Figure 1. An updated model of O₂ sensing by carotid body glomus cells. Hypoxia inhibits K⁺ channels such as TASK via an O₂ sensor. The depolarization opens voltage-gated Ca²⁺ channels and elevates [Ca²⁺]_i. Rise of [Ca²⁺]_i begins to increase Na⁺ influx via the 20-pS channel, and initiates a feed-forward mechanism to maintain an optimal level of [Ca²⁺]_i during moderate to severe hypoxia. Further increase in [Ca²⁺]_i is prevented by activation of BK that opposes depolarization.