Signalling of static and dynamic features of muscle spindle input by external cuneate neurones in the cat

1. The present experiments examined the capacity of external cuneate nucleus (ECN) neurones in the anaesthetized cat to respond to static and vibrotactile stretch of forearm extensor muscles. The aim was to compare their signalling capacities with the known properties of main cuneate neurones in order to determine whether there is differential processing of muscle spindle inputs at these parallel relay sites. 2. Static stretch (<= 2 mm in amplitude) and sinusoidal vibration were applied longitudinally to individual muscle tendons and responses recorded from single ECN neurones. The muscle-related ECN neurones that were sampled displayed a high sensitivity to both static and dynamic components of stretch, including muscle vibration at frequencies of 50-800 Hz, consistent with their dominant input being derived from primary spindle afferent fibres. 3. In response to ramp-and-hold muscle stretch, ECN neurones resembled their main cuneate counterparts in the pattern of their responses and in quantitative response measures. Their coefficients of variation in interspike intervals during steady stretch ranged from approximately 0.3 to 0.7, as they do in main cuneate responses, and their stimulus-response relations were graded as a function of stretch magnitude with low variability in responses at a fixed stretch amplitude. 4. In response to muscle vibration, ECN activity was tightly phase locked to the vibration waveform, in particular at frequencies of <= 150 Hz, where vector strength measures (R) were high (R >= 0.8) before declining as a function of frequency, with R values of approximately 0.6 at 300 Hz and <= 0.4 at 800 Hz. Both the qualitative and quantitative aspects of ECN responsiveness to the vibro-stretch disturbances were indistinguishable from those of the main cuneate neurones. 5. The results demonstrate a high transmission fidelity for muscle signals across the ECN and no evidence for differential synaptic transmission across the parallel main and external cuneate nuclei. Earlier limitations observed in the capacity of cerebellar Purkinje cells to respond to primary spindle inputs must therefore be imposed at synapses within the cerebellum.