Motoneuron properties after operantly conditioned increase in primate H-reflex

Abstract

1. Monkeys can increase (HRup conditioning mode) or decrease (HRdown conditioning mode) the triceps surae (TS) H-reflex in response to an operant conditioning task. This conditioning modifies the spinal cord. To define this spinal cord plasticity and its role in the behavioral change (H-reflex increase or decrease), we have recorded intracellularly from TS motoneurons in conditioned animals. The present report describes data from HRup animals and compares them with data from previously studied naive (NV; i.e., unconditioned) animals. 2. Thirteen monkeys (Macaca nemestrina, male, 3.8-7.1 kg) were exposed to the HRup conditioning mode, in which reward occurred when H-reflex size in one leg (i.e., the trained leg) was above a criterion value. Conditioning was successful (i.e., increase of > or = 20%) in 12 of the 13 animals. At the end of conditioning, H-reflex size in the trained leg averaged 188% of its initial value, whereas size in the control leg averaged 134% of its initial value. 3. Intracellular recordings were obtained from 136 TS motoneurons on trained (UT + motoneurons) and control (UC + motoneurons) sides of the successful animals. Measurements included axonal conduction velocity, input resistance, time constant, electrotonic length, rheobase, firing threshold to current injection, afterhyperpolarization duration and amplitude, and composite homonymous and heteronymous excitatory postsynaptic potential (EPSP) size and shape. Results were compared with intracellular data from NV animals.(ABSTRACT TRUNCATED AT 250 WORDS)