Glover, Isabel S;
Baker, Stuart N;
(2022)
Both Corticospinal and Reticulospinal Tracts Control Force of Contraction.
The Journal of Neuroscience
10.1523/JNEUROSCI.0627-21.2022.
(In press).
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Abstract
The control of contraction strength is a key part of movement control. In primates, both corticospinal and reticulospinal cells provide input to motoneurons. Corticospinal discharge is known to correlate with force, but there are no previous reports of how reticular formation (RF) activity modulates with different contractions. Here we trained two female macaque monkeys (body weight 5.9-6.9kg) to pull a handle which could be loaded with 0.5-6kg weights, and recorded from identified pyramidal tract neurons (PTNs) in primary motor cortex and RF cells during task performance. Population-averaged firing rate increased monotonically with higher force for the RF, but showed a complex profile with little net modulation for PTNs. This reflected a more heterogeneous profile of rate modulation across the PTN population, leading to cancellation in the average. Linear discriminant analysis (LDA) classified the force based on the time course of rate modulation equally well for PTNs and RF cells. Peak firing rate had significant linear correlation with force for 43/92 (46.7%) PTNs and 21/46 (43.5%) RF cells. For almost all (20/21) RF cells the correlation coefficient was positive; similar numbers of PTNs (22 vs 21) had positive vs negative coefficients. Considering the timing of force representation, similar fractions (PTNs: 61.2%; RF cells: 55.5%) commenced coding before the onset of muscle activity. We conclude that both corticospinal and reticulospinal tracts contribute to control of contraction force; the reticulospinal tract seems to specify an overall signal simply related to force, whereas corticospinal cell activity would be better suited for fine-scale adjustments.SIGNIFICANCE STATEMENTFor the first time, we compare coding of force for corticospinal and reticular formation cells in awake behaving monkeys, over a wide range of contraction strengths likely to come close to maximum voluntary contraction. Both cortical and brainstem systems coded similarly well for force, but whereas reticular formation cells carried a simple uniform signal, corticospinal neurons were more heterogenous. This may reflect a role in gross specification of a coordinated movement, versus more fine-grained adjustments around individual joints.
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