Milos Ljubisavljevic

Neuropeptide Y induces ischemic angiogenesis and restores function of ischemic skeletal muscles

Previously we showed that neuropeptide Y (NPY), a sympathetic vasoconstrictor neurotransmitter, stimulates endothelial cell migration, proliferation, and differentiation in vitro. Here, we report on NPYs actions, receptors, and mediators in ischemic angiogenesis. In rats, hindlimb ischemia stimulates sympathetic NPY release (attenuated by lumbar sympathectomy) and upregulates NPY-Y2 (Y2) receptor and a peptidase forming Y2/Y5-selective agonist. Exogenous NPY at physiological concentrations also induces Y5 receptor, stimulates neovascularization, and restores ischemic muscle blood flow and performance. NPY-mediated ischemic angiogenesis is not prevented by a selective Y1 receptor antagonist but is reduced in Y2(-/-) mice. Nonischemic muscle vascularity is also lower in Y2(-/-) mice, whereas it is increased in NPY-overexpressing rats compared with their WT controls. Ex vivo, NPY-induced aortic sprouting is markedly reduced in Y2(-/-) aortas and spontaneous sprouting is severely impaired in NPY(-/-) mice. NPY-mediated aortic sprouting, but not cell migration/proliferation, is blocked by an antifetal liver kinase 1 antibody and abolished in mice null for eNOS. Thus, NPY mediates neurogenic ischemic angiogenesis at physiological concentrations by activating Y2/Y5 receptors and eNOS, in part due to release of VEGF. NPYs effectiveness in revascularization and restoring function of ischemic tissue suggests its therapeutic potential in ischemic conditions.

Alterations in information transmission in ensembles of primary muscle spindle afferents after muscle fatigue in heteronymous muscle.

This study showed that fatigue of the ipsilateral medial gastrocnemius muscle caused a clear-cut reduction in the ability of ensembles of primary muscle spindle afferents from the lateral gastrocnemius muscle to discriminate between muscle stretches of varying amplitude. The results were probably caused by reflex-mediated effects from chemosensitive group III and IV afferents onto the gamma-motoneurons projecting to lateral gastrocnemius muscle spindles. The experiments were conducted on seven cats anaesthetized with alpha-chloralose and a total of 41 primary muscle spindle afferents from the lateral gastrocnemius were registered. Afferents were simultaneously recorded in ensembles of three to 10 afferents. A method based on principal component analysis and algorithms for quantification of stimulus discrimination in ensembles of muscle afferents was used prior to, immediately following and five or more minutes after muscle fatigue had been induced to the ipsilateral medial gastrocnemius muscle. It is well established that the primary muscle spindle afferents play an important role in proprioception and kinaesthesia. Therefore the decrease in the accuracy of the information transmitted by ensembles of primary muscle spindle afferents caused by fatigue in an ipsilateral muscle implies concomitant effects on proprioception and kinaesthesia.

Impairment of cortical inhibition in writer's cramp as revealed by changes in electromyographic silent period after transcranial magnetic stimulation.

Changes in silent period (SP) duration following transcranial magnetic stimulation (TMS) set at 20% above the motor threshold were studied in six subjects suffering from writers cramp, while performing dystonic movement and during voluntary isometric contraction of the muscles mostly involved in the dystonic movement. Dependency of SP duration on the intensity of preceding muscle contraction was compared on both affected and healthy side. In all subjects SP duration during dystonic contraction was shorter than during voluntary contraction of the similar strength performed with the same hand. Also, in five subjects, SP duration during dystonic contraction was shorter than during voluntary contraction of the similar strength performed with the healthy hand. In addition, the SP duration on the affected side was negatively associated with the intensity of the preceding contraction (i.e. the stronger contraction the shorter SP), while on the healthy side it was not the case. It is concluded that central inhibitory mechanisms are abnormal in writers cramp.

Sympathetic modulation of muscle spindle afferent sensitivity to stretch in rabbit jaw closing muscles

