Anatomy of the Muscle

1. Anatomy of the Muscle Mr. Neuberger

2. Types of Muscle • Skeletal Muscle • Defined as voluntary, striated muscle that’s attached to one or more bones • Striations are overlapping arrangement of the contractile proteins • Muscle cells are called fibers or myofibers due to their length • Smooth Muscle • Cardiac Muscle

3. Functions of Muscle • Movement • Allow us to move from place to place and move individual body parts • Also aid in the movement of body contents during breathing, circulation, digestion, etc • Stability • Help maintain posture, prevent unwanted movement • Antigravity muscles • Stabilize joints by constant tension on tendons and bones • Heat Production • Muscles are responsible for as much as 85% of the body’s heat production, vital to function of all enzymes and therefore to all metabolism • Control of Body Openings and Passages • Eye • Mouth • Urethra and Anus

4. Properties of Muscle • Excitability • Responsiveness • Property of all living cells, developed to the highest degree in muscle and nerve cells • Exhibit electrical and mechanical responses to stimuli • Conductivity • Local electrical excitation produced at the point o muscle stimulation is conducted throughout entire plasma membrane • Contractility • Shorten substantually when they are stimulated • Pull on bones(skeletal muscles) as a result • Extensibility • Can stretch as much as three times their contracted length • Most cells in body show little to no extensibilty • Elasticity • When tension in muscle fiber is released, it recoils to its original length

5. Connective Tissue • Endomysium • Thin sleeve of loose connective tissue that surrounds each muscle fiber • Creates room for blood capillaries and nerve fibers • Perimysium • Connective tissue sheath that wraps muscle fibers together in bundles called fascicles. Carries the larger nerves and blood vessels as well as stretch receptors called muscle spindles • Epimysium • Fibrous sheath that surrounds entire muscle • Grades into the fascia • Fascia • Sheet of connective tissue that separates neighboring muscle or muscle groups from each other and subcutaneous tissue

6. Muscle Shapes • Fusiform • Thick in the middle and tapered at the end • Biceps brachii is an example • Parallel • Fairly uniform width and parallel fascicles • Rectus abdominus • Span longer distances and shorten more than other muscles but produce limited force • Triangular(Convergent) • Fan-shaped, broad at origin and narrow at insertion • Relatively strong due to the number of fibers in the wide part of the muscle • Pectoralis major is an example

7. Muscle Shapes cont. • Pennate • Feather-shaped • Fascicles insert obliquely on a tendon that runs the length of the muscle • Unipennate- approach tendon from one side • Semimembranosus • Bipennate- approach tendon from both sides • Rectus femorus • Multipennate- multiple “feathers” with quills approaching one point • Deltoid • Circular • Form rings around body openings • Orbicularis oculi

8. Muscle Shapes

9. Muscle Attachments • Indirect • The muscle ends short of the bony destination • The gap is bridged by a fibrous band called a tendon • Strong continuity from muscle to bone • Aponeurosis- tendon is broad sheet, example is in the palm of the hand • Direct(Fleshy) • To the naked eye, appears as if muscle is attaching directly to bone • Small gaps filled by collagen fibers • Some muscle will insert on fascia or tendon of another muscle, not bone

10. Functional Groups of Muscles • Prime Mover(Agonist) • Muscle that produces the most force during a joint action • Synergist • Aids the prime mover • Stabilizer • Antagonist • Opposes action of prime mover • Antagonistic pair (triceps and brachialis) • Fixator • Prevents the bone from moving • Scapula during a dumbell curl

11. Intrinsic vs Extrinsic • Intrinsic muscles are entirely contained within a particular region • Extrinsic muscles act upon a designated region but has its origin elsewhere

12. Ultrastructure of Muscle Fibers • Sarcolemma • Plasma membrane of the muscle fiber • Sarcoplasm • Cytoplasm of the muscle fiber • Myofibrils • Sarcoplasm is occupied by long protein bundles called myofibrils • Myoglobin • Red pigment within the cells that stores oxygen needed for muscular activity • Sarcoplasmic Reticulum • Smooth ER, forms a network around each myofibril

13. Ultrastructure of Muscle Fibers cont. • Terminal cisternae • Dilated end-sacs cross the muscle fiber from one side to the other • Transverse (T) Tubules • Sarcolemma turns inward at many points to form tunnels called T tubules, penetrate through the cell and emerge on the other side

16. Myofilaments • Thick Filaments • Made of several hundred molecules called myosin • One half moves right, one half moves left • Thin Filaments • Combination of two intertwined strands of protein • Tropomyosin counteracts action of myosin by blocking it • Elastic Filaments • Made of titin • Flank each thick filament and anchor it to the Z disc

17. Contractile Proteins • The two main proteins are Myosin and Actin • They do the work of shortening the muscle fiber • Tropomyosin and Troponin are called the regulatory proteins • Act like a switch to determine when it can contract and when it cannot

18. Striations and Sarcomeres • A bands • Anisotropic • Consists of thick filaments lying side by side and partially overlap the thin filaments • I bands • Isotropic • Consists of thin filaments only, lying side by side • H band • Lighter region of the A band where thin filaments do not reach

