1. Muscle Tissue
Kristine Krafts, M.D.

2. Muscle Tissue Lecture Objectives
Describe the location, type of contraction (e.g., strong vs. weak), and histologic features of the three types of muscle.
Compare and contrast the sarcoplasmic reticulum structure, T-tubule location, and formation of diads/triads in skeletal vs. cardiac muscle.
Draw a sarcomere at rest, showing actin and myosin filaments, Z and M lines, and I, A , and H bands. Describe how the sarcomere changes as contraction occurs. Watch youtu.be/U2TSaz8-yNQ if you need a refresher.

3. Muscle Tissue Lecture Objectives
Describe the unique structural features of smooth muscle which distinguish it from skeletal and cardiac muscle.
Describe the connective tissue sheaths that surround skeletal muscle.
Compare the ability of the three types of muscle to regenerate.

4. Muscle Tissue Lecture Outline
Introduction
Skeletal muscle
Cardiac muscle
Smooth muscle

5. Muscle Tissue Lecture Outline
Introduction

6. Fun facts about muscle terminology
“Sarco-” is from the Greek sarx (flesh)
“Myo-” is from the Greek mys (muscle)
Muscle cells are longer than they are wide, so they are also called “fibers”
There are three types of muscle cells: smooth, skeletal, and cardiac. Smooth muscle cells are non- striated; skeletal and cardiac muscle cells are striated.

7. Don’t make this mistake!
“Striated muscle” and “skeletal muscle” are sometimes used interchangeably. This is wrong, wrong, wrong! Skeletal muscle is striated – but so is cardiac muscle. So if you mean skeletal, say skeletal, not striated.

8. Muscle types and activity

9. Summary that will make sense later
Skeletal
Cardiac
Smooth
Location
Near bones
Heart wall
Walls of hollow organs and blood vessels
Nuclei
Many. Flat.
Peripheral.
1-2 per cell. Plump. Central.
One per cell.
Central.
Cell diameter
Largest
Intermediate
Smallest
Striations
Yes No
Sarcoplasmic reticulum
T tubules
At junction of A-I bands. Form triads.
At Z lines.
Form diads.
None

10. More summary that will make sense later
Skeletal
Cardiac
Smooth
Motor control
Voluntary
Involuntary
Contraction
Quick and strong
Quick, strong, rhythmic
Slow, in waves
Blood supply
Moderate
Extensive
Less abundant
Other features
Prominent fascicles
Intercalated disks, branching cells
Cells overlap; can synthesize collagen and elastin

11. Muscle Tissue Lecture Outline
Introduction
Skeletal muscle

12. Transverse section
Longitudinal section
Skeletal muscle

13. Where do skeletal muscle cells come from?
Derived from mesenchymal cells, which give rise to myoblasts.
Myoblasts: spindle-shaped. Fuse to form multinucleated myotubes which elongate.
Mature skeletal muscle cells (or fibers): long, unbranched tubes with many flattened nuclei.
Sarcoplasm contains a ton of mitochondria, glycogen granules and myoglobin.
Mature skeletal muscle cells can’t divide.

14. Abundant eosinophilic cytoplasm
Peripheral nuclei
Skeletal muscle

15. Skeletal muscle Cells are long, unbranched tubes
Can just barely see striations
Peripheral, flattened nuclei
Skeletal muscle

16. What’s in an actual skeletal muscle?
A bunch of fascicles.
Each fascicle is composed of a bunch of muscle cells (fibers).
Each muscle cell contains a bunch of myofibrils.

17. Connective tissue surrounds the cells, fascicles, and entire muscle.
Epimysium
Dense layer of connective tissue that surrounds entire muscle
Perimysium
Connective tissue that surrounds each fascicle
Endomysium
Delicate connective tissue (reticular fibers and basal lamina) that surrounds each muscle cell (fiber)

18. So what’s in a myofibril? Myofilaments.
Thin (actin) filaments
Thick (myosin) filaments

19. Bands and sarcomeres
Each myofibril has repeating subunits called sarcomeres which have bands.
The sarcomeres in each myofibril are aligned with those of adjacent myofibrils so their bands are also aligned.
A sarcomere is defined as running from Z line to Z line.

