Neural Integration In The Vasomotor Center

The vasomotor center is located in the reticular substance of the medulla and a portion of the pons. Its anatomy and functional organization is complex and still not completely understood; however, it is considered to have at least four important functional areas: (1) the vasoconstrictor region (often referred to as C-l), which is located in the upper medulla and lower pons, sends fibers into the spinal cord where they activate sympathetic vasoconstrictor neurons; (2) the vasodilator region (often referred to as A-l), which is located in the lower half of the medulla, sends axons into the vasoconstrictor region that appear to inhibit neurons within C-l; (3) a sensory region, which is located in the tractus solitarius of the medulla and the pons, has neurons that receive sensory input from glossopharyngeal and vagal fibers; and (4) the cardiac center regulates heart rate and contractility. In terms of the cardiac center, direct electrical stimulation of one part of it, the cardiac accelerator area, increases heart rate and contractility via the sympathetic nerves; stimulation of another part of the cardiac center, the cardiac inhibitory area, decreases the heart rate via vagal fibers.

Within the vasomotor center, spontaneous activity is integrated with information from sensory receptors that monitor various cardiovascular parameters. Efferent commands are then generated as a result of this integration to regulate cardiac and vascular function in a purposeful manner. Neurons emanating from the vasoconstrictor region are always active. This ongoing activity produces a continuous repetitive firing of sympathetic vasoconstrictor fibers that sustains a partial state of vascular smooth muscle contraction, referred to as vasomotor tone. Interrupting that basal sympathetic activity by spinal anesthesia will reduce MAP from 100 mm Hg to about 50 mm Hg, as shown in Fig. 1. Vasomotor tone is important because the sympathetic constrictor fibers are usually not paired with corresponding dilator fibers. The vasomotor tone, however, provides the vasoconstrictor center with the ability to dilate blood vessels by simply withdrawing sympathetic tone.

Arterial Pressure Sensors

Baroreceptors are stretch receptors located within the walls of the heart and some blood vessels. Baroreceptors

FIGURE 1 Sympathetic maintenance of mean arterial pressure. Effects of spinal anesthesia and spinal anesthesia plus intravenous injections of norepinephrine.

provide the central nervous system with information regarding the blood pressure. When they are stretched by pressure within the vessel wall, they elicit action potentials in the sensory neurons that innervate them. The firing frequency of all baroreceptors is proportional to the pressure applied to them.

One set of baroreceptors is within the walls of the carotid sinuses near the bifurcation of the common carotid arteries. Sensory fibers from the carotid barore-ceptors enter the central nervous system via the sinus nerve (Hering's nerve), which is a branch of the glossopharyngeal nerve. These afferent nerves go to the sensory region of the vasomotor center, in particular the tractus solitarius.

Baroreceptor Reflex

Cutting the sinus nerves causes an immediate rise in MAP. Similarly, electrical stimulation of the central end of the bisected sinus nerve produces a reduction in MAP. These experiments, in conjunction with others that have examined associated alterations in autonomic control of the heart and the vasculature, have characterized the carotid baroreceptor reflex. As illustrated in Fig. 2, an increase in MAP stretches the walls of the carotid sinus. This stretch increases the firing rate of sensory fibers in the carotid sinus nerve, which informs the vasomotor center that arterial pressure is above 100 mm Hg, which seems to be the body's set point for MAP. The vasomotor center responds by increasing the firing rate of cardiac vagal fibers and decreasing the firing rate of cardiac sympathetic fibers so that heart rate and contractility are decreased. Simultaneously, the vasomotor center will decrease the firing rate of sympathetic vasoconstrictor fibers

Neural Integration in the Vasomotor Center Vasomotor center

Increased mean arterial pressure

Carotid sinus tCSN firing rate

Carotid sinus tCSN firing rate

Neural Integration in the Vasomotor Center Vasomotor center

I Cardiac sympathetic firing rate t Vagus firing rate

I Cardiac sympathetic firing rate t Vagus firing rate

I Sympathetic vasconstrictor nerves firing rate

Arterioles & veins

Vascular

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Responses

  • marco
    What is neural integration refer to for an action potential?
    6 years ago

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