Question

Explain how the sympathetic nervous system acts to directly regulate stroke volume. Include a detailed description of the mechanism (include the target cells, the proteins, signaling molecules, ions, and changes in membrane potential where appropriate). What effect does this have on cardiac output? This will take about half a page to describe.

Answer 1

Stimulation of cardiac nerves by Sympathetic nervous system causes release of norepinephrine (NE) at its neuromuscular junction. Norepinephrine acts on SA node via beta 1 receptors which are coupled to Gs protein. This shortens the repolarization period and increases the depolarization and contraction rate. The chemical or ligand gated sodium and calcium ion channels open up which causes influx of ions which are positively charged. Norepinephrine binds to the beta 1 recptors which increases heart rate and myocardial contractility. Cardiac output (CO) is the product of heart rate (HR) i.e number of time heart beats per minute and the stroke volume (SV) i.e volume of blood which is pumped out by the ventricles on every heart beat.

CO = HR x SV

As heart rate increases when norepinephrine binds to beta 1 receptors, the increase in heart rate causes increase in myocardial contractility which increases the stroke volume and since cardiac output is product of heart rate and stroke volume and since heart rate and stroke volume both are increasing due to sympathetic action the cardiac output also increases.