1. Muscles and Muscle Tissue9
P A R T A
Muscles and Muscle Tissue

2. Introduction to the Muscular System
Microanatomy
Electrical Stimulus
Physiology
Contraction Strength
Muscle Names

3. Muscle Tissue

4. The three types of muscle tissue are skeletal, cardiac, and smooth
These types differ in structure, location, function, and means of activation
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Anatomy Review: Skeletal Muscle Tissue, page 3

5. Muscle terminology is similar
Muscle Similarities
Skeletal and smooth muscle cells are elongated and are called muscle fibers or myocytes
Muscle contraction depends on two kinds of protein myofilaments – actin (thin) and myosin (thick)
Muscle terminology is similar
Prefixes – myo or mys = muscle, sarco = flesh
Sarcolemma – muscle plasma membrane
Sarcoplasm – cytoplasm of a muscle cell
Sarcoplasmic Reticulum – Network storing calcium

6. Skeletal Muscle Tissue
Packaged in skeletal muscles that attach to and cover the bony skeleton
Has obvious stripes called striations (made by repeating patterns of actin and myosin proteins)
Is controlled voluntarily (i.e., by conscious control)
Contracts rapidly but tires easily
Is responsible for overall body motility
Is extremely adaptable and can exert forces ranging from a fraction of an ounce to over 70 pounds

7. Cardiac Muscle Tissue
Occurs only in the heart
Is striated like skeletal muscle but is not voluntary
Contracts at a fairly steady rate set by the heart’s pacemaker
Neural controls allow the heart to respond to changes in bodily needs
Intercalated discs connect muscle cells, which allows the contraction to spread across the heart for a coordinated heartbeat

8. Smooth Muscle Tissue
Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages, as well as in arterioles and the iris of the eye
Forces food and other substances through internal body channels, controls blood flow to different organs, and controls the amount of light entering the eye
It is not striated and is involuntary

9. Functional Characteristics of Muscle Tissue
Excitability, or irritability – the ability to receive and respond to stimuli, which depends on the ability to change their electric charge
Contractility – the ability to shorten forcibly (active)
Extensibility – the ability to be stretched or extended (passive)
Elasticity – the ability to recoil and resume the original resting length (passive)

10. Muscle Function
Skeletal muscles are responsible for all locomotion
Cardiac muscle is responsible for coursing the blood through the body
Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs
Muscles also maintain posture, stabilize joints, and generate heat

11. Skeletal Muscle
Each muscle is a discrete organ composed of several tissues: muscle tissue, blood vessels, nerve fibers, and connective tissue
Examples include the biceps brachii, gastrocnemius, and all those other guys you’re so familiar with!

12. Skeletal Muscle Macroanatomy
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InterActive Physiology ®:
Anatomy Review: Skeletal Muscle Tissue, pages 4-6
Figure 9.2a

13. Skeletal Muscle Macroanatomy
Bundles within Bundles!
Muscle
Fascicle
Myocyte
Myofibril
Myofilaments
(Actin & Myosin)
Wrappings
Fascia & Epimysium
Perimysium
Endomysium & Sarcolemma
Sarcoplasmic Reticulum
Figure 9.2a

14. 1 2 3 9 4 5 8 6 7

15. Skeletal Muscle Connective Tissues
The three connective tissue sheaths are:
Endomysium – fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber (myocyte)
Perimysium – fibrous connective tissue that surrounds groups of muscle cells (aka muscle fibers) called fascicles
Epimysium – an overcoat of dense regular connective tissue that surrounds the entire muscle – this is usually covered with another thicker layer called fascia

16. Skeletal Muscle: Nerve and Blood Supply
Each muscle is served by one nerve, an artery, and one or more veins
Each skeletal muscle fiber (cell) is supplied with a nerve ending that controls contraction
Contracting fibers require continuous delivery of oxygen and nutrients via arteries
Wastes must be removed via veins

17. Skeletal Muscle: Attachments
Most skeletal muscles cross over joints and are attached to bone in at least two places: the origin and the insertion
When muscles contract, the movable bone (usually distal) moves toward the immovable bone (usually proximal) – this is described as moving the muscle’s insertion (usually distal) towards the muscle’s origin (usually proximal)
Origin
Insertion

18. Skeletal Muscle: Attachments
Muscles can attach either directly or indirectly (via a tendon) to a bone:
Directly – epimysium of the muscle is fused to the periosteum of a bone
Indirectly – connective tissue wrappings (fascia) extend beyond the muscle as a tendon or aponeurosis
Tendon = cord of connective tissue that joins muscle to bone
Aponeurosis = sheet of connective tissue that joins muscle to bone

