Type I muscle fiber

NMJs at type IIx and/or Ilb fibers. Moreover, the extent of overlap between pre- and postsynaptic components of the NMJ was lower at type I muscle fibers in hypothyroid compared to control animals. No studies have specifically examined the effects of hypothyroidism (either as a result of low-iodine diets or other conditions) on the ultrastructure of NMJs or the size of synaptic vesicle pools. 

Decreased diameter of soma and axons innervating type I muscle fibers, which would increase excitability and decrease conduction velocity. 

EXAMPLE 13.20 Type I muscle fibers are often referred to as red muscle fibers because they are colored a distinct reddish brown by their relatively large numbers of mitochondria that contain colored cytochromes in the electron transport chain (Chap. 10 Fig. 10-17). They are also characterized by their association with a dense blood supply and a high concentration of the red-colored oxygen-binding protein, myoglobin. On arrival of a nerve impulse, these cells contract relatively slowly, so they are often referred to as slow-twitch muscle fibers. Thus, type I fibers have evolved for slow, endurance contractions such as are used for maintaining posture. 

EXAMPLE 13.21 Type II muscle fibers are subdivided into type Ila and type Ilb. Type Ila can use both aerobic and anaerobic metabolism to produce ATP whereas type lib fibers use only anaerobic metabolism. In contrast to type I muscle fibers, type lib fibers contract rapidly after stimulation by a nerve impulse. They have evolved for short-lived, powerful contractions by the possession of characteristically active myosin ATPases and a dense packing of contractile filaments. So much of the cytoplasmic space is taken up with filaments that little exists for mitochondria. Similarly, these fibers are associated with a relatively poor blood supply. Type lib fibers are also known as white and fast-twitch muscle fibers and are adapted for short-lived but powerful contractions. The relative paucity of mitochondria and the poor blood supply impose obvious constraints on the generation of ATP during exercise. 

Type of muscle fiber Type I Red muscle Aerobic exercise Type II White muscle Anaerobic exercise... 

Fiber type. Skeletal muscle has several distinct fiber types. Type I is used primarily for aerobic activity, whereas type II is specialized for short, intense bursts of activity. How could you distinguish between these types of muscle fiber if you viewed them with an electron microscope ... 

Type El fibers (fetal, reticular and vascular collagen) are delicate compared with type I fibers. In the fetus, type IE collagen is incorporated within the type I collagen to impart the greater flexibility critical for fetal development. After birth, the delicate type El collagen fibers contribute to reticular fibers and also type I collagen fibers that are present in cardiovascular and lymphoid tissues and also beneath epithelial basal cell layers, muscles, and nervous tissue Schwann cells. 

Giardino etal. (2002) reported that hypothyroidism reduced the number of motoneurons, an effect that was prevented by hormonal replacement. Hypothyroidism also resulted in smaller somal diameters and surface areas of motoneurons innervating the rat soleus muscle (comprised of type S motor units primarily, with some type FR motor units), as well as a reduction in the dimensions of other motoneurons located in the lumbar spinal cord (Bakels et al., 1998 Giardino et al., 2002). This decrease in average somal size is consistent with the disappearance of type 11a muscle fibers (i.e., type FR motor units) from the hypothyroid soleus muscle (Gaiozzo et al., 1993). Based on the size principle, smaller motoneuron somata would make motor units more excitable and increase the likelihood of recruitment, which would clearly impact the force generated by the motor unit. 

These are postulated as adversely influencing mainly the type-2 fibers (or sub-preferentially the type-2B fibers). Hypothetically, "pan-denervations" are due to (a) abnormality of both type-2 and type-1 LMNs or of their intimately related, respectively type-2 and type-1 Schwann cells (which are nurturing the LMNs and being nurtured by them). This results in lack of trophic influence on "all" the muscle fibers, either (i) fully (in pan-denervations) or (ii) partially deficient - quantitatively or qualitatively - (in pan-dysinnervations), to which the type-2 muscle fibers (or sub-preferentially type-2B fibers) are more susceptible or (b) hypothetically, relatively selective abnormality at the level of the presumed type-2 LMNs, or of their closely associated Schwann cells that we designate as "type-2 (or type-2B) Schwann... 

