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Zusammenfassung:
Glutamatergic inputs clustered over ˜20−40 µm can elicit local N−methyl−D−aspartate (NMDA) spike/plateau potentials in terminal dendrites of cortical pyramidal neurons, inspiring the notion that a single terminal dendrite can function as a decision−making computational subunit. A typical terminal basal dendrite is ˜100−200 µm long: could it function as multiple decision−making subunits? We test this by sequential focal stimulation of multiple sites along terminal basal dendrites of layer 5 pyramidal neurons in rat somatosensory cortical brain slices, using iontophoresis or uncaging of brief glutamate pulses. There was an approximately sevenfold spatial gradient in average spike/plateau amplitude measured at the soma, from ˜3 mV for distal inputs to ˜23 mV for proximal inputs. Spike/plateaus were NMDA receptor (NMDAR) conductance−dominated at all locations. Large Ca2+ transients accompanied spike/plateaus over a ˜10− to 40−µm zone around the input site; smaller Ca2+ transients extended approximately uniformly to the dendritic tip. Spike/plateau duration grew with increasing glutamate and depolarization; high Ca2+ zone size grew with spike/plateau duration. The minimum high Ca2+ zone half−width (just above NMDA spike threshold) increased from distal (˜10 µm) to proximal locations (˜25 µm), as did the NMDA spike glutamate threshold. Depolarization reduced glutamate thresholds. Simulations exploring multi−site interactions based on this demonstrate that if appropriately timed and localized inputs occur in vivo, a single basal dendrite could correspond to a cascade of multiple co−operating dynamic decision−making subunits able to retain information for hundreds of milliseconds, with increasing influence on neural output from distal to proximal. Dendritic NMDA spike/plateaus are thus well−suited to support graded persistent firing
