Week 08 Lect 2 - Reflex Circulation Control

Question 1

What is the principle cardiovascular parameter controlled by reflex regulation?

Answer 1

systemic mean arterial pressure

Question 2

What is the main difference between reflex control and local control?

Answer 2

reflex - effects distributed throughout entire system

local - only affects local flow/pressure

Question 3

How does elimination of sympathetic innervation effect mean arterial pressure?

Answer 3

Causes it to drop

(from 100 mmHg to about 50 mmHg)

Question 4

What hemodynamic parameters does sympathetic innervation of veins affect?

Answer 4

sympathetic innervation of veins reduces their compliance

• so in the same volume, there is a higher venous pressure and a higher cardiac filling pressure
• in this way it also affects cardiac output

Question 5

How does autonomic innervation of the heart differ btwn sympathetic and parasympathetic?

Answer 5

sympathetic - SA/AV nodes + cardiac muscle

parasympathetic - only SA/AV nodes

Question 6

What is heterometric regulation in relation to the heart?

Answer 6

stroke volume increases in response to an increase in volume of blood filling the heart (Frank-Starling Law)

• this is because the increased volume of blood stretches the heart wall, causing the cardiac muscle to contract more forcefully

(hetero-metric refers to the change in length)

Question 7

What and where is the carotid sinus?

What kind of tissue makes it up mostly?

What is it sensitive to?

Answer 7

An extension of the internal carotid artery just past the bifurcation of the common carotid

• mostly elastic fibers (no collagen or smooth muscle)
• sensitive to transmural pressure via stretch receptors

Question 8

What two types of ion channels are present in the baroreceptors of the carotid sinus?

Answer 8

TRP (transient receptor potential) channels

and

Piezo channels

Question 9

What is the relationship between carotid sinus nerve firing rate and arterial blood pressure?

In what range is the relationship curve steepest?

Answer 9

the relationship is more or less linear within normal physiological BP ranges

• the curve is steepest around normal BP values to allow for high sensitivity within the physiological range

Question 10

What nerve receives information from the carotid sinus?

And what is it a branch of?

Answer 10

Hering’s Nerve

• a branch of the glossopharyngeal

Question 11

How do the pressure sensitivity ranges of the aortic and carotid baroreceptors differ?

And what are their values?

Answer 11

Carotid sinus receptors have a wider range of pressure sensitivity than aortic receptors.

Carotid - 50-200 mmHg

Aortic - 100-200 mmHg

Question 12

How does carotid sinus receptor potential (ie receptor cell membrane voltage) change in response to a sudden increase in pressure?

(hint: there are two phases)

Answer 12

the receptor potential has a dynamic depolarization peak that then settles to a more constant static depolarization plateau

Question 13

How do nervous signals from arterial baroreceptors code for different pressures they sense?

Answer 13

via AP frequency

• higher pressures result in higher frequency APs sent to medulla oblongata

Question 14

What is the relationship between isolated carotid sinus pressure and systemic arterial pressure?

Why?

Answer 14

the relationship is inversely proportional

• because as pressure increases in the carotid sinus, signals are sent to the medulla which induce a reduction in systemic pressure

Question 15

What two cardiovascular changes take place to counteract increased mean arterial pressure?

Answer 15

Bradychardia

and

Vasodilation

Question 16

What happens to arterial baroreceptor activity in conditions of sustained (as in, for weeks or more) hypertension?

Answer 16

the baroreceptor adapts to the higher pressure

• it now considers the higher pressures to be normal and fires APs at the set-point “normal” firing rate even when the pressure is higher than the normal, healthy range

Question 17

How does baroreceptor adaptation to hypertension effect a graph of AP firing rate vs. pressure?

Answer 17

the curve shifts to the right

• this indicates that the receptor now considers higher pressures to be ‘normal’

Question 18

How are the aortic baroreceptors innervated?

Answer 18

via the vagus nerve

Question 19

What is the pressure threshold for eliciting sinus nerve firing?

And the pressure at which the firing rate maxes out?

Answer 19

50 mmHg threshold

200 mmHg maximum

Question 20

How does a decrease in pulse pressure (but constant mean pressure) in the carotid sinus affect its baroreceptors’ function?

Answer 20

baroreceptor response is greater when pulse pressure is higher, so…

decreased pulse pressure reduces firing rate…

this in turn increases systemic arterial pressure

Question 21

What brainstem nucleus is involved in receiving baroreceptor information?

Answer 21

• Solitary Tract - from both CN IX + X for carotid and aortic receptors, respectively

Question 22

What two brainstem nuclei contain efferent neurons which affect the heart in relation to baroreceptor-mediated blood pressure regulation?

Together what are they called?

Answer 22

The cardioinhibitory area is made up of…

• Dorsal Motor Nucleus of Vagus
• Nucleus Ambiguus (CN X and IX)

Question 23

From where do sympathetic efferent signals affecting blood pressure originate in the brainstem?

Answer 23

from the vasomotor area

or rostroventrolateral medulla

Question 24

Where do signals from the vasomotor area go?

And what effects do they have?

Answer 24

First they travel through the spinal cord, synapsing in the lateral horn with pre-ganglionic sympathetic fibers.

After synapsing in a sympathetic ganglion, they then innervate…

• SA/AV nodes and atrial/ventricular myocytes - producing positive chronotrope/inotrope effects
• arterioles/venules - vasoconstriction, increased resistance/decreased compliance

Question 25

What area sends signals to inhibit the vasomotor area?