19. Microscopic Anatomy of a Skeletal Muscle Fiber I
Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma (muscle cell membrane)
Fibers are 10 to 100 m* in diameter, and up to hundreds of centimeters long
*m = microns, aka micrometers:
1 m = 1/1,000,000 of a meter

20. Microscopic Anatomy of a Skeletal Muscle Fiber II
Each cell (muscle fiber) is a syncytium (single cell with many nuclei). It is produced by the fusion of several embryonic cells whose membranes break down but whose nuclei remain.
Sarcoplasm (cytoplasm) has numerous glycosomes and a unique oxygen-binding protein called myoglobin – this allows aerobic respiration to continue beyond what is possible by breathing alone
Muscle fibers (myocytes) contain the usual organelles, plus myofibrils, sarcoplasmic reticulum, and T tubules

21. Structure and Organization of Skeletal Muscle
Table 9.1a

22. Myofibrils & Myofilaments
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Anatomy Review: Skeletal Muscle Tissue, pages 7-8
Figure 9.3b

23. Myofibrils
Myofibrils are bundles of actin & myosin proteins inside a muscle cell: densely packed, rodlike contractile elements
They make up most of the muscle volume
The arrangement of myofibrils within a fiber (cell) forms striations: perfectly aligned repeating series of dArk “A bands” and lIght “I bands” is evident

24. Sarcomeres (“Flesh-Part”)
The smallest contractile unit of a muscle
The region of a myofibril between two successive Z discs
Different parts of the sarcomere have names that use letters (I band, Z disc, H zone….)
Composed of myofilaments made up of contractile proteins
Myofilaments are of two types of protein:
thick myosin filaments and thin actin filaments

25. Sarcomeres I Band A Band 1 2 H Zone 3 M Line Z Disc 4 5 Sarcomere 6
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Figure 9.3c

26. Myofilaments: Banding Pattern
Thick myosin filaments – extend the entire length of an A band
Thin actin filaments – extend across the I band and partway into the A band
Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another
Thin filaments do not overlap thick filaments in the lighter H zone
M lines appear darker due to the presence of the protein desmin

27. Myofilaments: Formation of Striations
I Band is lightest because it only has thin filaments
A Band is dark because it has both thick and thin filaments
H Zone is lighter than the rest of the A Band because it only has thick filaments
Z Disc “iz” inside I Band!
HAM is a tasty skeletal muscle! (M Line is inside H Zone which is inside A Band!)
Ham Iz!

28. Myofilaments: Banding Pattern
Figure 9.3c,d

29. Ultrastructure of Myofilaments: Thick Filaments
Thick filaments are composed of many myosin proteins woven together
Each myosin molecule has a rod-like tail and two globular heads (like a double golf club)
Tails – two interwoven, heavy polypeptide chains
Heads – two smaller, light polypeptide chains called cross bridges – these reach out from the thick filament so they can bind to actin and pull it towards the center of the sarcomere when the muscle contracts

30. Ultrastructure of Myofilaments: Thick Filaments
Figure 9.4a,b

31. Ultrastructure of Myofilaments: Thin Filaments
Thin filaments are chiefly composed of the protein actin
Each actin molecule is a helical polymer of globular subunits called G actin
The subunits contain the active sites to which myosin cross bridges attach during contraction
Tropomyosin and troponin are regulatory molecules bound to actin

32. Ultrastructure of Myofilaments: Thin Filaments
Figure 9.4c

33. Arrangement of the Filaments in a Sarcomere
Longitudinal section within one sarcomere
Figure 9.4d

34. Structure and Organization of Muscle Filaments
Table 9.1b

36. Muscle Fibers / Myocytes / Muscle Cells6 8 5 7 9 1 2 10 3 4

37. Muscle Fibers / Myocytes / Muscle Cells
Cisternae
Myofibril
Mitochondrion
Endomysium (connective tissue)
Triad
Transverse Tubule
Sarcoplasmic Reticulum
Sarcolemma (cell membrane)
Nucleus
Muscle Fiber / Myocyte 6 8 5 7 9 1 2 10 3 4

38. Muscle Fibers / Myocytes / Muscle Cells
Muscle cells have the basic features of animal cells, but they have special features required for their function in movement.
Most of the muscle fiber is filled with myofibrils, which are bundles of thick and thin myofilaments that create the muscle contraction when signaled.
The nuclei are squeezed to the outer edge of the myocyte.
The myoctye has many mitochondria since it needs an abundant source of energy. They are scattered between myofibrils and along the outer edge of the cell.

