Autonomic Control of Blood Pressure

Question 1

Identify the 2 sensory receptors, 2 afferent pathways, the 1 central integrating center, efferent pathways, and effector organs that participate in the baroreceptor reflex negative feedback loop, including their locations.

Answer 1

high pressure sensory receptors = baroreceptors, which are stretch receptors as they are sensitive to stretch (MAP) in a frequency dependent manner (main ones @ the carotid sinus and aortic arch). Stretching (increasing MAP) results in a reflex of vasodilation and bradycardia to control MAP.

afferent pathways (mnemonic: A1”0” steak–remember vagus hides under arch of aorta):

• Signals from the “carotid baroreceptors” in the carotid sinus are transmitted through small Hering’s nerves to glossopharyngeal nerves (IX) in the high neck, and then to the nucleus tractus solitarius (NTS) in the medulla.
• Signals from the “aortic baroreceptors” in the aortic arch are transmitted through the vagus nerves (X) also to NTS in the medulla.

central integrating centers (nucleus tractus solitarius (NTS) in the medulla oblongata = set point = command center for regulation of cardiac function). NTS does both PNS & SNS messages.

efferent ANS pathways are used to turn on effector organs

effector organs: decrease in heart rate & dialate vessels

Question 2

How do changes in the afferent input from arterial baroreceptors influence the activity of the sympathetic and parasympathetic preganglionic fibers, and how do the sympathetic and parasympathetic outputs from the medullary cardiovascular centers change in response to changes (increase) in arterial pressure.

Answer 2

After the baroreceptor signals enter the tractus solitarius of the medulla (the set point location), secondary signals inhibit the vasoconstrictor center and excite the vagal parasympathetic center.

The net effects are (1) vasodilation of the veins and arterioles throughout the peripheral circulatory system by reduction of the SYMPATHETIC vasal tone and (2) decreased heart rate and strength of heart contraction via PARASYMPATHETIC activity.

Remember: MAP = CO x TPR
-The negative feedback loops act on both CO & TPR

Thus, 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, a decrease in MAP has the opposite effects and uses the sympathetic nervous system, reflexly causing the pressure to rise back toward normal (tachycardia & vasoconstriction).

Question 3

Diagram the chain of events that are initiated by the arterial baroreceptor reflex to compensate for a change in arterial pressure

Answer 3

An increase in MAP (stimulus) activates homeostatic negative feedback mechanisms to counteract this stimulus (increase in MAP).

The increase in MAP activates high-pressure baroreceptors (detectors on the high pressure/arterial portion of the CV system) whose signal is carried via afferent nerve pathways to a central coordinating center in the medulla oblongata (integrator/set point).

The coordinating center uses efferent ANS pathways to decrease heart rate and dilate vessels (effectors).

Thus, reflex bradycardia and vasodilation lead to a decrease in MAP, counteracting the initial stimulus (negative feedback). This response is referred to as a baroreceptor reflex.

Question 4

Explain what is meant by baroreceptor adaptation

Answer 4

The baroreceptor reflex ADAPTS to long-term changes in mean arterial pressure. For example, in hypertension, the set point is raised.

In hypertension, the curve is parallel and shifted to the right to adapt to a higher range of blood pressures.

See curve pg. 106

Question 5

Neural & hormonal mechanisms are often involved in regulating mean arterial pressure to maintain adequate tissue ______.

What is the equation for MAP (MUST MEMORIZE)

Answer 5

Perfusion

MAP = CO x TPR

CO = HR x SV

Question 6

MAP is monitored by by 3 diff mechanisms

Answer 6

1) High pressure arterial baroreceptors (arterial system)–most important
2) Renal juxtaglomerular apparatus
3) Low pressure baroreceptors (in venous system & atria, they are volume receptors)

Question 7

Adjustments to MAP are carried out by which effector

Answer 7

The autonomic nervous system

Question 8

The arterial baroreceptor reflex is the most important mechanism in providing ______ term regulation of ______ blood pressure.

Answer 8

short term! Via high pressure arterial baroreceptors.

arterial BP

Question 9

Which baroreceptor is more important, the carotid sinus or the aortic arch?

Answer 9

The carotid sinus since it is more sensitive! The aortic baroreceptor curve is shifted to the right & does not operate most sensitively over the pressures that the carotid sinus does.

Question 10

Discuss the role of arterial chemoreceptors in restoring MAP, where they are located, how they send afferent signals, & @ what pressure its activity is highest.

