Myogenic tone and reactivity: definitions based on muscle physiology

This paper suggests definitions of myogenic tone and reactivity in blood vessels on the basis of general muscle physiology and discusses some aspects of the cellular mechanisms involved. Tone is defined as a sustained state of contraction in a muscle. The word says nothing about the exact cellular events except that it is an active process in the contractile proteins. Myogenic implies that the stimulus for the tone originates in the muscle itself without involving nerves, hormones or specific autacoids. When working with isolated vascular smooth muscle we conclude that it has myogenic tone if we can demonstrate contractile force in standard physiological salt solution without the addition of vasoconstrictor agents or abnormal ion concentrations. Tone can occur in any type of muscle, principally through two different patterns of cellular activity: (1) The individual cells or motor units all contract in a tonic way, either by graded depolarization and contracture or by frequent action potentials and fused tetanus; and (2) the muscle as a whole shows tone as an integrated result of asynchronous phasic contractions in its separate units. These principles are discussed below in relation to vascular tone in various parts of the circulation. Myogenic reactivity in a blood vessel is defined as the ability of the vascular smooth muscle to contract in response to stretch or to an increase in transmural pressure. Many different components of the cellular machinery of the smooth muscle may respond to the applied force but the present discussion is concerned with excitation of the cell membrane by stretch (mechano-electrical coupling).(ABSTRACT TRUNCATED AT 250 WORDS)