Proprioception and myoclonus

This review focuses on sensory information originating from muscle spindles and its role in proprioception and motor control. The first part reminds of the structural and functional properties of these muscle mechanoreceptors, with arguments for an independent fusimotor command, i.e. the gamma-motoneurons, that would regulate spindle mechanical sensitivity in keeping with the requirements of ongoing motor action. The possibility that dysfunction of the fusimotor system might be responsible for clinical signs is discussed with respect to the hyperexcitability of the sensorimotor cortex that is observed in myoclonus of cortical origin. What is known about the spindle afferents projections into the spinal cord and about the dysfunction of the spinal sensorimotor networks in patients with neurological disorders, is put together in the second part. It is stressed on the significant complexity of the monosynaptic reflex in spite of its "simple" organization. The monosynaptic reflex constitutes the only possible way for testing the excitability of motoneurons and spinal networks. This method is extensively used clinically to examine changes in the nervous system with diseases. When studying changes from the norm, it is important to understand how the reflex functions in neurologically normal conditions. Different mechanisms such as pre-synaptic inhibition, post-activation depression and motoneuronal intrinsic properties are reviewed as they may induce changes in reflex amplitude and have therefore consequences for interpretation of spinal excitability.