Tissue-Specific Expression. In adult rodents, PPAR.a is expressed in liver, kidney, intestine, heart, skeletal muscle, retina, adrenal gland, and pancreas. In adult human, PPARa is expressed in the liver, heart, kidney, large intestine, skeletal muscle (mostly slow-twitch oxidative type I fibers), and in cells of atherosclerotic lesions (endothelial cells, smooth muscle cells, and monocytes/macrophages). Therefore, regardless of... 

Figure 3. Top panel Whole muscle force (x) and single fiber PCr (a, a) and ATP ( , ) concentrations at rest and after 10 and 20 sec of intermittent electrical stimulation at 50 Hz. Open symbols denote type I fibers closed symbols denote type II fibers. Bottom panel Glycogenolytic rates in type I and II fibers during the 20 sec stimulation period. The open bar denotes type I fibers the closed bar denotes type II fibers.

Another condition due to mutations in the RYRl gene is central core disease. This is a rare myopathy presenting in infancy with hypotonia and proximal muscle weakness. Electron microscopy reveals an absence of mitochondria in the center of many type I (see below) muscle fibers. Damage to mitochondria induced by high intracellular levels of Ca secondary to abnormal functioning of RYRl appears to be responsible for the morphologic findings. 

Different types of fibers have been detected in skeletal muscle. One classification subdivides them into type I (slow twitch), type IIA (fast twitch-oxidative), and type IIB (fast twitch-glycolytic). For the sake of simphcity, we shall consider only two types type I (slow twitch, ox-... 

Table 49-10. Characteristics of type I and type I fibers of skeletal muscle.

Some researchers reject the explanation that caffeine inhibits glycogen depletion during short term exercise54 but there is an increasing abundance of research supporting the notion of variable sensitivity to caffeine by muscle type. Muscles with higher ratios of type I fibers appear more sensitive than type II fibers, both in animal55-56 and human models.57... 

I realize that this is a side track issue, but relevant all the same. Cycle protocols were an approach intended to facilitate optimal growth of muscle tissue. Remember there are two main muscle fiber types Type I, which is endurance orientated, and Type II which is strength orientated. Type "Ha", "Hb", and Type "He" are responsible for most musculature size and have the greatest potential for growth. Testosterone increases the number of Type II fibers at the expense of the Type I transformation. Growth hormone, Insulin, IGF-1, and thyroid hormones effect growth and hyperplasia of both fiber types. This should be another key relating to protocols that were utilized and why. 

Lewis, M. K., Nahirney, P. C., Chen, V., Adhikari, B. B., Wright, J., Reedy, M. K., Bass, A. H., and Wang, K. (2003). Concentric intermediate filament lattice links to specialized Z-band junctional complexes in sonic muscle fibers of the type I male midshipman fish./ Struct. Biol. 143, 56-71. 

Mammalian skeletal muscle can be separated Into two distinct fiber populations, based on relative contraction characteristics, and are referred to as slow-twltch (Type I) or fast-twitch (Type II) fibers. The slow-twltch fiber type exhibits a relatively low shortening velocity (27), a low rate of tension development (27). a low myosin ATPase activity (28) and a low rate of calcium sequestration by the sarcoplasmic reticulum (29). The converse Is true for the fast-twitch fibers. Since contraction velocity highly correlates with myosin ATPase activity (30), It Is possible to easily Identify,... 

Similarly, measurements of blood flows to sections of muscle, which are primarily composed of a single fiber type, exhibit large differences consistent with the expected demands of oxygen supply based on mitochondrial content (32,33). Thus, mammalian skeletal muscle is typically comprised of three biochemically and functionally distinct fiber types slow-twitch red, fast-twitch red and fast-twitch white. These fiber types are also commonly referred to as Type I, Type Ila, and Type 11b, respectively (7 ). 

One can usually predict from these various characteristics whether or not a particular muscle would be more involved in endurance versus sprint activity as well as the fuel or fuel mixture used. For example, as discussed in the preceding chapter by Terjung type I and IIA fibers are more involved with endurance performance relying on a fuel mixture of both lipids and carbohydrate. On the other nand, type IIB fibers are more involved with short sprint-type of activity with a fuel dependence almost exclusively on carbohydrate. The fiber composition of muscle from a few animals and man is shown in Table 111. Animals raised for quick "stop and go" activity... 

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