Here is a review that, if you are interested in how the brain controls muscles, and you are prepared to put in some hard yards, you should read. Simon Gandevia works down the corridor from me, so do Janet Taylor and Jane Butler. Nicholas Peterson doesn’t but I once had a cup of tea from the same pot. So, I am, everyday, in the presence of corticospinal greatness. What one doesn’t realise however, because they are all such modest folk, is how corticospinally great they really are. I could do a full review of this review, but I really don’t have the expertise. Here are the things that I think are most relevant to what we do as clinicians when we help people to move:
i) Motor neurones get input from many different parts of the brain, not just primary motor cortex. I think this is a great thing to remember because it gives us a mechanism by which a gaggle of factors can influence how we move. I think this is really relevant to work I did with Paul ‘The Walking Cortex’ Hodges on postural adjustments, where the expectation of back pain changes trunk muscle activation and where beliefs about back pain seem to relate to altered postural adjustment strategies. That the primary motor neurone receives corticospinal projections from frontal cortex is a lovely possible route by which beliefs can change motor output.
ii) Many corticospinal projections run to many motoneurones. I think this is interesting too because it fits with the whole brain as an orchestra idea – each musician contributes to many tunes, just like brain motor cells contribute to lots of different movements.
iii) The whole thing is highly dynamic and can be relatively easily modified. This is, of course, what we are trying to do when we teach people new ways of moving or not moving. Sometimes it feels really really difficult to shift, but at least we know, from the likes of Nicholas, Jane, Janet and Simon, that the system is very shiftable. Anyway, if you are up for a challenge, read this paper. You won’t regret it.
Probing the corticospinal link between the motor cortex and motoneurones: some neglected aspects of human motor cortical function.
Petersen NC, Butler JE, Taylor JL, Gandevia SC.
This review considers the operation of the corticospinal system in primates. There is a relatively widespread cortical area containing corticospinal outputs to a single muscle and thus a motoneurone pool receives corticospinal input from a wide region of the cortex. In addition, corticospinal cells themselves have divergent intraspinal branches which innervate more than one motoneuronal pool but the synergistic couplings involving the many hand muscles are likely to be more diverse than can be accommodated simply by fixed patterns of corticospinal divergence. Many studies using transcranial magnetic stimulation of the human motor cortex have highlighted the capacity of the cortex to modify its apparent excitability in response to altered afferent inputs, training and various pathologies. Studies using cortical stimulation at ‘very low’ intensities which elicit only short-latency suppression of the discharge of motor units have revealed that the rapidly conducting corticospinal axons (stimulated at higher intensities) drive motoneurones in normal voluntary contractions. There are also major non-linearities generated at a spinal level in the relation between corticospinal output and the output from the motoneurone pool. For example, recent studies have revealed that the efficacy of the human corticospinal connection with motoneurones undergoes activity-dependent changes which influence the size of voluntary contractions. Hence, corticospinal drives must be sculpted continuously to compensate for the changing functional efficacy of the descending systems which activate the motoneurones. This highlights the need for proprioceptive monitoring of movements to ensure their accurate execution.
Full article at Acta Physiol (2010), 98 (4), 403-16
Petersen NC, Butler JE, Taylor JL, & Gandevia SC (2010). Probing the corticospinal link between the motor cortex and motoneurones: some neglected aspects of human motor cortical function. Acta physiologica (Oxford, England), 198 (4), 403-16 PMID: 20003100
Moseley GL, Nicholas MK, & Hodges PW (2004). Does anticipation of back pain predispose to back trouble? Brain : a journal of neurology, 127 (Pt 10), 2339-47 PMID: 15282214
Moseley GL, & Hodges PW (2006). Reduced variability of postural strategy prevents normalization of motor changes induced by back pain: a risk factor for chronic trouble? Behavioral neuroscience, 120 (2), 474-6 PMID: 16719709