Answer 10

Peripheral chemoreceptors are located in the carotid and aortic bodies & are in close contact with arterial blood (they are secondary in regulating MAP compared to high pressure arterial baroreceptors).

When arterial pressure falls below a critical level, the receptors become stimulated because diminished blood flow causes decreased oxygen, and excess buildup of carbon dioxide and hydrogen ions that are not removed by the slowly flowing blood (i.e., they detect pO2, pCO2, & pH).

Signals transmitted from the chemoreceptors, along with the baroreceptor fibers, pass through Hering’s nerves and the vagus nerves into the vasomotor center to elevate the arterial pressure back toward normal.

However, the chemoreceptor reflex is not a powerful arterial pressure controller until the arterial pressure falls below 80 mm Hg (e.g., hemorrhage)*. Thus, it is at the lower pressures that this reflex becomes important to help prevent further decreases in arterial pressure.

Question 11

The amplitude of the stretch receptor AP is

Answer 11

proportional to the degree of stretch, that is, the higher the stretch the higher the frequency of AP firing (frequency dependent)

Question 12

Baroreceptors respond _____ to changes in arteial pressures and are most sensitive in the normal operating range of _____mmHg (the normal MAP).

Answer 12

Rapidly

100 mmHg

See the sigmoidal curve on pg. 106

In the normal operating range of arterial pressure (~100 mm Hg) even a slight change in pressure causes a strong change in the baroreflex signal to readjust arterial pressure back toward normal. Thus, the baroreceptor feedback mechanism functions most effectively in the pressure range where it is most needed.

Question 13

If a person suffered from hypertension on the carotid baroreceptor curve, how would the curve change?

Answer 13

The baroreceptor reflex ADAPTS to long-term changes in mean arterial pressure.

In hypertension, the curve is parallel and shifted to the right to adapt to a higher range of blood pressures.

Question 14

Compare the deviation in BP in an animal whose baroreceptors do not function vs a normal animal

Answer 14

In summary, a primary purpose of the arterial baroreceptor system is to reduce the minute-by-minute variation in arterial pressure to about one-third that which would occur if the baroreceptor system was not present.

Without baroreceptors, there is a wide variation in mean arterial pressure.

Question 15

1) What events cause slight variations in MAP?
2) A change in posture from standing to lying down ______ venous return and thus increases _____ volume which ______ MAP.

Answer 15

1) lying down increases MAP, standing decreases MAP (dizziness occurs due to lack of blood to brain), excitement, & eating.
2) A change in posture from standing to lying down increases venous return and thus increases stroke volume which increases MAP.

Question 16

In most tissues, sympathetic nerve fibers innervate all vessels except the _______.

Answer 16

capillaries

Question 17

Discuss how sympathetic innervation of small arteries & arterioles is different from that of the veins.

Answer 17

The innervation of the small arteries and arterioles allows sympathetic stimulation to increase resistance to blood flow and thereby to decrease rate of blood flow through the tissues.

The innervation of the large vessels, particularly of the veins, makes it possible for sympathetic stimulation to decrease the volume of these vessels. This can push blood into the heart by decreasing compliance (veins hold most of the blood, so constriction pushes it into the heart & produces the starling effect) and thereby play a major role in regulation of cardiac output.

Question 18

1) Are blood vessels always partially constricted? What nervous system keeps them this way?
2) How do vasodilatation & vasoconstriction occur?

Answer 18

Yes! SNS impulses normally maintain a partial state of contraction in the blood vessels, called vasomotor tone, which keeps MAP around 100mmHg.

Without this vasomotor tone, like during anethesia, MAP drops to 50mmHg.

Therefore, we vasodialate by decreasing these SNS AP’s & vasomotor tone or vasoconstrict by increasing the SNS AP’s & vasomotor tone.

Question 19

Describe the Valsalva Maneuver

Answer 19

The Valsalva maneuver can be used to test the integrity of the baroreceptor reflex. The subject is asked to expire against a closed glottis (as during heavy lifting, coughing, defecating).

This causes an increase in intrathoracic pressure and a decrease in venous return to the heart, which decreases CO and MAP. Intact baroreceptors will sense the decrease in MAP and direct an increase in SNS and decrease in PNS outflow to the heart and vessels, the increase in HR is measured. A rebound decrease in HR is noted after release from the maneuver.

Question 20

Decreased MAP, _____ parasympathetic activity & ______ sympathetic activity

Answer 20

reduces PNS

increases SNS

Question 21

Describe the carotid massage

Answer 21

Carotid sinus massage (or release from a Valsalva maneuver) stimulates the baroreceptors and reflexly slows the heartrate in people with atrial tachycardia (e.g., atria flutter or fibrillation) which fight the tachycardia.

