. Principles of modern biology. Biology. 440 - Multicellular Animals, Especially Man keep on beating for more than a month after it has been excised from the body. This means that the contractions of the heart do not depend upon excitations from the cen- tral nervous system, although in the intact body, the strength and frequency of the heartbeat are altered considerably under the action of the cardiac nerves. Each beat of the heart represents a single twitch, and it is not possible for the heart to undergo a tetanic contraction, because the refractory period (p. 449) of cardiac mus- cle exten
. Principles of modern biology. Biology. 440 - Multicellular Animals, Especially Man keep on beating for more than a month after it has been excised from the body. This means that the contractions of the heart do not depend upon excitations from the cen- tral nervous system, although in the intact body, the strength and frequency of the heartbeat are altered considerably under the action of the cardiac nerves. Each beat of the heart represents a single twitch, and it is not possible for the heart to undergo a tetanic contraction, because the refractory period (p. 449) of cardiac mus- cle extends into the contraction period. Con- sequently a second stimulus never elicits a contraction unless it comes after the heart has started to relax. Sometimes visceral muscle also contracts and expands in rhythmic fashion, even in the absence of any apparent external stimula- tion. But the most important characteristic of visceral muscle is its capacity to remain in a contracted or semicontracted state even while "at ; A sustained contraction, or tetanus, in skeletal muscle involves a series of excitations coming in usually at the rate of about 50 per second, and skeletal muscle continues to expend extra energy so long as the tetanus continues. But in visceral muscle, a state of sustained contraction, which is re- ferred to as tonus, does not involve continu- ous excitation, nor does tonus demand any extra metabolism above that of the resting state. Thus any degree of tonus in the range between complete contraction and complete relaxation can be maintained with perfect ef- ficiency, and extra metabolic activity is de- manded only when the tonus is to be in- creased or decreased. Visceral muscle is not called upon for quick reversible movements, and consequently there is no need for visceral muscle to ready itself instantaneously foi another full-scale contraction. Thus visceral muscle is able to maintain itself at any given status, and visceral muscle expends energ
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