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PubMed2001

Jaw-muscle spindle afferent pathways to the trigeminal motor nucleus in the rat.

Neural pathways conveying proprioceptive feedback from the jaw muscles were studied in rats by combining retrograde and intracellular neuronal labeling. Initially, horseradish peroxidase was iontophoresed unilaterally into the trigeminal motor nucleus (Vmo). Two days later, 1-5 jaw-muscle spindle afferent axons located in the mesencephalic trigeminal nucleus were physiologically identified and intracellularly stained with biotinamide. Stained mesencephalic trigeminal jaw-muscle spindle afferent axon collaterals and boutons were predominantly distributed in the supratrigeminal region (Vsup), Vmo, dorsomedial trigeminal principal sensory nucleus (Vpdm), parvicellular reticular formation (PCRt), alpha division of the parvicellular reticular formation (PCRtA), and dorsomedial portions of the spinal trigeminal subnuclei oralis (Vodm), and interpolaris (Vidm). Numerous neurons retrogradely labeled with horseradish peroxidase from the trigeminal motor nucleus were found bilaterally in the PCRt, PCRtA, Vodm, and Vidm. Retrogradely labeled neurons were also present contralaterally in the Vsup, Vpdm, Vmo, peritrigeminal zone, and bilaterally in the dorsal medullary reticular field. Putative contacts between intracellularly stained mesencephalic trigeminal jaw-muscle spindle afferent boutons and trigeminal premotor neurons retrogradely labeled with horseradish peroxidase were found in the ipsilateral Vodm, PCRtA, and PCRt, as well as the contralateral Vsup, Vmo, Vodm, PCRt, and PCRtA. Thus, multiple disynaptic jaw-muscle spindle afferent-motoneuron circuits exist. These pathways are likely to convey long-latency jaw-muscle stretch reflexes and may contribute to stiffness regulation of the masticatory muscles.

Afferent Pathways↗

PubMed1993

Microneurography and applications to issues of motor control: Fifth Annual Stuart Reiner Memorial Lecture.

Among the hypotheses regarding fusimotor functions based on earlier animal experiments some are inconsistent, others are in conformity with microneurographic observations in man. The human data provide evidence against the following two theories: (1) the length follow-up servo theory; and (2) the theory that fusimotor neurons can be selectively activated to produce spindle sensitization and stretch reflex reinforcements. The human data support the theory of alpha-gamma coactivation. In particular, in the early phase of isometric voluntary contractions fusimotor-driven afferent spindle activity assists in autogenetic activation of alpha motoneurons and in reciprocal relaxation of antagonists. As muscle fatigue develops, the autogenetic reflex drive via the fusimotor route declines. The fusimotor bias during contraction provides for maintenance of spindle sensitivity to minute perturbations and for load-compensating reflex adjustments to such perturbations. Reflex overcorrections may lead to uncontrollable oscillations of the type seen in enhanced physiological tremor.

Afferent Pathways↗

PubMed1990

Specification of synaptic connections between sensory and motor neurons in the developing spinal cord.

Experimental studies of mechanisms underlying the specification of synaptic connections in the monosynaptic stretch reflex of frogs and chicks are described. Sensory neurons innervating the triceps brachii muscles of bullfrogs are born throughout the period of sensory neurogenesis and do not appear to be related clonally. Instead, the peripheral targets of these sensory neurons play a major role in determining their central connections with motoneurons. Developing thoracic sensory neurons made to project to novel targets in the forelimb project into the brachial spinal cord, which they normally never do. Moreover, these foreign sensory neurons make monosynaptic excitatory connections with the now functionally appropriate brachial motoneurons. Normal patterns of neuronal activity are not necessary for the formation of specific central connections. Neuromuscular blockade of developing chick embryos with curare during the period of synaptogenesis still results in the formation of correct sensory-motor connections. Competitive interactions among the afferent fibers also do not seem to be important in this process. When the number of sensory neurons projecting to the forelimb is drastically reduced during development, each afferent still makes central connections of the same strength and specificity as normal. These results are discussed with reference to the development of retinal ganglion cells and their projections to the brain. Although many aspects of the two systems are similar, patterned neural activity appears to play a much more important role in the development of the visual pathway than in the spinal reflex pathway described here.

Animals↗

PubMed1990

Postural maintenance during movement: simulations of a two joint model.

