Motor Cortex control of finely graded forces

Abstract

Discharges of primary motor cortex (MI) neurons were recorded in 2 [rhesus] monkeys trained to maintain postural stability in 2 or more positions at 2 or more loads. Of 821 neurons studied, 273 exhibited levels of tonic activity that depended on either steady torque or position and 75% of these were related to both torque and position. The majority (96%) of these torque and position-related neurons displayed a congruent relationship to position and torque that resembled the pattern of prime-mover muscles: units whose discharge rate increased with steady-state torque in a given direction (position held constant) also became more active when the steady-state forearm position was shifted in the corresponding direction (torque held constant). In 1 monkey, pyramidal tract neurons (PTN) were identified and their discharge frequencies were determined at 5 or more different external loads with special attention to changes of activity near zero external load. Of 132 PTN related to dynamic aspects of movement and tested for steady-state relations to load, 94 exhibited statistically significant changes of discharge frequency with different loads. PTN could be subdivided into 2 principal groups according to the relationship between static force and tonic firing rates. One group of PTN was related to magnitude and direction of load over the entire range of forces examined. A 2nd group of PTN exhibited graded frequency changes over a limited range of force that differed for each neuron, thus resulting in sets of s-shaped functions. These 2 groups of PTN differed with respect to axonal conduction velocity as inferred from antidromic response latency (ADL). Encoding of force by frequency gradation over the entire range of loads was more common in small PTN with long ADL, while the PTN that came into play with successively higher force levels were more likely to have short ADL. Smaller PTN tend to have lower recruitment thresholds for tonic firing and to maintain relatively higher discharge frequencies in the absence of external load. A large proportion of the sample of MI neurons as a whole was strongly related to relatively small changes of force, especially near zero external load, and it thus appears that MI outputs are especially important in controlling the early-recruited motoneurons that are involved in precise fine movements. Since most PTN are already recruited at low force levels, it follows that frequency modulation is a major mechanism by which PTN output controls levels of muscular contraction, and it may be via this mechanism that PTN activate later recruited motoneurons.