Electrical properties of phrenic motoneurons in the cat: correlation with inspiratory drive

Abstract

1. Resting membrane potential (Vmp), input resistance (Rn), rheobase (Irh), and after hyperpolarization duration (AHPdur) and amplitude (AHPamp) were measured in 38 phrenic motoneurons of anesthetized, paralyzed, and artificially ventilated cats during hypocapnic apnea. The mean +/- SD and range of values for these variables were as follows: Vmp, -68 +/- 5mV (range: -60 to -82); Rn, 1.3 +/- 0.6 M omega (0.6-2.4); Irh, 9.7 +/- 5 nA (2-20); AHPdur, 68 +/- 19 ms (37-134); AHPamp, 3.3 +/- 1.8 mV (1.0-8.5). In 31 motoneurons, the membrane potential level at which firing occurred (Vthr) during intracellular current injection was measured. The mean value of Vthr was -58 +/- 3 mV (range: -52 to -64). 2. A histogram of Rn revealed a bimodal distribution. Also a plot of Irh against Rn showed a grouping of the motoneurons into two subpopulations: 1) low-Rn and high-Irh cells, called type L neurons, and 2) high-Rn, low-Irh cells, called type H neurons. The overall negative linear correlation between Irh and Rn (r = -0.85; P less than 0.0001) resulted from this grouping rather than from a strictly linear relation between these two variables. 3. Electrical properties were compared for type L (n = 20) and type H (n = 18) phrenic motoneurons. The following mean values were found for each group, respectively: Rn, 0.8 and 1.8 M omega; Irh, 13.7 and 5.3 nA; AHPdur, 58 and 79 ms; AHPamp, 2.4 and 4.4 mV. All differences were significant (t test, P less than 0.001). Mean Vthr was the same for the two groups. 4. Comparison of these data with those available for lumbosacral motoneurons revealed that almost all investigated electrical properties of type L and type H phrenic motoneurons are similar to the analogous properties of type F (fast twitch) and type S (slow twitch) lumbosacral motoneurons, respectively. The apparent exception is the lower mean value of Irh for type L phrenic motoneurons compared with type F lumbosacral motoneurons. 5. For 13 cells, membrane potential was continuously monitored while spontaneous respiratory activity was restored by increasing CO2. It was found that at approximately the same end-tidal CO2 (about 7%) and a similar end-expiratory mean membrane potential level (approximately -70 mV), mean amplitude of peak inspiratory synaptic depolarization was higher in type H motoneurons (8.8 mV, n = 5) than in type L (2.9 mV, n = 8; P less than 0.001).(ABSTRACT TRUNCATED AT 400 WORDS)

Repetitive firing properties of developing rat brainstem motoneurones.

“…Since resting membrane potential did not vary significantly during development, this finding indicates that RN is a significant factor in determining motoneuronal repetitive firing threshold. This is also the case in adult cat spinal motoneurones (Kernell, 1966;Jodkowski, Viana, Dick & Berger, 1987) and is consistent with the higher excitability in the group of higher resitance, slower conducting type S spinal motoneurones (Henneman et al 1965;Kernell & Monster, 1981;Gustafsson &Pinter, 1984 (Kernell, 1965b;Schwindt, 1973), was not observed at any age in HMs. This agrees with the study of genioglossus motoneurones by Nuniez-Abades et al (1993).…”

The membrane properties and firing characteristics of rat jaw‐elevator motoneurones.

“…Systematic studies of the properties of other groups of motoneurones have been relatively rare (abducens: Grantyn & Grantyn, 1978; phrenic: Jodkowski, Viana, Dick & Berger, 1987neck and shoulder; Rose & Vanner, 1988) and so it has been difficult to ascertain the general significance of particular features of organization described for lumbosaeral motoneurones. Therefore the aims of this study were first to determine the electrical properties of a population of elevator motoneurones and then to compare these to those of hindlimb motoneurones.…”

Key aspects of phrenic motoneuron and diaphragm muscle development during the perinatal period

“…Motor unit composition is critically important in determining overall functional capacity of the DIAm in accomplishing different motor behaviors since the forces generated by a muscle during any motor behavior result from an orderly recruitment of motor units. Consistent with the "size principle," motor units comprising smaller motoneurons, smaller axons, and thus slower axonal conduction velocities are recruited before units comprising larger motoneurons with faster axonal conduction velocities (23,44,48). Thus motor unit recruitment results in activation of muscle fibers in agreement with their mechanical properties and fatigue resistance.…”