DEVELOPMENT OF PATTERNS OF ACTIVITY IN DIAPHRAGM OF FETAL LAMB EARLY IN GESTATION

To examine the development of respiratory motor activity early in mamm alian development and its relationship to nonrespiratory activity, we recorded spontaneous electromyogram activity from chronically instrume nted fetal lambs over the period from 45 to 65 days' gestation (G45 to G65, term = G147), Two distinct forms of motor behavior were observed at G45 in recordings made from the costal diaphragm and longissimus d orsi muscles, The predominant behavior consisted of cycles of sustaine d, coincident activity in the two muscles alternating with periods of inactivity, The incidence of this type of activity declined between G4 5 and G65 and the cyclic nature of the discharges disappeared in most animals, The second form of motor behavior at G45 consisted of episode s of repetitive bursting activity lasting up to 20 min that were confi ned to the diaphragm, These bursts had a duration of 97.5 +/- 8.3 ms ( mean +/- S.E.M.) and frequently occurred as doublets in which two burs ts were separated by an intervening period of 100-200 ms. The mean dur ation of these bursts declined to 69.7 +/- 7.7 ms at G65, doublets bec ame rare, and bursts evolved a stereotyped form by G65 that was charac terized by an abrupt onset and rapid decline in discharge intensity, R epetitive bursts of this form evolve into the mature respiratory motor pattern over the second half of gestation, At G45, episodes of repeti tive bursting were almost always linked with episodes of sustained dis charge, while at G65 these two forms of behavior were always segregate d, We conclude that the neurons responsible for generating the respira tory rhythm in the lamb are assembled into a functional rhythm generat or and make appropriate connections to motor output pathways as early as G45, The generation of the respiratory rhythm at G45 appears to be triggered by episodes of widespread motor activity that occur in both respiratory and nonrespiratory muscles. (C) 1996 John Wiley & Sons, In c.