Voluntary movements of the upper body are accompanied by anticipatory postural adjustments to the lower body in a standing subject. The long-standing hypothesis is that these anticipatory adjustments serve to counteract the perturbation to the body's center of gravity caused by the voluntary arm movement. This paper presents model simulations investigating the possible roles of anticipatory postural activity that accompanies a rapid, upward arm swing. The model incorporates two (idealized) antagonistic muscle pairs controlling the movements of a double-joint system, with a "shoulder joint" between the arm and stiff body links, and an "ankle joint" between the stiff body-leg segment and the ground. Each muscle is represented by a nonlinear viscoelastic element and also includes proprioceptive feedback. Four inputs to the model define the motor control signals for muscle force generation in both the arm and the postural muscle pairs. The neurological component of the model describes consequences of alternate strategies for cocontractions, stretch reflex activity, and anticipatory and synchronous postural activities (or combinations thereof). Simulations with this model show that: (1) none of the postural maintenance schemes considered in these simulations (including varying anticipation) could suppress the initial backward thrust on the body link; (2) the more important destabilizing perturbation is a subsequent forward sway that, left uncountered by postural activity, would eventually leave the body to fall flat on its face; and (3) anticipatory silencing of the postural extensor followed by a brief period of extensor activation (descending control) and synchronous reflex activity (feedback control) appears to be the most likely postural stabilizing strategy that inhibits the continuous forward sway and is consistent with the experimental evidence.

Arm↗

PubMed1990

Reductions in recruitment force thresholds in human single motor units by successive voluntary contractions.

Recruitment force thresholds of biceps brachii single motor units were studied in 4 male subjects before and after an isometric muscle contraction, passive muscle stretch, or following successive muscle contractions, muscle stretches or during alternations between muscle stretches and muscle contractions. Isometric muscle contractions of 5 s duration decreased subsequent single motor unit force thresholds. These force thresholds could usually be reset at or near precontraction force threshold values by passive muscle stretch induced by elbow extension. Single motor units showing reduced force thresholds following contraction were momentarily derecruited during and/or after muscle stretch. Successive muscle stretches alone did not significantly alter single motor unit force thresholds. In contrast, single motor unit recruitment force thresholds during successive weaker contractions were progressively lowered. Intercontraction muscle stretches maintained the single motor unit force thresholds at or near the initial force threshold level. The mechanism(s) underlying a muscle contraction-induced lowering of single motor unit force thresholds may reside in stretch reflex pathways.

Electrodes↗

PubMed1985

Visual and vestibular contributions to pitch sway stabilization in the ankle muscles of normals and patients with bilateral peripheral vestibular deficits.

Vestibular, visual, and proprioceptive influences on muscle activity correcting for backwards body tilt were investigated in normals and patients with bilateral peripheral vestibular deficits. Body tilt was induced by a dorsi-flexion rotation of the feet about the ankle joints while the subject stood on a force measuring platform. Ankle muscle activity and torque were monitored as upright stance was reestablished, and correlated with head angular accelerations and neck muscle activity. In normals with eyes closed, soleus stretch reflex activity at 50-80 ms was followed by two bursts of tibialis anterior (TA) EMG activity at ca 80 and 125 ms from the onset of 36 deg/s, 3 deg amplitude platform rotations. Neck muscle activity rotated the head backwards at the same time as TA activity rotated the body forwards about the ankle joints. Under the influence of vision, i.e. eyes open, slight increases in the second burst of TA activity, and ankle torque were observed. When the subjects sat, and were instructed to activate TA rapidly on onset of the platform movement, TA EMG activity increased gradually at ca. 150 ms and not as a burst. In patients with long-lasting bilateral vestibular deficits, both bursts of TA activity were significantly less than normal with eyes closed. Consequently sway correcting torques were abnormally low and all but one of the patients fell over backwards. With eyes open, TA activity was slightly less than, and ankle torques were approximately equal to normal values. In contrast to normals, TA responses obtained in standing and sitting positions were not significantly different. Neck EMG activity varied from normal, consisting of a long burst 100 ms in duration. The present data indicate that a coordinated pattern of ankle, and neck muscle activity occurs during the first 150 ms following induced backward tilt. Ankle muscle activity corrects for the body sway, and neck muscle activity attempts to stabilise the head with respect to earth fixed coordinates. It is proposed that the vestibulo-spinal reflex system predominantly underlies the genesis and coordination of this muscle activity.

Adult↗

PubMed1996

Effects of differently induced stretch loads on neuromuscular control in drop jump exercise.

