Transcript for Muscle Anatomy Review

Slide 1: Muscle Development

Slide 2: Objectives

• Describe the process by which skeletal muscles are formed in the fetal stages through the postnatal stage. • Identify the structural components of muscle and describe the functional organization of muscle • Explain the sliding filament theory • Describe the process by which skeletal muscle becomes stronger. • Describe the influence of hormones on muscle growth • Describe the impact of various types of exercise on muscle hypertrophy

Slide 3: This week we are going to review muscle anatomy, growth and development. Muscle grows by increasing in size, hypertrophy, and increasing in fiber number, hyperplasia. Some of these processes take place predominantly during embryonic development and others are predominantly postnatal, which means after birth.

Slide 4: There are three types of muscle, skeletal, cardiac and smooth. We'll be focusing on skeletal muscle in this class.

Slide 5: I'd like to review some terminology before we discuss muscle formation.

• mesoderm: primary germ layer in the embryo • somite: block of mesoderm • myogenesis: embryonic muscle cell formation • myoblasts: early muscle cells in the mesoderm • myocyte: multi-nucleated single cell composed of myofibrils. Also called a muscle fiber or a myofiber. • myofibril: basic rod like unit that contains long chains of sarcomeres • myofilaments: filaments within the myofibrils; there are two types, thick filaments myosin and thin filaments called actin.

Slide 6: This is an ultrasound at 8 weeks of gestation. Muscles begin to form in the fetus and the embryo is capable of motion as detected with muscular contractions by this age. By the 20th week of gestation, fetal movements are more discrete involving the feet, eyelids and individual fingers.

Slide 7: Myogenesis is the embryonic development of muscle. We'll be looking at this process in four steps.

• Step 1: Formation: The mesoderm, which is the primary germ layer in muscle, is formed • Step 2: Separation: The mesoderm separates into blocks, the somites. • Step 3: Proliferation: Muscle forming cells, called myogenic cells or myoblasts, begin to proliferate (which means they increase in number) • Step 4: Differentiation: the cells fuse and differentiate into myotubes and eventually mature to myofibers.

Myogenesis is an extremely complex process and controlled by a number of myogenic regulatory transcription factors and growth factors. I'm not going to review the regulatory factors, but we'll touch on the important hormones. I'll start with a more detailed review of the steps involved in embryonic muscle formation.

Slide 8: Step 1 (Formation). The mesoderm is one of the three germ layers present in the early embryo. The mesoderm forms the three muscle tissues, cardiac, skeletal and smooth muscle.

Slide 9: Step 2 (Separation). Once the mesoderm has been formed, the development into various tissues is dependent on whether the cells become dorsal or ventral cells. The dorsal mesoderm separates to form blocks called somites. The somites eventually form the vertebral column, the ribs, dermis and skeletal muscle.

Slide 10: Step 3 (Proliferation). To form just one muscle requires the creation of millions of cells that are ultimately joined together to form muscle fibers. Proliferation results in the myoblasts increasing in number. Myoblasts are myogenic progenitor cells and once the muscle has matured, the myogenic progenitor cells will enter quiescence (inactive) and reside as satellite cells. This is important because muscle is a renewing organ. When muscle cells need to be replaced, these satellite cells can be activated to differentiate into muscle fibers.

Slide 11: Step 4 (Differentiation). Differentiation means that a less specialized cell becomes a more specialized cell. Myoblasts differentiate to become myotubes, which are multi-nucleated fibers. To accomplish this, myoblasts align and then fuse together to form myotubes. Myotubes can synthesize myofilaments, which are proteins, actin and myosin. During this step, actin and myosin continue to increase in amount and the myotube enlarges. Myotubes eventually mature into myofibers. Fiber formation is complete at birth. As a general rule, hyperplasia is fixed. But, there are exceptions to this rule, which we'll discuss later.

Slide 12: This image from Penn State University, Pennsylvania Muscle Institute demonstrates the use of fluorescence microscopy, allowing for the visualization of myofibrillogenesis, the formation of myofibrils, one of the early and important steps of muscle cell formation. Myofibrils are rod-like units which make up muscle fibers. Myofibrils contain actin and myosin filaments.

