Muscle fiber-types

Some muscles may appear dark-red on macroscopical inspection while other muscles are much paler. Examples of such dark-red muscles are the long back muscles and also the soleus muscles in the hind-legs. A large majority of the fibers in these muscles are of the so-called type I, and these fibers derive their energy (to regenerate ATP from ADP) from the aerobic oxidation of glucose (which takes place in mitochondria). Glucose enters the myocytes from capillaries and the extracellular space. These muscles appear dark-red because the type I myocytes contain high levels of the deep red myoglobin molecules, which bind oxygen, and also because of the many capillaries around the muscle fibers. Glucose is readily replenished from the blood vessels during muscle contractions and therefore these muscles are able to produce contractions for long periods of time.

Other muscles are much paler. These muscles mainly contain type II muscle fibers (and a minority of type I fibers). In type II muscle fibers, the ATP is regenerated by the anaerobic catabolism of glycogen which is stored in the muscle. This reaction is much faster but energetically less efficient than the aerobic metabolism of glucose. Moreover, the supplies of glycogen are limited. As the recovery of these supplies takes a considerable period of time, the contractions of the fibers in these myocytes can only be brief. Other fibers within the same muscle will take over but, obviously, this has its limits. Two subtypes of type II muscle fibers can be distinguished. Type IIb fibers derive their energy exclusively from the anaerobic metabolism of glycogen and type IIa fibers have a mixed metabolism; these make use both of the anaerobic as well as the aerobic route to generate energy.

The mechanical force of single muscle fibers has been recorded after briefly stimulating the motor nerve. Such stimulation induces a so-called muscle-twitch which is a single contraction of the fiber. It has been demonstrated that the force produced by a twitch of type I muscle fibers increases relatively slowly, has a low peak value and also decreases slowly. When stimulating for sustained periods and with such frequencies that the separate twitches fuse into a tetanic contraction, it appears that the tension in type I muscle fibers can be maintained for longer periods. In other words, these ‘slow-twitch’ muscle fibers are ‘resistant to fatigue’ and this is related to their oxidative metabolism (see above). On the other hand, the type II muscle fibers, after brief electrical stimuli, show a much faster increase in their tension and also the peak value of tension is much higher than that in the type I fibers. This increased tension is explained by the formation of alternative cross bridges between the actin and mysosin molecules. The tension after a twitch declines rapidly in these fibers. When stimulated with high, constant frequency leading to a tetanic contraction, the tension in these ‘fast-twitch, fast-fatigable’ muscle fibers diminishes after a relatively short period of time. After such contractions, these fibers need a considerable time to replenish their glycogen stores and to wash out the lactic acid, which is produced by the metabolism of glycogen. Type IIb fibers produce the highest tension, and these fibers show signs of fatigue most rapidly, while type IIa muscle fibers take an intermediate position between the type I and type IIb fibers. After stimulation their force rises and falls rapidly (though, less than that of the type IIb fibers) and because of a mixed aerobic and anaerobic metabolism, they are more resistant to fatigue than the type IIb fibers.

The different types of muscle fibers can be visualized histologically. Post-mortem studies allow the investigation of the distribution of fiber types in sections taken perpendicular to the longitudinal axis of the muscle at several levels. Multiple biopsies only allow an approximation of such data. In a classical method of processing (Dubowitz & Brooke, 1973), the muscle material is preincubated at different pHs and the ATPase staining technique then enables differentiation of the muscle fiber types. More recent methods apply myosin-related antibodies to identify several subtypes of muscle fibers (Schiaffino et al., 1986). Investigations with these methods have demonstrated that all muscles consist of a mixture of type I, type IIa and type IIb muscle fibers, but the relative proportions of these fibers differ greatly. Extremity muscles and particularly the flexor muscles may contain higher proportions of type IIa and IIb fibers and fewer type I muscle fibers. The antigravity muscles in the extremities, in contrast, contain increased proportions of types I and IIa muscle fibers. This is understandable when realizing that these muscles have to carry the weight of the animal for long periods of time. The proportions of type I muscle fibers are particularly pronounced in the soleus muscle in the hind legs, and portions of the flexor carpi ulnaris muscle in the forelegs. In these muscles, the type I muscles fibers may be as high as 100% (e.g. in rats). It is noteworthy that the type I fibers both in the extensors and the flexor-muscles are mostly concentrated in the deeper portions of the muscles, close to the skeletal elements (Wang & Kernell, 2001). Similar data have been obtained in horses (Grotmol et al., 2002).

