How do nerves from the medulla influence heart rate?

Sourav PanNovember 8, 2024

Answered

The medulla oblongata plays a crucial role in regulating heart rate through its influence on the autonomic nervous system, specifically via the sympathetic and parasympathetic pathways. Here’s how it functions to control heart rate:

1. Location and Structure

• The medulla oblongata is part of the brainstem and contains the cardiovascular control center, which integrates sensory information related to blood pressure, blood chemistry, and other physiological parameters to modulate heart function.

2. Autonomic Nervous System

• The medulla communicates with the heart through two main branches of the autonomic nervous system:
  • Sympathetic Nervous System: This system is responsible for increasing heart rate. When activated, sympathetic neurons release the neurotransmitter noradrenaline (norepinephrine), which binds to receptors on the sinoatrial (SA) node (the heart’s natural pacemaker) and other cardiac tissues. This action increases the firing rate of the SA node, leading to an increased heart rate (positive chronotropic effect) and enhanced force of contraction (positive inotropic effect) .
  • Parasympathetic Nervous System: This system decreases heart rate. The vagus nerve, a major component of the parasympathetic system, releases the neurotransmitter acetylcholine at the SA node. This decreases the firing rate of the SA node, resulting in a slower heart rate (negative chronotropic effect) .

3. Feedback Mechanisms

• The medulla receives input from various sensory receptors that monitor changes in blood pressure and blood chemistry:
  • Baroreceptors: These are stretch-sensitive receptors located in the walls of blood vessels (such as the carotid arteries and aorta) that detect changes in blood pressure. When blood pressure rises, baroreceptors send signals to the medulla to increase parasympathetic activity and decrease sympathetic activity, leading to a reduction in heart rate. Conversely, if blood pressure drops, sympathetic activity increases while parasympathetic activity decreases, resulting in an increased heart rate .
  • Chemoreceptors: These receptors monitor levels of carbon dioxide (CO₂), oxygen (O₂), and pH in the blood. Changes in these parameters can also influence heart rate regulation through adjustments made by the medulla.

4. Integration of Signals

• The medulla integrates signals from both systems to maintain homeostasis. For instance:
  • During physical activity or stress, sympathetic stimulation predominates, increasing heart rate to supply more oxygenated blood to muscles.
  • During rest or relaxation, parasympathetic stimulation prevails, slowing down the heart rate.