Key points Acetylcholine receptors are aggregated in the central regions of intrafusal muscle fibres. Single unit muscle spindle afferent responses from isolated mouse extensor digitorum longus muscle were recorded in the absence of fusimotor input to ramp and hold stretches as well as to sinusoidal vibrations in the presence and absence of the acetylcholine receptor blockers d‐tubocurarine and α‐bungarotoxin. Proprioceptive afferent responses to both types of stretch were enhanced in the presence of either blocker. Blocking acetylcholine uptake and vesicular acetylcholine release by hemicholinium‐3 also enhanced stretch‐evoked responses. These results represent the first evidence that acetylcholine receptors negatively modulate muscle spindle responses to stretch. The data support the hypothesis that the sensory nerve terminal is able to release vesicles to fine‐tune proprioceptive afferent sensitivity. Abstract Muscle spindles are complex stretch‐sensitive mechanoreceptors. They consist of specialized skeletal muscle fibres, called intrafusal fibres, which are innervated in the central (equatorial) region by afferent sensory axons and in both polar regions by efferent γ‐motoneurons. Previously it was shown that acetylcholine receptors (AChR) are concentrated in the equatorial region at the contact site between the sensory neuron and the intrafusal muscle fibre. To address the function of these AChRs, single unit sensory afferents were recorded from an isolated mouse extensor digitorum longus muscle in the absence of γ‐motoneuron activity. Specifically, we investigated the responses of individual sensory neurons to ramp‐and‐hold stretches and sinusoidal vibrations before and after the addition of the competitive and non‐competitive AChR blockers d‐tubocurarine and α‐bungarotoxin, respectively. The presence of either drug did not affect the resting action potential discharge frequency. However, the action potential frequencies in response to stretch were increased. In particular, frequencies of the dynamic peak and dynamic index to ramp‐and‐hold stretches were significantly higher in the presence of either drug. Treatment of muscle spindle afferents with the high‐affinity choline transporter antagonist hemicholinium‐3 similarly increased muscle spindle afferent firing frequencies during stretch. Moreover, the firing rate during sinusoidal vibration stimuli at low amplitudes was higher in the presence of α‐bungarotoxin compared to control spindles also indicating an increased sensitivity to stretch. Collectively these data suggest a modulation of the muscle spindle afferent response to stretch by AChRs in the central region of intrafusal fibres possibly fine‐tuning muscle spindle sensitivity.

Key points Muscular dystrophy patients suffer from progressive degeneration of skeletal muscle fibres, sudden spontaneous falls, balance problems, as well as gait and posture abnormalities. Dystrophin‐ and dysferlin‐deficient mice, models for different types of muscular dystrophy with different aetiology and molecular basis, were characterized to investigate if muscle spindle structure and function are impaired. The number and morphology of muscle spindles were unaltered in both dystrophic mouse lines but muscle spindle resting discharge and their responses to stretch were altered. In dystrophin‐deficient muscle spindles, the expression of the paralogue utrophin was substantially upregulated, potentially compensating for the dystrophin deficiency. The results suggest that muscle spindles might contribute to the motor problems observed in patients with muscular dystrophy. Abstract Muscular dystrophies comprise a heterogeneous group of hereditary diseases characterized by progressive degeneration of extrafusal muscle fibres as well as unstable gait and frequent falls. To investigate if muscle spindle function is impaired, we analysed their number, morphology and function in wildtype mice and in murine model systems for two distinct types of muscular dystrophy with very different disease aetiology, i.e. dystrophin‐ and dysferlin‐deficient mice. The total number and the overall structure of muscle spindles in soleus muscles of both dystrophic mouse mutants appeared unchanged. Immunohistochemical analyses of wildtype muscle spindles revealed a concentration of dystrophin and β‐dystroglycan in intrafusal fibres outside the region of contact with the sensory neuron. While utrophin was absent from the central part of intrafusal fibres of wildtype mice, it was substantially upregulated in dystrophin‐deficient mice. Single‐unit extracellular recordings of sensory afferents from muscle spindles of the extensor digitorum longus muscle revealed that muscle spindles from both dystrophic mouse strains have an increased resting discharge and a higher action potential firing rate during sinusoidal vibrations, particularly at low frequencies. The response to ramp‐and‐hold stretches appeared unaltered compared to the respective wildtype mice. We observed no exacerbated functional changes in dystrophin and dysferlin double mutant mice compared to the single mutant animals. These results show alterations in muscle spindle afferent responses in both dystrophic mouse lines, which might cause an increased muscle tone, and might contribute to the unstable gait and frequent falls observed in patients with muscular dystrophy.

Introduction/Aims: The muscle relaxant methocarbamol and the antimyotonic drug mexiletine are widely used for the treatment of muscle spasms, myotonia, and pain syndromes. To determine whether these drugs affect muscle spindle function, we studied their effect on the resting discharge and on stretch-induced action potential frequencies of proprioceptive afferent neurons.Methods: Single unit action potential frequencies of proprioceptive afferents from muscle spindles in the murine extensor digitorum longus muscle of adult C57BL/6J mice were recorded under resting conditions and during ramp-and-hold stretches.Maximal tetanic force of the same muscle after direct stimulation was determined.High-resolution confocal microscopy analysis was performed to determine the distribution of Na v 1.4 channels, a potential target for both drugs.Results: Methocarbamol and mexiletine inhibited the muscle spindle resting discharge in a dose-dependent manner with IC 50 values around 300 μM and 6 μM, respectively. With increasing concentrations of both drugs, the response to stretch was also affected, with the static sensitivity first followed by the dynamic sensitivity.At high concentrations, both drugs completely blocked muscle spindle afferent output. Both drugs also reversibly reduced the specific force of the extensor digitorum longus muscle after tetanic stimulation. Finally, we present evidence for the presence and specific localization of the voltage-gated sodium channel Na v 1.4 in intrafusal fibers.Discussion: In this study we demonstrate that both muscle relaxants affect muscle spindle function, suggesting impaired proprioception as a potential side effect of both drugs. Moreover, our results provide additional evidence of a peripheral activity of methocarbamol and mexiletine.