39. Muscle Fibers / Myocytes / Muscle Cells
The cytoplasm of a muscle cell is known as sarcoplasm.
The cell membrane of the myoctye is known as the sarcolemma; it is covered with connective tissue called the endomysium.
The muscle fiber has special organelles needed to control muscle contractions, including sarcoplasmic reticulum and its cisternae, T tubules, and triads.

41. Sarcoplasmic Reticulum (SR)
SR is an elaborate, smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril
Paired terminal cisternae are swollen regions of the SR that run perpendicular to the myofibril (cistern: a storage tank)
SR functions in the regulation of intracellular calcium levels – SR releases Ca++ from its cisternae when signalled by a nerve impulse, then reabsorbs Ca++ when the contraction ends

42. Transverse Tubules
Elongated tubes called T tubules penetrate into the cell’s interior from the cell membrane at each A band–I band junction
T tubules are continuous with the sarcolemma – they form pits in the membrane that allow electrical impulses to reach from the surface of the cell deep inside it
T tubules are found with each pair of terminal cisternae where they form triads
The impulses signal the release of Ca2+ from adjacent terminal cisternae of the SR

43. T tubules and SR provide tightly linked signals for muscle contraction
Triad Relationships
T tubules and SR provide tightly linked signals for muscle contraction
A double zipper of integral membrane proteins protrudes into the intermembrane space
T tubule proteins act as voltage sensors
SR foot proteins are receptors that regulate Ca2+ release from the SR cisternae
PLAY
InterActive Physiology ®:
Anatomy Review: Skeletal Muscle Tissue, page 10

44. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

45. Muscle Fiber / Myocyte / Muscle Cell
Triad
Nucleus
Mitochondrion
Myofibril
Myofilaments / Actin & Myosin
Sarcolemma
Thick Filament / Myosin
Sarcoplasmic Reticulum
Thin Filament / Actin
Cisternae of SR
Transverse Tubule
Sarcomere
I Band
A Band
Z Disc
H Zone
M Line

46. Sarcomere
Myofilaments
Myosin
Actin
Nucleus
Figure 9.5

47. Sliding Filament Model of Contraction
Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree, which shortens the entire muscle
In the relaxed state, thin and thick filaments overlap only slightly
Upon stimulation from a nerve, myosin heads (aka, cross bridges) bind to actin and sliding begins

48. Sliding Filament Model of Contraction
Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere
As this event occurs throughout the sarcomeres, the muscle shortens

49. Sliding Filament Model of Contraction
Cross-bridges pull thin filaments towards H zone
1. Relaxed
2. Cross-Bridges Bind

50. Skeletal Muscle Contraction
In order to contract, a skeletal muscle must:
Be stimulated by a nerve ending
Propagate an electrical current, or action potential, along its sarcolemma
Have a rise in intracellular Ca2+ levels, the final trigger for contraction
Linking the electrical signal to the contraction is “excitation-contraction coupling”

51. Neuromuscular Junction

52. Nerve Stimulus of Skeletal Muscle
Skeletal muscles are stimulated by motor neurons of the somatic nervous system
Axons of these neurons travel in nerves to muscle cells
Axons of motor neurons branch profusely as they enter muscles
Each axonal branch forms a neuromuscular junction (NMJ) with a single muscle fiber
The NMJ is the point where the neuron sends a signal to the muscle cell – it is a specialized synapse

53. Neuromuscular Junction 1
The neuromuscular junction is a specialized synapse (a connection between cells where they communicate with chemical signals)
The NMJ includes the presynaptic membrane, the postsynaptic membrane, and the fluid-filled space between them (the synaptic cleft)

54. Neuromuscular Junction 2
The presynaptic membrane is part of the axon terminal, which has many small membranous sacs (called synaptic vesicles) filled with the neurotransmitter acetylcholine (ACh) – the ACh is released when the nerve is stimulated, and it acts as the chemical messenger (the neurotransmitter) between the neuron and the myocyte.
The postsynaptic membrane is called the motor end plate of a muscle fiber, which is a specific part of the sarcolemma that contains ACh receptors that bind to the ACh, receiving the message from the neuron and signaling the muscle to contract.
Though exceedingly close, axon terminals and muscle fibers are always separated by a fluid-filled space called the synaptic cleft – the ACh crosses the synaptic cleft by diffusion.

55. Neuromuscular Junction
Figure 9.7 (a-c)