Week 08 Lect 2 - Reflex Circulation Control Flashcards by Ben Dahlberg

What is the principle cardiovascular parameter controlled by reflex regulation?

systemic mean arterial pressure

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

reflex - effects distributed throughout entire system

local - only affects local flow/pressure

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How does elimination of sympathetic innervation effect mean arterial pressure?

Causes it to drop

(from 100 mmHg to about 50 mmHg)

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What hemodynamic parameters does sympathetic innervation of veins affect?

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

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

sympathetic - SA/AV nodes + cardiac muscle

parasympathetic - only SA/AV nodes

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What is heterometric regulation in relation to the heart?

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)

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What and where is the carotid sinus?

What kind of tissue makes it up mostly?

What is it sensitive to?

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

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

TRP (transient receptor potential) channels

and

Piezo channels

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

In what range is the relationship curve steepest?

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

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What nerve receives information from the carotid sinus?

And what is it a branch of?

Hering’s Nerve

a branch of the glossopharyngeal

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

And what are their values?

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

Carotid - 50-200 mmHg

Aortic - 100-200 mmHg

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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)

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

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

via AP frequency

higher pressures result in higher frequency APs sent to medulla oblongata

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

Why?

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

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

Bradychardia

and

Vasodilation

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

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

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

the curve shifts to the right

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

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How are the aortic baroreceptors innervated?

via the vagus nerve

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What is the pressure threshold for eliciting sinus nerve firing?

And the pressure at which the firing rate maxes out?

50 mmHg threshold

200 mmHg maximum

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

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

decreased pulse pressure reduces firing rate…

this in turn increases systemic arterial pressure

What brainstem nucleus is involved in receiving baroreceptor information?

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

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

Together what are they called?

The cardioinhibitory area is made up of…

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

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

from the vasomotor area

or rostroventrolateral medulla

Where do signals from the vasomotor area go?

And what effects do they have?

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

What area sends signals to inhibit the vasomotor area?

**Caudoventrolateral Medulla** or **Depressor Area**

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

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

What is the _pressor response_? How does it work?

- 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

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

in **skin** and **skeletal muscle**

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)

**splanchnic vessels** and **kidney vessels**

What happens to the major hemodynamic parameters of pressure and resistance under **_heat stress_**? How? And how is this corrected for?

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.

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

it **_fluctuates_** much more - the receptors' regulatory effects are removed

What other baroreceptors are found in the _high pressure_ system? What response do they initiate to regulate low blood pressure?

**Renal Artery Receptors** - initiate the **renin-angiotensin system** to restore blood pressure when it is low

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)

Where are the baroreceptors in the _low pressure system_ found? What do they mainly sense?

**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**

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

When atrial pressure increases, the baroreceptors induce **_Atrial Natriuretic Peptide (ANP)_** release (also _decreases renin-angiotensin system_ activity)

What effect does atrial baroreceptor-mediated _ANP release_ have on kidney function?

it dilates efferent arterioles and constricts afferent arterioles, increasing filtration rate... this results in _increased urination and blood volume loss_

What is the **Bainbridge reflex**? What purpose does it serve?

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

What is **respiratory sinus arrhythmia**? How does it occur?

- 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)

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

Increase - via _Bainbridge reflex_ (atrial baroreceptors) Decrease - via _baroreceptor reflex_ (carotid sinus + aorta)

What 3 mechanisms contribute to respiratory sinus arrhythmia?

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_

In what conditions do _chemoreceptors_ contribute to blood pressure control?

only in _emergency situations_ of _very low_ blood pressure

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

* **decreased PO2** * **increased PCO2** * **decreased pH**

What are the _medullary chemoreceptors_ mostly sensitive to?

changes in **P_CO2**

At what pressures do peripheral chemoreceptors induce vasoconstriction? And at what pressures do medullary chemoreceptors play a bigger role?

40-60 mmHg below 40 mmHg

What is the **Cushing reflex**?

Intracranial pressure increase leads to... 1. increased BP 2. bradycardia 3. irregular breathing

What is the mechanism for the **Cushing reflex**?

* 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_**

What is the Bezold-Jarisch reflex?

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