Slide 13: Myogenesis starts with the mesoderm. Cells destined to be muscle will become dorsally located and the dorsal mesodermal cells are also called myotome cells. Myotome cells (from the dorsal mesoderm) divide rapidly, multiplying in number, called proliferation (or multiplication). The cells align and then fusion, differentiating from myoblasts to a multinucleated myotubes. The myotube then matures to form a muscle fiber.

Slide 14: Just because muscle fibers have been created, it does not means that the muscle is a functional unit yet. The body lays out a plan, which results in the appearance and shape of each individual. Muscle masses group together to form individual muscles. Once individual muscles are formed, they are assembled within connective tissue with tendons at each end and connective tissue that groups the muscle fibers into bundles called fascicles. This image shows the soleus muscle, which lies underneath the gastrocnemius muscle (not shown) and the Achilles tendon which connects the muscle to bone. Slide 15: We can see the muscle attached to the bone via the tendon here. Within the muscle, there are groups of fascicles. The fascicles contain muscle fibers. Remember, muscle fibers are cells. Within the muscle fiber there are groups of rod shaped myofibrils. The myofibrils contain sarcomeres, which are made up of actin and myosin, the two main protein filaments in muscle.

Slide 16: Quiz Yourself!

Which of the following is a muscle fiber?

A. myofibril B. myofilament C. myocyte D. myoblast

Slide 17: Answer Slide

Which of the following is a muscle fiber?

A. myofibril B. myofilament C. myocyte D. myoblast

Answer: C. A Myocyte is a muscle cell and also called a muscle fiber or myofiber

Slide 18: Quiz Yourself!

Myoblasts increase in number during what stage of embryonic myogenesis?

A. Formation B. Separation C. Proliferation D. Differentiation

Slide 19: Quiz Yourself!

Myoblasts increase in number during what stage of embryonic myogenesis?

A. Formation B. Separation C. Proliferation D. Differentiation

Answer C: Proliferation

Slide 20: Quiz Yourself!

All of the following are true EXCEPT? A. Sarcomeres contain myofibrils B. Fascicles contain muscle fibers C. Muscle fiber contain myofibrils D. Sarcomeres contain actin and myosin

Slide 21: Quiz Yourself!

All of the following are true EXCEPT?

A. Sarcomeres contain myofibrils B. Fascicles contain muscle fibers C. Muscle fiber contain myofibrils D. Sarcomeres contain actin and myosin Answer A

Slide 22: Let's take a closer look at the sarcomere, the basic unit of muscle, for a better understanding of how muscle contracts. This will be review for those of you who've had physiology. If we look at the sarcomere, it contains two main proteins, actin and myosin. Actin is a thin filament and myosin is a thick filament. This image is of a relaxed sarcomere. The thin filaments are represented in blue and the thick filaments are represented in red. The I band is the area that only has thin filament. It extends from here, the end of the thick filament (this pink is a connecting protein, not myosin) to the thin filament in the next sarcomere and the H zone is the area of only thick filament, extending from here to here.

Slide 23: Here we can see the relaxed sarcomere. The actin is here, the myosin here. Note the width of the H zone, the area that is only myosin, with no overlapping actin. Note the I band, that is only actin. When the sarcomere contracts, the actin and myosin overlap. The H zone becomes smaller and the I band become smaller, but actin and myosin maintain their length, it's the overlap that changes.

Slide 24: The sliding filament theory describes how muscles contract. This is actin in grey and this is myosin in purple, bound to ADP and Phosphate. Tropomyosin is shown in green and it is blocking myosin from binding to actin. Troponin is bound to actin and has the ability to bind calcium.

