Sympathetic nervous system, cardiac innervation

Figure 15.5 Effects of sympathetic and parasympathetic nervous activity on mean arterial pressure. The parasympathetic nervous system innervates the heart and therefore influences heart rate and cardiac output. The sympathetic nervous system innervates the heart and veins and thus influences cardiac output. This system also innervates the arterioles and therefore influences total peripheral resistance. The resulting changes in cardiac output and total peripheral resistance regulate mean arterial pressure.

The heart responds constantly to hormonal and nervous system signals. Sympathetic nervous system terminals releasing norepinephrine are found in cardiac cells of the atria and ventricles. This allows for reflex regulation of heart muscle contractility. Sympathetic innervation is also present to the SA node and AV junction, where norepinephrine release acts to increase heart rate (enhanced phase 4 depolarization) and also to increase conduction velocity by reducing the AV junction impedance to conduction. Parasympathetic innervation is provided by cranial nerve 10, the vagus nerve, to the SA node and the AV junction. These fibers release acetylcholine, which slows SA node activity (decreasing the rate of phase 4 depolarization) and decreases conduction throughout the AV junction. 

Because cardiac muscle is myogenic, nervous stimulation is not necessary to elicit the heart beat. However, the heart rate is modulated by input from the autonomic nervous system. The sympathetic and parasympathetic systems innervate the SA node. Sympathetic stimulation causes an increase in heart rate or an increased number of beats/min. Norepinephrine, which stimulates ( -adrenergic receptors, increases the rate of pacemaker depolarization by increasing the permeability to Na+ and Ca++ ions. If the heart beat is generated more rapidly, then the result is more beats per minute. 

FIGURE 14.5 The autonomic nervous system innervates smooth muscle, cardiac muscles, and gland. ACh = acetylcholine N = nicotinic cholinergic receptors M = muscarinic cholinergic receptors S = sympathetic chain P = parasympathetic chain E = epinephrine D = dopamine NE = norepinephrine Sup. = superior Inf. = inferior. 

The sinoatrial (SA) node is innervated by both the sympathetic (beta and parasympathetic (vagus) nervous systems. Sympathetic activation increases the discharge rate of the SA pacemaker cells, and thereby increases heart rate (a positive chronotropic effect). Sympathetic nerves also innervate adrenergic receptors (betaj) on cardiac ventricular cells leading to an increase in stroke volume (a positive inotropic effect). Vagal activation, on the other hand, has the opposite effect and decreases heart rate and conduction velocity. In normal adults, cardiac vagal innervation is functionally predominant, so abolition of vagal activity results in a pronounced tachycardia (increased heart rate). 

The heart is innervated by both sympathetic and parasympathetic nerve fibers of the autonomic nervous system. Although the heart can generate its own heartbeat independently of nervous control, stress, exercise, and physical trauma make it advantageous to adjust cardiac contraction to meet the needs at the time. Thus, the cardiovascular control system (Figure 6.20.5), which is located in the brain, controls the contractility of the myocardium (the muscle of the heart), and produces both inotrophic (force of contraction) and chronotrophic (rate of contraction) effects. 

The autonomic nervous system guides the electrical and mechanical functions of the heart. The heart is innervated by both the sympathehc and parasympathetic systems, which have opposite effects and are activated reciprocally. They play important roles in arrhythmia susceptibility. Sympathetic shmulation originates from the intermediolateral column of the thoracic spinal cord. Its neurotransmitter, norepinephrine, is released from neurons of postganglionic fibers of stellate ganglia and epinephrine is released from the adrenal medulla. Both of these act on cardiac p-adrenergic receptors. Sympathetic nerves are predominantly on... 

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