My fascination for neurophysiology started in secondary school, when we were taught about action potentials. Whilst studying Biology, I was able to do two long neurophysiological research projects: connections within the brainstem reticular formation of the mallard (I am a biologist, after all) and their role in control of food uptake, and a second project, “experimental epilepsy”, investigating the effects of various antiepileptic drugs, making recordings of neurons in hippocampal slices. However, I encountered my real love during my PhD research in Daniel Kernell’s lab in the Academic Medical Center in Amsterdam. The research project focused on the “matching” between the properties of motor neurons and those of the muscle fibres they innervate. This research had some aspects which I found very attractive.
First, the motor neuron is the ideal “model” neuron. It is relatively straightforward to obtain in vivo intracellular recordings from individual neurons. Next, the motor neuron can be activated by injecting current through the electrode. This will then result in the contraction of only those muscle fibres innervated by the motor neuron (motor unit). Thus, it is possible to compare the “electrical” motor neuron properties with the “mechanical ”properties of its muscle fibres. In short, motor neurons probably are the only class of neurons of which not only the activity can be recorded, but also the exact outcome.
Second, the properties of motor neurons and their muscle fibres are rather precisely matched. The “size principle” (as originally proposed by Henneman et al 1965) described that with increasing force weak motor units (“S”) are recruited before stronger ones (“F”). S motor neurons are highly excitable and are consequently activated most easily (and therefore their muscle fibres). Motor neurons innervating the stronger “F” motor units have larger somata, which makes them less excitable. They are therefore activated less easily. Contractile properties covary. Thus, S units are not only weak, but are also slow and are able to maintain force for long periods. Therefore, the units activated most easily (and therefore most often) are at the same time highly fatigue-resistant.
The beauty of the size principle lies in the fact that the properties of the motor neurons themselves result in the initial activation of slow, weak and fatigue/resistant units and gradual additional recruitment of stronger, faster, but more fatiguable muscle fibres as more force is needed. The advantage is that the recruitment order occurs “automatically”; it does not require additional control by the central nervous system. Since Henneman’s 1965 paper a large amount of new and detailed knowledge about motor control has been collected, but motor unit recruitment according to the size principle still holds true.
Legend:
Figure 1 from Henneman’s first “size principle” publication (Somjen et al 1965). The pyramidal tract is stimulated in the brain stem with increasing intensity (frames 1-9) while recording activity of motor axons in the ventral root. With increasing stimulus intensity increasingly larger motor neurons are activated.