On the possible functional role of afferent pathways in skin sensation

Deactivation and reactivation of somatosensory cortex after dorsal spinal cord injury

Sensory stimuli to the body are conveyed by the spinal cord to the primary somatosensory cortex. It has long been thought that dorsal column afferents of the spinal cord represent the main pathway for these signals, but the physiological and behavioural consequences of cutting the dorsal column have been reported to range from mild and transitory to marked. We have re-examined this issue by sectioning the dorsal columns in the cervical region and recording the responses to hand stimulation in the contralateral primary somatosensory cortex (area 3b). Following a complete section of the dorsal columns, neurons in area 3b become immediately and perhaps permanently unresponsive to hand stimulation. Following a partial section, the remaining dorsal column afferents continue to activate neurons within their normal cortical target territories, but after five or more weeks the area of activation is greatly expanded. After prolonged recovery periods of six months or more, the deprived hand territory becomes responsive to inputs from the face (which are unaffected by spinal cord section). Thus, area 3b of somatosensory cortex is highly dependent on dorsal spinal column inputs, and other spinal pathways do not substitute for the dorsal columns even after injury.

Hair follicle discrimination dysfunction in multiple sclerosis patients

A method was developed of assessing somatosensory deficits quantitatively using hair follicle displacement as a stimulus within a psychophysical signal detection task paradigm. Multiple sclerosis patients with and without somatosensory disturbances could be differentiated and compared with normal subjects. This method may distinguish patients with somatosensory dysfunction, and dorsal funiculus neuropathology may underlie this distinction.

Impairments in limb actions after dorsal funiculi section in cats

A study of the forces exerted by cats against the ground when jumping from a platform equipped with force transducers, was carried out. The study, complemented by film analysis of the jump, attempted to disclose some of the mechanisms underlying the decrease in efficiency and accuracy in releasing a moving piece of liver observed after dorsal funiculi section above C1 level. The results showed that after the lesion, peak force and height of jump were significantly reduced. However, angle of take-off, an index of force distribution in the three axes of rectangular coordinates remained unchanged. Intact cats consistently extend their forelimbs in a smooth and progressive way toward the liver in order to release it. The distance between the tips of the forelimbs remained more or less constant during the flight. Postoperatively, instead of the regular pattern described, the extension of the limbs was interrupted by fast marked flexion movements and the distance between their tips increased significantly during the flight. Peak extension of the forelimbs was also decreased after surgery. The results are interpreted in terms of the different disruption of the various components (proprio and exteroceptive) that make up fasciculus gracilis and cuneatus of the dorsal funiculi.

Role of the dorsal funiculi in movement control

We trained monkeys to perform visuomotor tracking tasks in which the target's position was unpredictable, or in which the target jumped from one side of the screen to the other ('ballistic'). Section of the right dorsal funiculus at C2 resulted in transient deficits in both tasks, with full recovery of function. Subsequent removal of postcentral cortex in the left hemisphere produced a more intense and stable deficit in performance. We conclude that proprioceptive data on limb position and movement must be able to reach motor command structures by pathways paralleling the dorsal column-medial lemniscus system.