Autonomic Regulation of Heart Rate

Sympathetic and Parasympathetic Nervous Systems

The autonomic nervous system (ANS) plays a crucial role in regulating heart rate by balancing the actions of its two main divisions: the sympathetic and parasympathetic nervous systems. These systems modulate cardiac function through specific neural pathways and neurotransmitters.

• Sympathetic Nervous System (SNS): The SNS increases heart rate and the force of cardiac contractions. It prepares the body for 'fight or flight' responses.

• Parasympathetic Nervous System (PNS): The PNS decreases heart rate and promotes 'rest and digest' activities, conserving energy and maintaining baseline body functions.

Neural Pathways and Nerves Involved

• Sympathetic Pathway:

  • Originates from the thoracic and upper lumbar segments (T1–T5) of the spinal cord.

  • Preganglionic fibers synapse in sympathetic ganglia (such as the sympathetic chain ganglia).

  • Postganglionic fibers travel to the heart via cardiac nerves.

  • Neurotransmitter: Norepinephrine (noradrenaline) is released at the cardiac synapses, increasing heart rate and contractility.

• Parasympathetic Pathway:

  • Originates from the medulla oblongata in the brainstem.

  • Fibers travel to the heart primarily via the vagus nerve (cranial nerve X).

  • Neurotransmitter: Acetylcholine is released at the cardiac synapses, decreasing heart rate by acting on the sinoatrial (SA) and atrioventricular (AV) nodes.

Mechanisms of Heart Rate Modulation

• Sympathetic Stimulation: Increases the rate of depolarization at the SA node, shortens AV node delay, and enhances myocardial contractility.

• Parasympathetic Stimulation: Decreases the rate of depolarization at the SA node, prolongs AV node delay, and reduces myocardial contractility.

Summary Table: Autonomic Effects on the Heart

Example: Autonomic Response to Exercise

• During exercise, sympathetic activity increases, raising heart rate and cardiac output to meet the body's increased oxygen demand.

• After exercise, parasympathetic activity predominates, returning heart rate to resting levels.