<P> Even at resting levels of blood pressure, arterial baroreceptor discharge activates NTS neurons . Some of these NTS neurons are tonically activated by this resting blood pressure and thus activate excitatory fibers to the nucleus ambiguus and Dorsal nucleus of vagus nerve to regulate the parasympathetic nervous system . These parasympathetic neurons send axons to the heart and parasympathetic activity slows cardiac pacemaking and thus heart rate . This parasympathetic activity is further increased during conditions of elevated blood pressure . Note that the parasympathetic nervous system is primarily directed toward the heart . </P> <P> The baroreceptors are stretch - sensitive mechanoreceptors . At low pressures, baroreceptors become inactive . When blood pressure rises, the carotid and aortic sinuses are distended further, resulting in increased stretch and, therefore, a greater degree of activation of the baroreceptors . At normal resting blood pressures, many baroreceptors are actively reporting blood pressure information and the baroreflex is actively modulating autonomic activity . Active baroreceptors fire action potentials ("spikes") more frequently . The greater the stretch the more rapidly baroreceptors fire action potentials . Many individual baroreceptors are inactive at normal resting pressures and only become activated when their stretch or pressure threshold is exceeded . </P> <P> Baroreceptor action potentials are relayed to the solitary nucleus, which uses frequency as a measure of blood pressure . Increased activation of the solitary nucleus inhibits the vasomotor center and stimulates the vagal nuclei . The end - result of baroreceptor activation is inhibition of the sympathetic nervous system and activation of the parasympathetic nervous system . </P> <P> The sympathetic and parasympathetic branches of the autonomic nervous system have opposing effects on blood pressure . Sympathetic activation leads to an elevation of total peripheral resistance and cardiac output via increased contractility of the heart, heart rate, and arterial vasoconstriction, which tends to increase blood pressure . Conversely, parasympathetic activation leads to decreased cardiac output via decrease in heart rate, resulting in a tendency to lower blood pressure . </P>

When baroreceptors sense a drop in blood pressure the vasomotor center responds by
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