Week 08 Lect 2 - Reflex Circulation Control Flashcards
What is the principle cardiovascular parameter controlled by reflex regulation?
systemic mean arterial pressure
What is the main difference between reflex control and local control?
reflex - effects distributed throughout entire system
local - only affects local flow/pressure
How does elimination of sympathetic innervation effect mean arterial pressure?
Causes it to drop
(from 100 mmHg to about 50 mmHg)
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
How does autonomic innervation of the heart differ btwn sympathetic and parasympathetic?
sympathetic - SA/AV nodes + cardiac muscle
parasympathetic - only SA/AV nodes
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)
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
What two types of ion channels are present in the baroreceptors of the carotid sinus?
TRP (transient receptor potential) channels
and
Piezo channels
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
What nerve receives information from the carotid sinus?
And what is it a branch of?
Hering’s Nerve
- a branch of the glossopharyngeal
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
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
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
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
What two cardiovascular changes take place to counteract increased mean arterial pressure?
Bradychardia
and
Vasodilation
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
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’
How are the aortic baroreceptors innervated?
via the vagus nerve
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
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?
- direct inhibition of the vasomotor area
- 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
- Decreased pressure decreases baroreceptor firing rate
- Depressor area activity decreases
-
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
-
Venomotor** activity increase + **Venous Compliance decrease leading to…
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?
- Bainbridge reflex - decreased intrathoracic P increases venous return + atrial pressure
- Decreased Stroke Volume - decreased intrathoracic P decreases left ventricle filling via pulmonary veins
- 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 PCO2
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…
- increased BP
- bradycardia
- 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
