The specificity of synapses in the mammalian brain cannot possibly be accounted for by one-to-one biochemical matching. The alternative proposed in this article is that connections are genetically specified between classes of cells, but the final wiring pattern depends on the refinement of those collections by selective stabilisation during neuronal activity.

Selective stabilisation of developing synapses as a mechanism for the specification of neuronal networks.

The sections in this article are:


1
Motor Unit
1.1
Fibers per Motor Unit

1.2
Contractile Properties

1.3
Biochemical Basis for Differences in Twitch Properties

1.4
Histochemical Differentiation of Muscle Fibers

1.5
Ultrastructural Basis for Skeletal Muscle Fiber Typing

1.6
Maximal Contractile Force

1.7
Speed of Contraction

1.8
Fatigue Characteristics

1.9
Metabolic Characteristics

1.10
Ionic Composition of Skeletal Muscle

1.11
Summary




2
Muscle Fiber Composition in Human Skeletal Muscle

3
Motor-Unit Recruitment

4
Adaptive Response in Skeletal Muscle
4.1
Muscle Size

4.2
Metabolic Capacity




5
Connective Tissue

6
Capillaries
6.1
Methodology

6.2
Anatomy

6.3
Capillary Density

6.4
Capillary Length and Diameter

6.5
Use and Disuse

6.6
Regulation




7
Significance of Adaptation
7.1
Muscular Size

7.2
Substrate Stores

7.3
Enzyme Activities

7.4
Summary

Skeletal Muscle Adaptability: Significance for Metabolism and Performance

Enzyme changes in neonatal skeletal muscle: effect of thyroid deficiency

1. Five subjects trained for 8 weeks on a bicycle ergometer for an average of 40 min/day, four times a week at a work load requiring 80% of the maximal oxygen uptake (VO2 max.). VO2 max. determinations were performed, and muscle biopsies from the quadriceps femoris muscle (vastus lateralis) were taken before, as well as repeatedly during, the training period. The muscle biopsies were histochemically stained for fibre-types (myofibrillar ATPase) and capillaries (amylase-PAS method), and analysed biochemically for succinate dehydrogenase and cytochrome oxidase activities.

2. The training programme resulted in a 16% increase in VO2 max., a 20% increase in capillary density, a 20% increase in mean fibre area, and an approximately 40% increase in the activities of succinate dehydrogenase and cytochrome oxidase.

3. The capillary supply to type I, IIA and IIB fibres, expressed as the mean number of capillaries in contact with each fibre-type, relative to fibre-type area, increased equally.

4. The present study shows that endurance training constitutes a powerful stimulus for capillary proliferation in human skeletal muscle.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1977.sp011975

Capillary supply of the quadriceps femoris muscle of man: adaptive response to exercise.

Skeletal muscle undergoes profound changes in morphological, physiological, and biochemical character when subjected to prolonged periods of increased use. Although increased use may be brought about in a variety of ways, the results show consistent features. In particular, endurance exercise and chronic stimulation differ only in degree: the properties which change in response to exercise are also those which change at an early stage of stimulation; the properties which are resistant to change under exercise conditions change only after prolonged stimulation. There is therefore a hierarchy of stability in the properties of skeletal muscle which is revealed in its response to changing functional demands. The adaptive potential of muscle provides a logical framework for understanding neural influences on the emergence of fiber types during muscle development. It is also relevant to the study of pathological conditions which may involve a sustained departure from normal postural and locomotor patterns of activity.

The adaptive response of skeletal muscle to increased use

When rabbit fast muscles were chronically stimulated at a frequency naturally occurring in nerves to slow muscles (10 Hz), there was a transformation towards a slow muscle type such as an increase of capillary density, increased activity of the oxidative enzyme, succinic dehydrogenase, and a decrease of muscle fibre diameters. After 28 days the intensity and distribution of SDH and the capillary density were similar to those of soleus. The increases in capillary density preceded the changes in activity of SDH; there was a significantly greater capillary/muscle fibre ratio and number of capillaries/mm2 in muscles stimulated for only 4 days at which time no change could be detected in SDH. These changes were induced by slow frequency stimulation only, and not by an overall increase of activity. Stimulation of fast muscles for 4 days at a higher frequency naturally occurring in the nerves to fast muscles (short bursts of tetani), with the same total number of stimuli as that used in slow frequency stimulation did not produce any changes in capillary density, activity of SDH or contraction times. No changes were observed in either fast or slow muscles stimulated with short bursts of tetani (and lower total number of impulses) up to 28 days. Activation of fast muscles at 5 Hz continuously or 10 Hz intermittently also caused an increase in capillary density. It is therefore concluded that only low frequency activation of fast muscles brings about a transformation of the muscle fibres towards a slow type and that the first noticeable change is an increase in the capillary density.

