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.

Reflex effects evoked by graded electrical stimulation of the posterior articular nerves (PAN) of the ipsi- and contralateral knee joints were investigated using both micro-electrode recordings from 7 - motoneurones and recordings from single muscle muscle spindle afferents. Spindle afferent responses were also recorded using natural stimulation of different types of receptors, to elucidate if the articular reflexes onto the y -motoneurones were potent enough to significantly alter the muscle spindle afferent activity. Stretches of the ipsilateral posterior (PCL) and anterior (ACL) cruciate ligaments, pressure on the ipsi- and contralateral knee and ankle joint capsules, and passive flexion/extension movements of the joints in the contralateral hind limb were performed. The occurrance of different sensory endings in the ACL and PCL was examined using gold chloride staining for neuronal elements. All experiments were performed on chloralose anaesthetized cats. More than 90% of the static and dynamic y -motoneurones were responsive to electrical stimulation of the PAN. Most 7-cells responded to low intensity electrical stimulation. Excitatoiy reflex effects predominated on both static and dynamic posterior biceps-semitendinosus (PBSt) 7 -cells, while excitatory and inhibitory effects occurred with an about equal frequency on triceps-plantaris (GS) 7-cells. The fastest segmental route for excitatory PAN effects on hind limb 7-motoneurones seems to be di- or trisynaptic, while the path for inhibitory effects seems to be at least one synaps longer. Physiological stimulations of ipsi- and contralateral joint capsules and of ipsilateral cruciate ligaments were all found to evoke frequent and potent changes in spindle afferent responses from the GS and PBSt muscles. It was shown that these effects were due to reflexes onto dynamic and static fusimotor neurones caused by physiological activation of articular sensory endings. Both ipsi- and contralateral joint receptor stimulation evoked excitatory as well as inhibitory fusimotor effects. The highest responsiveness was found during stimula­tion of the cruciate ligaments, i.e. 58% for GS and 47% for PBSt primary spindle afferents to PCL stimula­tion, and 73% for GS and 55% for PBSt primary spindle afferents to ACL stimulation. Significant altera­tions in spindle afferent activity was encountered at very low traction forces applied to the cruciate ligaments (5-10 N). The low thresholds, the tonic character of the stimuli, and the fact that different types of sensory endings were demonstrated in the cruciate ligaments (i.e. Ruffini endings, Pacinian corpuscles, Golgi ten­don organ like endings and free nerve endings), indicate that the fusimotor effects observed were caused by activation of slowly adapting mechanoreceptors, most likely Ruffini endings and/or Golgi tendon organ like endings. The potent reflex effects on the muscle spindle afferents elicited by increased tension in the cruciate ligaments indicate that these ligaments may play a more important sensory role that hitherto believed, and it is suggested that they may be important in the regulation of the stiffness of muscles around the knee joint, and thereby for the joint stability. The possible clinical relevance and the mechanisms by which joint receptor afferents, via adjustment of the muscle stiffness, may control joint stability are discussed. / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 1989, härtill 7 uppsatser.</p> / digitalisering@umu

