Studies on the phrenic nerve and diaphragmatic plexus in rabbits and gerbils

Abstract

Studies on nerve fibres within the phrenic nerve and in the diaphragmatic plexus were carried out in one species, the gerbil, a small desert rodent. The resistance of this small mammal to surgical insult enabled experimental intrathoracic transection of the phrenic nerve to be carried out so that the results in 10 animals could be observed within the diaphragmatic plexus, the phrenic nerve, and the phrenic nucleus. The findings in the diaphragmatic plexus and in the nerve trunk strongly resemble corresponding results for other animals with regard to 'persisting' fibres. The phrenic nucleus, however, is much more extensive in this mammal than has been found in any other species; it is represented within the second to sixth cervical segment, its rostral extremity occupying the intermediolateral part of the spinal grey matter. The possible functional significance of these findings is discussed in the light of the mixed functions of the diaphragm, either voluntary or involuntary.An extensive search of the literature reveals that almost all of the relevant studies during the past century have considered only small circumscribed aspects of diaphragmatic nerve supply. Thus, while Kohnstamm (1898), Hirako (1928), Keswani, Groat, and Hollinshead (1954) and Samuel and Warwick (1955) gave their full attention to the 'phrenic nucleus', Sherrington (1894), Kidd (1911), and Botha (1957) placed more emphasis on the peripheral features of diaphragmatic innervation. Although limited conclusions can be drawn from these records, species differences in the various studies preclude direct comparisons, thus diminishing the collective significance of the results.These criticisms have been forestalled in this investigation by applying reliable methods used in cats and rodents (Wilson, 1968) to one species, the gerbil. The principal aim here is to obtain a more comprehensive overview of the phrenic nerve-diaphragm apparatus in gerbils by including spinal cord connexions, fibre spectrum of phrenic nerve. and structural features of the diaphragmatic plexus. MATERIAL AND METHODS INVESTIGATIONS IN NORMAL ANIMALSTOPOGRAPHICAL STUDIES Three adult gerbils, killed with a lethal dose of intraperitoneal sodium pentobarbitone, were perfused with formol saline and immersed in formalin 5 % for several days. The chest cage was opened widely and the phrenic nerve on each side was traced centripetally from its conspicuous diaphragmatic extremity to its source. In each case the level of origin in the cervical spinal segments was determined by reference to the bodies of adjacent cervical vertebrae.HISTOLOGICAL STUDIES In three gerbils and three rabbits (all adults), selected at random without reference to sex or feeding habits, anaesthesia was induced by intraperitoneal injection of sodium pentobarbitone, 48 mg/kg, and each animal was perfused through the abdominal aorta with formol saline 10% until the return flow through the inferior vena cava was clear. This method of fixation was standard for all histological studies which, from this point onwards, v...