E. Gutmann

Fast and Slow Motor Units in Ageing

The differences in onset and degree of old age changes in different muscles are explained by the differentiation and different reactivity of fast and slow motor units with respect to physiological, structural and biochemical characteristics. The main changes in the motor units in old age are described. The general basic change is a progressive random disturbance of neuromuscular contact ascribed to a decrease of the trophic function of the neuron. The main motor disturbances in old age, i.e; slowness, decrease of muscle strength and lack of fine coordination are explained in terms of physiological changes in senescent motor units. The reactions of senescent motor units differ from one unit to another as shown, e.g. in denervation, reinnervation and regeneration (transplantation) studies. The trend to a shift from a heterogeneous to a more uniform muscle fibre pattern and the defficiencies in recovery of the original muscle fibre pattern in reinnervation and regeneration of senescent muscle is demonstrated and explained by a decrease of the differentiating capacity of different motor units. The changes in the heterogeneous fibre pattern of skeletal and the homogeneous fibre pattern of the papillary heart muscle in old age are contrasted.

Contraction properties and ATPase activity in fast and slow muscle of the rat during denervation

A different reaction to denervation is observed in contraction properties or the fast extensor digitorum longus (EDL) and the slow soleus muscle in 1- and 6-month-old rats. Changes in contraction properties were observed from 12 hr until 75 days after unilateral or bilateral section of the sciatic nerve. The denervated EDL muscle shows a prolongation of contraction time in young and adult rats. The slow soleus muscle of young rats shows a prolongation of contraction time until 7 days after denervation followed by a shortening, whereas the soleus muscle of adult rats shows a shortening of contraction time beginning 2–3 days after nerve section. Bilateral denervation in young and adult animals confirms the trend of results of unilateral nerve section, i.e., prolongation of contraction time in the fast muscle and shortening in the soleus muscle. Compensatory changes in the “control” muscle are thus of minor importance. Ca2+-activated myosin ATPase activity of the EDL muscle of adult rats decreases until 3–7 days after nerve section, but changes very little in the soleus muscle. It is stressed that in denervation studies, animal species, age of the animal, and time of denervation have to be considered.

Differentiation of myosin in soleus and extensor digitorum longus muscle in different animal species during development

SummaryCa2+-ATPase activity and light chains of myosin prepared from fast and slow muscles of rat, guinea-pig and rabbit were studied during development from embryonic to old age to establish further correlation with the well-known developmental changes in contraction properties of these muscles. The changes involve the slow soleus muscle much more than the fast extensor digitorum longus muscle. Myosin-ATPase activity of the soleus muscle before or at birth is higher than in the muscle of adult animals. Myosin from the soleus muscle of embryos or newborn animals reveales light chains of myosin of both fast and slow type (with a preponderance of light chains of fast type in 26-days-old rabbit embryos). During postnatal development the amount of light chains of the fast type decreases, that of the slow type increases. Myosin from the soleus muscle of adult animals contains only light chains of the slow type. However, myosin from the soleus muscle of 30-months-old rats exhibits high myosin ATPase activity and contains light chains of myosin of both slow and fast type as in perinatal development. This is in agreement with the shortening of contraction time observed in this muscle in very old age. Thus developmental differentiation of myosin in the soleus muscle is followed by a trend of levelling out of the differences between fast and slow muscles of senescent animals. No such “biphasic” development is observed with respect to the fast extensor digitorum longus muscle.

Contractile and Histochemical Properties of Regenerating Cross-Transplanted Fast and Slow Muscles in the Rat*

SummaryThe soleus (SOL) or extensor digitorum longus (EDL) muscles of month-old rats were denervated for 14 days and then cross-transplanted so that the fast muscle was placed into the bed of the slow muscle and vice versa. At 17, 30, 60, and 90 days the transplants were tested for certain contractile and histochemical properties. By 90 days the cross-transplanted SOL showed complete conversion of the full contraction time and nearly complete conversion of the half relaxation time to those of the normal EDL. In contrast, the contraction and relaxation times of the cross-transplanted EDL became considerably slowed, but did not attain the values of the normal SOL. Histochemical staining for ATPase and SDH activity demonstrated similar transformations of fiber types. The degree of transformation of twitch and histochemical characteristics in cross-transplanted muscles was greater than the values reported after cross-innervation of the same muscles. The cross-transplantation model has certain advantages over nerve cross-union experiments because the cross-transplanted muscle is placed in the normal functional environment of the other muscle.