Previous reports showed that sympathetic stimulation affects the activity of muscle spindle afferents (MSAs). The aim of the present work is to study the characteristics of sympathetic modulation of MSA response to stretch: (i) on the dynamic and static components of the stretch response, and (ii) on group Ia and II MSAs to evaluate potentially different effects. In anaesthetised rabbits, the peripheral stump of the cervical sympathetic nerve (CSN) was stimulated at 10 impulses s−1 for 45–90 s. The responses of single MSAs to trapezoidal displacement of the mandible were recorded from the mesencephalic trigeminal nucleus. The following characteristic parameters were determined from averaged trapezoidal responses: initial frequency (IF), peak frequency at the end of the ramp (PF), and static index (SI). From these, other parameters were derived: dynamic index (DI = PF ‐ SI), dynamic difference (DD = PF ‐ IF) and static difference (SD = SI ‐ IF). The effects of CSN stimulation were also evaluated during changes in the state of intrafusal muscle fibre contraction induced by succinylcholine and curare. In a population of 124 MSAs, 106 units (85.4 %) were affected by sympathetic stimulation. In general, while changes in resting discharge varied among different units (Ia vs. II) and experimental conditions (curarised vs. non‐curarised), ranging from enhancement to strong depression of firing, the amplitude of the response to muscle stretches consistently decreased. This was confirmed and detailed in a quantitative analysis performed on 49 muscle spindle afferents. In both the non‐curarised (23 units) and curarised (26 units) condition, stimulation of the CSN reduced the response amplitude in terms of DD and SD, but hardly affected DI. The effects were equally present in both Ia and II units; they were shown to be independent from gamma drive and intrafusal muscle tone and not secondary to muscle hypoxia. Sympathetic action on the resting discharge (IF) was less consistent. In the non‐curarised condition, IF decreased in most Ia units, while in II units decreases and increases occurred equally often. In the curarised condition, IF in group II units mostly increased. The results have important functional implications on the control of motor function in a state of ‘high’ sympathetic activity, like excessive stress, as well as in certain pathological conditions such as sympathetically maintained pain.

Detrended fluctuation analysis of time series of a firing fusimotor neuron

We study the interspike intervals (ISI) time series of the spontaneous fusimotor neuron activity by applying the detrended fluctuation analysis that is a modification of the random walk model analysis. Thus, we have found evidence for the white noise characteristics of the ISI time series, which means that the fusimotor activity does not possess temporal correlations. We conclude that such an activity represents the requisite noisy component for occurrence of the stochastic resonance mechanism in the neural coordination of muscle spindles.

Changes in discharge rate of fusimotor neurones provoked by fatiguing contractions of cat triceps surae muscles.

1. Changes in discharge rate of thirty‐one fusimotor neurones to triceps surae muscles during long‐lasting, fatiguing contractions of these muscles were studied in decerebrate cats. Discharges of fusimotor neurones were recorded from the nerve filaments. Muscle contractions were elicited by electrical stimulation of either the muscle nerves (twenty‐one neurones) or the corresponding ventral roots (ten neurones), until the muscle tension fell to about 30% of its initial value. 2. Early and late changes could be recognized in fusimotor discharge rate during long‐lasting muscle contraction. The early changes obviously not related to muscle fatigue, consisted of an initial increase at the onset of muscle contraction and a subsequent decrease to or below the resting discharge level. The late change in discharge rate, supposedly related to muscle fatigue, was an increase developing gradually towards the end of muscle contraction, ranging at its peak from 2 to 15 impulses/s (mean value 5.5 impulses/s, n = 31) and outlasting the contraction for 20‐320 s. 3. When the contracting muscle was made ischaemic the late increase in fusimotor discharge rate started earlier and was maintained until the arterial clamp was removed. After severing the muscle nerves distal to the site of stimulation no changes, a slight sustained increase, or else a decrease in fusimotor discharge rate occurred during electrical stimulation of either muscle nerves or ventral roots. At its cessation the spontaneous firing rate was reassumed immediately. Stimulation of the distal stumps of the severed nerves elicited no changes in fusimotor discharge rate. 4. It is proposed that the late increase in fusimotor discharge rate may appear due to autogenetic excitation of fusimotor neurones by discharge from group III and IV muscle afferent fibres provoked and/or enhanced by metabolic products liberated in muscle tissue during the fatiguing contraction. The fusimotor firing was estimated to remain elevated to a level twice that of the spontaneous activity on average for approximately 120 s after the muscle contraction. Its functional role in muscle fatigue is discussed.

Central changes in muscle fatigue during sustained submaximal isometric voluntary contraction as revealed by transcranial magnetic stimulation

Changes in responses to transcranial magnetic stimulation (TMS) during submaximal isometric voluntary contraction (60% of maximal voluntary contraction (MVC) of the adductor pollicis muscle and the subsequent recovery period have been studied in healthy volunteers. TMS at twice the motor threshold was applied during the sustained contraction, as well as at rest and during short-lasting (2 s) 60% MVCs before and immediately after the sustained contraction, and at 5 min intervals during the recovery period. Both motor evoked potential (MEP) magnitude (peak and area) and silent period (SP) duration in electromyographic activity (EMG) of the adductor pollicis muscle showed a gradual decrease up to the endurance point and an increase thereafter. MEPs elicited at rest immediately after the fatiguing contraction were larger, whereas those elicited later on during the recovery period did not differ significantly from the controls. It is suggested that the changes in responses to TMS, divergent from those in ongoing voluntary EMG during the sustained 60% MVC, indicate complex processes at levels preceding the motor cortex output cells in an attempt to maintain a submaximal contraction of the fatigued muscle. The increase in MEP magnitude after the sustained 60% MVC may indicate residual changes in cortical activity after fatiguing contraction.

