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13.6: Introduction to Muscle Tissue - Biology

13.6: Introduction to Muscle Tissue - Biology


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Learning Objectives

  • Explain the organization of muscle tissue
  • Describe the function and structure of skeletal, cardiac muscle, and smooth muscle
  • Explain how muscles work with tendons to move the body
  • Describe how muscles contract and relax
  • Define the process of muscle metabolism
  • Explain how the nervous system controls muscle tension
  • Relate the connections between exercise and muscle performance
  • Explain the development and regeneration of muscle tissue

When most people think of muscles, they think of the muscles that are visible just under the skin, particularly of the limbs. But there are two other types of muscle in the body, with distinctly different jobs.

Cardiac muscle, found in the heart, is concerned with pumping blood through the circulatory system. Smooth muscle is concerned with various involuntary movements, such as having one’s hair stand on end when cold or frightened, or moving food through the digestive system. This chapter will examine the structure and function of these three types of muscles.


13.6: Introduction to Muscle Tissue - Biology

Figure 1. Tennis Player. Athletes rely on toned skeletal muscles to supply the force required for movement. (credit: Emmanuel Huybrechts/flickr)

When most people think of muscles, they think of the muscles that are visible just under the skin, particularly of the limbs. These are skeletal muscles, so-named because most of them move the skeleton. But there are two other types of muscle in the body, with distinctly different jobs.

Cardiac muscle, found in the heart, is concerned with pumping blood through the circulatory system. Smooth muscle is concerned with various involuntary movements, such as having one’s hair stand on end when cold or frightened, or moving food through the digestive system. This chapter will examine the structure and function of these three types of muscles.


13.6: Introduction to Muscle Tissue - Biology

The amount of satellite cells present within in a muscle depends on the type of muscle. Type I or slow-twitch oxidative fibers, tend to have a five to six times greater satellite cell content than Type II (fast-twitch fibers), due to an increased blood and capillary supply (2). This may be due to the fact that Type 1 muscle fibers are used with greatest frequency, and thus, more satellite cells may be required for ongoing minor injuries to muscle.

Immunology
As described earlier, resistance exercise causes trauma to skeletal muscle. The immune system responds with a complex sequence of immune reactions leading to inflammation (3). The purpose of the inflammation response is to contain the damage, repair the damage, and clean up the injured area of waste products.
The immune system causes a sequence of events in response to the injury of the skeletal muscle. Macrophages, which are involved in phagocytosis (a process by which certain cells engulf and destroy microorganisms and cellular debris) of the damaged cells, move to the injury site and secrete cytokines, growth factors and other substances. Cytokines are proteins which serve as the directors of the immune system. They are responsible for cell-to-cell communication. Cytokines stimulate the arrival of lymphocytes, neutrophils, monocytes, and other healer cells to the injury site to repair the injured tissue (4).

The three important cytokines relevant to exercise are Interleukin-1 (IL-1), Interleukin-6 (IL-6), and tumor necrosis factor (TNF). These cytokines produce most of the inflammatory response, which is the reason they are called the “inflammatory or proinflammatory cytokines” (5). They are responsible for protein breakdown, removal of damaged muscle cells, and an increased production of prostaglandins (hormone-like substances that help to control the inflammation).

Growth Factors
Growth factors are highly specific proteins, which include hormones and cytokines, that are very involved in muscle hypertrophy (6). Growth factors stimulate the division and differentiation (acquisition of one or more characteristics different from the original cell) of a particular type of cell. In regard with skeletal muscle hypertrophy, growth factors of particular interest include insulin-like growth factor (IGF), fibroblast growth factor (FGF), and hepatocyte growth factor (HGF). These growth factors work in conjunction with each other to cause skeletal muscle hypertrophy.

Insulin-Like Growth Factor
IGF is a hormone that is secreted by skeletal muscle. It regulates insulin metabolism and stimulates protein synthesis. There are two forms, IGF-I, which causes proliferation and differentiation of satellite cells, and IGF-II, which is responsible for proliferation of satellite cells. In response to progressive overload resistance exercise, IGF-I levels are substantially elevated, resulting in skeletal muscle hypertrophy (7).

