Glomus Cell Hypoxia Transduction

Arterial Blood Gases

PaO₂ (arterial O₂) 95 mmHg

PaCO₂ (arterial CO₂) 40 mmHg

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Membrane potential V_m

−68 mV

Intracellular Ca²⁺

0.05

Afferent nerve firing rate

2.1 Hz

How it works

The carotid body's glomus (type I) cells sense falling blood oxygen through background O₂-sensitive K⁺ channels (TASK-like leak channels). Their open probability follows a steep Hill function of oxygen tension:

g_K(PaO₂) = g_min + (g_max − g_min) · PaO₂ⁿ / (PaO₂ⁿ + K½ⁿ)
   n ≈ 4, K½ shifted lower by PaCO₂ (CO₂/pH potentiates O₂ sensing)

Less K⁺ conductance means less hyperpolarizing outward current, so the resting membrane depolarizes:

V_m ≈ V_hyperpolarized + (V_depolarized − V_hyperpolarized) · (1 − g_K / g_max)

Once V_m crosses roughly −45 mV, voltage-gated Ca²⁺ channels open and Ca²⁺ enters, triggering dopamine-vesicle release onto the afferent petrosal ganglion fiber:

d[Ca²⁺]/dt = k_in · max(0, V_m − V_thr) − [Ca²⁺] / τ_Ca
firing rate = f_max · [Ca²⁺] / ([Ca²⁺] + K_m)   (saturating, Michaelis–Menten-style)

PaO₂ slider — arterial oxygen tension; below ~60 mmHg the K⁺-channel Hill curve drops steeply and firing accelerates.
PaCO₂ slider — CO₂/pH shifts the K½ of the O₂ curve, so hypercapnia sensitizes the same PaO₂ to a stronger response (a real synergy seen in carotid body recordings).
K⁺ channel blocker — mimics pharmacological/metabolic block (e.g. cyanide-like poisoning of channel gating) by forcing g_K near zero regardless of oxygen, proving the channel — not oxygen itself — gates the signal.

Real-world relevance: this cascade is why breathing rate rises within one circulation time of acute hypoxia (high altitude, apnea, lung disease) — it is the fastest chemical sensor in the human respiratory control loop, feeding the nucleus tractus solitarius directly via the glossopharyngeal nerve.

Open K⁺ channel Closed K⁺ channel Ca²⁺ influx Nerve impulse

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