Stretch-sensitive baroreceptors in the carotid sinus and aortic arch fire more action potentials as arterial wall stretch (pressure) rises. The relationship between mean arterial pressure P and afferent firing rate F is a classic sigmoid ("logistic") stimulus-response curve, not a straight line:
F(P) = Fmin + (Fmax − Fmin) / (1 + e^(−G·(P − P0)))
G = reflex gain (sensitivity, %/mmHg at the steepest point)
P0 = set-point — the pressure at the curve's midpoint / inflection
Fmin, Fmax = 0% and 100% (saturating floor and ceiling)
The brainstem (nucleus tractus solitarius) reads this firing rate and adjusts vagal (parasympathetic) and sympathetic outflow to the heart and vessels — higher firing raises vagal tone and lowers heart rate; lower firing does the opposite:
HR(P) = HRmax − (HRmax − HRmin) · F(P) / 100
Local gain: dF/dP = G·(Fmax−Fmin)·e^(−G(P−P0)) / (1 + e^(−G(P−P0)))²
- MAP slider — moves the operating point left/right along the fixed sigmoid; the marker, nerve traffic and heart rate all update instantly.
- Gain slider — steepens or flattens the curve around P0, matching how baroreflex sensitivity (BRS) is measured clinically (ms/mmHg or %/mmHg) and how it declines with age or autonomic disease.
- Set-point P0 slider — shifts the whole curve left or right without changing its shape, the mechanism behind baroreflex resetting: in sustained hypertension the operating range re-centers around a higher "normal" pressure over days, so the reflex still functions but defends the wrong target.
- Firing / Heart-rate curve toggle — switches which output the 3D graph plots: the raw afferent sigmoid, or the resulting heart-rate response curve (its mirror image).
- The local gain readout is the analytic slope of F(P) at the current pressure — it is largest near P0 and flattens toward the floor/ceiling, exactly why the reflex is most effective for defending pressure near its set-point and saturates during large swings.
Real-world relevance: sigmoid baroreflex curves like this are how physiologists quantify hypertension, autonomic failure and baroreflex resetting after chronic disease, and how baroreflex-activation-therapy devices are tuned in cardiology.