1. The first step is an electrical signal sent from the brain in the form of an action potential. 2. This causes a release of calcium from the sarcoplasmic reticulum. The sarcoplasmic reticulum is an interconnected network of membranous sacs surrounding the muscle fiber that contain stores of calcium. 3. Calcium binds to the troponin. 4. Once calcium has bound to troponin, the tropomyosin moves out of the way to uncover a binding site on actin. 5. Myosin binds to the uncovered binding sites on actin 6. Myosin slides the actin in a movement called the power stroke, resulting in contraction of the muscle. 7. ADP and phosphate are released. 8. ATP (our energy currency) binds to myosin which allows it to release actin.

If ATP was not available, the muscle would remain contracted. This is called rigor mortis and this is why people are stiff when they die. There is no energy available to bind myosin to initiate relaxation. If calcium is continuously available (the brain is sending an electrical signal to contract) then the power stroke is repeated. I have a great animation in the reading material which describes muscle fibers and explains muscle contractions.

Slide 25: Let's return back to muscle development and growth. Once a fetus is born, the muscles begin to enlarge. In fact, the muscle grows to about 20 times it's size during childhood and puberty. Muscle grows in length and size due to the formation of new sarcomeres which are added to the ends of the growing muscle fibers. The myofibrils thicken and then split, forming new sarcomeres which are then attached to the ends of the muscle, thus lengthening the muscle fiber. Remember, hyperplasia, the increase in muscle cell fiber number, occurs mainly during fetal development.

Slide 26: In this illustration, you can see the myofibril hypertrophy. Remember, myofibrils are the rod-like units which contain sarcomeres. Growth hormone, insulin, good nutrition, and insulin-like growth factor (IGF) are critical for forming new sarcomeres. When there is a deficiency of growth hormone in childhood, growth of muscle is impaired. Another factor contributing to the growth of muscle is the response of the muscle to increasing bone length. As bone increases in length, the muscles are placed under a sustained stretch. This sustained stretch stimulates an increase in the number of sarcomeres, which align end to end, increasing the muscle length.

Slide 27: Remember the GH-IGF axis we discussed when covering endochondral ossification? The interaction between growth hormone and insulin like growth factor regulates body size in growing children. Growth hormone has direct effects on growth. Growth hormone stimulates the liver (and other tissues) to produce IGF and IGF stimulates growth.

IGF stimulates: • differentiation and proliferation of myoblasts (this occurs mostly during embryonic development) • amino acid uptake • protein synthesis in muscles GH and IGF work together as anabolic factors by:

• increasing amino acid uptake • increasing protein synthesis • decreasing oxidation of proteins

Slide 28: Although not part of the GH-IGF axis, testosterone is another hormone involved in muscle growth. Testosterone stimulates myofibril hypertrophy and inhibits protein degradation. Testosterone also stimulates the release of growth hormone and the promotion of satellite cell activation. Exercise enhances the action of testosterone, which means that it promotes the training response. Myofibril hypertrophy is the thickening of the myofiber, which is the result of the production of the proteins actin and myosin. Insulin and thyroid hormone are both necessary for the muscle to not only grow in length and size, but also to reach maturation.

Slide 29: Before we move on, I'd like to review some research from Dr. Clemens at John Hopkins University. His group performed studies on mice by creating mice without receptors for growth hormone or IGF. If there are no receptors, this means the hormone cannot bind to it's target organ and cannot carry out it's action. They compared these mice to a group of control mice. What they found was:

• Mice without a receptor for growth hormone had decreased muscle strength. This was measured by grip strength. This is how grip strength is tested in mice. The mice are held by their tail and gently lowered to the screen until they grip. Then brought almost horizontal. The grip strength meter records their strength. The implication is that growth hormone influences muscle strength. • Mice without a receptor for IGF had a lower force of contraction. This means that IGF affects muscle contractility. • Mice that lacked the growth hormone receptor had difficulty with balance, muscle contraction, strength and neuromuscular reactivity. When placed on a rotarod, which is a slowly rotating device that allows for gripping, the mice without the receptor for growth hormone fell off at a greater rate than the control mice. • In addition, older mice without growth hormone receptors started to gain weight. What's interesting is that the energy expenditure and energy intake was the same. They gained fat mass without losing lean body mass or bone density. • The mice that lacked growth hormone receptors had fewer type 1 fibers (slow twitch) and had a decrease in the diameter of the individual fibers (which is the thickness of the fiber) compared with the control mice. • Dr. Clemen's research demonstrates in animals that GH and IGF together to increase muscle strength and contractility, fiber thickness and neuromuscular reactivity while influencing fat mass and fiber type.