Fibroblast Growth Factor
FGF is stored in skeletal muscle. FGF has nine forms, five of which cause proliferation and differentiation of satellite cells, leading to skeletal muscle hypertrophy. The amount of FGF released by the skeletal muscle is proportional to the degree of muscle trauma or injury (8).

Hepatocyte Growth Factor
HGF is a cytokine with various different cellular functions. Specific to skeletal muscle hypertrophy, HGF activates satellite cells and may be responsible for causing satellite cells to migrate to the injured area (2).
Hormones in Skeletal Muscle Hypertrophy
Hormones are chemicals which organs secrete to initiate or regulate the activity of an organ or group of cells in another part of the body. It should be noted that hormone function is decidedly affected by nutritional status, foodstuff intake and lifestyle factors such as stress, sleep, and general health. The following hormones are of special interest in skeletal muscle hypertrophy.

Growth Hormone
Growth hormone (GH) is a peptide hormone that stimulates IGF in skeletal muscle, promoting satellite cell activation, proliferation and differentiation (9). However, the observed hypertrophic effects from the additional administration of GH, investigated in GH-treated groups doing resistance exercise, may be less credited with contractile protein increase and more attributable to fluid retention and accumulation of connective tissue (9).

Cortisol
Cortisol is a steroid hormone (hormones which have a steroid nucleus that can pass through a cell membrane without a receptor) which is produced in the adrenal cortex of the kidney. It is a stress hormone, which stimulates gluconeogenesis, which is the formation of glucose from sources other than glucose, such as amino acids and free fatty acids. Cortisol also inhibits the use of glucose by most body cells. This can initiate protein catabolism (break down), thus freeing amino acids to be used to make different proteins, which may be necessary and critical in times of stress.
In terms of hypertrophy, an increase in cortisol is related to an increased rate of protein catabolism. Therefore, cortisol breaks down muscle proteins, inhibiting skeletal muscle hypertrophy (10).

Testosterone
Testosterone is an androgen, or a male sex hormone. The primary physiological role of androgens are to promote the growth and development of male organs and characteristics. Testosterone affects the nervous system, skeletal muscle, bone marrow, skin, hair and the sex organs.
With skeletal muscle, testosterone, which is produced in significantly greater amounts in males, has an anabolic (muscle building) effect. This contributes to the gender differences observed in body weight and composition between men and women. Testosterone increases protein synthesis, which induces hypertrophy (11).

Fiber Types and Skeletal Muscle Hypertrophy
The force generated by a muscle is dependent on its size and the muscle fiber type composition. Skeletal muscle fibers are classified into two major categories slow-twitch (Type 1) and fast-twitch fibers (Type II). The difference between the two fibers can be distinguished by metabolism, contractile velocity, neuromuscular differences, glycogen stores, capillary density of the muscle, and the actual response to hypertrophy (12).

Type I Fibers
Type I fibers, also known as slow twitch oxidative muscle fibers, are primaritly responsible for maintenance of body posture and skeletal support. The soleus is an example of a predominantly slow-twitch muscle fiber. An increase in capillary density is related to Type I fibers because they are more involved in endurance activities. These fibers are able to generate tension for longer periods of time. Type I fibers require less excitation to cause a contraction, but also generate less force. They utilize fats and carbohydrates better because of the increased reliance on oxidative metabolism (the body’s complex energy system that transforms energy from the breakdown of fuels with the assistance of oxygen) (12).
Type I fibers have been shown to hypertrophy considerably due to progressive overload (13,15). It is interesting to note that there is an increase in Type I fiber area not only with resistance exercise, but also to some degree with aerobic exercise (14).

Type II Fibers
Type II fibers can be found in muscles which require greater amounts of force production for shorter periods of time, such as the gastrocnemius and vastus lateralis. Type II fibers can be further classified as Type IIa and Type IIb muscle fibers.

