Question

A 78 year-old man gets up too quickly from his Lazyboy and nearly falls to the floor after experiencing light-headedness, but luckily falls backward into the chair.

a. Briefly, what are the essential components of a visceral reflex?

b. What is the cardiovascular response to this drop in blood pressure?

c. What location in the central nervous system coordinates this response?

Answer 1

A) the visceral reflex components are

1. sensory receptor

2. sensory neuron

3. integration center

4. Efferent pathway

5. effector target

B) the entire vasoconstrictor and cardioaccelerator functions of the sympathetic nervous system are stimulated together. At the same time, there is reciprocal inhibition of parasympathetic vagal inhibitory signals to the heart. Thus, three major changes occur simultaneously, each of which helps to increase arterial pressure. They are as follows:

1. Most arterioles of the systemic circulation are constricted. This greatly increases the total peripheral resistance, thereby increasing the arterial pressure.

2. The veins especially (but the other large vessels of the circulation as well) are strongly constricted. This displaces blood out of the large peripheral blood vessels toward the heart, thus increasing the volume of blood in the heart chambers. The stretch of the heart then causes the heart to beat with far greater force and therefore to pump increased quantities of blood. This, too, increases the arterial pressure.

3. Finally, the heart itself is directly stimulated by the autonomic nervous system, further enhancing cardiac pumping. Much of this is caused by an increase in the heart rate, the rate sometimes increasing to as great as three times normal. In addition, sympathetic nervous signals have a significant direct effect to increase contractile force of the heart muscle, this, too, increasing the capability of the heart to pump larger volumes of blood. During strong sympathetic stimulation, the heart can pump about two times as much blood as under normal conditions. This contributes still more to the acute rise in arterial pressure.

C-

By far the best known of the nervous mechanisms for arterial pressure control is the baroreceptor reflex. Basically, this reflex is initiated by stretch receptors, called baroreceptors located at specific points in the walls of several large systemic arteries. A rise in arterial pressure stretches the baroreceptors and causes them to transmit signals into the central nervous system. “Feedback” signals are then sent back through the autonomic nervous system to the circulation to reduce arterial pressure downward toward the normal level.

It is important when a person stands up after having been lying down. Immediately on standing, the arterial pressure in the head and upper part of the body tends to fall, and marked reduction of this pressure could cause loss of consciousness. However, the falling pressure at the baroreceptors elicits an immediate reflex, resulting in strong sympathetic discharge throughout the body. This minimizes the decrease in pressure in the head and upper body.

After the baroreceptor signals have entered the tractus solitarius of the medulla, secondary signals inhibit the vasoconstrictor center of the medulla and excite the vagal parasympathetic center. The net effects are

(1) vasodilation of the veins and arterioles throughout the peripheral circulatory system and

(2) decreased heart rate and strength of heart contraction. Therefore, excitation of the baroreceptors by high pressure in the arteries reflexly causes the arterial pressure to decrease because of both a decrease in peripheral resistance and a decrease in cardiac output. Conversely, low pressure has opposite effects, reflexly causing the pressure to rise back toward normal.