The effect of the activation of the sympathetic cholinergic vasodilator fibres has been studied by a technique permitting an analysis of the reactions in the pre‐ and postcapillary resistance vessels and the capacitance vessels of a skeletal muscle region. The experiments indicate that the dilator fibres are distributed essentially to the bigger precapillary resistance vessels of the muscles, where they can induce almost maximal dilatation, while it is likely that other consecutive vascular sections more or less completely lack such an innervation. The data do not support the view that the dilator fibres open up non‐nutritional specific ‘shunt vessels’, but the above‐mentioned fibre distribution may in certain experimental situations create a ‘functional shunting’ by causing an uneven distribution of capillary flow. A central excitation of these fibres in alarm situations, such as has been suggested, would thus mean an immediate massive blood flow increase in the skeletal muscles, with little or no regional blood ‘pooling’ or opening up of closed capillaries. However, as soon as the skeletal muscles are brought into action, the local production of vasodilator metabolites will open up precapillary sphincters, allowing the already established big flow to be distributed over the entire capillary network.
Folkow et al. (1961) studied this question.