The effects of different patterns of muscle activity on capillary density, mechanical properties and structure of slow and fast rabbit muscles.

Chronic stimulation of fast rabbit muscles (tibialis anterior, extensor digitorum longus and the peroneal muscle group) at a frequency naturally occurring in nerves to slow muscles increased their ability to withstand fatigue. Isometric tension decreased during a 10-min period of contractions at 4 Hz by 75% in control muscles, but only 55% in muscles chronically stimulated for 4 days, and 23% in muscles stimulated for 28 days. Chronic stimulation had little effect on resting blood flow, oxygen or glucose consumption. The output or consumption of lactate and free fatty acids (FFA) at rest were also unaffected. The glycogen content was regularly increased, and was apparent after only 2 days of stimulation. The activity of fatty acid activating enzyme was increased after 28 days. During a 10-min period of isometric contractions at 4 Hz, there was a markedly greater increase in blood flow and oxygen consumption in muscles stimulated for 14–28 days than in control muscles; lactic acid output was lower in muscles stimulated for 28 days, and the uptake of FFA was significantly higher. It is therefore suggested that muscles chronically stimulated for 14–28 days use fats as the main source of energy during isometric contractions. The predominantly oxidative metabolism is probably facilitated by the higher density of capillaries. The latter also enables more efficient delivery of oxygen, and therefore smaller fatiguability, already after 4 days of chronic stimulation.

The effect of long-term stimulation of fast muscles on their blood flow, metabolism and ability to withstand fatigue

SINCE Buller et al. demonstrated1 that the contraction speed of skeletal muscle is determined by the nature of its innervation, there have been several reports that the biochemical composition and properties of muscle are likewise influenced. When the motor nerve to a slow muscle is replaced by a nerve that normally innervates a fast muscle, changes in phosphorylase, succinic dehydrogenase and myosin ATPase activities, such as would be expected if the biochemical characteristics of the fibres were changing from those of type I to those of type II, can be demonstrated histochemically2. This is also true in the opposite situation, when the nerve to a fast muscle is replaced with one that normally innervates a slow muscle. The histochemical evidence implies that a change in concentration of the enzymes studied has occurred in response to the new type of innervation and although it would not be evident in such investigations, the possibility of change in isoenzyme complement cannot be excluded. In the case of myosin, however, which has been isolated after cross innervation, the changes have been analysed with more precision. These indicate that when the contraction speed of skeletal muscle is altered by either cross innervation or long term electrical stimulation, there are changes in Vmax of the myosin ATPase, in its light chain components and in its electrophoretic mobility3–7. These changes imply that new myosin molecules of different primary and quaternary structure are being synthesised as a consequence of the altered activity pattern of the muscle resulting from these procedures. They correlate well with the widely accepted view that the Vmax of the myosin ATPase rises with increasing muscle speed8 and the differences in light chain components known to exist in myosin from fast and slow skeletal muscle9–12.

Cross innervation and the regulatory protein system of rabbit soleus muscle

1.

The role of innervation of the differentiation of contractile speeds was studied in the slow anterior latissimus dorsi (ALD) and fast posterior latissimus dorsi (PLD) muscle of the chick.




2.

These muscles become innervated during the 12th and 15th day of embryonic development. At this time both muscles contract and relax extremely slowly and their contractile speeds are very similar. By the 18th day their contraction and relaxation becomes more rapid. It is at this time that the contractile characteristics of both muscles also become very different from each other, ALD being about 3 times slower than PLD. Thus innervation percedes differentiation of contractile speeds by several days.




3.

The influence of innervation on the contractile characteristics of developing slow and fast muscles was studied during muscle regeneration in adults. When a slow ALD muscle was minced and implanted in place of a fast PLD the newly regenerated ALD became innervated by a PLD nerve and resembled a fast PLD. Conversely, when PLD muscles were minced and replaced ALD the regenerated PLD was innervated by ALD nerve and became slow.




4.

Histological examination revealed that the regenerated ALD became focally innervated, and the regenerated PLD multiply innervated.




5.

Thus, the contractile speeds are not predetermined properties of the muscle fibre. Both contractile characteristic and the pattern of innervation of developing muscles are determined by the motor nerve.

Gerta Vrbová

Papers

The effects of different patterns of muscle activity on capillary density, mechanical properties and structure of slow and fast rabbit muscles.

The effect of long-term stimulation of fast muscles on their blood flow, metabolism and ability to withstand fatigue

Cross innervation and the regulatory protein system of rabbit soleus muscle

The influence of innervation on the differentiation of contractile speeds of developing chick muscles.

Changes of capillary density and enzyme pattern in fast rabbit muscles during long-term stimulation.