Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Abstract

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (P ab ) and intra-thoracic (P th ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effectively generation of both a negative P th for ventilation of the lungs and a positive P ab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. Didactic SynopsisThe DIAm separates abdominal and thoracic cavities; thus, it is a partition, and its evolution reflects that important role in isolating organs into separate thoracic and abdominal cavities. However, the DIAm is also a muscle, and is most often described as the principal pump muscle of inspiration. However, the DIAm also serves as a pump for generating both negative P th and positive P ab in other motor behaviors. Accordingly, the evolution of the DIAm is more complex and should be considered in the context of its dual physiological roles as a partition and muscular pump. In considering DIAm evolution, we adopt the guiding concept of symmorphosis or economy of design, where biological structures are not over designed for their functional roles. Thus, this is a tale of the evolution of two diaphragms, a partition and a muscular pump that separates thoracic and abdominal cavities but also affects generation of P th and P ab .

Size‐Dependent Vulnerability of Lumbar Motor Neuron Dendritic Degeneration in SOD1G93A Mice

“…Furthermore, in mouse lumbar MNs, the smaller MNs tend to have less elaborate dendritic arbors than larger MNs, which have more extensive dendritic arborization (Ma and Vacca‐Galloway, ; Leroy et al, ). The increased dendritic arbor of larger MNs seen in past studies and in the present study, and the consequent increase in relative capacitance of large soma surface area MNs to smaller soma surface area MNs is entirely consistent with the Henneman's size principle (Henneman, ; Henneman et al, ; Somjen et al, ; Cullheim and Ulfhake, ; Cullheim et al, ; Heckman and Enoka, ; Fogarty and Sieck, ).…”

“…Importantly, the dendritic length and membrane surface area is larger in F compared to S type motor units (Cullheim et al, ; Leroy et al, ). The capacitance of MNs is directly related to their orderly recruitment, where smaller surface area MNs of low capacitance require less unitary synaptic input to shift the membrane potential to discharge threshold, compared to larger MNs—the size principle (Henneman, ; Henneman et al, ; Somjen et al, ; Sieck and Fournier, ; Heckman and Enoka, ; Fogarty and Sieck, ).…”

Impact of sarcopenia on diaphragm muscle fatigue

“…In a recent study, we reported that in old Fischer 344 rats there is marked (∼25%) phrenic motor neuron (MN) loss compared with young rats (Fogarty, Omar, Zhan, Mantilla, & Sieck, ). This loss affects the larger phrenic MNs disproportionately (Fogarty et al., ), which are likely to innervate IIx and/or IIb fibres (Fogarty & Sieck, ; Sieck, ; Sieck & Fournier, ) most affected by sarcopenia. Thus, it is likely that there is a denervation of DIAm fibres in sarcopenic rats.…”

“…Obviously, in old age, resilience of the force‐generating capacity of the DIAm to accomplish ventilatory behaviours ranging from eupnoea to airway occlusion without fatigue is essential to sustain life (Belman & Sieck, ; Fogarty & Sieck, ; Mantilla & Sieck, ; Sieck, ; Sieck & Fournier, ). Previously, we reported a model for DIAm motor unit recruitment to accomplish P di generation across a range of motor behaviours (Fogarty, Mantilla, & Sieck, ; Mantilla & Sieck, ; Sieck, ; Sieck & Fournier, ).…”

The Fascial Breath

“…The diaphragm muscle falls within the fascial continuum. Breath is part of a concept of symmetry, that is, maximum capacity adaptive to multiple functional questions in a defined biological context [25]. The act of breathing determines and defines our holobiont: how we react and who we are.…”