Adaptive transformation of rat soleus motor units during growth: Histochemistry and contraction speed

doi:10.1016/0022-510X(76)90001-0

Journal of the Neurological Sciences

Volume 27, Issue 3, March 1976, Pages 269-289

https://doi.org/10.1016/0022-510X(76)90001-0 Get rights and content

Abstract

Soleus motor units of 5 and 34 week-old rats, weighing 150 and 520 g respectively, were examined for contraction time at 36°C, innervation ratio as well as cross-sectional area, ATPase and SDH activity of fibres. The muscle was continuously adjusted to the growth of the animal. Between 5 and 34 weeks the proportion of Type II fibre units with 15–26 msec contraction time decreased from 33 to 10% and Type I fibre units with 27–40 msec contraction time thus increased from 67 to 90%. This was parallelled by approximately the same relative changes in number of Type II and Type I fibres.

The average innervation ratio of Type I fibre units appeared unchanged between 5 and 34 weeks, 83 ± 16, resp. 83 ± 17 per unit (N = 18) and about 99% of the fibres of the units were histochemically uniform. Moreover, at 5 weeks there occurred continuous transitions between Type II and Type I fibre units as regards all tested markers including contraction time, intensity of alkali-, formaldehydeand acid-stable ATPase, SDH activity and fibre area.

It is concluded that the increase in Type I fibre units with growth results from transformation in toto of Type II fibre units to Type I owing to changing properties of their motoneurones from phasic to tonic. Thus one and the same motoneurone determines not only the histochemical and related functional characteristics of its muscle fibres at a specific level but may also alter them qualitatively.

With increasing contraction time of units transformation of fibres was manifested first by an increase in intensity of acid-stable ATPase, followed by a decrease in formaldehyde-stable and later by a decrease in alkali-stable ATPase. This led to the following sequence of relative intensities of ATPase at pH 9.4 and 4.35, respectively: dark-light (Type II A) → dark intermediate (Type II C) → (dark-dark) → intermediate-dark → light-dark (Type I). Fibres under transformation (ATPase intermediate fibres) were retarded in growth and showed within the unit variations in intensity of ATPase reactions which to some extent were topographically orientated.

Transformation from Type II to Type I was complemented at a slower rate by a decrease in SDH activity, which is opposite to that in larger mammals indicating that myofibrillar ATPase and mitochondria-bound oxidative enzymes are independently controlled by the motoneurone.

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^☆: This study was supported by a grant from the Swedish Medical Research Council.

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Adaptive transformation of rat soleus motor units during growth: Histochemistry and contraction speed☆

Abstract

Exp. Neurol.

Exp. Neurol.

J. neurol. Sci.

Exp. Neurol.

J. neurol. Sci.

Exp. Neurol.

Motor units of the first superficial lumbrical muscle of the cat

J. Neurophysiol.

Polyneuronal innervation of kitten skeletal muscle

J. Physiol. (Lond.)

The properties of motor units in a fast and a slow twitch muscle during post-natal development in the kitten

J. Physiol. (Lond.)

Effect of hind-limb immobilization on contractile and histochemical properties of skeletal muscle

Pflügers Arch. ges. Physiol.

Some comments on the histochemical characterization of muscle adenosine triphosphatase

J. Histochem. Cytochem.

Muscle fiber types — How many and what kind?

Arch. Neurol. (Chic.)

The behaviour of four fiber types in developing and reinnervated muscle

Arch. Neurol. (Chic.)

Differentiation of fast and slow muscles in the cat hind limb

J. Physiol (Lond.)

Physiological types and histochemical profiles in motor units of the cat gastrocnemius

J. Physiol. (Lond.)

A recording microplanimeter

Acta morph. neerl.-scand.

Dynamic properties of fast and slow skeletal muscles of the rat during development

J. Physiol. (Lond.)

Properties of motor units in fast and slow skeletal muscles of the rat

J. Physiol. (Lond.)

Postnatal changes in the histochemical fibre types of porcine skeletal muscle

J. Anat. (Lond.)

On the nature of postural reflexes

The action potentials of the alpha motoneurones supplying fast and slow muscles

J. Physiol. (Lond.)