Human walking displays an impressive precision and the ability to adapt to different
terrains. It is suggested that both spinal as well as supra-spinal structures play an important
role in this rhythmic task. The current thesis focuses on the role the motor cortex plays in
this task. Although there are experimental findings and clinical observations suggesting
that one of the supraspinal structures involved in walking is the motor cortex, it is not
generally accepted that the motor cortex is of major importance in human locomotion.
The motor cortex is generally considered to be important in voluntary movement and the
choice to term walking as non-voluntary movement may be more than just semantically
important and this matter is discussed in the thesis.
The thesis includes five original research papers which show that output from the motor
cortex is integrated with contributions from spinal structures in rhythmic tasks such as
walking and hopping. In study I it is shown how the motor cortex contributes to
motoneuronal drive in conjunction with sensory feedback mediated by spinal reflex loops.
The integration between afferent feedback and motor cortex is further displayed in study
II, in which we show that afferent input may relay to corticospinal neurons from where it
is fed back to the muscle and may produce a large functional directed response during
walking. Study II thereby displays a role for the motor cortex in error correction during
walking. Study III shows not only that afferent feedback from agonist muscles is relayed
to corticospinal neurons during walking but also that feedback from the antagonist is
relayed to the motor cortex where signals from both muscles increase the excitability of
the motor cortex. This organisation is somewhat reversed from the organisation of
antagonist afferent input to the spinal cord, but it may be hypothesised that both spinal
and supra-spinal structures contribute to a balanced response to a perturbation. The
motor cortex also plays a role during normal, unperturbed walking. In study IV we show
that output from the motor cortex may be suppressed during walking and standing by
exciting intracortical inhibitory interneurons using subthreshold transcranial magnetic
stimulation. The results may be explained by the suggestion that there is less corticospinal
output during walking. Alternatively, it may be suggested that it is more difficult to stop
the corticospinal neurons or motoneurons from firing during walking. The final study
showed that muscle activity can still be suppressed during a dynamic contraction during
sitting and it also suggests that the observed effects cannot be unambiguously related to
changes in corticospinal output.
In conclusion, the studies in the thesis further confirm a role for the primary motor cortex
in driving the muscle during rhythmic tasks like walking and hopping. It shows a role for
the motor cortex in mediating afferent feedback to both agonists as well as antagonists,
underlining the integrative nature of the neural system. Indirect measurements of the
ability of intracortical inhibitory neurons to suppress corticospinal neurons suggest that
the motor cortex is involved in both walking and standing, but that the control is very
different.
Human Locomotion and
the Motor Cortex