Operant control of precentral neurons in monkeys: Evidence against open loop control

Four normal monkeys were operantly conditioned to change the firing pattern of 111 precentral neurons from phasic to tonic using an operant paradigm which quantifies the control of single neurons. Two monkeys then had their contralateral pyramidal tract (PT) sectioned and one monkey had C_5-7 ventral rhizotomies. Post-lesion data were: 1. (1) contralateral C_1-2 PT lesions did not encumber the monkeys' control of precentral PTNs; 2. (2) contralateral C_5-7 ventral rhizotomies completely abolished accurate control of precentral neurons which received proprioceptive feedback from flaccid arm regions. These results indicate that precentral neurons are operantly controlled through proprioceptive feedback from peripheral mechanoreceptors. The output of the mechanoreceptors is probably dependent upon discrete joint angles and/or muscle tension which is maintained through non-PT pathways. These data do not support the concept that precentral neurons are operantly controlled directly from a central, 'open loop', pathway.