Fig. 7.

Action potential propagation along parallel fibers is not compromised. A, Loose cell-attached recording of a granule cell soma during molecular layer stimulation. The latency between the stimulus and arrival of the antidromic action potential (mostly conduction) in this cell was 5.3 msec (top trace, from left triangle toasterisk marking the antidromic action potential). Paired stimuli show that the refractory period was ∼1 msec. The intervals between the two stimuli are 0.9 msec (top trace, one action potential) and 1.0 msec (bottom trace, two action potentials). B, Paired pulse somatic stimuli similarly reveal an orthodromic refractory period of 0.9 msec. The intervals between the two stimuli are 0.8 msec (top trace, one action potential) and 0.9 msec (bottom trace, two action potentials). C, So that annihilation by collision with the orthodromic action potential (somatic stimulation) can be avoided, an antidromic action potential (molecular layer stimulation) must be elicited >6.3 msec later. The intervals between the two stimuli in the panel are 6.3 msec (top trace, antidromic action potential absent) and 6.4 msec (bottom trace, antidromic action potential present). This interval indicates that the orthodromic action potential is conducted to the site of antidromic stimulation, beyond the point at which Purkinje cells would have been recorded. The surprising form of the loose cell-attached action potential signal (e.g., inA) corresponds to an intracellular action potential with an inflection on the rising phase. This is because the loose cell-attached signal is essentially a capacity current, proportional to the first derivative of the membrane potential (Barbour and Isope, 2000); as such, it is quite sensitive to changes of action potential shape. The inflection probably reflects a delay between the arrival of the axonal antidromic action potential and invasion of the soma. Most of the orthodromic action potentials elicited in the soma are obscured in part by the stimulus artifacts (which were not subtracted). All action potentials were identified with respect to known failures of excitation.