Abstract
Motor units are generally recruited from the smallest to the largest following the size principle, while cutaneous stimulation has the potential to affect spinal motor control. We aimed to examine the effects of stimulating transient receptor potential channel sub-family M8 (TRPM8) combined with exercise on the modulation of spinal motor neuron (MN) excitability. Mice were topically administrated 1.5% icilin on the hindlimbs, followed by treadmill stepping. Spinal cord sections were immunostained with antibodies against c-fos and choline acetyltransferase. Icilin stimulation did not change the number of c-fos+ MNs, but increased the average soma size of the c-fos+ MNs during low-speed treadmill stepping. Furthermore, icilin stimulation combined with stepping increased c-fos+ cholinergic interneurons near the central canal, which are thought to modulate MN excitability. These findings suggest that TRPM8-mediated cutaneous stimulation with low-load exercise promotes preferential recruitment of large MNs and is potentially useful as a new training method for rehabilitation.
Highlights
Motor neurons (MNs) are generally recruited from the smallest to the largest with an increasing load following the size principle [1]
To confirm whether transient receptor potential channel sub-family M8 (TRPM8)-mediated Skin cooling (SC) stimulation is induced by the cutaneous afferents, we examined the expression of c-fos in the mouse spinal cord dorsal horn following application of a specific TRPM8 agonist, icilin [17] on the hindlimbs
We demonstrated that TRPM8-mediated cutaneous stimulation combined with low-load treadmill stepping facilitates the activation of large MNs with the activation of cholinergic INs near the central canal
Summary
Motor neurons (MNs) are generally recruited from the smallest to the largest with an increasing load following the size principle [1]. There are some exceptions [3]; for example, the order of MU recruitment can be changed by electrically stimulating the cutaneous afferents. The afferent stimulation generates inhibitory responses in the small MUs and excitatory responses in the large MUs [4,5,6]. These findings suggest that cutaneous afferent input modulates MN excitability via somatosensory reflex pathways.
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