Influence of Higher Centres on Vasomotor Tone

The vasomotor centres also respond to higher centres in the brain. These include:

• Hypothalamus—stimulation of the anterior hypothalamus decreases blood pressure and heart rate, while stimulation of the posterolateral hypothalamus increases blood pressure and heart rate. The hypothalamus also controls cutaneous vasodilatation and vasoconstriction in response to environmental or body temperature changes

• Cerebral cortex—stimulation of the motor and pre-motor areas is associated with increased blood pressure and heart rate

• Limbic system—emotional stimuli can produce depressor responses such as fainting and blushing Baroreceptors

Baroreceptors are irregularly branched and coiled nerve endings located in the walls of the carotid sinus, the aorta and the heart. The carotid sinus is an enlargement of the internal carotid artery just above its origin. They respond to the degree of stretch in the vessel or heart wall and, hence, to the pressure (or more strictly the transmural pressure) in the vessel or heart. When intraluminal pressure increases, wall stretch increases and the frequency of impulses discharged by baroreceptors increases. If stretch decreases baroreceptor output frequency decreases. The baroreceptor impulses exert an inhibitory influence on the pressor centre and, thus, baroreceptor control represents a negative feedback control system to maintain cardiovascular stability. The baroreceptor response curve is sigmoidal but is linear over a pressure range of 80-180 mmHg (Figure CR.26).

Baroreceptors not only respond to pressure magnitude but also rate of change of pressure. The impulse discharge rate is, thus, greater during early systole than diastole. At low pressures there are few discharges during the upstroke of arterial pressure. At higher mean arterial pressures, discharges are present throughout more of the cycle. The frequency remains higher early in the cycle. At very high mean arterial pressures, discharges occur continuously, achieving maximum inhibition of the vasomotor centre (Figure CR.27). This means that baroreceptors not only respond to changes in pressure but also to changes in pulse pressure and heart rate.

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Figure CR.26 Baroreceptor response curve

Maori artoriût prasiurs — SO mmHg

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Mann arlnrial pfflSiLtnt - 1 75 mmHg

Figure CR.27 Baroreceptor activity during the cardiac cycle

Baroreceptors can reset their working range and sensitivity, or are 'adaptive', in response to sustained pressure changes. During chronic hypertension, the baroreceptors adapt to higher pressures and the response curve is shifted to the right. These changes are reversible.

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