LES calyx terminals cannot follow APs at higher frequencies. AP trains were triggered at 100 Hz (A), 300 Hz (B), and 1 kHz (C) at room temperature (24°C, A and B) and at physiological temperature (PT; 37°C, C). The calyx of Held of both LE and LES rats can follow 100 Hz stimulation, but at higher stimulation frequencies (300 Hz and 1 kHz) presynaptic APs of the LES calyx terminals experience intermittent failures, whereas normal LE calyx terminals can routinely follow 300 Hz (at room temperature) and even 1 kHz at 37°C without AP failures. Recordings were from P16 LE and LES rats (black and red traces, respectively). The horizontal dashed line indicates the resting membrane potential of −80 mV. Arrowheads in B and blue arrows in C indicate AP failures. LES rats showed more AP failures and the timing of the APs showed more “jitter.”