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Changes in fusimotor activity during repetitive lengthening muscle contractions in decerebrate cats
Umeå University, Faculty of Medicine, Department of Surgical and Perioperative Sciences.
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1998 (English)In: Neuroscience, ISSN 0306-4522, E-ISSN 1873-7544, Vol. 86, no 4, 1337-1342 p.Article in journal (Refereed) Published
Abstract [en]

Responses of fusimotor neurons to lengthening vs isometric contractions have been studied in decerebrate cats. Spike discharges of fusimotor neurons to the medial gastrocnemius muscle were recorded from this muscle nerve filament during sequences of contractions and/or stretches of the lateral gastrocnemius and soleus muscles. The sequences lasted for 250-450s (duty cycle 4:2 s). Isometric contractions were elicited by electrical stimulation (40 Hz, 1.3 times motor threshold) of the muscle nerves. Lengthening contractions were elicited in the same way while the muscles were stretched by 4 mm at a velocity of 1 mm/s. Of 25 fusimotor neurons studied, 23 responded to muscle contractions with an increase in firing rate, subsiding towards the end of the sequence. The increase was either modulated with each subsequent contraction or smooth throughout the sequence. Approximately 64% of fusimotor neurons, responding to muscle contractions, responded in a similar way to the sequences of muscle stretches, applied alone. Responses to sequences of the lengthening contractions were significantly larger, on average, than those to the isometric ones, but smaller than the sum of the responses to the contractions and stretches applied separately. On the other hand, they were also larger in fusimotor units, showing no overt responses to muscle stretches alone.

Place, publisher, year, edition, pages
1998. Vol. 86, no 4, 1337-1342 p.
Keyword [en]
fusimotor neurons, muscle contraction, muscle stretch
National Category
Medical and Health Sciences
URN: urn:nbn:se:umu:diva-4151DOI: 10.1016/S0306-4522(98)00122-5PubMedID: 9697137OAI: diva2:143133
Available from: 2004-10-08 Created: 2004-10-08 Last updated: 2010-06-29Bibliographically approved
In thesis
1. Experimental studies of spinal mechanisms associated with muscle fatigue
Open this publication in new window or tab >>Experimental studies of spinal mechanisms associated with muscle fatigue
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Muscle fatigue is ubiquitous in every day life.Muscle fatigue might be considered as an altered state of motor behaviour, which impairs motor performance. By contrast, muscle fatigue could also be considered a positive phenomenon, which protects muscle tissue from damage that might be incurred to it by overuse.

The principal aim of the thesis was to explore some of the mechanisms of muscle fatigue at the spinal level in animal models.The activation of multiple motor units of a single calf muscle may influence contractile properties of its neighbouring, otherwise inactive units, providing evidence for spatial spreading of fatigue between different muscle parts. The release of metabolites, their action on inactive muscle units and the effects of local hypoxia are the most likely causes. Fatigue-induced metabolite shift in the interstitium provokes excitation and/or sensitisation of high-threshold afferent fibers, with complex effects on the spinal premotoneuronal network involved in the modulation of motoneuronal output. This was examined by studing the intrasegmental lamellar distribution of the lumbar spinal interneurons following fatiguing contractions of the triceps surae muscle. Furthermore, fatigue of calf muscles enhanced the activity of fusimotor neurons to these muscles irrespective of the regime of muscle activity (isometric vs. lengthening) in conditions that simulate locomotion. Altered fusimotor activity, through increased or maintained muscle spindle afferent responsiveness may be advantageous, providing support to the skeletomotor activity and enhanced information about muscle periphery to higher nervous centres. The particular effects of interneuronal network at motor input (presynaptic inhibition system) and output (recurrent inhibition system) stages were then addressed. Fatigue of triceps surae muscle induced a suppression of the monosynaptic reflex. The intensity of presynaptic inhibition increased, while the intensity of recurrent inhibition decreased. Post fatigue-evoked changes in monosynaptic reflexes and presynaptic inhibition indicate the possibility that high-threshold afferents inhibit group Ia terminals pre-synaptically, which would allow fatigue-induced signals from the muscle to reduce the relevance of proprioceptive feedback. Besides intrasegmental, intersegmental spreading of nociceptive signals was explored. Activation of sensory afferents from dorsal neck muscles by capsaicin induces powerful activation of interneurons located in the cervical spinal cord, as well as a widespread activation of cells in lumbar spinal cord segments.

The results confirm the pivotal role of small diameter muscle afferents in the orchestration of segmental responses to fatigue and show complex interactions that may lead to limited accuracy of motor output. They also depict processes that may be related to, and even become precursors of chronic muscle pain.

Umeå University medical dissertations, ISSN 0346-6612 ; 910
muscle fatigue, monosynaptic reflex, presynaptic inhibition, recurrent inhibition, fusimotor system, Fos-immunoreactivity, referral pain
Research subject
urn:nbn:se:umu:diva-332 (URN)91-7305-703-7 (ISBN)
Public defence
2004-11-12, Stora salen, Arbetslivsinstitutet, 13:00
Available from: 2004-10-08 Created: 2004-10-08 Last updated: 2010-06-28Bibliographically approved

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