Transcranial magnetic stimulation and the motor learning-associated cortical plasticity

It has been well established that repetitive motor performance and skill learning alter the functional organization of human corticomotoneuronal system. Over the past decade, transcranial magnetic stimulation (TMS) has helped to demonstrate motor practice and learning-related changes in corticomotoneuronal excitability and representational plasticity. It has also provided some insights into the mechanisms underlying such plasticity. TMS-derived indices show that motor practice, skill acquisition and learning are associated with an increase in cortical excitability and a modulation of intracortical inhibition partly related to the amount of GABA-related inhibition. It has been suggested that these changes in excitability might be related to learning and motor memory formation in the motor cortex. However, it has proved difficult to relate different aspects of TMS-derived representational plasticity with specific behavioral outcomes. A better understanding of the relationship between TMS measurements of practice-related cortical plasticity and underlying mechanisms, in the context of associated changes in behavior, will facilitate the development of techniques and protocols that will allow predictable modulation of cortical plasticity in health and disease.

Comparison of brain activation after sustained non-fatiguing and fatiguing muscle contraction: a positron emission tomography study

The concept of fatigue refers to a class of acute effects that can impair motor performance, and not to a single mechanism. A great deal is known about the peripheral mechanisms underlying the process of fatigue, but our knowledge of the roles of the central structures in that process is still very limited. During fatigue, it has been shown that peripheral apparatus is capable of generating adequate force while central structures become insufficient/sub-optimal in driving them. This is known as central fatigue, and it can vary between muscles and different tasks. Fatigue induced by submaximal isometric contraction may have a greater central component than fatigue induced by prolonged maximal efforts. We studied the changes in regional cerebral blood flow (rCBF) of brain structures after sustained isometric muscle contractions of different submaximal force levels and of different durations, and compared them with the conditions observed when the sustained muscle contraction becomes fatiguing. Changes in cortical activity, as indicated by changes in rCBF, were measured using positron emission tomography (PET). Twelve subjects were studied under four conditions: (1) rest condition; (2) contraction of the m. biceps brachii at 30% of MVC, sustained for 60 s; (3) contraction at 30% of MVC, sustained for 120 s, and; (4) contraction at 50% of MVC, sustained for 120 s. The level of rCBF in the activated cortical areas gradually increased with the level and duration of muscle contraction. The fatiguing condition was associated with predominantly contralateral activation of the primary motor (MI) and the primary and secondary somatosensory areas (SI and SII), the somatosensory association area (SAA), and the temporal areas AA and AI. The supplementary motor area (SMA) and the cingula were activated bilaterally. The results show increased cortical activation, confirming that increased effort aimed at maintaining force in muscle fatigue is associated with increased activation of cortical neurons. At the same time, the activation spread to several cortical areas and probably reflects changes in both excitatory and inhibitory cortical circuits. It is suggested that further studies aimed at controlling afferent input from the muscle during fatigue may allow a more precise examination of the roles of each particular region involved in the processing of muscle fatigue.

Changes in movement final position associated with agonist and antagonist muscle fatigue.

Abstract We have tested the hypothesis that agonist and antagonist muscle fatigue could affect the final position of rapid, discrete movements. Six subjects performed consecutive elbow flexion and extension movements between two targets, with their eyes closed prior to, and after fatiguing the elbow extensor muscles. The results demonstrate that elbow extension movements performed in the post-test period systematically undershot the final position as compared to pre-test movements. However, attainment of the aimed final position in elbow flexion movements was unaffected by fatiguing of the extensor muscles. Undershoot of the final position obtained in extension movements was associated with agonist muscle fatigue, a result that was expected from the point of view of current motor control theories, and that could be explained by a reduced ability of the shortening muscle to exert force. On the other hand, the absence of the expected overshoot of the final position when the antagonist is fatigued, indicates the involvement of various reflex and/or central mechanisms operating around the stretched muscle that could contribute to returning the limb to the standard final position after a brief prominent overshoot.