behavior/neurological
• mice show a flexed posture when inverted
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• mice exhibit a wide-based, ataxic gait
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nervous system
• only rudimentary muscle spindles with a single Egr3-positive myofiber are seen in tibialis anterior muscles at P3 in contrast to spindles in controls which have multiple Egr3+ intrafusal myofibers
• on P4 muscle spindles, few mutant spindles have the complex annulospiral afferent terminal seen around the equatorial region of intrafusal myofibers in control muscle; muscle spindle afferent terminal morphology is simple compared to controls
• spindle development fails to progress beyond E18 and mature spindles do not form
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• in tibialis anterior (TA) muscle, muscle spindle numbers are decreased 70% from those of controls
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• innervation of muscle spindles is simpler than observed in controls, but spindle afferent projection to the ventral spinal cord shows normal pattern
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• monosynaptic inputs from muscle spindle afferent to motor neurons are functional but reduced in amplitude; EPSPs evoked by dorsal root stimulation or evoked by muscle nerve stimulation are reduced from those of controls by similar amounts (to about 22-26% of wild-type amplitudes)
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muscle
• only rudimentary muscle spindles with a single Egr3-positive myofiber are seen in tibialis anterior muscles at P3 in contrast to spindles in controls which have multiple Egr3+ intrafusal myofibers
• on P4 muscle spindles, few mutant spindles have the complex annulospiral afferent terminal seen around the equatorial region of intrafusal myofibers in control muscle; muscle spindle afferent terminal morphology is simple compared to controls
• spindle development fails to progress beyond E18 and mature spindles do not form
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• in tibialis anterior (TA) muscle, muscle spindle numbers are decreased 70% from those of controls
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