Amiloride-sensitive, or acid-sensing, ion channels (ASICs) are highly expressed in all primary mechanosensory nerve endings examined. However, their role there is unclear, since an involvement in neurosensory mechanotransduction is disputed1-3. Nevertheless, ASICs are one of the few putative mechanosensory channel families with a selective conductance for Na+, the predominant current carrier of the receptor potential in muscle spindles4. We have shown high expression of ASIC2a in muscle spindle primary sensory terminals3. We therefore generated knockout (Asic2a-/-) mice and examined the effect on mechanosensory responses of parvalbumin-expressing sensory neurones innervating muscle spindles. Soleus nerve-muscle preparations were removed from Asic2a-/- mice or their wild-type littermates (n=5 each genotype), after humane killing. Their afferent firing was then recorded in response to stretch-hold-release (ramp) and saw-tooth length changes of 0.2, 1 and 5 Hz. The number of action potentials (APs) fired, or the firing rate (APs/sec) in the whole muscle nerve during particular phases of the stretch profile were quantified and compared between genotypes. The sensitivity of firing to amiloride, a broad-spectrum blocker of ASICs, was also examined. Data presented as mean ± SEM, and WT vs KO. The significance of differences were ascertained by ANOVA, threshold P=0.05. ASIC2a knock out enhanced both hold-phase spindle afferent firing rate (178.9±8.4 vs 219.1±10.0 APs/sec; P<0.01), and inhibition of firing during release (release deficit, -34.1±2.3 vs -47.6±3.0 APs; P<0.001). Dynamic firing was enhanced at low stretch rates (dynamic excess; 0.2Hz – 195.0±1.0 vs 255.8±1.2 APs, P<0.0001. 1Hz 72.8±0.3 vs 76.1±0.3 APs, P<0.0001; data normalised to means) and inhibited at low release rates (release deficit; 0.2Hz – 156.8±1.2 vs 148.6±1.2 APs; P<0.0001; 1Hz – 25.6±0.3 vs -21.9±0.3 APs; P<0.0001; data normalised to means). Similar trends were found at 5Hz. Interestingly, amiloride (1mM) was more effective in inhibiting spindle responses in Asic2a-/- soleus both during hold phase-firing (91.3±3.07 vs 62.1±5.1 APs/sec; P<0.0001; data normalised to final control value) and dynamic sawtooth stretches (e.g. 1Hz dynamic excess – 85.9±2.4 vs 68.2±3.4 APs; P<0.0001; data normalised to final control value). In behavioural tasks, Asic2a-/- mice had significant deficits in balance beam walking. We conclude that ASIC2a is a molecular determinant regulating mechanosensitive firing rates in mouse proprioceptors.