Summation of slow motor unit forces at constant and variable interpulse intervals in rat soleus muscle

The temperature sensitivity of motor units in rat soleus

Temperature has a significant impact on the performance of the neuromuscular system and motor control processes. The smallest functional components of these systems are motor units (MUs), which may differ significantly between different muscles. The influence of temperature on the contractile properties of slow-twitch (S) MUs from soleus (SOL) muscles in rats was investigated under hypothermia (25 °C), normothermia (37 °C), and hyperthermia (41 °C). Hypothermia prolonged the twitch time parameters, decreased the rate of force development, increased the twitch-to-tetanus ratio, enhanced twitch force, and abolished post-tetanic depression. In contrast, hyperthermia did not alter twitch time parameters. Moreover, there was no effect on force despite the noted increase in post-tetanic depression and the twitch-to-tetanus ratio. Therefore, hypothermia induced more profound changes in S MUs compared with hyperthermia. The temperature effects in SOL MUs were compared to the effects previously reported for S MUs in the medial gastrocnemius (MG). The major differences between the S MUs of both muscles were the effects of temperature on twitch force, post-tetanic force modulation, twitch-to-tetanus ratio, and the slope of the force-frequency curve under hypothermia. Hyperthermia shortened twitch time parameters solely in the MG. In contrast, post-tetanic depression, twitch-to-tetanus ratio, and the slope of the force-frequency curve were influenced by hyperthermia only in SOL MUs. The different temperature effects of S MUs probably corresponded to differences in muscle architecture and their diverse functional tasks and enzyme activity. In summary, S MUs in SOL are more thermal-sensitive than their counterparts in MG.

Non-Linearity in Motor Unit Velocity Twitch Dynamics: Implications for Ultrafast Ultrasound Source Separation

Ultrasound (US) muscle image series can be used for peripheral human-machine interfacing based on global features, or even on the decomposition of US images into the contributions of individual motor units (MUs). With respect to state-of-the-art surface electromyography (sEMG), US provides higher spatial resolution and deeper penetration depth. However, the accuracy of current methods for direct US decomposition, even at low forces, is relatively poor. These methods are based on linear mathematical models of the contributions of MUs to US images. Here, we test the hypothesis of linearity by comparing the average velocity twitch profiles of MUs when varying the number of other concomitantly active units. We observe that the velocity twitch profile has a decreasing peak-to-peak amplitude when tracking the same target motor unit at progressively increasing contraction force levels, thus with an increasing number of concomitantly active units. This observation indicates non-linear factors in the generation model. Furthermore, we directly studied the impact of one MU on a neighboring MU, finding that the effect of one source on the other is not symmetrical and may be related to unit size. We conclude that a linear approximation is partly limiting the decomposition methods to decompose full velocity twitch trains from velocity images, highlighting the need for more advanced models and methods for US decomposition than those currently employed.

Robust estimation of average twitch contraction forces of populations of motor units in humans

The characteristics of motor unit force twitch profiles provide important information for the understanding of the muscle force generation. The twitch force is commonly estimated with the spike-triggered averaging technique, which, despite the many limitations, has been important for clarifying central issues in force generation. In this study, we propose a new technique for the estimation of the average twitch profile of populations of motor units with uniform contractile properties. The method encompasses a model-based deconvolution of the force signal using the identified discharge times of a population of motor units. The proposed technique was validated using simulations and tested on signals recorded during voluntary activation. The results of the simulations showed that the proposed method provides accurate estimates (relative error <25%) of the main parameters of the average twitch force when the number of identified motor units is between 5% and 15% of the total number of active motor units. It is discussed that current detection and decomposition methods of multi-channel surface EMG signals allow decoding this relative sample of the active motor unit pool. However, even when this condition is not met, our results show that the estimates provided by the new method are anyway always superior to those obtained by the spike triggered average approach, especially for high motor unit synchronization levels and when a relatively small number of triggers is available. In conclusion, we present a new method that overcome the main limitations of the spike-triggered average for the study of contractile properties of individual motor units. The method provides a new reliable tool for the investigation of the determinants of muscle force.

Motor control by precisely timed spike patterns

A fundamental problem in neuroscience is understanding how sequences of action potentials ("spikes") encode information about sensory signals and motor outputs. Although traditional theories assume that this information is conveyed by the total number of spikes fired within a specified time interval (spike rate), recent studies have shown that additional information is carried by the millisecond-scale timing patterns of action potentials (spike timing). However, it is unknown whether or how subtle differences in spike timing drive differences in perception or behavior, leaving it unclear whether the information in spike timing actually plays a role in brain function. By examining the activity of individual motor units (the muscle fibers innervated by a single motor neuron) and manipulating patterns of activation of these neurons, we provide both correlative and causal evidence that the nervous system uses millisecond-scale variations in the timing of spikes within multispike patterns to control a vertebrate behavior-namely, respiration in the Bengalese finch, a songbird. These findings suggest that a fundamental assumption of current theories of motor coding requires revision.