The following chapters discuss methods that allow the selective investigation of different spinal pathways. Whatever the pathway investigated, its activation produces changes in the excitability of spinal motoneurones, ‘the final common path’ in the motor system. A prerequisite for any investigation of changes in the spinal circuitry in human subjects is therefore to be able to assess changes in motoneurone excitability quantitatively, using valid reproducible methods. Several non-invasive methods have been developed, and these are considered in this chapter with their advantages and disadvantages. All are, of course, indirect, and valid conclusions can only be obtained if congruent results are obtained with different methods relying on different principles. All may be, and many have been, used in studies on patients, but here the methodology should be simple and rapid.

This initial chapter is technical and non-specialist readers could bypass it, referring back if they need to clarify how results were obtained or understand the advantages and limitations of a particular technique. However, the chapter is required reading for those who want to understand fully the particular techniques used for the different pathways and how to use those techniques.

The monosynaptic reflex: H reflex and tendon jerk

The ‘monosynaptic reflex’ forms the basis of the first technique available to investigate spinal pathways in animals and humans. The principle is based on the apparent simplicity of the monosynaptic projection of Ia afferents to homonymous motoneurones. Subsequent studies have shown that the so-called monosynaptic reflex is not as simple as was initially thought.