The neuromuscular characteristics of the triceps surae and vastus lateralis muscles and interactions between the pre-activation of these muscles and the muscle output itself during ground contact were investigated during various types of stretch-shortening cycle muscle loading. The loading of the muscles was effected by using three different types of drop jump exercise. These jumps allowed separate modifications of the loading of the leg extensor muscles by changing the velocity of the centre of gravity (CG) or by changing directly the body mass, which was also affected by changing artificially the acceleration of the CG. It was found that the eccentric peak angular velocity of the ankle joint was related to the various precontact and eccentric parameters in all the different types of jumping exercise. The correlations were higher for the gastrocnemius muscle (P < 0.001) than for the soleus muscle (n.s., P < 0.01). In all the experimental conditions, the pre-activation of the measured muscles started well before the impact of contact with the ground. However, the duration of the pre-activation phase depended on the type of stretch exercise. The results would suggest a clear interaction between the pre-activation of the muscles and that part of the muscle output which is effected by the segmental stretch reflex system. The control mechanism of the pre-activation itself appears to be multiple in character. It seems reasonable to assume that the pre-activation is preprogrammed, but which can, however, be modified by proprioceptive, vestibular and visual inputs. Thus, the possibility of conscious modification of the expected muscle load must be considered.

Adult↗

PubMed1986

Dominance of the short-latency component in perturbation induced electromyographic responses of long-trained monkeys.

The effects of prolonged training of adult monkeys subjected to random, brief perturbations of alternating elbow flexions and extensions were studied over a period of four years. The training was intensive at first, for about one year, and then irregular, with long pauses, during the following three years. As a consequence of the prolonged training with the brief perturbations, the M2 component of the electromyographic (EMG) response of the biceps and triceps muscles became gradually smaller, and finally disappeared. The M1 component, on the other hand, progressively increased in amplitude and continued to do so after the loss of the M2, until it finally dominated the EMG response. The training had similar effects on the response of the biceps muscle to longer perturbations, but, only under certain conditions, did it affect the triceps muscle response. All changes occurred at earlier stages of the training in the flexor than in the extensor muscle. These observations demonstrate a long-term functional plasticity of the sensorimotor system of adult animals and suggest a growing role for fast segmental mechanisms in the reaction to external disturbances as motor learning progresses. Changes at various levels of the stretch reflex system could underlie the enlargement of the M1 component, while the lack of the M2 component should, at least partially, reflect a reduced cortical effect on alpha-motoneurones and/or changes in spinal systems processing afferent information.

Animals↗

PubMed1984

Mandibular postural tremor during relaxation and during volitional innervation of the jaw muscles.

Jaw tremor was recorded as the changes in the vertical distance between mandible and maxilla. Two states of motor performance of the jaw muscles were chosen: relaxed and actively positioning of the mandible close to toothcontact, referred to as respectively rest- and activity tremor. Rest tremor contained a strong ballistocardiac component (bc power = 50%) and was of small amplitude (9.4 micron rms). Activity tremor revealed increased amplitudes (19.2 micron rms) with a predominant neuromuscular component (nm power = 75%). Bc tremor patterns differed significantly from nm tremor patterns. A negative linear relationship, which apparently was similar for rest and for activity tremor (P greater than 0.10), existed between the estimated bc contribution and tremor amplitude. As tremor amplitude increased neuromuscular activity appeared increasingly time locked to bc activity, which should be attributed to synchronization of spindle activity, either through the stretch reflex or through vascular pulsation. The average bc contribution to tremor amplitude remained of almost constant magnitude (7.0 micron rms), with no differences between rest and activity for the group as a whole. During resting conditions, complete relaxation of the jaw muscles was not attained by the majority of the subjects. The latter revealed a heart rate arrhythmia that was strongly correlated with motor (nm) activity (P less than 0.001). During activity conditions no such correlation was present. The present results demonstrate that for the understanding and the interpretation of the underlying mechanisms of jaw tremor genesis, both the recording technique, i.e., differential jaw displacements, and the signal analysis procedure, i.e., autocorrelograms and power spectra over extended record length, were of crucial importance.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1992

Independent control of joint stiffness in the framework of the equilibrium-point hypothesis.

In the framework of the equilibrium-point hypothesis, virtual trajectories and joint stiffness patterns have been reconstructed during two motor tasks practiced against a constant bias torque. One task required a voluntary increase in joint stiffness while preserving the original joint position. The other task involved fast elbow flexions over 36 degrees. Joint stiffness gradually subsided after the termination of fast movements. In both tasks, the external torque could slowly and unexpectedly change. The subjects were required not to change their motor commands if the torque changed, i.e. "to do the same no matter what the motor did". In both tasks, changes in joint stiffness were accompanied by unchanged virtual trajectories that were also independent of the absolute value of the bias torque. By contrast, the intercept of the joint compliant characteristic with the angle axis, r(t)-function, has demonstrated a clear dependence upon both the level of coactivation and external load. We assume that a template virtual trajectory is generated at a certain level of the motor hierarchy and is later scaled taking into account some commonly changing dynamic factors of the movement execution, for example, external load. The scaling leads to the generation of commands to the segmental structures that can be expressed, according to the equilibrium-point hypothesis, as changes in the thresholds of the tonic stretch reflex for corresponding muscles.

