Inspiratory muscle motoneurone pools are linked via many reflex connections. These reflexes are largely inhibitory and can operate over several segments. Much of our knowledge of these reflex connections comes from studies in animals (Marlot et al. 1988, Speck & Revelette. 1987). In humans, we know that stimulation of the phrenic nerve powerfully inhibits the motor output to the diaphragm, intercostal and scalene muscles (Butler et al. 2003). In the present study, our aims were to determine whether specific reflex connections operate between afferent nerve fibres from intercostal muscles and the scalene muscles, and whether these connections operate after a clinically-complete injury of the cervical spinal cord.
Surface electromyographic recordings were made from scalene muscles bilaterally, in seven able-bodied participants and seven participants with motor- and sensory-complete cervical spinal cord injury. We recorded the reflex responses produced by electrical stimulation of the eighth or tenth left intercostal nerve.
WHAT DID WE FIND?
A short-latency inhibitory reflex occurred in able-bodied participants, in both the ipsilateral and contralateral scalene muscles, in response to intercostal nerve stimulation. It was elicited during inspiratory efforts, and the onset of the reflex inhibition occurred at ~40 ms. This bilateral intersegmental inhibitory reflex occurred more commonly at higher stimulus intensities, i.e. when the stimulus level was up to nine times that of motor threshold. The size of the reflex did not increase with larger stimulus intensities and thus it is likely to be mediated by large-diameter intercostal muscle afferents, such as those innervating tendon organs (De Troyer, 1998).
In participants with clinically-complete cervical spinal cord injury, the reflex was absent, presumably because the ascending sensory pathways underlying the reflex were disrupted.
These findings describe a new inhibitory intersegmental reflex connection between inspiratory motoneurone pools in humans. We have termed it the intercostal-to-scalene reflex.
SIGNIFICANCE AND IMPLICATIONS
The absence of this reflex in participants with complete cervical spinal cord injury clearly demonstrates the importance of intact spinal connections for these intersegmental reflexes. The intercostal-to-scalenes reflex could be used in conjunction with routine sensory tests to monitor progressive changes after injury in intersegmental neural connectivity across the injury site or changes produced by treatment. Further work is needed to describe these reflexes more fully.
McBain RA, Taylor JL, Gorman RB, Gandevia SC, Butler JE (2016). Human intersegmental reflexes from intercostal afferents to scalene muscles. Exp Physiol, 101, 1301-1308. doi: 10.1113/EP085907
Butler JE, McKenzie DK, Gandevia SC (2003). Reflex inhibition of human inspiratory muscles in response to contralateral phrenic nerve stimulation. Respir Physiol and Neurobiol, 138, 87-96. doi: 10.1016/S1569-9048(03)00161-7
De Troyer AD (1998). The canine phrenic-to-intercostal reflex. J Physiol, 508, 919-927. doi: 10.1111/j.1469-7793.1998.919bp.x
Marlot D, Macron JM, Duron B (1988). Effects of ipsilateral and contralateral cervical phrenic afferents stimulation on phrenic motor unit activity in the cat. Brain Res, 450, 373-377. doi: 10.1016/0006-8993(88)91578-8
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