The relation between tension and axonal conduction velocity for motor units in the medial gastrocnemius muscle of the cat

We tested whether the muscle innervated may influence the expression of motoneuron electrical properties. Properties of individual motor units were examined following cross-reinnervation (X-reinnervation) of cat lateral gastrocnemius (LG) and soleus muscles by the medial gastrocnemius (MG) nerve. We examined animals at two postoperative times: 9-10 wk (medX) and 9-11 mo (longX). For comparison, normal LG and soleus motoneuron properties were also studied. Motor units were classified on the basis of their contractile responses as fast contracting fatigable, fast intermediate fast contracting fatigue resistant, and slow types FF, FI, FR, or S, respectively) (9, 21). Motoneuron electrical properties (rheobase, input resistance, axonal conduction velocity, afterhyperpolarization) were measured. After 9-11 mo, MG motoneurons that innervated LG muscle showed recovery of electrical properties similar to self-regenerated MG motoneurons. The relationships between motoneuron electrical properties were largely similar to self-regenerated MG. For MG motoneurons that innervated LG, motoneuron type (65) predicted motor-unit type in 74% of cases. LongX-soleus motoneurons differed from longX-LG motoneurons or self-regenerated MG motoneurons in mean values for motoneuron electrical properties. The differences in overall means reflected the predominance of type S motor units. The relationships between motoneuron electrical properties were also different than in self-regenerated MG motoneurons. In all cases, the alterations were in the direction of properties of type S units, and the relationship between normal soleus motoneurons and their muscle units. Within motor-unit types, the mean values were typical for that type in self-regenerated MG. Motoneuron type (65) was a fairly strong predictor of motor-unit type in longX soleus. MG motoneurons that innervated soleus displayed altered values for axonal conduction velocity, rheobase, and input resistance, which could indicate incomplete recovery from the axotomized state. However, although mean afterhyperpolarization (AHP) half-decay time was unaltered by axotomy (25), this parameter was significantly lengthened in MG motoneurons that innervated soleus muscle. There were, however, individual motoneuron-muscle-unit mismatches, which suggested that longer mean AHP half-decay time may also be due to incomplete recovery of a subpopulation of motoneurons. Those MG motoneurons able to specify soleus muscle-fiber type exhibited motoneuron electrical properties typical of that same motoneuron type in self-regenerated MG.(ABSTRACT TRUNCATED AT 400 WORDS)

Download Full-text

Activity and motor unit size in partially denervated rat medial gastrocnemius

Journal of Applied Physiology ◽

10.1152/jappl.1994.76.6.2663 ◽

1994 ◽

Vol 76 (6) ◽

pp. 2663-2671 ◽

Cited By ~ 29

Author(s):

L. J. Einsiedel ◽

A. R. Luff

Keyword(s):

Motor Unit ◽

Gastrocnemius Muscle ◽

Motor Units ◽

Ventral Root ◽

Treadmill Walking ◽

Medial Gastrocnemius ◽

Unit Size ◽

Partial Denervation ◽

Motoneuron Activity ◽

Medial Gastrocnemius Muscle

The aim of the study was to determine whether increased motoneuron activity induced by treadmill walking would alter the extent of motoneuron sprouting in the partially denervated rat medial gastrocnemius muscle. An extensive partial denervation was effected by unilateral section of the L5 ventral root, and it is very likely that all units remaining in the medial gastrocnemius were used in treadmill walking. Rats were trained for 1.5 h/day and after 14 days were walking at least 1 km/day. Motor unit characteristics were determined 24 days after the partial denervation and were compared with units from partially denervated control (PDC) animals and with units from normal (control) animals. In PDC rats, force developed by slow, fast fatigue-resistant, and fast intermediate-fatigable motor units increased substantially compared with control animals; that of fast-fatigable units did not increase. In partially denervated exercised animals, force developed by slow and fast-fatigue-resistant units showed no further increase, but fast-intermediate- and fast-fatigable units showed significant increases compared with those in PDC animals. The changes in force were closely paralleled by changes in innervation ratios. We concluded that neuronal activity is an important factor in determining the rate of motoneuron sprouting.

Download Full-text

Fast-to-Slow Conversion Following Chronic Low-Frequency Activation of Medial Gastrocnemius Muscle in Cats. II. Motoneuron Properties

Journal of Neurophysiology ◽

10.1152/jn.1997.77.5.2605 ◽

1997 ◽

Vol 77 (5) ◽

pp. 2605-2615 ◽

Cited By ~ 48

Author(s):

John B. Munson ◽

Robert C. Foehring ◽

Lorne M. Mendell ◽

Tessa Gordon

Keyword(s):

Gastrocnemius Muscle ◽

Low Frequency ◽

Motor Units ◽

Input Resistance ◽

Medial Gastrocnemius ◽

Excitatory Postsynaptic Potential ◽

Chronic Stimulation ◽

General Hypothesis ◽

Slow Type ◽

Medial Gastrocnemius Muscle

Munson, John B., Robert C. Foehring, Lorne M. Mendell, and Tessa Gordon. Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. II. Motoneuron properties. J. Neurophysiol. 77: 2605–2615, 1997. Chronic stimulation (for 2–3 mo) of the medial gastrocnemius (MG) muscle nerve by indwelling electrodes renders the normally heterogeneous MG muscle mechanically and histochemically slow (type SO). We tested the hypothesis that motoneurons of MG muscle thus made type SO by chronic stimulation would also convert to slow phenotype. Properties of all single muscle units became homogeneously type SO (slowly contracting, nonfatiguing, nonsagging contraction during tetanic activation). Motoneuron electrical properties were also modified in the direction of type S, fatigue-resistant motor units. Two separate populations were identified (on the basis of afterhyperpolarization, rheobase, and input resistance) that likely correspond to motoneurons that had been fast (type F) or type S before stimulation. Type F motoneurons, although modified by chronic stimulation, were not converted to the type S phenotype, despite apparent complete conversion of their muscle units to the slow oxidative type (type SO). Muscle units of the former type F motor units were faster and/or more powerful than those of the former type S motor units, indicating some intrinsic regulation of motor unit properties. Experiments in which chronic stimulation was applied to the MG nerve cross-regenerated into skin yielded changes in motoneuron properties similar to those above, suggesting that muscle was not essential for the effects observed. Modulation of group Ia excitatory postsynaptic potential (EPSP) amplitude during high-frequency trains, which in normal MG motoneurons can be either positive or negative, was negative in 48 of 49 chronically stimulated motoneurons. Negative modulation is characteristic of EPSPs in motoneurons of most fatigue-resistant motor units. The general hypothesis of a periphery-to-motoneuron retrograde mechanism was supported, although the degree of control exerted by the periphery may vary: natural type SO muscle appears especially competent to modify motoneuron properties. We speculate that activity-dependent regulation of the neurotrophin-(NT) 4/5 in muscle plays an important role in controlling muscle and motoneuron properties.

Download Full-text