Stimulation and immobilization effects on contractile and histochemical properties of denervated muscle

Summary1.Chronic stimulation of the denervated tibialis anterior muscle of 6-month-old rats through implanted electrodes over a period of 3 days results in a decrease of wet weight loss and shortening of contraction time, maximal rate of isometric tension development and relaxation time. This effect is, however, observed only if stimulation is started immediately after denervation.2.Hind limb immobilization (carried out in 3-month-old rats) over periods of 7 or 14 days results in prolongation of contraction time of the denervated extensor digitorum longus (EDL) muscle when performed in the extended limb position, but shortening of contraction time when performed in the flexed position.3.Immobilization in extension (stretching) results histochemically in a decrease of myosin ATPase activity and an increase of oxidative SDH enzyme activity. Atrophy of EDL muscle fibres is more marked after immobilization in the flexed position of the limb.4.Hind limb immobilization over periods of 7 or 14 days does not result in significant changes in contractile properties in the “one joint” soleus muscle, whatever the position of immobilization.5.When evaluating the effects of stimulation and immobilization of denervated muscles, the time of onset of treatment, type of muscle and type of immobilization have to be considered.

Testosterone-induced changes of choline acetyl-transferase and cholinesterase activities in rat levator ani muscle

One year after castration the activities of choline acetyltransferase (ChAc) and of cholinesterase (ChE) in the levator ani (LA) muscle of male rats were lowered by 42 and 79% respectively. The weight of the muscle corresponded to 15% of control values. These changes were not accompanied by a decrease in the number of the muscle fibres. Treatment with testosterone rapidly increased the activity of ChAc and the weight of the muscle near to control values; the restoration of ChE was less complete. Testosterone produced an increase in the size of the muscle fibres and increased the histochemically observed activity of ChE in the postsynaptic part of the motor end-plates. In non-castrated rats the administration of testosterone increased the weight ofthe LA muscle, but was not accompanied by an increase of ChAc above control values.

Changes in the neuromuscular system during old age

Abstract Evidence is cited, that the decline of efficiency of motor functions during ageing is due to reduction of number of muscle fibres as a result of random degeneration of some motor end-plates, decrease of the size of the motor unit, increased degradation of sarcoplasmatic proteins of muscle, relative increase of sarcoplasmatic and decrease of contractile proteins of muscle, reduction of ChE activity in axons above site of nerve crush suggesting reduced proximo-distal axonal transport, deficiencies of the regeneration process after nerve interruption, disuse atrophy with a neuronal and a hormonal component, the latter suggested by the analogy of endplate changes during old age and after castration and finally decreased rate of incorporation of aminoacids into the neuronal proteins. Decrease of substrate supply rather than of turnover rate which remains unaltered (at least for the neuronal proteins with a slow turnover) appears to be responsible for the disturbance of protein metabolism of the spinal motoneurones. The mechanism especially responsible for the disorder of the neuro-muscular system in old age is thought to be the impairment of intercellular neurotrophic relations.

Changes in speed of contraction and ATPase activity in striated muscle during old age

Abstract Contraction time of fast and slow muscles is rather constant in rat up to 2 years after birth but increases in both muscles between the second and third year of life. This corresponds to a decrease of myofibrillar and myosin ATPase activity observed in muscles of 3-year-old rats.

Differential response of myosin--ATPase activity and contraction properties of fast and slow rabbit muscles following denervation.

Nachweis unterschiedlicher Kontraktionsgeschwindigkeiten bei schnellen und langsamen Kaninchenmuskeln sowie entsprechende Unterschiede in der Myosin-ATPase-Aktivität und im Spektrum niedermolekularer Proteine nach Denervation werden gezeigt.

Regeneration in Spinal Neurons: Proteosynthesis Following Nerve Growth Factor Administration

Incorporation of H3-leucine into dorsal root ganglion cells in rats was markedly increased over that of controls following section of sciatic and femoral nerves. Crush lesion of dorsal roots did not increase the H3-leucine uptake of these cells except in animals which had received nerve growth factor after the operation.