Type IIa Fibers
Type IIa fibers, also known as fast twitch oxidative glycolytic fibers (FOG), are hybrids between Type I and IIb fibers. Type IIa fibers carry characteristics of both Type I and IIb fibers. They rely on both anaerobic (reactions which produce energy that do not require oxygen), and oxidative metabolism to support contraction (12).
With resistance training as well as endurance training, Type IIb fibers convert into Type IIa fibers, causing an increase in the percentage of Type IIa fibers within a muscle (13). Type IIa fibers also have an increase in cross sectional area resulting in hypertrophy with resistance exercise (13). With disuse and atrophy, the Type IIa fibers convert back to Type IIb fibers.

Type IIb Fibers
Type IIb fibers are fast-twitch glycolytic fibers (FG). These fibers rely solely on anaerobic metabolism for energy for contraction, which is the reason they have high amounts of glycolytic enzymes. These fibers generate the greatest amount of force due to an increase in the size of the nerve body, axon and muscle fiber, a higher conduction velocity of alpha motor nerves, and a higher amount of excitement necessary to start an action potential (12). Although this fiber type is able to generate the greatest amount of force, it is also maintains tension for a shortesst period of time (of all the muscle fiber types).
Type IIb fibers convert into Type IIa fibers with resistance exercise. It is believed that resistance training causes an increase in the oxidative capacity of the strength-trained muscle. Because Type IIa fibers have a greater oxidative capacity than Type IIb fibers, the change is a positive adaptation to the demands of exercise (13).

Conclusion
Muscular hypertrophy is a multidimensional process, with numerous factors involved. It involves a complex interaction of satellite cells, the immune system, growth factors, and hormones with the individual muscle fibers of each muscle. Although our goals as fitness professionals and personal trainers motivates us to learn new and more effective ways of training the human body, the basic understanding of how a muscle fiber adapts to an acute and chronic training stimulus is an important educational foundation of our profession.


Table 1. Structural Changes that Occur as a Result of Muscle Fiber Hypertrophy
Increase in actin filaments
Increase in myosin filaments
Increase in myofibrils
Increase in sarcoplasm
Increase in muscle fiber connective tissue
Source: Wilmore, J.H. and D. L. Costill. Physiology of Sport and Exercise (2nd Edition).Champaign, IL: Human Kinetics, 1999.

References

1. Russell, B., D. Motlagh,, and W. W. Ashley. Form follows functions: how muscle shape is regulated by work. Journal of Applied Physiology 88: 1127-1132, 2000.

2. Hawke, T.J., and D. J. Garry. Myogenic satellite cells: physiology to molecular biology. Journal of Applied Physiology. 91: 534-551, 2001.

3. Shephard, R. J. and P.N. Shek. Immune responses to inflammation and trauma: a physical training model. Canadian Journal of Physiology and Pharmacology 76: 469-472, 1998.

4. Pedersen, B. K. Exercise Immunology. New York: Chapman and Hall Austin: R. G. Landes, 1997.

5. Pedersen, B. K. and L Hoffman-Goetz. Exercise and the immune system: Regulation, Integration, and Adaptation. Physiology Review 80: 1055-1081, 2000.

6. Adams, G.R., and F. Haddad. The relationships among IGF-1, DNA content, and protein accumulation during skeletal muscle hypertrophy. Journal of Applied Physiology 81(6): 2509-2516, 1996.

7. Fiatarone Singh, M. A., W. Ding, T. J. Manfredi, et al. Insulin-like growth factor I in skeletal muscle after weight-lifting exercise in frail elders. American Journal of Physiology 277 (Endocrinology Metabolism 40): E135-E143, 1999.

8. Yamada, S., N. Buffinger, J. Dimario, et al. Fibroblast Growth Factor is stored in fiber extracellular matrix and plays a role in regulating muscle hypertrophy. Medicine and Science in Sports and Exercise 21(5): S173-180, 1989.

