Functional and structural changes of rat plantaris motoneurons following compensatory hypertrophy of the muscle

Abstract

The conduction velocity and histological structure of motoneurons innervating normal and hypertrophied rat plantaris muscles were investigated. Hypertrophy was produced by ablation of synergist muscles. Single motor units were obtained by ventral root dissection and conduction velocities measured. The structure of neurons was investigated following retrograde labeling with horseradish peroxidase. A combined silver, gold and cholinesterase staining method was developed to study the motor endplate. In addition, the peripheral nerve was fixed, embedded in Araldite, and sectioned for determination of axonal size and myelin thickness. Conduction velocity of motor axons decreased following hypertrophy of the skeletal muscle (control CV = 75.8 +/- 8.9 m s-1, n = 94, hypertrophy CV = 69.0 +/- 12.3 m s-1, n = 84). However, no alteration in the size of motor axons or myelin thickness could account for this alteration in conduction velocity. Mean motoneuronal soma size decreased following muscle hypertrophy (soma diameter: control 36.1 +/- 4.6 microns, n = 283, hypertrophy 32.9 +/- 4.5 microns, n = 294). The complexity of the motor endplate increased following hypertrophy with an increased occurrence of nodal sprouts. In addition, the area of cholinesterase staining increased following hypertrophy (control 588.1 +/- 297.2 microns 2, n = 269, hypertrophy 857.7 +/- 357.0 microns 2, n = 269). This study found that both the morphological and physiological parameters of motoneurons innervating a hypertrophied muscle were shifted toward those of normal rat slow motor units.

Endurance training alters the biophysical properties of hindlimb motoneurons in rats

“…Adaptations on the part of the fast motoneurons were accompanied by an increased mean cell capacitance, suggesting an increase in soma volume. Previous investigations on motoneuron adaptations to increased activity have found either no change34, 43, 44 or a decrease12 in histologically measured cell size. Nonetheless, in the present study, mean input resistance and rheobase were not influenced by activity, in spite of apparent changes in cell capacitance (Table 1 and Fig.…”

Adaptations of motoneuron properties to chronic compensatory muscle overload

“…Previous studies evaluating motoneuron size in response to compensatory overload have provided contradictory results. Finkelstein et al (1991) indicated decreased motoneuronal soma size following muscle hypertrophy in plantaris motoneurons, but in research of Roy et al (1999) the mean motoneuron soma area was not affected after functional overload of the same muscle.…”

Characteristics of compensatory hypertrophied muscle in the rat: II. Comparison of histochemical and functional properties

“…The shift to slower muscle characteristics in the hypertrophied PLA muscle has also been reported by other investigators. Thus, a slower velocity of shortening (Binkhorst and Van'tHof, 1973;, reduction of conduction velocity of motoneuron with reduced cell body size (Finkelstein et al, 1991), reduction of biochemically measured myosin Noble et al, 1983) and myofibrillar ATPase activity (Baldwin et al, 1977Tsika et al, 1987) have been described. In present study, the twitch and tetanus of the OP and CON-L muscles shifted to a slow muscle pattern in a manner similar to these reports ( Figs.…”