Slide 30: Quiz Yourself! When muscle contracts, which part of the sarcomere shortens? Choose all that apply A. I-band B. H-zone C. myosin D. actin

Slide 31: Answer Slide When muscle contracts, which part of the sarcomere shortens? Choose all that apply

A. I-band B. H-zone C. myosin D. actin Answer: A & B

Slide 32: Quiz Yourself!

Muscle increase in length in response to bone increasing in length True or False?

Slide 33: Answer Slide

Muscle increase in length in response to bone increasing in length True or False?

Answer: True As bone increases in length, the muscles are placed under a sustained stretch which stimulates an increase in the number of sarcomeres aligning end to end, increasing the muscle length.

Slide 34: Quiz Yourself!

GH and IGF work together as anabolic factors by:

A. increasing protein synthesis B. increasing amino acid uptake C. decreasing oxidation of proteins D. a and b E. a, b and c

Slide 35: Quiz Yourself!

GH and IGF work together as anabolic factors by:

A. increasing protein synthesis B. increasing amino acid uptake C. decreasing oxidation of proteins D. a and b E. a, b and c

Answer: E

Slide 36: In addition to hormones, there are other factors that influence muscle growth. Mechanical load is an important factor in stimulating the muscle fiber to grow longer by forming new sarcomeres. Before we can discuss load, we need to review the types of muscle contractions. When the muscle contracts and it's shortening, which is demonstrated by lifting a weight up, it's a concentric contraction. If the man in this picture, was lifting this dumbbell up, his bicep would be contracting and shortening. If he slowly lowered the weight down, this is called an eccentric biceps dumbbell curl, an eccentric contraction. Lengthening and contracting. If he just held the weight in this position, the muscle would be neither shortening nor lengthening, the muscle length stays constant. This is called an isometric contraction. In all three types of contractions, the muscle is actively working.

Slide 37: Muscle soreness is associated eccentric contraction and eccentric contractions are thought to produce greater gains in muscle strength. Most activities involve both eccentric and concentric contractions, but walking downhill involves greater eccentric loading. I recently hiked Mt. Whitney, which ended up being 14 hour day trip for us, and this is an image of me hiking down 99 switch backs. Although it is more work going up, because your muscles must concentrically contract to lift your body weight against gravity, I find going downhill more difficult, one reason is I have 40+ year old knees! The other is because going downhill involves more eccentric loading, particularly steep downhills. Eccentric contractions are associated with more muscle injury, and we'll discuss this topic in a couple weeks. A lengthening contraction, eccentric contraction, favors the formation of new sarcomeres and new myofilaments.

Slide 38: In an isometric contraction, some sarcomeres are lengthening and some are shortening. Thus, there is some new sarcomere formation. Essentially, muscle hypertrophy occurs with eccentric and isometric contractions more than with concentric contractions. A maintained passive stretch, an isometric exercise, stimulates the formation of new sarcomeres and new myofilaments (myofibrillogenesis), they align end to end, thus lengthening the muscle fiber. A forceful intermediate stretch, however, results in new myofilaments around the myofibrils that have already been formed; the result is increased size but not length.

Slide 39: When sarcomeres are increased in number and align in series (end to end), this lengthens the muscle, but also increases the contraction velocity. When sarcomeres are aligned in parallel, force increases.

Slide 40: When an overload stimulus is applied to muscle, that results in an increase in actin and myosin, increasing the number of sarcomeres aligned in parallel, this increases the muscle fiber diameter and increases the cross sectional area of the muscle.

Slide 41: Lynn and Morgan tested the hypothesis that sarcomeres increase in length with an eccentric load by exercising rats on a treadmill on an incline or a decline and then measuring fiber amount and sarcomere length in the vastus intermedius muscles. More total sarcomeres were found in the rats declining on the treadmill. More sarcomeres in series were also found in the rats declining, demonstrating that eccentric loading produces greater muscle length and sarcomere number.