9. Frisch, H. Growth hormone and body composition in athletes. Journal of Endocrinology Investigation 22: 106-109, 1999.

10. Izquierdo, M., K Hakkinen, A. Anton, et al. Maximal strength and power, endurance performance, and serum hormones in middle-aged and elderly men. Medicine and Science in Sports Exercise 33 (9): 1577-1587, 2001.

11. Vermeulen, A., S. Goemaere, and J. M. Kaufman. Testosterone, body composition and aging. Journal of Endocrinology Investigation 22: 110-116, 1999.

12. Robergs, R. A. and S. O. Roberts. Exercise Physiology: Exercise, Performance, and Clinical Applications. Boston: WCB McGraw-Hill, 1997.

13. Kraemer, W. J., S. J. Fleck, and W. J. Evans. Strength and power training: physiological mechanisms of adaptation. Exercise and Sports Science Reviews 24: 363-397, 1996.

14. Carter, S. L., C. D. Rennie, S. J. Hamilton, et al. Changes in skeletal muscle in males and females following endurance training. Canadian Journal of Physiology and Pharmacology 79: 386-392, 2001.

15. Hakkinen, K., W. J. Kraemer, R. U. Newton, et al. Changes in electromyographic activity, muscle fibre and force production characteristics during heavy resistance/power strength training in middle-aged and older men and women. Acta Physiological Scandanavia 171: 51-62, 2001.


Muscle Tissue of Animals: Origin and Functions (With Diagram)

Muscular tissue in general develops from the mesoderm of the embryo but the muscles of the iris of the eye and myoepithelial cells of the salivary, mammary and sweat glands arise from the ectoderm of the embryo.

General Structure of Muscle Tissue:

Myo, sarco and motor are concerned with muscles. This tissue constitutes the muscles, made up of cells, which are in the form of contractile fibres varying in lengths. The fibres are bound together by connective tissues but they have no intercellular substance.

Myoblasts give rise to muscle fibres. Myocytes (= sarcocytes) are muscle cells. Each fibre consists of fine fibrils called myofibrils, present in the cytoplasm known as sarcoplasm. Sometimes the muscle fibre is externally covered by a membrane, termed as sarcolemma.

Special Properties of Muscle Tissue:

The special property of muscular tissue is contractility i.e., the cells of muscular tissue can shorten considerably and return to the original relaxed state. The muscle cells contract in a definite direction. Another property of muscle is the electrical excitability. It is due to the energy stored in the electrical potential difference across the plasma membrane.

Functions of Muscle Tissue:

1. It brings about movements of the body parts and locomotion of the individual.

2. Muscles are responsible for peristalsis in tubular viscera, heartbeat, production of sound, etc.

3. Facial expression also depends on muscles.

4. It supports the bones and other structures.

5. Muscles are required for delivering a baby.

A whole muscle (Fig. 7.30) is covered by a connective tissue sheath, the epimysium. Beneath the epimysium each skeletal muscle consists of many muscle fibres arranged in bundles called fasciculi (sing, fasciculus or fascicle). Each bundle or fasciculus is surrounded by a connective tissue sheath, the perimysium and each muscle fibre or cell, is surrounded by a thin connective tissue sheath, the endomysium.

Types of Muscle Tissue:

There are present three types of muscle tissue:

1. Striped or striated or skeletal or voluntary muscles.

2. Un-striped or non-striated or visceral or smooth or involuntary muscles.

1. Skeletal Muscle Tissue (Striated or Striped Muscles):

These muscles are found in the limbs, body walls, tongue, pharynx and beginning of oesophagus and are under the control of animal’s will.

These muscle fibres occur in bundles and are normally attached to the skeleton. Each muscle fibre is an elongated cell surrounded externally by a delicate membrane, the sarcolemma.

Just beneath the sarcolemma in each fibre many nuclei occur at irregular intervals. Thus, these fibres are multi-nucleated or syncytial in nature. The cytoplasm of each fibre (sarcoplasm) has a large number of myofibrils which are tightly packed.

