Normal postexercise facilitation and depression of motor evoked potentials in postpolio patients

Genetic variation in insulin-induced kinase signaling

Individual differences in sensitivity to insulin contribute to disease susceptibility including diabetes and metabolic syndrome. Cellular responses to insulin are well studied. However, which steps in these response pathways differ across individuals remains largely unknown. Such knowledge is needed to guide more precise therapeutic interventions. Here, we studied insulin response and found extensive individual variation in the activation of key signaling factors, including ERK whose induction differs by more than 20-fold among our subjects. This variation in kinase activity is propagated to differences in downstream gene expression response to insulin. By genetic analysis, we identified cis-acting DNA variants that influence signaling response, which in turn affects downstream changes in gene expression and cellular phenotypes, such as protein translation and cell proliferation. These findings show that polymorphic differences in signal transduction contribute to individual variation in insulin response, and suggest kinase modulators as promising therapeutics for diseases characterized by insulin resistance.

Effect of passive whole body heating on central conduction and cortical excitability in multiple sclerosis patients and healthy controls

Heat stress is associated with increased fatigue perception and decrements in function for individuals with multiple sclerosis (MS). Similarly, healthy individuals experience decrements in exercise performance during hyperthermia. Alterations in central nervous system (CNS) function during hyperthermia include reduced voluntary activation of muscle and increased effort perception. The purpose of this investigation was to test the hypothesis that passive heat exposure in MS patients will produce increased subjective fatigue and impairments in physiological measures of central conduction and cortical excitability compared with healthy individuals. Eleven healthy individuals and 11 MS patients completed a series of transcranial magnetic stimulation studies to examine central conduction and cortical excitability under thermoneutral and heat-stressed (HS) conditions at rest and after a fatiguing thumb abduction task. Passive heat stress resulted in significantly greater fatigue perception and impairments in force production in MS patients. Central motor conduction time was significantly shorter during HS in controls; however, in MS patients normal increases in conduction velocity with increased temperature were not observed centrally. MS patients also exhibited decreased cortical excitability during HS, evidenced by significant increases in resting motor threshold, decreased MEP amplitude, and decreased recruitment curve slope. Both groups exhibited postexercise depression of MEP amplitude, but the magnitude of these decrements was amplified in MS patients during HS. Taken together, these results suggest that CNS pathology in MS patients played a substantial role in reducing cortical excitability during HS.

Motor-evoked potentials recorded from lumbar erector spinae muscles: a study of corticospinal excitability changes associated with spinal manipulation

OBJECTIVE

The purpose of this study was to determine if high-velocity, low-amplitude spinal manipulation (SM) altered the effects of corticospinal excitability on motoneuron activity innervating the paraspinal muscles. In a previous study using transcranial magnetic stimulation (TMS), augmentation of motor-evoked potentials (MEPs) from the gastrocnemius muscle after lumbar SM was reported. To date, there is no known report of the effect of SM on paraspinal muscle excitability.

METHODS

The experimental design was a prospective physiologic evaluation of the effects of SM on corticospinal excitability in asymptomatic subjects. The TMS-induced MEPs were recorded from relaxed lumbar erector spinae muscles of 72 asymptomatic subjects. The MEP amplitudes were evaluated pre-SM and post-SM or conditions involving prethrust positioning and joint loading or control.

RESULTS

There was a transient increase in MEP amplitudes from the paraspinal muscles as a consequence of lumbar SM (F([6,414]) = 8.49; P < .05) without concomitant changes after prethrust positioning and joint loading or in control subjects (P > .05). These data findings were substantiated by a significant condition x time interaction term (F([12,414]) = 2.28; P < .05).

CONCLUSIONS

These data suggest that there is a postsynaptic facilitation of alpha motoneurons and/or corticomotoneurons innervating paraspinal muscles as a consequence of SM. It appears that SM may offer unique sensory input to the excitability of the motor system as compared to prethrust positioning and joint loading and control conditions.

Fatigue of muscles weakened by death of motoneurons

Weakness is a characteristic of muscles influenced by the postpolio syndrome (PPS), amyotrophic lateral sclerosis (ALS), and spinal cord injury (SCI). The strength deficits relate to changes in muscle use and to the chronic denervation that can follow the spinal motoneuron death common to these disorders. PPS, ALS, and SCI also involve variable amounts of supraspinal neuron death, the effects of which on muscle weakness remains unclear. Nevertheless, weakness of muscle itself defines the functional consequences of these disorders. A weaker muscle requires an individual to work that muscle at higher than usual intensities relative to its maximal capacity, inducing progressive fatigue and an increased sense of effort. Little evidence is available to suggest that the fatigue commonly experienced by individuals with these disorders relates to an increase in the intrinsic fatigability of the muscle fibers. The only exception is when SCI induces chronic muscle paralysis. To reduce long-term functional deficits in these disorders, studies must identify the signaling pathways that influence neuron survival and determine the factors that encourage and limit sprouting of motor axons. This may ensure that a greater proportion of the fibers in each muscle remain innervated and available for use.

