What is the vasomotor center? Discuss its role in helping to maintain cardiovascular homeostasis.

Outline:

·        Anatomical parts:

- vasoconstrictor (C1)

- vasodilator (A1)

- sensory area (A2)

·        Tonic control of:

- vasoconstrictor tone

- heart rate

·        Afferents from:

- cortex via hypothalamus

- carotid and aortic chemoreceptors

- carotid, aortic and cardiopulmonary baroreceptors

- lungs

Essay:

            The vasomotor center is the center of autonomic control of blood pressure. It is located bilaterally in the reticular substance of the medulla and lower third of the pons. The center transmits parasympathetic impulses through the vagus to the heart and sympathetic impulses through the sympathetic preganglionic neurons in the intermediolateral gray column of the spinal cord and the peripheral sympathetic nerves to the blood vessels of the body.

            The vasomotor center can be divided functionally into a vasoconstrictor, vasodilator and sensory area. The vasoconstrictor area is located in the anterolateral portions of the upper medulla. The neurons in this area are known as the C1 group of cells. Their fibers are distributed throughout the spinal cord, where they excite the vasoconstrictor neurons of the sympathetic nervous system.

            The vasodilator area is located in the anterolateral portions of the lower half of the medulla. The neurons in this area are known as the A1 group of cells. Their fibers project upward to the vasoconstrictor area and mediate inhibition of vasomotor discharge, thus causing vasodilation.

            The sensory area contains the A2 groups of cells. It is located in the tractus solitarius in the posterolateral portion of the medulla and lower pons. The neurons of this area receive afferent nerve signals from the vagus and glossopharyngeal nerves. The output signals from this sensory area then helps to control the activities of both the vasoconstrictor and vasodilator areas, thus providing reflex control of many circulatory functions.

            Under normal conditions, the vasoconstrictor area of the vasomotor center transmits signals continuously to the sympathetic nerves over the entire body. These impulses maintain a partial state of contraction in the blood vessels called vasomotor tone which plays an important role in maintaining the blood pressure and cardiac output. The medial portion of the vasomotor center transmit impulses continuously through the vagus nerve to the heart to decrease heart rate (vagal tone).

            The vasomotor center receives sensory input from the peripheral baroreceptors and chemoreceptors, the cortex, hypothalamus, pain pathways and muscles. However, it is the baroreceptors which play the pivotal role in daily cardiovascular homeostasis – maintaining the blood pressure within a narrow range. The baroreceptors fire impulses that inhibit the discharge of the vasoconstrictor area while stimulating the vasodilator area at the same time. Their rate of firing increases with the degree of stretch of the arterial wall within which they are located. The correspondent vasodilation in the blood vessels prevents any excessive rise in blood pressure.