Detailed Structure of Striated Muscle Fibre:

A myofibril has dark and light bands. The dark bands are also called A-bands (Aniso­tropic bands). The light bands are also called I-bands (Isotropic bands). At the centre of А-band, a comparatively less dark zone called H—Zone (= Hensen zone, named after Hensen who first described) is present.

In the centre of the H-zone is the M-line: The letter ‘M’ is from the German word Mittleschiebe (mittle = middle). Each I-band has at its centre a dark membrane called Z-line. The letter Z’ is from the German word Zwischenschiebe (zwischen = between, schiebe = disc). The Z-line is also called Z-disc, or Krause s mem­brane or Dobie’s line.

The part of the myofibril between two successive Z-lines is called sarcomere. Therefore, the sarcomere comprises А-band and half of each adjacent I-band. The sarcomere is the functional unit of myofibril. In fact each sarcomere is a bundle of thick and thin myofilaments. The thick myofilaments have diameters of about 150A, whereas the thin myofilaments have diameters of about 70A.

They consist mainly of myosin protein. They form cross bridges.

They are composed of three different proteins- actin, tro­pomyosin and troponin.

Skeletal muscles are un­der the control of animal’s will, Calcium is an essential element for the contraction of muscles. In the presence of calcium ions and energy from ATP, actin and myo­sin interact forming actomyosin which causes contraction of muscles.

2. Smooth Muscle Tissue (Non-striated Muscles):

Non-striated muscles are found in the posterior part of oesopha­gus, stomach, intestine, lungs, urinogenital tract, urinary bladder, blood vessels, iris of eye, dermis of skin and arrestor pili muscle of hair. Smooth muscles never connect with skeleton.

These muscle fibres or cells are elongated and spindle shaped. Each fibre contains a single oval nucleus surrounded by the cytoplasm (sarcoplasm). In the cytoplasm the myofibrils are arranged longitudinally. There is no sarcolemma, however, the fibre is enclosed by plasma membrane.

These muscles help in peristalsis which happens in tubular viscera. Action of these muscles is controlled by autonomic nervous system and hence they are not under the control of the animal’s will.

Functionally smooth muscles are of two types — single-unit smooth muscles and multi-unit smooth muscles.

(i) Single-unit smooth muscles are composed of muscle fibres closely joined together. All the fibres of the single smooth muscle contract simultaneously as a single unit. These muscles are found in the walls of hollow visceral organs like gastrointestinal tract and urinary bladder.

(ii) Multi-unit smooth muscles are composed of independent muscle fibres and are not closely joined together. Their fibres contract more or less independently as separate units. Arrector pili muscles of skin dermis, ciliary and iris muscles in the eyes, and muscles of the walls of the large blood vessels are some examples of multi-unit smooth muscles.

3. Cardiac Muscle Tissue:

The cardiac muscles are found in the wall of the heart and in the wall of large veins (e.g., pulmonary veins and superior vena cava) where these veins enter the heart.

These fibres show the char­acters of both un-striped and striped muscle fibres. Each fibre is a long and cylindrical structure which has a definite sarcolemma. The fibres are uninucleate and the nuclei lie near the centre. The fibres have some lateral branches, known as oblique bridges to form a contractile network. The myo­fibrils have transverse faint dark and light bands, which alternate with each other.

In this way cardiac muscle fibres are also striped, but having dark intercalated discs at intervals. The intercalated discs are specialized regions of cell membrane of two adjacent fibres. The intercalated discs function as boosters of contraction wave and permit the wave of muscle contraction to be transmitted from one cardiac fibre to another.

Cardiac muscle fibres are supplied with both central and autonomic nervous system and are not under the control of the will of the animal. However, these muscles never get fatigued. Thus they are immune to fatigue. Blood capillaries penetrate the cardiac muscle fibres. They have very rich blood supply. They have the property of con­traction, even when they are isolated from the body temporarily.