Answer 25

Caudoventrolateral Medulla

or

Depressor Area

Question 26

In what two ways does the caudal ventrolateral medulla (CVLM) counteract the vasomotor area’s effects?

Answer 26

1. direct inhibition of the vasomotor area
2. spinal cord inhibition - nerves leaving CVLM inhibit sympathetic signals from the lateral horn

Question 27

What is the pressor response?

How does it work?

Answer 27

• the body’s “anti-hypotensive” response to decreased blood pressure

1. Decreased pressure decreases baroreceptor firing rate
2. Depressor area activity decreases
3. Vasomotor area activity increases, causing…
   
   • Venomotor** activity increase + **Venous Compliance decrease leading to…
     
     • higher atrial filling (heterometric regulation)
   • HR ^ + Contractile Force ^
     
     • higher cardiac output (homometric regulation)
   • Vasoconstriction - increases BP

Question 28

In the case of a normal pressor response (as in when standing after a long period of sitting), where does most vasoconstriction occur?

Answer 28

in skin and skeletal muscle

Question 29

In the case of a strong challenge to the body’s blood pressure-regulating systems (such as severe hemorrhage)…

in what organs do vessels constrict?

(which normally do not under less severe circumstances)

Answer 29

splanchnic vessels

and

kidney vessels

Question 30

What happens to the major hemodynamic parameters of pressure and resistance under heat stress?

How?

And how is this corrected for?

Answer 30

Arterial pressure and total peripheral resistance decrease due to vasodilation which occurs as the body attempts to dissipate excess heat.

The pressor response initiates, increasing sympathetic and decreasing parasympathetic activity.

Question 31

What happens to blood pressure in the absence of high pressure baroreceptors?

Answer 31

it fluctuates much more

• the receptors’ regulatory effects are removed

Question 32

What other baroreceptors are found in the high pressure system?

What response do they initiate to regulate low blood pressure?

Answer 32

Renal Artery Receptors

• initiate the renin-angiotensin system to restore blood pressure when it is low

Question 33

How does a graph of arterial pressure vs. time displaying the effects of the renin-angiotensin system after hemorrhage look?

(including the same event without renal baroreceptor regulation)

Answer 33

Question 34

Where are the baroreceptors in the low pressure system found?

What do they mainly sense?

Answer 34

Atrial Baroreceptors

• A-type fibers in the right atrium
• B-type fibers at the inlet of the two venae cavae
• mainly sense changes in blood volume

Question 35

What response do atrial baroreceptors induce in response to blood volume increase?

Answer 35

When atrial pressure increases, the baroreceptors induce Atrial Natriuretic Peptide (ANP) release

(also decreases renin-angiotensin system activity)

Question 36

What effect does atrial baroreceptor-mediated ANP release have on kidney function?

Answer 36

it dilates efferent arterioles and constricts afferent arterioles, increasing filtration rate…

this results in increased urination and blood volume loss

Question 37

What is the Bainbridge reflex?

What purpose does it serve?

Answer 37

an increase in heart rate due to increased right atrial pressure

(detected by atrial baroreceptors which communicate with the medullary control centers)

• acts to decrease atrial pressure by pumping blood out of the atrium

Question 38

What is respiratory sinus arrhythmia?

How does it occur?

Answer 38

• a transient increase in heart rate during inhalation

increased venous return triggered by decreased thoracic pressure during inhalation triggers the Bainbridge reflex

(an increase in heart rate due to atrial pressure increase)

Question 39

How do both an increase and a decrease in blood volume induce an increase in heart rate?

Answer 39

Increase - via Bainbridge reflex (atrial baroreceptors)

Decrease - via baroreceptor reflex (carotid sinus + aorta)

Question 40

What 3 mechanisms contribute to respiratory sinus arrhythmia?

Answer 40

1. Bainbridge reflex - decreased intrathoracic P increases venous return + atrial pressure
2. Decreased Stroke Volume - decreased intrathoracic P decreases left ventricle filling via pulmonary veins
3. Respiratory Center Activity Increase

Question 41

In what conditions do chemoreceptors contribute to blood pressure control?

Answer 41

only in emergency situations of very low blood pressure

Question 42

What conditions do chemoreceptors in the aortic arch and carotid sinus respond to?

Answer 42

• decreased PO2
• increased PCO2
• decreased pH

Question 43

What are the medullary chemoreceptors mostly sensitive to?

Answer 43

changes in P_CO2

Question 44

At what pressures do peripheral chemoreceptors induce vasoconstriction?

And at what pressures do medullary chemoreceptors play a bigger role?

Answer 44

40-60 mmHg

below 40 mmHg

Question 45

What is the Cushing reflex?

Answer 45

Intracranial pressure increase leads to…

1. increased BP
2. bradycardia
3. irregular breathing

Question 46

What is the mechanism for the Cushing reflex?

Answer 46

• increased intracranial pressure overcomes arterial pressure, reducing cerebral flow
• cerebral ischemia affects medullary control centers
• vasomotor area AND cardioinhibitory area activated leading to bradycardia** and **vasoconstriction

Question 47

What is the Bezold-Jarisch reflex?

Answer 47

Temporary bradycardia and hypotension to protect heart in case of O2 deficiency

(as in heart attack + ischemia)

• triggered by chemo/mechanoreceptors in L ventricle decreasing vasomotor and increasing cardioinhibitory