The canine phrenic-to-intercostal reflex

• Published in: The Journal of physiology Vol. 508; no. 3; pp. 919 - 927
• Main Author: Troyer, André
• Format: Journal Article
• Language: English
• Published: Oxford, UK The Physiological Society 01-05-1998; Blackwell Science Ltd; Blackwell Science Inc
• Subjects: Animals; Diaphragm - innervation; Dogs; Electric Stimulation; Electromyography; Intercostal Muscles - innervation; Neurons, Afferent - physiology; Original; Phrenic Nerve - cytology; Phrenic Nerve - physiology; Reflex - physiology; Respiration - physiology; Spinal Nerve Roots - physiology
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1111/j.1469-7793.1998.919bp.x

Paralysis of the diaphragm in the dog causes a non-vagal, non-chemical increase in the activity of the inspiratory intercostal
muscles. In the present studies, the hypothesis was tested that phrenic afferent fibres may elicit a reflex inhibition of
inspiratory intercostal activity.
The electrical activity of the three groups of inspiratory intercostal muscles (parasternal intercostals, external intercostals,
levator costae) was recorded in twenty vagotomized, spontaneously breathing dogs, and the proximal end of one or both C5 phrenic
nerve roots was stimulated during inspiration.
Stimulation of the ipsilateral and contralateral C5 phrenic roots caused an immediate reduction in inspiratory intercostal
activity. This reduction was abolished when phrenic stimulation was repeated after section of the C5 dorsal roots.
The reduction in external intercostal and levator costae activity during bilateral C5 afferent stimulation appeared when the
stimulus strength was 3 times the motor threshold and it increased in magnitude when stimulus intensity was increased further.
In contrast, the reduction in parasternal intercostal activity occurred only when the stimulus strength was 12 times the motor
threshold.
These observations confirm the hypothesis that diaphragmatic receptors may reflexly inhibit efferent activity to the inspiratory
intercostal muscles, in particular the external intercostals and levator costae. This inhibition appears to be primarily mediated
by small myelinated fibres.