Autonomic Nervous System

The autonomic nervous system provides moment-to-moment control of the heart and the circulatory system.

Effect of activation of the autonomic nervous system

Activation of the sympathetic nervous system (SNS) leads to:

• Increased heart rate
• Increased speed of conduction
• Decreased refractory period
• Increased contractility
• Increased distensibility

Activation of the parasympathetic system (PNS) leads to:

• Decreased heart rate
• Decreased contractility
• Reduced speed of conduction
• Increased refractory period
• Inhibits the release of and therefore effect of sympathetic action at the level of the heart

Messengers of the SNS and PNS

For the SNS it is epinephrine and norepinephrine (catecholamines).

• The source of epinephrine is mainly from the secretion of the adrenal gland
• The source of norepinephrine is mainly from the nerve terminals and the adrenal gland

For the PNS it is acetylcholine.

How does the SNS mediate its action?

The catecholamines act on specific receptors called adrenergic receptors that are located on the cell surface of the target organ.

Types of adrenergic receptors:

• Alpha 1: mainly located on blood vessels; stimulation causes vasoconstriction (arterial vasoconstriction or venoconstriction). They are mainly located post-synaptically.
• Alpha 2: mainly located pre-synaptically on the nerve terminal and activation inhibits the release of norepinephrine. Thus they reduce the effect of Alpha 1 activity (negative feedback on norepinephrine). However, distal to the synaptic cleft alpha 2 receptors promote vasoconstriction. Vasoconstriction is the dominant effect.
• Beta 1: mainly located on the heart; stimulation causes increase in heart rate, contractility, and relaxation.
• Beta 2: mainly located on the arteries (mainly coronary and skeletal muscle); stimulation causes vasodilation; no specific innervation to these receptors thus stimulated by circulating catecholamines.

How does the PNS mediate its action?

Acetylcholine acts on specific receptors called cholinergic receptors.

There are two main types of cholinergic receptors: muscarinic and nicotinic receptors. Only muscarinic receptors are involved with activation of cardiovascular activity.

Types of muscarinic receptors:

• M2: located on the heart; stimulation causes decreased heart rate and contractility. Acetylcholine acts on the M2 receptor.
• M3: located on the arterial tree; stimulation causes vasodilation. Nitric oxide acts via the M3 receptor activation.