20. Myofibril, bands and sarcomere

21. Skeletal muscle with A bands, I bands, Z lines

22. Sarcomere and bands

23. When muscles contract, sarcomeres shorten!
Thin (actin) filaments
Thick (myosin) filaments
Actin filaments get pulled toward each other. The distance between the Z lines shortens, and the H band disappears.

24. Check out this video to see how the sarcomere shortens. https://youtu

25. Summary: bands and lines
A bands are composed of thick (myosin) filaments overlapping thin (actin) filaments.
H bands are in the center of A bands. They lie between the free ends of the thin filaments and contain only myosin filaments.
M lines bisect H bands. They are where adjacent thick filaments connect.
I bands contain only thin filaments.
Z lines (disks) bisect I bands. Alpha actinin attaches actin filaments to the Z line.

26. Sarcoplasmic reticulum
Sarcoplasmic reticulum is the smooth endoplasmic reticulum in muscle cells.
It is specialized to sequester & release calcium ions.
It surrounds each myofibril.

27. Transverse (T) tubules
At each A-I band junction, a tubular invagination of the sarcolemma termed a transverse (T) tubule penetrates the muscle fiber and lies next to the surface of myofibrils.

28. Terminal cisterna and triad
An expansion of the sarcoplasmic reticulum called a terminal cisterna lies on each side of the T tubule.
A triad is a complex of 2 terminal cisternae with a T tubule in between.

30. Muscle Tissue Lecture Outline
Introduction
Skeletal muscle
Cardiac muscle

31. Cardiac muscle

32. Structure of cardiac muscle cells
Cardiac muscle cells are branched and striated.
Intercalated disks glue adjacent cells together.
Sarcoplasmic reticulum is kind of sparse.
T-tubules occur at Z lines and form diads with adjacent sarcoplasmic reticulum.

33. Cardiac Muscle Cells

34. Cardiac Muscle Cells

35. Intercalated Disks
Intercalated disks are complexes of several types of cell-cell junctions!
Fascia adherens anchor actin filaments.
Macula adherens (spot desmosomes) prevent cardiac muscle fibers from tearing apart during contraction.
Gap junctions allow electrical stimuli to be passed from cell to cell.

36. Intercalated Disk
Macula
adherens
Fascia adherens
Gap junction

37. Cardiac muscle with plump nuclei (N), intercalated discs (I), and striations (S)

38. Muscle Tissue Lecture Outline
Introduction
Skeletal muscle
Cardiac muscle
Smooth muscle

39. Transverse section
Longitudinal section
Smooth muscle

40. Location and function of smooth muscle
Most smooth muscle is present in walls of hollow organs (such as intestine and uterus).
Smooth muscle is also present in walls of larger blood vessels and in the eye.
In addition to their contractile properties, smooth muscle cells produce collagen, elastin and proteoglycans.

41. Smooth Muscle Morphology
Smooth muscle cells (fibers) are fusiform (spindle-shaped) with single central nucleus.
The width of the fiber or cell is only slightly greater than the width of the nucleus.
They are packed together tightly.

42. More smooth muscle morphology
Abundant mitochondria, some rough ER and large Golgi complex.
Each cell is surrounded by basal lamina and reticular fibers.
Thin (actin) and thick (myosin) filaments are not tightly arranged but just crisscross through cell.
Intermediate filaments (desmin and vimentin) also present in cytoplasm.

43. Smooth muscle: thin and thick filaments

44. Wait, what are dense bodies?
Anchor sites for actin-myosin filament bundles.
Comparable to Z-disks of skeletal and cardiac muscle.
Located along inside of sarcolemma and scattered throughout cytoplasm.

45. Contraction of smooth muscle
Relaxed smooth muscle cell
Contracted smooth muscle cell

46. Regeneration of muscle tissue
Cardiac muscle has no regenerative ability. Death of cardiac muscle leads to replacement by dense connective tissue scar. Skeletal muscle can undergo limited regeneration. Satellite cells are inactive myoblasts that become activated and proliferate and fuse after injury. Smooth muscle is capable of active regeneration. Muscle fibers undergo mitosis and replace damaged tissue.

47. Muscle Tissue Lecture Outline
Introduction
Skeletal muscle
Cardiac muscle
Smooth muscle