Question

Describe the sympathetic and parasympathetic control of cardiac function. What are the neurotransmitters employed and their signaling and physiological functions?

Answer 1

The ANS is responsible for controlling many physiological functions: inducing the force of contraction of the heart, peripheral resistance of blood vessels and the heart rate. The ANS has both sympathetic and parasympathetic divisions that work together to maintain balance.

Parasympathetic

The parasympathetic input into the heart is via the vagus nerve (CN X). The vagus nerve forms synapses with postganglionic cells in SA Node and AV Node. When stimulated, acetylcholine which binds on to M₂ receptors, which acts to decrease the slope of the pacemaker potential, leading to a decrease in heart rate (a negative chronotropic effect).

Sympathetic

The sympathetic input into the heart is via the postganglionic fibres from the sympathetic trunk which innervate the SAN and AVN. The post ganglionic fibres release noradrenaline, which acts on B₁ adrenoreceptors to increase the slope of the pace maker potential, thereby increasing the heart rate (a positive chronotropic effect), as well as increasing the force of contraction (positive inotropic effect).

The parasympathetic input on the SAN dominates at rest, to give a normal resting heart rate of around 60bpm. Any initial increases in heart rate are brought about by a reduction in parasympathetic outflow, and increasing the heart rate over 100bpm is via an increase in sympathetic outflow.