19. Striations and Sarcomeres cont. • M line • Line where the thick filaments originate • Z disc(line) • Bisects the I band • Dark, narrow, and provides anchorage for the thin filaments and elastic filaments • Sarcomere • Segment of the myofibril from one Z disc to the next

22. The Nerve-Muscle Relationship • Skeletal muscles are innervated by somatic motor neurons • Cell body lies in the brain stem and spinal cord and their axons, somatic motor fibers, lead to the skeletal muscles • Neuromuscular Junction • The nerve fiber branches and establishes several points of contact • Point where the nerve and muscle fiber meet, there is an indentation • Synaptic Knob- bulbous swelling at each synapse • Space between is called synaptic cleft

23. The Synaptic Knob • Contains spheroid organelles called synaptic vesicles • Filled with a chemical called acetylcholine (ACh) • When a nerve signal arrives at the synaptic knob, some of the vesicles release their ACh by exocytosis • Diffuses across the cleft and binds to membranes called ACh receptors on the sarcolemma • Sarcolemma has infoldings called junctional folds that increase the membrane surface area

24. Basal Lamina • Thin layer composed partly of collagen and glycoproteins, which separates it from the surrounding connective tissue • Passes through the synaptic cleft and virtually fills it, also covers the Schwann cell of the NMJ • Acetylcholinesterase is found in both the sarcolemma and basal lamina • Breaks down the ACh after it has stimulated the muscle cell, this is important for turning off the muscle contraction

25. The Motor Unit • The muscle fibers innervated by a singular nerve fiber all function in unison and they are called a Motor Unit • The muscle fibers of a single motor unit are not all clustered together but they are dispersed throughout a muscle • When they are stimulated, they cause a weak contraction over a wide area, not just localized to a twitch • Roughly 200 muscle fibers per motor neuron • Small motor units have 3-6 muscle fibers per nerve fiber • Large motor units have roughly 1000 muscle fibers per nerve fiber

26. The Blood Supply • The muscular system, as a whole, receives about 1.25L of blood per minute at rest (1/4 of the blood pumped by the heart) • Heavy exercise will cause the muscles to receive 11.6L/min (3/4 heart output) • Working muscle has a great demand for glucose and oxygen • Blood capillaries will run through the endomysium to reach every muscle fiber • These capillaries will coil when the muscle is contracted so that they have enough slack for when the muscle stretches back out

27. Contraction and Relaxation • 4 stages, 10 steps • Excitation • Excitation-Contraction Coupling • Contraction • Relaxation

28. Excitation • (1) A nerve signal arrives at the synaptic cleft • (2) Synaptic vesicles release ACh , which diffuses across the synaptic cleft and binds to receptors on the sarcolemma • Opens ion gates in the sarcolemma, resulting in sodium and potassium movements through membrane that electrically excites the muscle fiber • (3) A self-propagating wave of excitation spreads down the length of the fiber and into the T tubules

29. Excitation-Contraction Coupling • (4) Electrical events in the T tubules lead to the opening of calcium gates in the sarcoplasmic reticulum • Releases calcium into the cytosol • (5) Ca²+ attaches to the troponin molecules in the thin filaments. Troponin induces the tropomyosin to shift position to fall into the groove between the two F actin filaments and exposing the active sites on the G actin • (6) Myosin has been “waiting” in a flexed position with a molecule of ATP bound to its head. • ATP + myosin ATPase = ADP + P

30. Contraction • (7) Myosin forms a link, cross-bridge, with one of the active sites of actin, and releases ADP and P • (8) Myosin flexes and tugs on the thin filament. The thin filament slides a short distance along the thick filament, called the power stroke. • Myosin binds to a new ATP, lets go of the thin filament, breaks down the ATP, and recocks. • As long as the muscle is contracting, the thin filament is never entirely released, other myosin heads hold on • Steps 6-8 will occur repeatedly and produce the sliding filament theory • Can shorten up to 40%

31. Relaxation • (9) Stimulation of the muscle must stop in order for the muscle to relax. • The motor neuron stops firing and releasing ACh, so the muscle fiber is no longer electrically excited • (10) The sarcoplasmic reticulum reabsorbs the calcium ions and stores them until the next time the muscle is stimulated • Without calcium, troponin moves back into the position that blocks the active sites and prevents myosin-actin cross-bridges from closing

32. Muscle Growth • Skeletal muscle fibers are incapable of mitosis. • Exercise stimulates the muscle to produce more myofilaments, thus myofibrils grow thicker • At a certain point a large myofibril splits longitudinally • Well-condition muscles also develop more mitochondria, myoglobin and glycogen, and a greater density of blood capillaries

33. Slow twitch (Type I) Fibers • Abundant mitochondria, myoglobin, and glycogen • Well adapted to aerobic respiration • Means for making ATP but does not generate lactic acid as byproduct • Take longer to contract, but do not fatigue easily

34. Fast Twitch (Type II) Fibers • Rich in enzymes for anaerobic fermentation • Process that is independent of oxygen but produces lactic acid • React to a stimulus faster but fatigue easily • Quick responses, not endurance