Epidural electrical stimulation at C4 suppresses external intercostal EMG activity

Breathing ability is impaired after cervical spinal cord injury, and there are currently no therapeutics to recover natural, independent respiration. Electrical stimulation of the dorsal surface of the spinal cord, or epidural electrical stimulation, has emerged as a promising strategy to restore central control over spinal motor circuits after injury. Recently, we have shown that electrical stimulation targeting the diaphragm at spinal C4 restores phasic EMG activity in acutely hemisected, anesthetized rats (Mickle et al., 2024). Beyond directly recruiting diaphragm EMG during the stimulation period, we have further found that epidural stimulation at C4 indirectly activates endogenous respiratory circuits, including those responsible for shaping diaphragm EMG patterning as well as spinal and sensory reflex pathways connecting the phrenic and hypoglossal motor nuclei (unpublished observations). However, while this stimulation approach is effective at recruiting the diaphragm, accessory muscles also have significant impacts on respiratory function. In particular, the intercostals can play an increased role in breathing after injury, have known spinal reflex connections with the diaphragm, and as such may change their degree of activity in response to C4 stimulation. Indeed, electrical stimulation at C4 increases neuronal activity at T1, as determined by greater expression of the immediate early gene FOS, particularly within inhibitory thoracic interneurons (unpublished observations); however, the functional consequence or lack thereof of this altered neuronal activity on intercostal motor output remains unknown. As an accessory muscle, the recruitment of inspiratory intercostal activity varies widely under different postural and respiratory drive conditions. Therefore, we first established the degree of intercostal EMG activity under a variety of experimental settings. Rats (n = 3) were urethane anesthetized before implantation of EMG recording electrodes bilaterally in the diaphragm and the external intercostals in the second intercostal space. 3-minute recordings of diaphragm and intercostal EMG were taken while rats were free-breathing or on a cycle-triggered ventilator during 40/60 O 2 /N 2 gas mix as well as during administration of a 40/5/55 O 2 /CO 2 /N 2 gas mix. Mechanical ventilation greatly blunted both diaphragm and intercostal EMG vs during free breathing conditions. While the addition of inspired CO 2 during mechanical ventilation recovered a large degree of diaphragm EMG activity, hypercapnic challenge did not restore intercostal EMG amplitude. As such, further experiments were conducted in free breathing animals to prevent ventilator suppression of intercostal activity. To evaluate the effects of C4 electrical stimulation on intercostal EMG, a further set of rats (n = 7) were anesthetized and implanted with EMG recording electrodes, as well as cervical epidural stimulating electrodes. In intact rats, electrical stimulation during the inspiratory period increased diaphragm EMG while suppressing intercostal EMG bilaterally, likely reflecting increased activation of the inhibitory phrenic-to-intercostal reflex. C2 hemisection injury initially abolished all ipsilesional intercostal EMG, with a modest degree of recovery in the minutes post-injury. While electrical stimulation was able to restore ipsilesional diaphragm EMG, ipsilesional intercostal EMG was also not increased by stimulus post-injury. With these opposite effects on motor output in these two respiratory motor pools, it will be particularly critical to evaluate the net outcome of C4 stimulation on downstream functional respiratory metrics such as inspiratory pressure generation. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.