BISTABILITY IN SPINAL MOTONEURONS IN-VIVO - SYSTEMATIC VARIATIONS IN RHYTHMIC FIRING PATTERNS

Ln the presence of the monoamines serotonin and norepinephrine, spinal motoneurons can exhibit bistable behavior, in which a brief period of excitatory input evokes prolonged self-sustained firing. A brief inhi bitory input returns the cell to the quiescent state. To determine whe ther motoneurons differ in their capacity for bistable behavior, intra cellular recordings were obtained in the decerebrate cat preparation. To enhance the Likelihood of encountering bistable behavior; the norad renergic alpha(1) agonist methoxamine was applied to the ventral surfa ce of the cord. The capacity of the cells to produce bistable behavior was assessed from the duration of self-sustained firing evoked by a b rief (1.5 s) excitatory synaptic input from muscle spindle la afferent s. About 35% (17 of 49) of the cells produced steady self-sustained fi ring for >3 s and were considered fully bistable. The other 32 cells ( similar to 65%) were partially bistable, with self-sustained firing la sting <3 s. Fully bistable cells tended to have lower current threshol ds for spike initiation and slower axonal conduction velocities than d id partially bistable cells. This suggests that fully bistable motoneu rons innervate fatigue resistant muscle fibers. The frequency-currrent (F-I) relations of the motoneurons were characterized with slow trian gular current ramps. Fully bistable cells displayed an acceleration in firing rare immediately on initiation of rhythmic firing. The F-l gai n after completion of the acceleration was positive. Fully bistable ce lls also displayed a hysteresis in the current level for firing thresh old with the ascending threshold occurring at substantially higher cur rent level than the descending one. Additionally. these current thresh olds usually were centered about zero current, so that the ascending c urrent threshold was positive while the descending current threshold w as negative. This negative offset meant that fully bistable cells coul d exhibit tonic firing without depolarizing injected current. Partiall y bistable cells exhibited very different F-I characteristics. Firing rate acceleration was just as large as in fully bistable cells but did not occur until well above the current level needed to initiate rhyth mic firing. F-I gain after acceleration was negative, there was little to no hysteresis between the ascending and descending firing threshol ds, and both thresholds were above the zero current level. These prope r-ties of partially bistable cells suggest their functional role is in tasks requiring relatively brief, high forces. The low thresholds of fully bistable cells mean they will be readily recruited in low force tasks like posture, where their prolonged self-sustained firing would be advantageous.