Adult↗

PubMed2005

Electromechanical delay of the knee extensor muscles is not altered after harvesting the patellar tendon as a graft for ACL reconstruction: implications for sports performance.

Although the scar tissue, which heals the donor site defect, has different elasticity from the neighbouring patellar tissue, it remains unclear if this scar tissue can lead to the changes of the electromechanical delay (EMD) of the knee extensor muscles. If such changes do exist, they can possibly affect both the utilization of the stored energy in the series elastic component, as well as the optimal performance of the knee joint movement. The purpose of this study was to investigate the influence of harvesting the patellar tendon during anterior cruciate ligament (ACL) reconstruction and the associated patellar tendon scar tissue development on the EMD of the rectus femoris (RF) and vastus medialis (VM) muscles. Seventeen patients who underwent an ACL reconstruction using the medial third of the patellar tendon were divided in two groups based upon their post-operative time interval. Maximal voluntary contraction from the knee extensors, surface EMG activity, and ultrasonographic measurements of the patellar tendon cross-section area were obtained from both knees. Our results revealed that no significant changes for the maximal voluntary contraction of the knee extensors and for the EMD of the RF and the VM muscles due to patellar scar tissue development after harvesting the tendon for ACL reconstruction. The EMD, as a component of the stretch reflex, is important for the utilization of the stored energy in the series elastic component and thus, optimal sports performance. However, from our results, it can be implied that the ACL reconstruction using a patellar tendon graft would not impair sports performance as far as EMD is concerned.

Adult↗

PubMed2005

The influence of perturbation duration and velocity on the long-latency response to stretch in the biceps muscle.

Different neural pathways are proposed to mediate the long-latency stretch reflex response (M2) in muscles spanning distal and proximal joints of the upper limb. The M2 at the wrist joint is present only if the duration of the perturbation exceeds a critical time. Lee and Tatton put forward a converging input hypothesis, requiring an interaction of excitatory volleys at the spinal cord, to account for this feature. The goal of the present study was to examine the influence of the duration of perturbation on M2 responses elicited in a muscle spanning the elbow joint. Reflex responses were induced in the biceps brachii muscle by applying ramp-and-hold position displacements to the elbow. It was found that the M2 was strongly dependent on the duration of the perturbation. On average, responses were not elicited following perturbations of less than 35+/-5 ms. Using a novel double-movement paradigm, we were unable to provide support for the converging input hypothesis. The effect of the duration of perturbation on the M2 may account for the conflicting characteristics of the M2 that have been provided by previous studies applying mechanical or electrical perturbations of varying time durations.

Adult↗

PubMed2004

Botulinum toxin in post-stroke patients: stiffness modifications and clinical implications.

OBJECTIVE: To objectively quantify stiffness and clinical changes in the upper limb of poststroke patients following botulinum toxin (BT) injection. METHODS: Eighteen consecutive chronic post-stroke spastic patients were injected Botulinum toxin A in the forearm flexor spastic muscles. Spasticity was clinically evaluated with the Ashworth scale. Stiffness was measured with indices (passive stiffness index (ISI) and total stiffness index (TSI) obtained by mechanical wrist displacements induced by a torque motor,which could also provide the stretch reflex threshold speed (SRTS) from flexor muscles. Functional status was measured with the Barthel index and a specific hand ability scale, pain with a visual analogue scale (VAS). The ranges of voluntary wrist extension (EROM) and flexion (FROM) and wrist isometric extension and flexion (IE-IF) strength were also calculated. RESULTS: IE and EROM significantly increased, being respectively p < 0.01 and p < 0.05; also SRTS was augmented (p < 0.001),while TSI showed lower values (p < 0.001); the Ashworth score decreased at least one point. Hand function for selected tasks improved in 50% of patients, the Barthel index only in 4 (22 %), forearm pain was completely relieved in 3 patients (17 %). CONCLUSIONS: BT can be considered a valid therapeutic tool in all spastic patients, because of immediate advantages: reduction of muscle hypertonia, pain relief, improvement in selected motor performances.

Adult↗

PubMed2004

Model of head-neck joint fast movements in the frontal plane.

