Location and Structure of the Vasomotor Center
The vasomotor center is located in the reticular formation of the medulla oblongata. Electrical stimulation of these medullary reticular structures elicits specific vascular responses.
Functionally, the vasomotor center consists of two primary interacting regions:
- Depressor area — inhibits the activity of the pressor area.
- Pressor area — increases sympathetic outflow to blood vessels and the heart.
Blood Pressure Regulation at Rest
Under physiological resting conditions, the vasomotor center maintains normal systemic arterial blood pressure (approximately 120/80 mmHg).
- Afferent input: Baroreceptors located in vascular reflexogenic zones (aortic arch and carotid sinus) continuously generate action potentials at a baseline frequency. This tonic firing constantly updates the CNS on current arterial pressure.
- Central processing: The vasomotor center maintains equilibrium between sympathetic and parasympathetic influences.
- Efferent output: Continuous baseline firing of sympathetic fibers maintains basal vascular tone and normal heart rate (HR).
During emotional stress, moderate activation of the sympathoadrenal system occurs. However, homeostasis is preserved because increased blood pressure stimulates baroreceptors to send inhibitory afferent signals back to the vasomotor center, counterbalancing excitation.
Mechanism of the Depressor (Baroreceptor) Reflex
This is a self-regulatory negative feedback mechanism designed to lower elevated arterial blood pressure.
- Stimulus: Elevation of arterial blood pressure.
- Reception: Baroreceptors in the aortic arch and carotid sinus are stretched, increasing their impulse firing rate.
- Afferent pathway: Action potentials travel via the depressor nerve (n. depressor, a branch of the vagus nerve) and glossopharyngeal nerve (n. glossopharyngeus) to the medulla oblongata.
- Central processing: The depressor area of the vasomotor center is activated, inhibiting the tonic activity of the pressor area.
- Efferent pathway: Parasympathetic vagal nuclei (n. vagus) are activated while sympathetic spinal centers are inhibited.
- Effector response: Heart rate and myocardial contractility decrease, vascular smooth muscle relaxes, resulting in systemic vasodilation. The outcome is a return of blood pressure toward baseline.
Clinical Significance and Pharmacological Effects
- General Anesthesia: CNS structures display differential sensitivity to general anesthetics. Synaptic transmission in the cerebral cortex and reticular formation is depressed first, whereas the medullary vasomotor and respiratory centers are depressed last. Overdose of volatile anesthetics, such as halothane, can depress the vasomotor center, leading to severe hypotension.
- Tetanus Infection: Tetanospasmin exotoxin reaches the CNS via retrograde axonal transport and transsynaptic spread into the reticular formation. Disruption of reticular neurons can compromise the vasomotor center, posing a risk of cardiovascular collapse.
- Acute Radiation Syndrome: In early stages of radiation sickness, analeptic agents that stimulate the respiratory and vasomotor centers must be used cautiously due to a narrowed therapeutic index. In later stages, paradoxical responses to CNS stimulants may occur, requiring dose reductions for most medications except antimicrobials.
- Nicotinic Receptor Agonists: Cytisine strongly stimulates the respiratory center and exerts a pure pressor effect, producing a sustained rise in blood pressure without a secondary depressor response.