Physiological measures of therapeutic response to interferon beta-1a treatment in remitting-relapsing MS

OBJECTIVE

This pilot study was designed to determine the effects of interferon beta-1a (IFNB) therapy (Avonex) on cortically evoked motor potentials (MEPs) during resting and fatigued states in individuals with multiple sclerosis (MS).

METHODS

Eight women with relapsing-remitting MS (mean age 36) and mean Expanded Disability Status Scale (EDSS) score of 3.1 were evaluated before and after 3, 6, and 12 months of IFNB therapy. At each test period, MEPs were recorded at rest and following a fatigue paradigm (3 min maximal contraction). Effects of IFNB on neurological and functional (7.7 m walk and 10 s finger tapping) status and fatigue were also examined.

RESULTS

Recovery from post-exercise depression of MEP amplitudes (PED) was 41, 43, and 43.5% faster at 3, 6, and 12 months, respectively, compared to baseline (P < 0.05). Percent reduction of MEP amplitude was significantly less at 6 months (P < 0.05) The majority of subjects (5/8 at 3 months; 6/8 at 6 and 12 months) reported decreased physical fatigue. Functional improvements were observed for walk and finger tapping scores after 3 months of IFNB treatment. MEP latencies were unchanged over the course of the intervention.

CONCLUSIONS

Results indicate that IFNB therapy may improve the rate of recovery from central fatigue.

SIGNIFICANCE

Transcranial magnetic stimulation (TMS) may have promise as an objective physiological tool to evaluate disease activity and treatment responses in MS.

Increased post-exercise facilitation of motor evoked potentials in multiple sclerosis

OBJECTIVES

To study post-exercise facilitation following a non-fatigue exercise in a homogenous group of multiple sclerosis (MS) patients with complaints of muscle fatigue.

METHODS

In 15 MS patients and matched controls motor evoked potentials (MEP) were recorded from the biceps brachii muscle at time delays of 0.5-30s after an isometric contraction with a torque of 50% of maximal voluntary contraction (MVC) maintained for 2-6s. In addition, MEP was recorded after isometric torque of 25, 50, and 100% of MVC maintained for 6s.

RESULTS

Isometric non-fatigue contraction induced significant post-exercise increase in MEP amplitude in MS patients compared with controls being most pronounced after a contraction for 6s. The post-exercise increase in MEP lasted for longer than 30s in MS patients.

CONCLUSIONS

Post-exercise increase of MEP amplitude following a non-fatigue exercise was significant in MS patients with complaints of muscle fatigue compared with healthy subjects.

Postexercise facilitation of motor evoked potentials following transcranial magnetic stimulation: a study in normal subjects

The size of the motor evoked potential (MEP) elicited by transcranial magnetic stimulation increases soon after a nonexhaustive voluntary contraction of the target muscle (postexercise facilitation). Our aim was to determine whether the duration or intensity of voluntary muscle contraction influenced postexercise facilitation in normal subjects. We recorded the MEP from the thenar muscles following contractions of different durations (5, 15, and 30 s) and intensities (10%, 25%, and 50% of maximal voluntary contraction). We found that every combination of the tested intensities and durations of physical effort could induce postexercise MEP facilitation. Although the degree of postexercise MEP facilitation was comparable across the different durations and intensities, the maximal facilitation was observed with the shortest and strongest muscle contraction. Our study thus defines the optimal setting to study postexercise facilitation for clinical purposes.

Transcranial magnetic stimulation and human muscle fatigue

During exercise, changes occur at many sites in the motor pathway, including the muscle fiber, motoneuron, motor cortex, and "upstream" of the motor cortex. Some of the changes result in fatigue, which can be defined as a decrease in ability to produce maximal muscle force voluntarily. Transcranial magnetic stimulation (TMS) over the human motor cortex reveals changes in both motor evoked potentials (MEPs) and the silent period during and after fatiguing voluntary contractions in normal subjects. The relationship of these changes to loss of force or fatigue is unclear. However, during a sustained maximal contraction TMS evokes extra force from the muscle and thus demonstrates the development of suboptimal output from the motor cortex, that is, fatigue at a supraspinal level. In some patients with symptoms of fatigue, the response to TMS after exercise is altered, but the changed MEP behavior is not yet linked to particular symptoms or pathology.