Muscle spindles are complex sensory organs that have been strongly implicated in the control and perception of movements. Human muscle spindles in relaxed muscles behave as stretch receptors, responding to the length and velocity of their parent muscles. However, it has been unclear how they discharge during active movements since their discharges are also affected by fusimotor activity and extrafusal contractions. The vast majority of neurophysiological recordings of muscle afferents have been obtained under passive conditions, or active but behaviourally restricted conditions. These restrictions prevent predictions of human muscle afferent activity during purposeful multi-joint movements, naturally occurring during tasks such as hand shaping, grasping or key-pressing. An experimental protocol was therefore developed which allowed recordings of muscle receptor afferent activity using microneurography during unrestrained wrist and digit movements. Along with single afferent discharges, recordings were obtained of electromyographic activity of major forearm muscles and the kinematics of the wrist and digits. This approach allowed investigations of the factors shaping afferent discharge during everyday manual tasks, i.e., block-grasping and pressing sequences of keys, and during active sinusoidal joint movements. The afferents’ ability to encode information concerning the state of the muscle and joint kinematics during these tasks was also assessed. The responses of spindle afferents from load-bearing muscles were approximatelly 90 degrees more phase-advanced than expected on the length of their parent muscles. That is, the discharges of primary muscle spindle afferents were significantly affected by both velocity and acceleration, the discharges of secondary afferents by velocity, and neither afferent type was particularly affected by static muscle length. Accordingly, these afferents failed to encode length, encoded velocity well and acceleration poorly. The representation of muscle length and velocity was, however, significantly improved when the discharge activity of Golgi tendon afferents was taken into consideration along with muscle spindle activity. The discharge of primary afferents during both key-pressing and block-grasping was best correlated to the muscle velocities observed ~100-160 ms in the future. This predictive ability went beyond what could be expected from the spindles’ simultaneous sensitivity to velocity and acceleration, and could thus only be explained by implicating the fusimotor drive. In addition, evidence is presented that the fusimotor control of spindles was contingent on entire movement sequences during the key-pressing task. It is proposed that the phase relationship between the discharge rate of spindle afferents and the length of their parent muscles is load dependent. Moreover, muscle spindles seem to act as forward sensory models of their parent muscle, which makes sensorial feedback control possible despite neural delays.

The response patterns of human stretch receptors in the finger extensor muscles of the forearm were studied using the microneurography technique. Single-unit recordings were obtained from one-hundred and twenty-four afferents. A procedure was developed to classify the units in muscle spindle primary afferents, secondary afferents, and Golgi tendong organ afferents. The procedure allows an objective and reproducible classification on the basis of the afferents’ responses to a series of tests which individually are non-conclusive. It was demonstrated that maximal twitch contractions can be elicited in the finger extensor muscles of the forearm, without causing undue discomfort to the subjects, or hazarding the single-unit recording. The response of the units to this test allowed, in most cases but not always, a separation in muscle spindle and tendon organ afferents. Thus the test was not adequate for an unequivocal classification. Three discrete response parameters were extracted from ramp-and-hold stretches, viz. the presence or absence of an initial burst and a deceleration response, and prompt silencing at slow muscle shortening. The distributions of the parameters were significantly different among the three unit types. These parameters which were pair-wise independent constituted a set of considerable discriminative power. It was shown that human muscle spindles have about the same static position sensitivity to fractional muscle stretch as previously found in animals. Stretch sensitization was demonstrated by rapid, repeated stretches of the muscle which enhanced the réponse to subsequent slow stretches of muscle spindles. Sensitization was different with primary and secondary muscle spindle afferents whereas Golgi tendon organ afferents never displayed stretch sensitization. One-to-one driving with small-amplitude sinusoidal stretches superimposed on ramp-and- hold stretches was almost exclusively seen with primary muscle spindle afferents, whereas secondaries seldom and tendon organ afferents never displayed driving. The afferent responses during slowly increasing isometric contractions and rapid relaxations were analysed. An increased discharge rate on relaxation was common among spindle afferents whereas it was never seen in tendon organs afferents. Two separate groups of spindles afferents were found with regard to fusimotor recruitment. The largest group was recruited at rather low and variable contractile forces whereas the smaller group was not recruited at all. The proportions of the three unit types, spindle primary, spindle secondary, and Golgi tendon organ afferents were estimated from a preliminary classification and the distribution of the eight response features were analyzed for each class of afferents. On the basis of these estimates and the response pattern of the individual unit Bayes’ theorem was used to calculate the probabilities that the unit was a spindle primary, a spindle secondary, or a tendon organ afferent. Estimates indicate that about 19 out of 20 muscle afferents are correctly classified when all eight features are analyzed. / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 1988, härtill 6 uppsatser.</p> / digitalisering@umu