A systematic review of how cannabinoids affect motoneuron output.

Spinal motoneurons contain many ion channels and receptors upon which various cannabinoids are known to act. This scoping review involved the synthesis of evidence from literature published before August 2022 about the effects of cannabinoids on quantifiable measures of motoneuron output. Four databases (MEDLINE, Embase, PsycINFO, and Web of Science CoreCollection) were queried and 4,237 unique articles were retrieved.

The Task at Hand: Fatigue-Associated Changes in Cortical Excitability during Writing.

Measures of corticospinal excitability (CSE) made via transcranial magnetic stimulation (TMS) depend on the task performed during stimulation. Our purpose was to determine whether fatigue-induced changes in CSE made during a conventional laboratory task (isometric finger abduction) reflect the changes measured during a natural motor task (writing). We assessed single-and paired-pulse motor evoked potentials (MEPs) recorded from the first dorsal interosseous (FDI) of 19 participants before and after a fatigue protocol (submaximal isometric contractions) on two randomized days.

Estimates of persistent inward current in human motor neurons during postural sway.

Persistent inward current (PIC) plays a critical role in setting the gain of spinal motor neurons. In humans, most estimates of PIC are made from plantarflexor or dorsiflexor motor units in a seated position. This seated and static posture negates the task-dependent nature of the monoaminergic drive and afferent inhibition that modulate PIC activation.

On the horizon of aging and physical activity research.

This viewpoint is the result of a Horizon Round Table discussion of Exercise and Aging held during the 2017 Saltin International Graduate School in Exercise and Clinical Physiology in Gatineau, Quebec. This expert panel discussed key issues and approaches to future research into aging, across human physiological systems, current societal concerns, and funding approaches. Over the 60-min round table discussion, 3 major themes emerged that the panel considered to be "On the Horizon" of aging research.

Understanding exercise-dependent plasticity of motoneurons using intracellular and intramuscular approaches.

Spinal motoneurons (MN) exhibit exercise-dependent adaptations to increased activity, such as exercise and locomotion, as well as decreased activity associated with disuse, spinal cord injury, and aging. The development of several experimental approaches, in both human and animal models, has contributed significantly to our understanding of this plasticity. The purpose of this review is to summarize how intracellular recordings in an animal model and motor unit recordings in a human model have, together, contributed to our current understanding of exercise-dependent MN plasticity.

Chronic exercise mitigates disease mechanisms and improves muscle function in myotonic dystrophy type 1 mice.

Key Points: Myotonic dystrophy type 1 (DM1), the second most common muscular dystrophy and most prevalent adult form of muscular dystrophy, is characterized by muscle weakness, wasting and myotonia. A microsatellite repeat expansion mutation results in RNA toxicity and dysregulation of mRNA processing, which are the primary downstream causes of the disorder. Recent studies with DM1 participants demonstrate that exercise is safe, enjoyable and elicits benefits in muscle strength and function; however, the molecular mechanisms of exercise adaptation in DM1 are undefined.

On task: Considerations and future directions for studies of corticospinal excitability in exercise neuroscience and related disciplines.

Over the last few decades, transcranial magnetic stimulation (TMS) has emerged as a conventional laboratory technique in human neurophysiological research. Exercise neuroscientists have used TMS to study central nervous system contributions to fatigue, training, and performance in health, injury, and disease. In such studies, corticospinal excitability is often assessed at rest or during simple isometric tasks with the implication that the results may be extrapolated to more functional and complex movement outside of the laboratory.

Cortical Mechanisms of Central Fatigue and Sense of Effort.

The purpose of this study was to investigate cortical mechanisms upstream to the corticospinal motor neuron that may be associated with central fatigue and sense of effort during and after a fatigue task. We used two different isometric finger abduction protocols to examine the effects of muscle activation and fatigue the right first dorsal interosseous (FDI) of 12 participants. One protocol was intended to assess the effects of muscle activation with minimal fatigue (control) and the other was intended to elicit central fatigue (fatigue).

Cortical hypoexcitability persists beyond the symptomatic phase of a concussion.

Primary Objective: The purpose of this research was to assess cortical excitability, voluntary activation of muscle and force sensation beyond the initial highly symptomatic period post-concussion (1-4 weeks post-injury). It was hypothesized that reduced excitability of the motor cortex may impair muscle activation and alter perceptions of force and effort.

Research Design: Eight concussed varsity football players were age- and position-matched with eight healthy teammates to control for training and body size.

Cortical mechanisms of mirror activation during maximal and submaximal finger contractions in Parkinson's disease.

Background: Mirror movements are often reported in the early stages of Parkinson's disease (PD) and have been attributed to bilateral activation of the primary motor cortex; however, the precise cortical mechanisms are still unclear. Subclinical mirror activation (MA) that accompanies mirror movement has also been reported in healthy aging adults.

Objective: To characterize mirror activation and determine the cortical mechanisms of MA in individuals with PD who demonstrate mirror movements.

An evaluation of paired motor unit estimates of persistent inward current in human motoneurons.

Persistent inward current (PIC) plays an important role in setting the input-output gain of motoneurons. In humans, these currents are estimated by calculating the difference between synaptic input at motor unit recruitment and derecruitment (ΔF) derived from paired motor unit recordings. The primary objective of this study was to use the relationship between reciprocal inhibition (RI) and PIC to estimate the contribution of PIC relative to other motoneuron properties that result in nonlinear motor unit firing behavior.

