PHYSIOLOGICAL-PROPERTIES OF PRIMATE LUMBAR MOTONEURONS

1. Intracellular recordings were obtained from 149 motoneurons innervating triceps surae (n = 109) and more distal muscles (n = 4b) in 14 pentobarbital-anesthetized monkeys (Macaca nemestrina). The variables evaluated were resting membrane potential, action potential amplitude, conduction velocity (CV), input resistance (R(N)), membrane time constant (tau(m)), electrotonic length (L), whole-cell capacitance (C(tot)), long current pulse threshold (rheobase), short current pulse threshold (I(short) after-hyperpolarization ( AHP) maximum amplitude (AHP(max)), AHP duration (AHP(dur)), time to half maximum AHP amplitude (AHP(t1/2)), depolarization from resting potential to elicit action potential (V(dep)), and threshold voltage for action potential discharge (V(thr)). 2. Mean values +/- SD for the entire sample of motoneurons are as follows: resting membrane potential -67 +/- 6 mV; action potential amplitude 75 +/- 7 mV; CV 71 +/- 6 m/s; RN 1.0 +/- 0.5 OMEGA; tau(m) 4.4 +/- 1.5 ms; L 1.4 +/- 0.2 lambda; C(tot) 7.1 +/- 1.8 nF; rheobase 13 +/- 7 nA; I(short) 29 +/- 14 nA; AHP(max) 3.5 +/- 1.3 mV; AHP(dur) 77 +/- 26 ms; AHP(t1/2) 21 7 ms; V(dep) 11 +/- 4 mV; and V(thr) -56 +/- 5 mV. CV is lower in soleus than in either medial or lateral gastrocnemius motoneurons, and R(N) is lower and tau(m) is longer in soleus than in lateral gastrocnemius motoneurons. 3. RN is higher in motoneurons with longer tau(m) and slower CV. A linear relationship exists between log(CV) and log(1/R(N)) with a slope of 1.8-2.2 (depending on the action potential amplitude acceptance criteria used), suggesting that membrane resistivity (R(N)) does not vary systematically with cell size. 4. Rheobase is higher in motoneurons with lower R(N), longer tau(m), shorter AHP time course, and higher CV. I(short) and normalized rheobase (i.e., rheobase/C(tot)) vary similarly with these motoneuron properties, except that I(short) is independent of tau(m) and normalized rheobase is independent of CV. 5. V(thr) tends to be more depolarized in motoneurons with large C(tot), but the relationship is sufficiently weak so that any systematic variation in V(thr) according to cell size probably contributes only minimally to recruitment order. V(thr) does not vary systematically with CV, AHP time course, R(N), tau(m). 6. Quantitative differences between macaque and cat triceps surae motoneurons are apparent in CV, which is slower in macaque than in cat, and to a lesser extent in tau(m) and R(N), which are lower in macaque than in cat. These data, along with available anatomic data, suggest that R(m) may be lower in macaque motoneurons than in cat motoneurons. 7. Comparison of the present primate data with those from cat suggests that intraspecies relationships between motoneuron properties are qualitatively similar. However, excitability of macaque motoneurons appears to vary systematically with motoneuron properties related to both R(m) and cell size.