Illustration of the experimental rationale. (A) Simulated stimulus function for a sperm exposed to repetitive pulses of resact: a time interval Δt = 1 s each. Stimulus function was calculated using Eq. 3. (B) Stimulus function for paired resact stimuli with longer Δt. (C and D) Time course of bound resact molecules per flagellum. (C, black line) Repetitive pulses of resact. (C, colored lines) Sperm, swimming on a circular path in a chemoattractant gradient, at different distances from the egg (in millimeter): 2.1, dark yellow; 2.2, red; 2.3, blue; 2.4, magenta; 2.5, green; 2.6, navy; 2.7, violet; 2.8, purple; 2.9, wine. (D) Two resact pulses at long Δt. (E) A second stimulus delivered during the latency (interrupted lines) of the Ca²⁺ signal probed temporal sampling. Latency of the Ca²⁺ signal (blue) matches the time to peak of the hyperpolarization V_m signal (red). (F) Complete time course of a typical V_m (red) and a corresponding Ca²⁺ signal (blue). In paired stimulus experiments, the second stimulus was delivered at different phases of the Ca²⁺ and V_m signals. Arrows indicate time of flashes.