The Heart

The heart is innervated by both sympathetic and parasympathetic neurons; however, their distribution in the heart is quite different. Postganglionic noradrener-gic fibers from the stellate and inferior cervical ganglia innervate the sinoatrial (S-A) node and myocardial tissues of the atria and ventricles. Activation of the sympathetic outflow to the heart results in an increase in rate (positive chronotropic effect), in force of contraction (positive inotropic effect), and in conductivity of the atrioventricular (A-V) conduction tissue (positive dro-motropic effect).

The postganglionic cholinergic fibers of the para-sympathetic nervous system terminate in the S-A node, atria, and A-V conduction tissue. Cholinergic fibers do not innervate the ventricular muscle to any significant degree. Activation of the parasympathetic outflow to the heart results in a decrease in rate (negative chronotropic effect) and prolongation of A-V conduction time (negative dromotropic effect). There is a decrease in the contractile force of the atria but little effect on ventricular contractile force.

The effect of a drug on the heart depends on the balance of sympathetic and parasympathetic activity at the time the drug is administered. An example is the effect of the ganglionic blocking agents (see Chapter 14), which nonselectively inhibit transmission in both sympathetic and parasympathetic ganglia. Normally, during rest or mild activity, the heart is predominantly under the influence of the vagal parasympathetic system. Blockade of the autonomic innervation of the heart by the administration of a ganglionic blocking agent accelerates the heart rate. Conversely, if sympathetic activity is dominant, as in exercise, ganglionic blockade will decrease the heart rate and also reduce ventricular contractility. Likewise, the magnitude of effect of a drug antagonist of sympathetic activity will depend upon how much sympathetic activity exists at the time it is given. A similar relationship exists between parasympathetic antagonists and the level of parasympathetic activity.

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