The objective of this work is to develop a model representing the physiological systems driving fast head movements in frontal plane. All the contributions occurring mechanically in the head movement are considered: damping, stiffness, physiological limit of range of motion, gravitational field, and muscular torques due to voluntary activation as well as to stretch reflex depending on fusal afferences. Model parameters are partly derived from the literature, when possible, whereas undetermined block parameters are determined by optimising the model output, fitting to real kinematics data acquired by a motion capture system in specific experimental set-ups. The optimisation for parameter identification is performed by genetic algorithms. Results show that the model represents very well fast head movements in the whole range of inclination in the frontal plane. Such a model could be proposed as a tool for transforming kinematics data on head movements in 'neural equivalent data', especially for assessing head control disease and properly planning the rehabilitation process. In addition, the use of genetic algorithms seems to fit well the problem of parameter identification, allowing for the use of a very simple experimental set-up and granting model robustness.

Adult↗

PubMed2006

Concurrent changes in shortening reaction latency and reaction time of forearm muscles in post-stroke patients.

The objective was to confirm the hypothesis that shortening reaction (ShoRe), normally occurring on forcibly shortening a muscle, is depressed or delayed in post-stroke patients. Eight post-stroke patients and ten age-matched normal subjects had a 50 degrees wrist extension or flexion induced by a torque-motor in the affected and the non-affected upper limb. Patients were instructed either not to intervene or to assist displacement (reaction-time condition, RT). Frequency of occurrence and latency of stretch reflex (SR) and ShoRe, and RTs were measured from the electromyograms (EMG) of wrist flexor (FCR) and extensor (ECR) muscles. SR had higher than normal frequency in both muscles. ShoRe disappeared in ECR on the affected side but had normal frequency in FCR of both sides. ShoRe latency was prolonged in FCR and ECR, in both affected and unaffected sides. RTs were prolonged in both FCR and ECR, in both affected and unaffected sides. Across all patients, RTs and ShoRe latencies in the FCR were correlated. Neither RTs nor ShoRe latencies were correlated to Ashworth score. RTs were inversely correlated to Medical Research Council scores. The decreased and delayed ShoRe in post-stroke patients supports a role for the cortico-spinal pathway in its production or modulation. Monitoring of ShoRe can give insight into the recovery of the descending control of spinal reflexes.

Adult↗

PubMed1992

Phase resetting and frequency entrainment of essential tremor.

Step and sinusoidal forcings were delivered to the wrists of 10 patients with advanced essential tremor using a computer-controlled torque motor. Consistent (type 0) phase resetting was produced in only 5 patients, whereas all 10 patients exhibited harmonic and 2:1 subharmonic entrainment of essential tremor. These results indicate that the oscillator of essential tremor is coupled to the stretch-reflex arc. Irregularity in the tremor rhythm and postperturbation mechanical-reflex oscillations obscured the phase of essential tremor, thereby impeding the computation of phase resetting curves. Sinusoidal forcings circumvented these problems, but frequency entrainment was produced only by forcings at frequencies within +/- 1 Hz of the tremor frequency and its second harmonic. Resonance accompanied frequency entrainment and probably plays a role in the clinical expression of tremor.

Adult↗

PubMed1990

Memory traces in spinal cord.

The complexity and inaccessibility of the vertebrate CNS impede the localization and description of memory traces and the definition of the processes that create them. Recent work has shown that the spinal stretch reflex (SSR), which is produced by a monosynaptic two-neuron pathway, can be operantly conditioned, and that memory traces responsible for this behavioral change reside in the spinal cord. The probable locations are the terminal of the Ia affernt neuron on the motoneuron and/or the motoneuron itself. Because it modifies a simple well-defined and accessible pathway, SSR conditioning may be a valuable experimental model for studying vertebrate memory.

Animals↗

PubMed1987

Rigidity in rats due to radio frequency decerebration and effects of chlorpromazine and mephenesin.

Bilateral radio frequency (RF) lesions of the midbrain were produced in an attempt to establish a non-bleeding method of producing decerebrate rigidity in rats. Marked extensor rigidity occurred with a high reproducibility in hindlimbs without the appearance of voluntary movement. Stretch reflex tension induced by repetitive dorsiflexion of the hindfeet was employed as a measure of the intensity of rigidity. Chlorpromazine-HCl (0.1-1 mg/kg, i.v.) and mephenesin (10-50 mg/kg, i.v.) which depress intercollicular decerebrate rigidity, reduced the RF decerebrate rigidity. These results suggest that RF decerebrate rigidity can be used instead of intercollicular transection rigidity in the pharmacological and physiological studies.

Animals↗