The trunk muscles also contain elevated percentages of type I and IIa muscle fibers. This holds particularly for the multifidus and the longissimus muscles in the back (Gramsbergen et al., 1996). Similar results have been obtained in horses (Gellman et al., 2002). The high percentage of type I and IIa muscle fibers make these particular muscles fatigue resistant and therefore these muscles are ideally suited for postural functions, such as keeping the trunk straight and carrying the body weight (for a review on regional differences in the distribution of type I and type II muscle fibers, see Kernell (1998)).

Horse breeds differ in the size and composition of their muscles. Horses of some breeds are heavily built with bulky muscles that have a large cross-sectional area and these horses are kept in order to sustain enduring agricultural or transportation tasks. Their muscles contain large proportions of type I and type IIa muscle fibers which are able to produce forces for long-lasting periods (see Miyata et al., 1999). Horses of other breeds are characterized by a slender and athletic build that is appropriate for speed racing. Their muscles have increased proportions of type II muscle fibers that allow them to produce immense forces but only for relatively short periods of time.

Investigations in man with EMG recordings and, in addition, power spectral analysis of the EMG and the calculation of EMG-fatigue indices, indicated that loading for long periods induces the recruitment of satellite cells in the muscle (see below), their development into muscle cells and a relative increase in type II muscle fibers (Mileva et al., 2009). Investigations, e.g. in rats, have shown that per individual muscle the number of type I muscle fibers remains unchanged (or, is genetically determined; see Wilson et al., 1988; see also, Mileva et al., 2009) and Mileva’s results, therefore, point towards an absolute increase in type II fibers. Other investigators suggest, in contrast, that training may influence the fiber type distribution of muscles by shifts in the numbers of both type I and type II fibers (e.g. Duclay et al., 2009).

Other training effects are hypertrophy of muscle fibers as indicated by an increase in their cross sectional area, increases in mitochondrial content (important for aerobic glycolysis) (Grünheid et al., 2009), changes in muscular architecture (which might be secondary to the muscle fiber hypertrophy) (Duclay et al., 2009), as well as vascular changes by angiogenesis and an increase in arteriolar density (for review, Laughlin and Roseguini (2008)). Short-term effects of training aim at increasing the storage of large quantities of glycogen for rapid use in type II muscle fibers.

Muscular development

Muscle cells or myocytes develop by the fusion of a longitudinal array of mesenchymal cells. The first generation of these multinuclear myoblasts, extending from tendon to tendon, are the primary myotubes. Each of these is soon innervated by a multitude of motoneuronal axons. At a later stage, a second generation of secondary myotubes develops in the vicinity of the middle region of the primaries. The secondary myotubes also are initially innervated by multiple motoneuronal axons (Ontell & Dunn, 1978; Goldspink & Ward, 1979; Bennett, 1983). During further development, the contractile proteins actin and myosin increase enormously in the myotubes of both types. The motor endplate develops its folded appearance and from a certain stage of development these cells have acquired the characteristics of the mature myocyte. It is generally accepted that the primary myotubes are the precursors of type I muscle fibers and the secondary myotubes of the type II muscle fibers (see Wilson et al., 1988).

Wilson and coworkers studied in rats the effects of severe under-nutrition from early gestation and through the lactation period on muscular development. The number of primary myotubes remained unaffected but they detected a dramatic decrease in the number of secondary myotubes. These and other results has led them to hypothesize that the development of primary myotubes is genetically determined while that of the secondary myotubes is susceptible to environmental factors, such as under-nutrition, hormonal factors and also training (Wilson et al., 1988). In addition, non-differentiated satellite cells develop and these cells may differentiate at a later stage into myocytes after damage to the muscle or after training.

