It has been shown that the primary and secondary somatosensory cortex, as well as the supplementary motor area (SMA), are involved in central processing of proprioceptive signals during passive and active arm movements. However, it is not clear whether different cortical areas are involved in processing of different proprioceptive inputs (skin, joint, muscle receptors), what their relative contributions might be, where kinesthetic sensations are formed within the CNS, and how they interact when the full peripheral proprioceptive machinery acts. In this study we investigated the representation of the brain structures involved in the perception of passive limb movement and illusory movement generated by muscle tendon vibration. Changes in cortical activity as indicated by changes in regional cerebral blood flow (rCBF) were measured using positron emission tomography (PET). Twelve subjects were studied under four conditions: (1) passive flexion-extension movement (PM) of the left forearm; (2) induced illusions of movements (VI) similar to the real PM, induced by alternating vibration of biceps and triceps tendons (70-80 Hz) at the elbow; (3) alternating vibration of biceps and triceps tendons (with 20-50 Hz) without induced kinesthetic illusions (VN); and (4) rest condition (RE). The results show different patterns of cortex activation. In general, the activation during passive movement was higher in comparison with both kinds of vibration, and activation during vibrations with induced illusions of movement was more prominent than during vibrations without induced illusions. When the PM condition was contrasted with the other conditions we found the following areas of activation -- the primary motor (MI) and somatosensory area (SI), the SMA and the supplementary somatosensory area (SSA). In conditions where passive movements and illusory movements were contrasted with rest, some temporal areas, namely primary and associative auditory cortex, were activated, as well as secondary somatosensory cortex (SII). Our data show that different proprioceptive inputs, which induce sensation of movement, are associated with differently located activation patterns in the SI/MI and SMA areas of the cortex. In general, the comparison of activation intensities under different functional conditions indicates the involvement of SII in stimulus perception generation and of the SI/MI and SMA areas in the processing of proprioceptive input. Activation of the primary and secondary auditory cortex might reflect the interaction between somatosensory and auditory systems in movement sense generation. SSA might also be involved in movement sense generation and/or maintenance.

Intramuscular injection of hypertonic saline (HS) is a procedure widely adopted to experimentally induce deep muscle pain in humans. This study was undertaken to test whether intramuscular injections of HS (5%) influence the activity of primary and secondary muscle spindle afferents (MSAs) from homonymous as well as heteronymous muscles. The experiments were performed on six cats anaesthetised with alpha-chloralose. Usually responses of two to nine MSAs from gastrocnemius medialis (GM) and/or gastrocnemius lateralis (GL) muscles were recorded simultaneously, while HS was injected either into the receptor-bearing muscle (homonymous responses) or into a close (GM/GL) or remote synergistic muscle (posterior biceps, PB, heteronymous responses). The mean rate of discharge and the depth of modulation of the MSA responses to sinusoidal stretching of the receptor-bearing muscle were calculated. Out of the 42 afferents tested (7 from GM and 35 from GL), 38 (90%) exhibited statistically significant responses to injections of HS into homonymous and/or heteronymous muscles. With injections into the homonymous muscle, the average maximal increase in mean rate of discharge was 74% and the average decrease in depth of modulation was --18%. The mean duration of the effects was 2.1 min. The corresponding values for heteronymous injections into a close synergist were 87%, -17% and 2.1 min (GM or GL), and for injections into PB 52%, -11%, and 1.8 min. The majority of the responses (72%) were compatible with reflex action on static fusimotor neurones, whereas 20% of the responses could be attributed to mixed static and dynamic fusimotor action. The remaining 8% of the responses were attributed to inhibition of fusimotor activity. There were no statistically significant differences between the responses following injections into homonymous or heteronymous muscles. Injections of Tyrode's solution did not induce any significant alterations in MSA responses, implying that they were not induced by direct and/or injury effects of the injections. HS-related changes in MSA activity were completely abolished after the nerves to corresponding muscles were cut, confirming the reflex nature of the effects. Thus, intramuscular injections of HS induce reflex changes in MSA activity from both homonymous and heteronymous muscles, most likely via fusimotor reflexes. Predominantly static fusimotor neurones were activated. The possible role of the fusimotor-muscle spindle system in altered motor control during experimentally induced muscle pain is discussed.

Diagnosis of MF was associated with low AS and neonatal encephalopathy, whereas EA was only associated with low AS and not with neonatal encephalopathy. The found associations might be a result of confounding by indication, which is difficult to assess in a registry-based population study.

The aim of the present study was to establish if there exists reflex connections from ligamentous structures in cervical facet joints and the fusimotor system of dorsal neck muscles. In seven cats, anaesthetized with alpha-chloralose, bradykinin (BK) of concentrations between 12 and 50 microg was injected into the facet joint between C1 and C2. Recordings were made from single muscle spindle afferents (MSA) originating in contralateral trapezius and splenius muscles (TrSp). Fusimotor induced changes in the sensitivity of the muscle spindle afferents were assessed by recording the responses to sinusoidal stretches of the TrSp muscles. The mean rate of discharge and the depth of modulation of a fitted sine were taken as quantitative estimates of the response. A total of 25 MSAs were recorded, and 21 of these showed clear-cut alterations in their responses to the sinusoidal stretches following Bk. injections into contralateral facet joint. The majority of the responding afferents (13/21) showed changes in their responses indicating an increased activity of static fusimotoneurones, although responses of dynamic and mixed static and dynamic nature were also seen. Local anaesthetics applied to the intraarticular receptors abolished the effects. Injection (i.v.) of a general anaesthetic (pentobarbital) abolished the effects. The results show that there exist reflex connections between receptors in cervical facet joints and fusimotoneurones of dorsal neck muscles, and this might be of importance in the pathophysiology behind whiplash associated disorders (WAD).