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Chapter 8. Somatosensory Neurotransmission: Touch, Pain, & Temperature

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A 28-year-old man was seen by a neurologist because he had experienced prolonged episodes of tingling and numbness in his right arm. He underwent a neurologic exam to evaluate the integrity of his sensory nervous system, including a measure of the ability to discern that two pointed objects touching the skin are indeed two distinct points. What does this two-point discrimination test measure and what central nervous system pathway is being evaluated?

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A. Nociceptive sensitivity and the spinothalamic pathway

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B. Stereognosis and medial lemniscal pathway

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C. Stereognosis and spinoreticular pathway

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D. Tactile acuity and the dorsal column pathway

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E. Tactile acuity and the spinothalamic pathway

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The correct answer is D. The two-point threshold test measures the size of the receptive field for light touch which is a measure of tactile acuity. It is used to test the integrity of the dorsal column (medial lemniscus) system, the central pathway for touch and proprioception (rules out option E). Stereognosis is the perception of the form and nature of an object without looking at it (rules out options B and C) and also requires an intact dorsal column. Option A is ruled out since pain sensitivity, which requires an intact spinothalamic tract, is measured by asking a patient, with their eyes closed, if a stimulus to the skin is sharp (needle) or dull (brush).

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A 62-year-old man had a gunshot wound to the left side of his head that damaged the top portion of the lateral postcentral gyrus very close to the midline of the brain (posterior portion of the paracentral lobule). What sensory modalities will likely be compromised by his injury?

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A. He will lose the sense of touch and the discriminative aspect of pain over his left hand and wrist.

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B. He will lose the sense of touch, pressure, and vibration over his right forearm.

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C. He will lose the sense of touch and the discriminative aspect of pain over his right leg and hip.

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D. He will lose the sense of touch over his left leg and the discriminative aspect of pain over his right leg.

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E. He will lose the sense of touch, pressure, and vibration but maintain the discriminative aspect of pain over his right leg.

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The correct answer is C. The lateral postcentral gyrus within the parietal lobe is the location of the primary somatosensory cortex, the sensory ...

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