The muscle-fiber type distribution within muscles is, to some extent, related to their topological origin. The muscles in the trunk arise from the segmentally arranged myotomes. The intercostal muscles (together with the rib and the spinal cord segment innervating these muscles) have clearly retained this segmental origin in all mammals. But, the long muscles in the neck and in the back, the abdominal muscles and the diaphragm are also of axial origin. These muscle are particularly important for the stabilization of the trunk and the head and, therefore, for postural control. At adult age, the trunk muscles contain high proportions of slow-twitch, fatigue-resistant muscle fibers.

The muscles in the four legs arise from limb buds. The limb buds develop at the ventrolateral aspect of the body axis and these originate from groups of mesenchymal cells which migrated from the trunk. The extensor muscles in the limbs (e.g. the gastrocnemius and the quadriceps muscles in the hind leg) develop from the ventral plates in the buds and they are involved in ‘anti-gravity’ tasks such as standing, and also in the heel strike phase of walking and running. These muscles later contain considerable percentages of type I muscle fibers. The flexor muscles develop from the dorsal plate in the limb bud, they are particularly active during the swing phase of the leg and these muscles contain increased proportions of type II muscle fibers.

Development of muscular innervation

At early stages of development, muscle cells are innervated by more than one axon (see above) and this is known as polyneural innervation. Later, and already in early development, this multiple innervation of muscle cells is replaced by mononeural innervation (for reviews, Bennett, 1983; Jansen & Fladby, 1990). Research from many groups has indicated that this regressive process is near completion when the behavioral function in which the muscle is involved has developed (Gramsbergen et al., 1997). Rats are a precocious species and they are born at an early stage of brain development. Still, mononeural innervation of respiratory muscles in this species has already been reached at birth when the animals start breathing continuously. On the other hand, in the psoas muscle in the hind leg, this regression of polyneural innervation is only completed around the 16th postnatal day (IJkema-Paasen & Gramsbergen, 1998). This is precisely the point at which they start the adult-like pattern of walking. Horses are a precocious species, born with a nervous system at an advanced stage of development. They are able to stand and walk shortly after their birth and, therefore, it is safe to postulate that most of their muscles are mononeurally innervated from birth.

It has been suggested that the great excess of axons innervating the primitive muscle fibers at early stages and the selective regression of supernumerary nerve fiber-endings thereafter plays a role in matching the properties of muscle fibers to those of the motoneurons (O’Brien et al., 1978; Greensmith & Vrbova, 1991). More recent research has indicated, however, that even at the earliest stages ingrowing axons follow a trajectory towards a specific muscular region with a preponderance of particular muscle fiber types (Wang et al., 2002). Therefore, the polyneural innervation at first and its regression thereafter should rather be regarded as a fine-tuning in the matching-process.

Size principle

The motoneurons within a motoneuronal pool differ in their sizes (see above). Henneman & Mendell (1981) have demonstrated that within a motoneuronal pool, the motoneurons are recruited in an orderly fashion from small to large, with increasing intensities of neuronal drive (e.g. from higher brain centers). During motor tasks which require only low muscle forces only the smallest motoneurons are activated (these motorneurons innervate groups of type I-muscle fibers). With increasing forces, additional motoneurones with larger sizes are recruited, first those with FR properties, later those with Fint properties and finally the FF units are activated with their large numbers of type IIb muscle fibers. This orderly recruitment of motoneurons is related to differences in the membrane properties of the smaller and larger motoneurons. Interestingly, the relative force-increase by recruiting additional motor units at adult age is more or less the same over a wide range of forces (Milner-Brown et al., 1973) and this leads to a smooth increase of force with increasing intensities of neuronal drive.

Dendrite bundles

Apart from their size, motoneurons also differ in the morphology of their dendritic tree. The motoneurons in particular pools have their dendrites running in bundles both in a longitudinal direction as well as in transverse directions, while in other motoneuronal pools such organization is absent. Motoneurons with dendrite bundles have been described in cats (Scheibel & Scheibel, 1970), in rats (Gramsbergen et al., 1996) and in the human spinal cord (Schoenen, 1982) and because of their wide occurrence it seems highly likely that dendrite bundles also occur in the horse’s spinal cord. Dendrite bundles consist of up to 15 dendrites in close vicinity for several hundreds of millimeters (Fig. 4.1), and connected via long-stretched gap-junctions (Matthews et al., 1971; van der Want et al., 1998

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