Chemical activation of group I and II muscle afferents has no cardiorespiratory effects

Although stimulation of group III and IV muscle afferents is known to cause reflex changes in cardiorespiratory function, it has not been resolved whether group I and II afferents contribute to this reflex activation. Therefore, we measured the effects of intra-arterial nonparalyzing doses of succinylcholine (50-100 micrograms/kg) on the firing of muscle afferents from the gastrocnemius muscle in one group of cats, and heart rate, blood pressure, and integrated phrenic nerve activity in a second group of cats. In nonparalyzed cats, succinylcholine injections caused muscular fasciculations and firing of all four groups of muscle afferents. However, succinylcholine stimulated only group I and II afferents after paralysis with gallamine triethiodide. Succinylcholine caused increases in blood pressure, heart rate, and phrenic nerve activity before paralysis. After paralysis, succinylcholine had no effects on any of the cardiorespiratory measures. We conclude that activation of only group I and II afferent fibers from the gastrocnemius muscle has no reflex effects on blood pressure, heart rate, or phrenic nerve activity. These afferents, therefore, are unlikely to play a role in increasing cardiorespiratory function during exercise.