Increases the PR interval

-Action potentials are conducted more slowly from the atria to the ventricles.

Table 3-1. Autonomic Effects on the Heart and Blood Vessels



Conduction velocity (AV node)


Heart rate


Receptor Effect


Muscarinic Muscarinic

├či i (atria only) Muscarinic

Vascular smooth muscle

Skin, splanchnic

Constriction a, Constriction a, Relaxation p2

Skeletal muscle

AV = atrioventricular.

- The mechanism is decreased inward Ca2+ current and increased inward K+ current.

3. Sympathetic effects on heart rate and conduction velocity

- Norepinephrine is the neurotransmitter, acting at pi receptors.

a. Positive chronotropic effect

-increases heart rate by increasing the rate of phase 4 depolarization.

- More action potentials occur per unit time because threshold potential is reached more quickly and more frequently.

- The mechanism is increased If, the inward Na+ current that is responsible for phase 4 depolarization in the SA node.

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