Anatomy and Organization of the Vasomotor Center
The vasomotor center (VMC) is localized in the medulla oblongata.
The center includes two main zones:
- Pressor zone (P).
- Depressor zone (D) — upon activation, it inhibits the pressor zone via inhibitory interneurons and decreases cardiovascular activity.
The VMC participates in the reflex regulation of hemodynamics by receiving signals from vascular and cardiac receptors and transmitting efferent commands through the autonomic nervous system.
How the VMC Lowers High Blood Pressure
This homeostatic self-regulatory mechanism is known as the depressor (baroreceptor) reflex. Its primary role is to return arterial blood pressure to baseline levels when it rises.
- Stimulus and Reception: An increase in blood pressure stretches the baroreceptors located in the aortic arch and carotid sinus.
- Afferent Pathway: The frequency of action potentials (AP) increases along afferent fibers. The primary pathway includes the depressor nerve (N. depressor, a branch of the vagus nerve), which transmits the signal to the medulla oblongata.
- Central Processing: Within the VMC, the depressor area is excited, which in turn inhibits the activity of the pressor area.
- Efferent Pathway: Activation of the parasympathetic nervous system (via the vagus nerve nuclei, N. vagus) occurs alongside inhibition of the sympathetic nervous system.
- Effect: Sympathetic nerve activity (NE) and cardiac output decrease — heart rate and contractility drop, resulting in bradycardia. Smooth muscle tone in the blood vessels decreases, leading to vasodilation. As a result, systemic blood pressure drops back to baseline.
How the VMC Responds to Increased Venous Pressure
To adapt the heart to increased venous return, the unloading Bainbridge reflex (atrial stretch reflex) is activated. Its function is to return venous pressure (VP) to baseline levels.
- Stimulus and Reception: Elevated venous pressure and increased blood volume at the junctions of the venae cavae and the right atrium stimulate mechanoreceptors (stretch receptors).
- Afferent Pathway: Increased action potential frequency (AP) travels via vagal nerve fibers to the VMC.
- Central Processing: In the medulla oblongata, the pressor area is excited while the depressor area is inhibited. The tonic activity of the vagal nuclei decreases.
- Efferent Pathway: Parasympathetic activity (N. vagus) is suppressed, while the sympathetic nervous system (N. sympaticus) is activated. Sympathetic signals travel through the thoracic segments of the spinal cord (TH — Thoracic) to the heart.
- Effect: Nerve activity (NE) increases — heart rate and contractility rise (tachycardia). The heart pumps the excess blood volume from the venous to the arterial system more rapidly, preventing congestion and lowering venous pressure.
Effectors and Neurotransmitters
Key neurotransmitters involved in this reflex regulation include:
- Acetylcholine (ACh).
- Norepinephrine (NE).