In cortical pyramidal neurons, action potentials (APs) are initiated in the axon and back-propagate into their dendritic arbors both in vitro and in vivo (Stuart and Sakmann, 1994 Larkum and Zhu, 2002 Waters et al., 2003 Bereshpolova et al., 2007) APs back-propagate into the basal (Antic, 2003 Kampa and Stuart, 2006 Nevian et al., 2007), oblique (Antic, 2003 Frick et al., 2003), and apical main and tuft dendrites (Schiller et al., 1995 Williams and Stuart, 2000 Larkum and Zhu, 2002). Our data support models in which the interaction of synaptic input with action potential output is a function of the timing, rate and pattern of action potentials, and dendritic location. Various Ca 2+ channel types contributed to the enhanced calcium signals during high-frequency firing activity, whereas A-type K + and BK Ca channels strongly suppressed it. In contrast to the main apical dendrite, the more passive properties of the short basal and apical tuft dendrites prevented an efficient back-propagation. Both the critical frequency enabling action potential trains to invade efficiently and the dendritic calcium profile changed during postnatal development. Our data provide evidence that the temporal structure of physiological action potential patterns is crucial for an effective invasion of the main apical dendrites up to the major branch point. We used a combination of whole-cell recordings and Ca 2+ imaging techniques in vitro to explore the specific dendritic signaling role of physiological action potential patterns recorded in vivo in layer 5A pyramidal neurons of the whisker-related ‘barrel cortex’. Pyramidal neurons of layer 5A are a major neocortical output type and clearly distinguished from layer 5B pyramidal neurons with respect to morphology, in vivo firing patterns, and connectivity yet knowledge of their dendritic properties is scant.
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