Far-field somatosensory evoked potentials in response to selective stimulation of small diameter myelinated fibers in the cat.

Five far-field components were identified preceding the cortical wave in response to the stimulation of the A beta fibers of the sural nerve. When A delta activity was also included in the afferent volley, a second cortical wave and additional far-field potentials were recorded. Upon blocking the A beta fiber activity and thus isolating the peripheral input to A delta fibers alone, the initial cortical wave and the far-field potentials preceding it disappeared completely, leaving the A delta evoked potentials intact.

The effect of the selective activation of different peripheral nerve fiber groups on the somatosensory evoked potentials in the cat.

Contralateral and ipsilateral cortical somatosensory evoked potentials to selective activation of different diameter sural nerve fiber groups were recorded in cats. Activation of alpha fibers evoked a well defined positive-negative primary potential at the contralateral somatosensory area II. Activation of delta fibers in addition to alpha fibers resulted in another positive-negative wave of comparable amplitude with a latency approximately twice that of the early component. Isolation of the peripheral input to the delta fibers alone, by blocking the alpha fiber activity with the use of triangular pulses, caused the complete disappearance of the initial wave, leaving the second with no latency or amplitude changes. This demonstrated that the latter potential was evoked by delta fibers. Ipsilateral responses followed the same pattern, however with higher peripheral thresholds, longer latencies and smaller amplitudes. No cortical potentials could be recorded to activation of C fibers. The results of this study are at variance with previous observations about the cortical responses to small myelinated nerve fiber activity. The rationale of the use of triangular pulses for selectively activating nerve fiber groups is also discussed.

Combined light- and electron-microscopic study of the rat saphenous nerve.

Using light- and electron-microscopic techniques, a quantitative histological study of the rat saphenous nerve has been carried out. The distribution of myelinated fibre diameters was bimodal; while that of the unmyelinated axons was unimodal. The variation of myelin sheath thickness with axon diameter was non-linear; two straight lines were found to fit the data best. The variation of g (axon diameter/total fibre diameter) with total fibre diameter was also nonlinear, being best fitted by two lines -- one straight, the other slightly curved. The number of unmyelinated axons per Schwann cell process was examined. the ration of unmyelinated axons to myelinated fibres was 3.42:1.