Fig. 1. (A–D) Bicarbonate induction of increase in linear velocity of boar spermatozoa. Samples of washed spermatozoa were pre-incubated in Tyrode's medium before the addition of 15 mmol l^–1 bicarbonate. Subsamples were taken for video recording of motility shortly before (`zero time') and at intervals after bicarbonate addition. Motility parameter values were obtained by analysis of individual sperm tracks using the Hobson Sperm Tracker. These are presented as scatterplots (VAP vs LIN) (average path velocity vs linearity) of individual spermatozoa from a combination of two representative boars; each point represents a single sperm trajectory. In the absence of bicarbonate/CO₂ (A,C) most spermatozoa exhibit low VAP and LIN, although a small number of solubilised apical plasma membrane protein fraction (sAPM)-treated spermatozoa show high velocity (>60 µm s^–1) and straight tracks (LIN >60%); (highlighted in the box in the upper right corner). 7 min after the addition of bicarbonate/CO₂, a sizeable proportion of spermatozoa show activation (B,D); the boxes in the upper right of these panels also highlight spermatozoa showing high velocity (>60 µm s^–1) and straight tracks (LIN >60%). The density of spermatozoa within the upper right hand box is higher in the absence than in the presence of sAPM (compare B and D). (E) Representative trajectories of spermatozoa activated in the absence of sAPM. Track 3 represents the most activated trajectory (fast and linear), while tracks 1 and 2 represent subpopulations that would be classified as slow and/or non-linear. (F) Two representative fast-linear tracks activated in the presence of sAPM. These tracks show high linearity because there is relatively little deviation from the average path. Black dots represent x–y coordinates measured at 20-ms intervals; red lines are fitted spline curves.