Slide 42: These studies show how adaptable muscle is. In fact, skeletal muscle is the most adaptable tissue in the human body! Think about how quickly muscle can visibly change, increasing in size or decreasing in size! But, size isn't everything. This is a picture of a body builder from Venice Beach! In body-building, strength is not an important factor. The goal is to have well defined, symmetric musculature. For athletes, strength and performance is very important. The key for athletes is to build muscle that results in an increases in strength and power. Some call this optimal hypertrophy. Extra bulk that does not have significant strength and power contributions, can be detrimental to performance.

Slide 43: Myofibrillar hypertrophy (also known as sarcomere hypertrophy) is the increase in the contractile units of muscle. Sarcoplasmic hypertrophy is the increase in size of the sarcoplasm. In this image, the sarcoplasm is shown in pink and the muscle fibers are shown in red. Sarcoplasm is necessary to support muscle growth and supply the muscle fibers with ATP and the two will increase together, to some extent. But a difference in training methods can increase sarcoplasm more than the sarcomeres or vice versa. This distinction is important because an increase in sarcoplasm is an increase in non-contractile units, meaning there is an increase in size of the muscle without an increase in strength or power. There is a lot of controversy surrounding myofibrillar versus sarcoplasmic hypertrophy.

Slide 44: How do we promoting muscle hypertrophy? There are three proposed mechanisms that promote muscle hypertrophy from exercise: mechanical tension, muscle damage and metabolic stress. Exercise intensity increases tension which stimulates growth. Muscle damage, generally from eccentric exercises, initiates an inflammatory response that activates satellite cells. Metabolic stress is the build-up of hydrogen ions, inorganic phosphate and lactate, from anaerobic metabolism associated with resistance training, which stimulates a hormonal response.

Slide 45: We've been talking about increases in muscle size based on hypertrophy. What about hyperplasia? Hyperplasia is an increase in cell number and as I mentioned earlier, this occurs mostly during embryonic development. But, there are some exceptions. Recall during step 3, the proliferation process of embryonic myogenesis that myoblasts are increasing in number. Once the muscle has matured, the myogenic progenitor cells (myoblasts) will enter quiescence and become inactive. We call these cells satellite cells. When muscle is damaged and needs to be repaired and new cells need to be replaced, these satellite cells can be activated and differentiate into muscle fibers. This increases the number of muscle fibers (essentially increasing the cell number). Satellite cells not only differentiate into muscle fibers, but also facilitate hypertrophy by increasing the ability of the muscle cell to synthesize more actin and myosin. There is actually quite a bit of debate about muscle hypertrophy versus hyperplasia in the exercised muscle. You have a reading assignment on the topic by Jose (Joey) Antonio, who is the president and founder of the International Society of Sports Nutrition.

Slide 46: Quiz Yourself! When a muscle shortens and contracts, what type of contraction is this? A. Eccentric B. Isometric C. Concentric D. A and C

Slide 47: Quiz Yourself! When a muscle shortens and contracts, what type of contraction is this? A. Eccentric B. Isometric C. Concentric D. A and C Answer: C Concentric

Slide 48: Quiz Yourself! When sarcomeres increase and they are aligned in parallel, this increases the velocity of contraction.

True or False?

Slide 49: Quiz Yourself! When sarcomeres increase and they are aligned in parallel, this increases the velocity of contraction.

True or False?

Answer: False. When sarcomeres align end to end, the velocity of contraction increases, when they align in parallel, the force of contraction increases.

Slide 50: Quiz Yourself!

Proposed mechanism of muscle hypertrophy are: (choose all that apply)

A. muscle damage B. mechanical tension C. metabolic stress D. controlled growth

Slide 51: Quiz Yourself!

Proposed mechanism of muscle hypertrophy are: (choose all that apply)

A. muscle damage B. mechanical tension C. metabolic stress D. controlled growth Answer: A, B and C