Neural and Humoral Regulation of Cardiac Function
The efferent innervation of the heart is controlled by both the sympathetic nervous system and the parasympathetic nervous system. Afferent fibers accompany the efferents of both systems. The sympathetic fibers have positive chronotropic (rate-increasing) effects and positive inotropic (force-increasing) effects. The parasympathetic fibers have a negative chronotropic effect and may be somewhat negatively inotropic (but small and masked) in the intact circulatory system by the increased filling that occurs when diastolic filling time is increased.
The heart is normally
under the restraint of vagal inhibition, and thus bilateral vagotomy increases
the heart rate. Vagal stimulation not only slows the heart but also slows
conduction across the A-V node. Sectioning of the cardiac sympathetics does not
lower heart rate under normal circumstances.
The totally denervated
heart loses some (but surpris-ingly little) of its capacity to respond to changes
in its load. The denervated heart still responds to humoral influences, more
slowly and less fully, but it is remarkable how well the secondary mechanisms,
such as the suprarenal medullary output of catecholamines, can substitute for the
primary mechanism that controls heart rate in exercise.
The nervous mechanisms
controlling heart rate include the baroreceptor reflexes, with afferent
arms from the carotid sinus, the arch of the aorta, and other pressoreceptor zones
operating as negative feedback mechanisms to regulate pressure in the arteries.
These reflexes affect not only heart activity but also the caliber of the resistance
vessels in the vascular system.
The heart is also affected reflexively by afferent impulses via the autonomic nervous system. The response may be tachycardia or bradycardia, depending on whether the sympathetic or parasympathetic system is activated more strongly in the individual patient. Ta hycardia is the common response in excitement.
