway that are involved in processing pain information send their axons to the cingulate gyrus and insular cortex. Consequently, damage limited to the somatosensory parietal cortex will not result in the loss of all pain sensation. |
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Summary
From this chapter, you should have learned how the somatosensory system is organized from the skin, muscles and joints to the cortex. You have learned that stimulus features extracted by the somatosensory receptors are kept segregated in separate “information channels” and processed in parallel by different chains of neurons. Information coded and carried by thousands of spinal cord and cranial ganglion cells are distributed to millions of cortical neurons in the parietal lobe. The perceptions of coherent somatosensory stimuli and body image are recomposed out of these fragments of information by the simultaneous activation of large areas of cortex. You have learned how to use the somatotopic organization and the modality specificity of the different somatosensory pathways to determine the location and extent of damage to the somatosensory structures.
| Test Your Knowledge
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A. Insula |
| 2. Make the BEST match between the below cortical functional area and cortical structure area. Match Secondary somatosensory cortex |
A. Insula |
| 3. Make the BEST match between the below cortical functional area and cortical structure area. Match Somatosensory association cortex |
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| 4. Select the best answer: Electrical stimulation of the posterior paracentral lobe will result in the perception of a somatosensory stimulus at the |
| 5. Select the best answer: Damage to the posterior funiculus at spinal cord level T6 produces a loss... |
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