EXPERIMENTAL FIGURE 11-23 Ion flux through individual Na⁺ channels can be calculated from patch-clamp tracings. Two inside-out patches of muscle plasma membrane were clamped at a potential of slightly less than the resting membrane potential. The patch pipette contained NaCl. The transient pulses of electric current (in picoamperes), recorded as large downward deviations (blue arrows), indicate the opening of a Na⁺ channel and the movement of positive charges (Na⁺ ions) inward across the membrane. The smaller deviations in current represent background noise. The average current through an open channel is 1.6 pA, or 1.6 × 10⁻¹² amperes. Since 1 ampere = 1 coulomb (C) of charge per second, this current is equivalent to the movement of about 9900 Na⁺ ions per channel per millisecond: (1.6 × 10⁻¹² C/s)(10⁻³ s/ms)(6 × 10²³ molecules/mol) ÷ 96,500 C/mol. See F. J. Sigworth and E. Neher, 1980, Nature 287:447.