Dynamic stability and steering control following a sport-induced concussion.

Loss of balance control is one of the cardinal symptoms following a concussion; however, the ability to detect the duration of these balance impairments seems to largely depend on task type and complexity. Typical balance assessment tools are simplistic and do not challenge dynamic balance control. Changing direction represents an internal perturbation that challenges the balance control system.

Recovery of static stability following a concussion.

The purpose of this study was to use centre of pressure (COP) measurements to determine if static balance deficits had recovered when concussed athletes were cleared to return to play. Nine concussed varsity football players were matched with nine teammates who served as controls. Static balance in the anterior-posterior (A/P) and medial-lateral (M/L) directions was assessed during quiet stance with eyes open and eyes closed.

Modulation of cortical excitability and interhemispheric inhibition prior to rhythmic unimanual contractions.

The objective of this study was to investigate premotor modulation of motor cortical excitability between rhythmic unimanual finger contractions. Applying TMS at rest prior to an anticipated contraction provides a measure of cortical excitability that reflects premotor modulatory drive and is uncontaminated by the alterations in spinal and cortical excitability that occur during muscle activation. We hypothesized that premotor structures contribute to unimanual movement through the modulation of intracortical and interhemispheric inhibitory circuits within the primary motor cortex and that this premotor modulation would be evident at rest between contractions.

Whole-body hypothermia has central and peripheral influences on elbow flexor performance.

The superimposed twitch technique was used to study the effect of whole-body hypothermia on maximal voluntary activation of elbow flexors. Seven subjects [26.4 ± 4 years old (mean ± SD)] were exposed to 60 min of either immersion in 8°C water (hypothermia) or sitting in 22°C air (control).

Caloric restriction does not offset age-associated changes in the biophysical properties of motoneurons.

Age-associated changes in neuromuscular function may be due to a loss of motor neurons as well as changes in their biophysical properties. Neuronal damage imposed by reactive oxygen species may contribute to age-related deficits in CNS function. Thus we hypothesized that aging would alter the functional properties of motoneurons and that caloric-restriction would offset these changes.

Does elimination of afferent input modify the changes in rat motoneurone properties that occur following chronic spinal cord transection?

The purpose of this study was to determine the effects of 6-8 weeks of chronic spinal cord isolation (SI, removal of descending, ascending and afferent inputs), compared with the same duration of spinal cord transection (ST, removal of descending input only) on hindlimb motoneurone biophysical properties. Adult female Sprague-Dawley rats were placed into three groups: (1) control (no removal of inputs), (2) ST and (3) SI. The electrophysiological properties from sciatic nerve motoneurones were recorded from deeply anaesthetized rats.

Spike frequency adaptation of rat hindlimb motoneurons.

The objective of our study was to resolve two issues pertaining to motoneuron (MN) spike frequency adaptation (SFA): 1) to develop an index of SFA that is sensitive to a wide range of adaptation patterns and would correlate well with MN excitability and 2) to determine whether SFA pattern is stimulus current dependent. Sprague-Dawley rats (250-350 g) were anesthetized (ketamine-xylazine) before electrophysiological properties from sciatic nerve MNs located in the lumbar spinal cord were recorded. SFA was measured by 30-s square-wave current injections at 1.

The influence of caffeine on voluntary muscle activation.

Caffeine is a very common CNS stimulant that has been of interest to physiologists because of its direct effects on skeletal muscle in vitro, as well as ergogenic effects on laboratory tests of human performance. While in vitro studies have clearly demonstrated the effects of the drug on the CNS, the effects of caffeine on the voluntary activation of muscle in humans are less defined. Voluntary as well as involuntary supraspinal input, alpha motor neuron membrane properties, and afferent feedback to spinal and supraspinal neurons all modulate voluntary muscle activation, and caffeine may therefore alter muscle activation at several sites along the motor pathway.

Caffeine: a valuable tool to study central fatigue in humans?

This review focuses on caffeine's effects on the central modulation of muscle activation in humans. The drug's effects on voluntary muscle activation, the Hoffman reflex, motor-evoked potentials, self-sustained firing, pain, and sensation are discussed, and the possibility that caffeine maybe useful in the study of central fatigue is explored.

Central fatigue and transcranial magnetic stimulation: effect of caffeine and the confound of peripheral transmission failure.

In this experiment, we attempt to replicate the fatigue-induced decline in voluntary surface electromyography (EMG) and motor evoked potentials (MEPs) observed in previous studies and determine: (1) if this decline can be attributed to central failure, and (2) whether this failure is offset by caffeine. Seven subjects each attended two sessions (caffeine and placebo). Central excitability was estimated using transcranial magnetic stimulation (TMS), and surface EMG and twitch interpolation were used to estimate voluntary activation before, during and after fatigue of the first dorsal interosseous (FDI).

Effect of caffeine on self-sustained firing in human motor units.

This study examined the effect of caffeine on self-sustained firing (SSF) of human motor units. At physiological doses, caffeine acts as a competitive antagonist to the inhibitory effects of adenosine. This antagonism has many possible effects on the central nervous system.