How does the vasomotor center work?

The vasomotor center is a diffuse area of ​​the medulla oblongatain the brainstem, composed of a neural network that regulates blood pressure and other homeostatic processes along with the heart center and the respiratory center. It exerts its action through the Autonomic Nervous System and has two separate areas, a vasopressor area and a vasodepressor area.

Blood pressure is regulated by various mechanisms that can be local, in the blood vessels themselves, or central, through the vasomotor center which belongs to the Central Nervous System. They can also be short-term mechanisms of effect, such as the effects of the vasomotor center, or long-term, such as the effects of the renin-angiotensin-aldosterone hormonal system.

Regulation of blood pressure by the vasomotor center

The vasomotor center receives information from sensory receptors located in the arteries and that respond to blood pressure. These receptors are known as baroreceptors and they are the main sensory feedback component in blood pressure control.

The vasomotor center also receives information from higher brain structures and with all the information it makes a decision to regulate systemic blood pressure. Based on this decision, it modulates the Autonomic Nervous System and coordinates its response through the Sympathetic System or the Parasympathetic System. This regulation is short-term, from seconds to minutes.

baroreceptors

The baroreceptors are neurons whose cell body is in contact with vascular walls, mainly in the aortic arch and in the carotid sinus. From here, the baroreceptors project their axons to the vasomotor center.

Baroreceptors are classified as mechanical receptors and its membrane potential is modulated by local changes in tension. When blood pressure rises, the baroreceptors are mechanically deporalized and it generates a nerve impulse that is transmitted to the vasomotor center. The higher the blood pressure, the higher frequency of action potentials in the baroreceptors.

In various experiments it has been observed that baroreceptors hardly generate action potentials below a pressure of 60 mmHg. From this value, the frequency of action potentials increases until it reaches a maximum of 180 mmHg. This increase follows a sigmoidal curve with the steepest slope around 100 mmHg.

The value of 100mmHg is the value of Mean arterial pressure considered normal. The fact that the greatest slope of the response curve of the baroreceptors is around this level implies that they are more sensitive to deviations from the normal value, which may be a evolutionary physiological characteristic very important to keep blood pressure within the optimal range.

When the vasomotor center receives impulses from the baroreceptors at a low frequency, the arterial pressure is low. Conversely, high frequency of impulses from the baroreceptors indicates that the blood pressure is high. With this information, the vasomotor center acts through the autonomic nervous system, coordinating the sympathetic and parasympathetic response to maintain homeostasis.

baroreceptors-and-vasomotor-centre

In a situation of low pressure, the vasomotor center activates the sympathetic nervous system and a response of arterial vasoconstriction and increased heart rate that raises blood pressure. In the opposite case, in a situation of high pressure, the vasomotor center inhibits the sympathetic nervous system and increases the action of the parasympathetic nervous system to produce vasodilation and decreased heart rate.

The sympathetic nervous system appears to be much more important in maintaining blood pressure than the parasympathetic system. The simple inhibition of the sympathetic system translates into a decrease in systemic blood pressure, but when the sympathetic system is activated, norepinephrine is released, the main responsible for arterial vasoconstriction and the increased peripheral resistance which causes an increase in blood pressure.

Norepinephrine also causes vasoconstriction in the veins. By reducing the caliber of the veins, the blood is displaced towards the heart, increasing the intraventricular pressure and with it the outflow of blood from the heart (Frank-Starling mechanism), which also contributes to the increase in blood pressure. In addition, the action of the Sympathetic Nervous System also increases the heart rate.

medical implications

Alterations in the vasomotor center cause hypotension, defined by a systolic blood pressure less than 90 mmHg. Hypotension manifests with symptoms such as dizziness, nausea, and fatigue. When blood pressure is very low, blood circulation is inefficient and if maintained it can cause loss of vasomotor tone and dysautonomia (Imbalance of the Autonomic Nervous System).

Some diseases that can cause dysautonomia include diabetes and epilepsy. Other prominent conditions that present with an alteration of the Autonomic Nervous System are Parkinson's, postural tachycardia syndrome, mitral valve prolapse or multiple system atrophy (Shy-Drager syndrome).

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