# Activity in the pontine reticular nuclei scales with handgrip force in humans

## TL;DR

This study shows that brain activity in the pontine reticular nuclei increases with handgrip force, suggesting a role in force modulation in humans.

## Contribution

The study provides direct evidence that the reticulospinal tract contributes to force modulation in humans.

## Key findings

- PRN activation increased with handgrip force for both contralateral and ipsilateral nuclei.
- Linear scaling of PRN activity with force was observed bilaterally at the group level.
- Ipsilateral PRN showed significant scaling after correcting for multiple comparisons.

## Abstract

The neural pathways that contribute to force production in humans are currently poorly understood, as the relative roles of the corticospinal tract and brainstem pathways, such as the reticulospinal tract (RST), vary substantially across species. Using functional magnetic resonance imaging (fMRI), we aimed to measure activation in the pontine reticular nuclei (PRN) during different submaximal handgrip contractions to determine the potential role of the PRN in force modulation. Thirteen neurologically intact participants (age: 28 ± 6 yr) performed unilateral handgrip contractions at 25%, 50%, 75% of maximum voluntary contraction during brain scans. We quantified the magnitude of PRN activation from the contralateral and ipsilateral sides during each of the three contraction intensities. A repeated-measures ANOVA demonstrated a significant main effect of force (P = 0.012, ηp2 = 0.307) for PRN activation, independent of side (i.e., activation increased with force for both contralateral and ipsilateral nuclei). Further analyses of these data involved calculating the linear slope between the magnitude of activation and handgrip force for each region of interest (ROI) at the individual-level. One-sample t tests on the slopes revealed significant group-level scaling for the PRN bilaterally, but only the ipsilateral PRN remained significant after correcting for multiple comparisons. We show evidence of task-dependent activation in the PRN that was positively related to handgrip force. These data build on a growing body of literature that highlights the RST as a functionally relevant motor pathway for force modulation in humans.

NEW & NOTEWORTHY In this study, we used a task-based functional magnetic resonance imaging (fMRI) paradigm to show that activity in the pontine reticular nuclei scales linearly with increasing force during a handgrip task. These findings directly support recently proposed hypotheses that the reticulospinal tract may play an important role in modulating force production in humans.

Listen to this article’s corresponding podcast at https://jneurophysiol.podbean.com/e/jnp-micro-podcast-pontine-reticular-nuclei-scales-with-handgrip-force/.