Fig. 4. Voltage dependence of macroscopic activation and inactivation of T-current. *A*, The cells were held at −120 mV and stepped every 5 sec to −10 to −80 mV for 500 msec to elicit T-current. La^{3+}-sensitive currents were used to eliminate capacity transient and facilitate analysis of the currents. The rising phase of the macroscopic currents (from the beginning to the peak of each current) are fitted by monoexponential functions. The time constants from the fits are 5.6 ± 1.0, 3.2 ± 0.2, 1.9 ± 0.2, 1.3 ± 0.1, 0.89 ± 0.11, 0.61 ± 0.08, 0.48 ± 0.1, and 0.40 ± 0.07 msec for step potentials −80, −70, −60, −50, −40, −30, −20, and −10 mV, respectively (all *n* = 4; note that the*vertical axis* is in logarithmic scale).*B*, The decaying phase of the current sweeps in*A* are also fitted by monoexponential functions, and the time constants are 43.9 ± 8.0, 24.9 ± 4.2, 18.8 ± 2.3, 16.7 ± 1.7, 15.5 ± 1.2, 15.0 ± 0.8, 14.8 ± 0.8, and 15.7 ± 0.9 msec for step potentials −80, −70, −60, −50, −40, −30, −20, and −10 mV, respectively. *C*, A representative inactivation curve of T-channels from four type II thalamic neurons. The cells are held at −120 mV and stepped every 5 sec to the inactivating pulse (−50 to −150 mV) for 500 msec. The channels that remained available after each inactivating pulse were assessed by the peak currents during a following short pulse to −50 mV for 500 msec. The fraction available is defined as the normalized peak current (relative to the peak current evoked with an inactivating pulse at −150 mV) and is plotted against the voltage of the inactivating pulse. The line is a fit to the data with a Boltzmann function: fraction available = 1/(1 + exp((*V* + 96.8)/7.3)), where *V* denotes the voltage of the inactivating pulse in millivolts.