Action of the sympathetic system on skeletal muscle

The action exerted by the sympathetic system on skeletal muscle is briefly reviewed with regard to the effects induced by the adrenergic mediator on extra-fusal muscle fibre contractility and on muscle spindle receptor organs. In particular, the different responses of slow - and fast - contracting muscles to catecholamine administration and to sympathetic stimulation are reported. Also, the sympathetically--induced effects on muscle spindle afferent activity are discussed on the basis of recent anatomical and functional data suggesting the existence of a direct action exerted by noradrenergic axons on intrafusal muscle fibres. Some of the possible implications of these data are then briefly mentioned.