Light Touch and Pressure

The mechanoreceptors associated with light touch include the RA and SA I. The sensory nerve fibers are large and myelinated. The central course of the fibers subserving tactile sensation is more diffuse than that of the other large fiber modalities with fibers coursing in both the anterior spinothalamic tract and the dorsal columns. It has long been recognized that a morbid process confined to the dorsal column will give no clear-cut loss of simple tactile sensibility. Conditions are similar for lesions affecting the anterolateral sensory tracts. It appears that if one of the two systems subserving touch sensibility is damaged, the reduction is so mild that it is of no practical clinical relevance. y

Tactile sensory nerve fibers have their first-order neurons in the DRG and enter the spinal cord through the medial division of the dorsal root. These fibers are largely thought to traverse the medial strand of the dorsal roots and enter the dorsal columns ascending to the contralateral thalamus (see previous discussion of course of dorsal columns). Other fibers bifurcate into ascending and descending fibers that synapse within a few segments in the dorsal horn and laminae I, IV, V, and some of VI and VII. Some of these second-order neurons decussate in the anterior commissure and ascend in the contralateral anterior spinothalamic tract to the VPL nucleus of the thalamus. Within the thalamus the tactile sensory fibers are placed slightly caudad to those conveying pain. These third-order neurons then ascend to somatosensory cortex. The organization of this modality at the cortical level is not well understood.

Pressure is related to tactile sense but involves the perception of pressure sensations other than light touch from subcutaneous structures. It is closely related to proprioception and is mediated through the dorsal columns.


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