Effects of "partial axotomy" upon synaptic function.

A lesion limited to the dorsal columns, at the level of L3-L4, was carried out in chronic cats. This operation produced a "partial axotomy" type of lesion on the ascending branches of Ia hind limb afferents. Two to six months after this operation, intracellular studies on L7-S1 motoneurons were carried out. Similar studies were done in normal animals. The peak amplitude and the rate of rise (dV/dt) of heteronymous EPSP's were studied during control conditions (sampling at 1 Hz) and during the post-tetanic potentiation produced by a 500 Hz tetanus (for 3 s). The analysis of these synaptic potentials makes us conclude that: The amplitude of the enlarged EPSP's, observed during PTP, seems to be linearly dependent on their amplitude during control (i.e., pre-tetanus) conditions. Judging by their amplitude, there is no difference between potentiated EPSP's of operated and normal animals. There is also a linear relationship between the rate of rise of EPSP's and their peak amplitude. The slope of this relationship becomes steeper after "partial axotomy", i.e., for a given EPSP amplitude, the dV/dt of its rising part is steeper in operated cats. This steeper slope is also present in EPSP's studied during PTP. The sharper rate of rise of EPSP's, induced by the "partial axotomy" of Ia fibers, would be the mechanism behind the larger monosynaptic reflex previously observed in these operated cats.

The response of single guard and down hair mechanoreceptors to moving air-jet stimulation.

The response properties of single guard (G) and down (D) hair afferent nerve fibers innervating the hairy skin of the hindlimb were studied in acute barbiturate-anesthetized cats. The purpose of the study was to identify and analyze the relative contribution of those stimulus features determining the discharge patterns evoked in single afferents by a fine air-jet stimulus moving across the skin and varying in force, velocity, position, direction and orientation. The response of single G hair afferents to moving air-jet stimuli reveals that the responsiveness of each fiber to stimuli with arbitrary orientation, direction and position within the receptive field (RF) displays an optimum velocity sensitivity which is not predictable from punctate data. Although the response pattern is remarkably consistent for each moving stimulus condition, there are significant differences in response as a function of stimulus orientation, direction and velocity. RF 'maps' constructed from the responses evoked as the air-jet traverses the skin reveal multiple zones of high and low sensitivity. The distribution of sensitive zones is remarkably consistent for maps constructed with stimuli varying in orientation, direction and velocity. It is apparent that the principal determinant of the response for a given stimulus traverse is the spatial distribution of sensitive spots throughout the RF. Although noticeably more uniform in sensitivity, the RFs of D hair afferents demonstrate similar properties. These findings indicate that G and D hair afferent nerve fibers respond more vigorously to moving stimuli than to stationary displacement and display complex RF inhomogeneities which must be taken into account for the study of central neuronal information processing and feature extraction.