Figure 6.
Adult-type AChR channels in ectopic postsynaptic-like membrane. (A) Application of short pulses of ACh (upper trace) at ectopic site of innervated fiber produces rapid membrane depolarization (trace a). No response was detected at a site 100 μm away (trace b). Membrane potential was −62 mV. Calibration bars (see B) represent 5 nA, 2 mV, and 10 ms. (B) Functional properties of ectopic AChR channels. Upper trace shows membrane current evoked by prolonged application of ACh (bar above trace) at same sensitive site as shown in A. The fiber was voltage-clamped locally to −70 mV. Lower trace shows ac-coupled current record as in upper trace to illustrate increase in membrane current fluctuations during ACh application. Calibration bars represent 20 nA, 2.5 nA, and 10 s. (Lower) Autocorrelation function of ACh-induced current fluctuations shown above. Data points represent value of autocorrelation function φ(T) for different time intervals, T. Exponential function φ(T) = σ2·exp(−T/τ) is fitted to data points where σ2 represents the total variance and τ represents the decay time constant. The decay of the autocorrelation function is described by a single exponential with τ = 1.5 ms (marked by arrow; −70 mV, 18°C), indicating short AChR channel open bursts as measured for AChRs in normal adult neuromuscular junctions.