Question 22

Describe carotid sinus syndrome

Answer 22

Patients with carotid sinus syndrome have hyper-sensitive baroreceptors such that even mild external pressure to the neck elicits a strong reflex, even stopping the heart for 5-10 seconds = fainting.

Question 23

You are given three catecholamine solutions of known concentration: norepinephrine, epinephrine, and isoproterenol (a beta-adrenergic receptor agonist). Each solution is injected separately into a conscious subject at a concentration of 2 mg/kg of body weight. You can assume that, at this dose, isoproterenol stimulates only beta-adrenergic receptors. What response would you obtain from the norepinephrine injection.

Answer 23

a decrease in heart rate and an increase in arterial pressure that is more marked than those obtained from the other solutions.

Norepinephrine produces a more marked increase in the total systemic peripheral resistance than do the other catecholamines. It performs this function by virtue of being a strong stimulator of the α-receptors of arterioles and, at these concentrations, either not a stimulator or a very weak stimulator of the β-receptors of smooth muscle. Although norepinephrine also stimulates the β-adrenergic receptors of the heart to increase heart rate, the increase in arterial pressure that it produces acts through the arterial baroreceptors to mask this action reflexly and produce a slowing of the heart.

In other words, the vasoconstriction norepi produces, increases TPR & MAP so much, that the heartrate decreases reflexively to decrease MAP.

Question 24

A cardiovascular disturbance that decreases MAP will reflexly result in _______ baroreceptor discharge, which will lead to an ________ in SNS activity, which includes ________ heart rate, total peripheral resistance (vasoconstriction), and myocardial contractility.

Answer 24

A cardiovascular disturbance that decreases MAP will reflexly result in decreased baroreceptor discharge, which will lead to an increase in SNS activity, which includes increased heart rate, total peripheral resistance (vasoconstriction), and myocardial contractility.

Question 25

Diarrhea causes a loss of extracellular fluid volume, which produces a _______ in arterial pressure. The _______ in arterial pressure activates the baroreceptor mechanism, which produces an _______ in heart rate when the patient is supine. When she stands up, blood pools in her leg veins and produces a ______ in venous return, a ______ in cardiac output (by the Frank- Starling mechanism), and a further ______ in arterial pressure. The further ________ in arterial pressure causes further activation of the baroreceptor mechanism and a further _______ in heart rate.

Answer 25

Diarrhea causes a loss of extracellular fluid volume, which produces a decrease in arterial pressure. The decrease in arterial pressure activates the baroreceptor mechanism, which produces an increase in heart rate when the patient is supine. When she stands up, blood pools in her leg veins and produces a decrease in venous return, a decrease in cardiac output (by the Frank- Starling mechanism), and a further decrease in arterial pressure. The further decrease in arterial pressure causes further activation of the baroreceptor mechanism and a further increase in heart rate.

Question 26

When a person stands upright, blood _____ in the lower extremities. ______ venous return causes right and left ventricular preload to fall, which _______ ventricular stroke volume and cardiac output. Arterial pressure begins to fall as a result, which is sensed via a _______ in arterial (aortic and carotid sinus) baroreceptor firing rate. If the pressure drop is severe, cerebral blood flow can be compromised. Normally compensating individuals tolerate the upright position by initiating a baroreceptor reflex, which includes an _______ in systemic vascular resistance.

Answer 26

When a person stands upright, blood pools in the lower extremities. Decreased venous return causes right and left ventricular preload to fall, which reduces ventricular stroke volume and cardiac output. Arterial pressure begins to fall as a result, which is sensed via a decrease in arterial (aortic and carotid sinus) baroreceptor firing rate. If the pressure drop is severe, cerebral blood flow can be compromised. Normally compensating individuals tolerate the upright position by initiating a baroreceptor reflex, which includes an increase in systemic vascular resistance.

Question 27

When a person moves from a supine position to a standing position, which of the following compensatory changes occurs?

Answer 27

There is an increase in heart rate (resulting in a decreased PR interval), contractility, and total peripheral resistance (TPR). Because both rate and contractility are increased, cardiac output will increase toward normal.

Question 28

What is the mechanism of the normal physiologic response to hypotension?

Answer 28

Decreased baroreceptor afferent firing in the carotid sinus leads to increased sympathetic efferent firing. This results in systemic vasoconstriction, increased heart rate, increased contractility, and increased blood pressure.