Arterial Baroreceptors and Control of Blood Pressure Flashcards
What is an open loop and a closed loop experiment?
Open loop does not allow the controlled variable to feed back to the sensors. Open loop allows quantification of a certain stimulus in terms of it’s effect.
What did Angell James show in 1971?
That aortic nerve activity was similar whether transmural pressure is increased by either decreasing extramural pressure or by increasing intra-aortic pressure. Therefore baro-R are stretch-R.
What effect does pulsatility have on baro-R firing? Is this seen in all baro-R?
Pulsatile blood flow increased CSN firing cf to continuous pressure. However, not seen in AA
What are the two “types” of baro-R?
With myelinated axons, A fibres, greater rate of increase in firing and maximum firing rate.
With unmyelinated axons, C fibres, much higher threshold.
A fibres moderate small increases in BP, C involved with larger increases in BP?
Why is the baro-R response curve sigmoidal?
Increased firing of individual baro-R progressively recruits fibres with higher thresholds.
Are Baro-R slow adapating? Why?
Yes. Rapid increase in CSP to above threshold increased CSN firing which then progressively decays to a level which usually remains above zero. This is due to viscoelastic relaxation of the arterial wall.
Resetting occurs through this and also central resetting.
Effects of baroreflex?
Increase in HR, via reduction in PS increase in S.
Vasoconstriction, same mechanism as HR.
Increase in LV contractility, as assessed by dP/dt in a paced, anaesthetised dog.
Baroreflex has strong and weak control over which vascular beds?
Strong = skeletal muscle, kidney, splanchnic
Weak = Skin, coronary, cerebral
MSNA is pulsatile due to effect arterial baroreceptor control
Who developed original neck chamber? +/-
Ernsting and Parry, 1957.
Non-invasive therefore suitable for use in a concious man. Allows study of vascular response as well as HR. Fairly specific stimulus to CS baro-R (+/- ?)
Pressure transduction is not perfect and varies suction/pressure and corrections must therefore be applied. Does not stimulate AA baro-R, which will buffer the reflex effects from the CS. Produces unphysiological square wave BP stimulus.
How does cardiac phase and resp cycle influence effect of baro-R stimulus?
Greatest effect if stimulus arrives at SAN soon after peak of P wave and during expiration.
What is the bolus method of studying the baroreflex? +/-
Injection of vasoactive drug, eg phenylepherine. Plot systolic BP against ECG R-R intervals. Correlations systolic and the simultaneous or +1 R-R.
Simple to perform, easy to blind, simultaneous activation of both CS and AA baro-R.
May affect baro-R by other mechanism other than BP changes, can only use it to study HR response, not BP or vascular response, drugs may have unwanted effects on other receptors.
What is spontaneous baroreflex analysis? +/-
Sequences of at least 4 beats in which systolic BP is seen to either rise or fall together with PI. Regression lines are calculated and their slope is taken as an index of baro-R sensitivity.
Only requrie continuous, often non-invasive, BP monitoring. HR/PI measured from ECG or derived from BP trace. No need to use a stimulus. Onset of BRS measurement is blind. High reproducibility. Only requires 4 beats to get a value of BRS, can therefore be used to monitor changes minute-to-minute.
Interpretation of results given rise to some debate - meaning of low frequency power in the frequency domain methods and are the PI changes always due to the baroreflex.
