Modifications of afferent activities from Tibialis anterior muscle in rat by tendon vibrations, increase of interstitial potassium or lactate concentration and electrically-induced fatigue.

Although previous experiments with a partially similar objective have been described in dogs, cats and rabbits, the purpose of this study was to identify and characterize mechanosensitive and chemosensitive muscle afferents in the anaesthetized rat since it is a widely used laboratory animal. The peroneal nerve innervating the tibialis anterior muscle was studied. Measurement of conduction velocities from compound action nerve potentials evoked by peripheral nerve stimulation allowed identification of group I-II (10.79+/-1.02 m/s), group III (2.96+/-0.58 m/s) and group IV (0.46+/-0.07 m/s) afferent fibers. Computation of the different compound potential areas showed that afferents I and II arising from spindles and tendon organs represented 9.65+/-2.2%, whereas afferents III and IV arising from free nerve endings in muscle represented 90.35+/-2.2% (III, 46.66+/-2.71% and IV, 43.69+/-2.52%). Action potentials were recorded from teased nerve filaments. Mechanical tendon vibrations (10 to 90 Hz) were used to activate mechanoreceptors. Peak increase in afferent discharge (fimpulses) was measured at 50 Hz (n = 12/19 units) or 70 Hz (n = 7/19 units). Intra-arterial bolus injections of different concentrations of potassium chloride (KCl: 1 to 20 mM) or lactic acid (LA: 0.5 to 3 mM) elicited marked activation of III and IV afferents (n = 124). Enhancement of fimpulses was not proportional to the increase in [KCl] or [LA]. Activation of afferents plateaued when [KCl] was equal or greater than 5 mM while fimpulses peaked, then decreased, when [LA] was 1 mM. Muscle fatigue induced by direct electrical muscle stimulation (EIF) markedly activated group III-IV (n = 17/18) afferents (176.9+/-29.7% of control) which persisted for the 3 minutes of recovery from fatigue. Maximal fimpulses increases in response to LA (+67%) and KCl (+46.9%) injections and to EIF (+76.9%) were similar. This procedure for characterizing the functional properties of sensory nerve endings in a skeletal muscle may be used to assess further changes in sensory muscle paths in experimental rodent pathophysiological systems.