Similarities between Cardiac and Skeletal Muscles:

Both cardiac and skeletal muscles are made up of elongated fibres which have numerous myofibrils. The myofibrils of cardiac muscle have the same structure as those skeletal muscle and are made up of actin and myosin filaments. The cardiac and skeletal muscle fibres have dark and light bands. The connective tissue framework and the capillary network around cardiac muscle fibres are similar to those in skeletal muscle.

Similarities between Cardiac and Smooth Muscles:

Both cardiac and smooth muscles are uninucleate containing nucleus at the centre and are involuntary in function.

Comparison between Striated, Non-striated and Cardiac Muscle Fibres:


Muscular Tissue Quiz

5. The mineral released from the _______into a fiber that activates _______ to cause muscle contraction is_______.

a. t-tubules, ATP, Na + b. SR, DNA, Ca +2 c. SR, ATP, Ca +2 d. t-tubules, DNA, Ca +2 e. SR, ATP, K +

6. Two abundant organelles in skeletal muscle are

a. cilia and lysosomes b. ATP and glucose c. mitochondria and ATP d. myofibrils and mitochondria

7. Motor unit a. Variations in the degree of muscle contraction.

8. Myogram b. The graphic recording of a muscle's contraction.

9. Graded responses c. The condition where a muscle's power gradually drops and may reach zero.

10. Tetanus d. A motor neuron and all the muscle fibers it innervates (supplies).

11. Fatigue e. A smooth, sustained muscle contraction.

__________________________________________________________________________________

12. Glucose a. Can cause fatigue as it builds up in muscle under anaerobic conditions.

13. Creatine phosphate b. A muscle fiber's immediate source of energy for contraction.

14. Adenosine triphosphate c. A muscle's storage form of energy for contraction.

15. Lactic acid d. The aerobic, large energy-generating part of cellular respiration.

16. ETS (electron transport system) e. The body's energy source circulating in the blood.

Multiple Choice

17. The hypertrophy of muscles is due to

a. increased numbers of myofibrils and the increased amount of connective tissue in the muscle

b. increased numbers of fibers inside the muscle

c. increased numbers of both fibers and myofibrils inside the muscle

d. increased storage of glucose, myoglobin, ATP and creatine phosphate in the muscle

18. Muscle fascicles are surrounded by

a. epimysium b. perimysium c. endomysium d. fascia e. tendon

19. An electrical stimulus to a muscle that fails to bring about contraction is called a

a. maximal stimulus b. minimal stimulus c. tetanic stimulus d. subliminal stimulus

20. I f you are exercising and you are breathing normally (not out-of-breath), then your muscles are working

a. very rapidly b. aerobically c. anaerobically d. very slowly e. both c and d

21. Wave summation a. Increased force of contraction as more and more fibers in a muscle contract.

22. Multiple motor unit summation b. Lowest voltage that brings about muscle contraction.

23. Minimal stimulus c. Contraction without shortening.

24. Isotonic contraction d. Contraction with shortening.

25. Isometric contraction e. More rapid contractions due to more rapidly delivered stimuli.

26. Glycolysis a. The anaerobic breakdown of glucose to pyruvic acid.

27. Kreb's cycle b. An aerobic pathway that generates the greatest number of ATP molecules.

28. Electron transport chain c. A pathway that generates electrons, hydrogen ions and CO2.

29. Mitochondria d. An O2-binding molecule in some muscles.

30. Myoglobin e. An organelle where the Kreb's cycle and the electron transport chain operate.

31. Prime movers (agonists) a. Muscles that prevent a bone from moving while other muscles are working.

32. Synergists b. Muscle pairs that produce opposite movements across a given joint.

33. Antagonists c. Muscles that help prime movers do their job more powerfully and efficiently.

34. Fixators d. Muscles producing forces opposite to that of the prime movers.

35. Antagonistic muscles e. Muscles that produce most of the force in a particular joint movement.

Essays - Use the space below the questions to write your answers and draw your diagrams.

1. Explain how a whole muscle is constructed. Use diagrams to support your answer. (15)

2. Explain how actin and myosin interact to bring about muscle contraction. Use a diagram to support your answer.


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