At the thalamic level, the general quality of sensation is perceived (e.g., pain, cold, pleasant and unpleasant), but fine distinctions are not made. For full perception, a third group of sensory neurons sends impulses from the thalamus to the sensory cortex of the brain. Here stimuli are localized and discriminations made among them.
Lesions at different points in the sensory pathways produce different kinds of sensory loss. Patterns of sensory loss, together with their associated motor findings, help you to identify where the causative lesions might be. A lesion in the sensory cortex may not impair the perception of pain, touch, and position, for example, but does impair finer discrimination. A person so affected cannot appreciate the size, shape, or texture of an object by feeling it and therefore cannot identify it. Loss of position and vibration sense with preservation of other sensations points to disease of the posterior columns, while loss of all sensations from the waist down, together with paralysis and hyperactive reflexes in the legs, indicates transection of the spinal cord (see Table 16-5, p. 603). Crude and light touch are often preserved despite partial damage to the cord because impulses originating on one side of the body travel up both sides of the cord.
A knowledge of dermatomes also aids in localizing neurologic lesions. A dermatome is the band of skin innervated by the sensory root ofa single spinal nerve. Dermatome patterns are mapped in the next two figures. Their lev
C3 Front of neck
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