The monosynaptic reflex: a tool to investigate motor control in humans. Interest and limits.

The principle of the monosynaptic reflex used as a tool to explore the excitability of the motoneurones (MNs) is explained and the general methodology of the H reflex is described. The different drawbacks inherent in the technique are then considered: mechanisms other than the monosynaptic la excitation of MNs contributing to the H reflex size (limitation of the H reflex size by disynaptic IPSPs, presynaptic inhibition of la terminals, post-activation depression); non-linearity and changes in the 'recruitment gain' in the MN pool; and poor time resolution of the method. Despite these drawbacks, it is emphasized that the H reflex is the only available technique enabling one to investigate changes in transmission in spinal pathways during motor tasks.