Anatomy and physiology of phrenic afferent neurons

Abstract

Large-diameter myelinated phrenic afferents discharge in phase with diaphragm contraction, and smaller diameter fibers discharge across the respiratory cycle. In this article, we review the phrenic afferent literature and highlight areas in need of further study. We conclude that ) activation of both myelinated and nonmyelinated phrenic sensory afferents can influence respiratory motor output on a breath-by-breath basis;) the relative impact of phrenic afferents substantially increases with diaphragm work and fatigue; ) activation of phrenic afferents has a powerful impact on sympathetic motor outflow, and) phrenic afferents contribute to diaphragm somatosensation and the conscious perception of breathing. Much remains to be learned regarding the spinal and supraspinal distribution and synaptic contacts of myelinated and nonmyelinated phrenic afferents. Similarly, very little is known regarding the potential role of phrenic afferent neurons in triggering or modulating expression of respiratory neuroplasticity.

Ageing of the Diaphragm Muscle

“…The diaphragm plays an extraordinary role in the movement, muscle coordination, and posture. The phrenic nerve has high diameter (Ia, Ib, II) fibers, afferent fibers with a smaller diameter of non-myelinated type IV (C fibers), and type III myelinated free nerve endings [31]. The first type of afference is activated during the contraction of the diaphragm (inspiration), while the thinnest afferent type is activated during the inspiratory and exhalatory phase [31].…”

“…The phrenic nerve has high diameter (Ia, Ib, II) fibers, afferent fibers with a smaller diameter of non-myelinated type IV (C fibers), and type III myelinated free nerve endings [31]. The first type of afference is activated during the contraction of the diaphragm (inspiration), while the thinnest afferent type is activated during the inspiratory and exhalatory phase [31]. In particular, Ia-type fibers are activated during contractile fatigue, while Ib-type fibers are silenced during muscle fatigue of the diaphragm [31].…”

Neuromodulation as a new avenue for resuscitation in hemorrhagic shock

“…Severe trauma and hemorrhage can be rapidly lethal; more than 50% of trauma-related deaths due to HS occur during the prehospital period, and 93% of post-hospitalization deaths occur within the first 24 h period (Alam 2017;Holcomb et al 2013). The current standard of care for HS consists of timely hemostasis, volume replacement, and whole blood or blood component therapy (Gann and Drucker 2013;Jacob and Kumar 2014;Nair et al 2017). In the last 10 years, advances in hemorrhage pathogenesis and treatment have led to only modest improvements in survival, with fundamental issues such as timing and volume of fluid intervention remaining controversial (Kwan et al 2003).…”

Spinal decision making for respiratory muscle recruitment?

“…), and respiratory muscle afferents can modulate propriospinal neuron discharge (reviewed in Nair et al . ). Thus, a neuroanatomical substrate is in place by which spinal networks, in conjunction with the size principle and descending synaptic inputs, could shape respiratory motor unit recruitment to the mechanical needs of a given task.…”

“…How this happens is uncertain, but activation of a spinal network, which in turn activates respiratory motor units at 'normal' physiological rates, is plausible. Propriospinal neurons are synaptically coupled to IC motoneurons (Kirkwood et al 1993), and respiratory muscle afferents can modulate propriospinal neuron discharge (reviewed in Nair et al 2017). Thus, a neuroanatomical substrate is in place by which spinal networks, in conjunction with the size principle and descending synaptic inputs, could shape respiratory motor unit recruitment to the mechanical needs of a given task.…”