Autonomic Nervous System Controls

Sympathetic Division

Stimulation by the sympathetic division increases heart rate by increasing the population of open calcium channels.

Adenylcyclase and Cyclic AMP

Adenylcyclase (ACase circle) is a membrane-bound enzyme. When activated, it converts cytoplasmic adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP). Cyclic AMP is rapidly decomposed into its noncyclic form (AMP) by the enzyme phosphodiesterase (PDase circle). The half-life of cAMP is brief and adenylcyclase (ACase circle) must be continually re-activated to maintain cAMP in the cytoplasm. Cytoplasmic cyclic AMP equilibrates (double-headed dashed arrow) with sites on calcium channels enabling more of them to open when stimulated.

Sympathetic Division

Norepinephrine (NE), released from cardiac nerves (solid line,C)-- and epinephrine from the adrenal cortex (not shown)-- bind to and activate beta 1 receptors (B1 triangle). The associated G-stimulatory protein (Gs triangle) is composed of three subunits--alpha, beta and gamma. The activation of the receptor causes the alpha subunit (á hexagon) to detach from the other two. The alpha subunit diffuses through the membrane (shaded block arrow) to bind with and activate adenylcyclase (ACase circle). The resulting free cAMP binds to sites on calcium channels (double-headed dashed arrow.) Calcium channels with cAMP are more responsive to the signal to open.

The dashed curve in the action potential indicates that depolarization(section b) will be steeper and reach its peak sooner. After 100-150 msec the calcium channels revert to their closed state stopping the influx of calcium. Repolarization (section c) begins immediately. As a result the next prepotential (section a) will reach threshold sooner thus increasing heart rate.

Parasympathetic Division

Stimulation by the parasympathetic division decreases heart rate by:

increasing the population of open potassium channels, and
decreasing the population of open calcium channels.

Affect on Calcium Channels

Acetylcholine (ACh), released by the vagus nerve (V) (dashed line) binds with muscarinic 2 receptors (M2 triangle). The subunits of the associated G-inhibitory protein (Gi triangle) disassociate. The alpha subunit (á hexagon) diffuses through the membrane (shaded block arrow) to bind with and inhibit (dashed arrow) adenylcyclase (ACase circle).

The resulting decrease in cyclic AMP (cAMP)--see top map this page-- means fewer calcium channels will open and will not remain open as long. The reduced calcium influx results in a decreased depolarization slope (section b) of the action potential.

Affect on Potassium Channels

The beta/gamma subunit of the dissociated G-protein migrate (shaded block arrow) to potassium channels (oval). This increases the population of open potassium channels which will increase the repolarization slope (section c). The membrane becomes hyperpolarized--more negative than -55 mV. When the channels do close and potassium begins to accumulate in the cell, the prepotential (section a) requires more time to reach the threshold level (-40 mV.) This lengthens the time between each round of calcium channel openings and decreases the heart rate.

Last updated:7/16/2005