1. Introduction
The Muscular System

2. Striated = striped. Skeletal muscle appears striped under a microscope
Muscle Tissue
Elongated cells, specialized for contraction
Three types of muscle tissue:
Skeletal muscle – striated/voluntary
Cardiac muscle – striated/involuntary
Smooth muscle – not striated/involuntary
Striated = striped. Skeletal muscle appears striped under a microscope

3. Skeletal Muscle Functions
Produces movement of the skeleton
Maintains posture and body position
Supports soft tissues
Guards entrances and exits
Maintains body temperature by generating heat
Skeletal muscles pull on tendons attached to bones and thereby result in movement.
2. Continuous muscle contractions maintain posture. Without this constant action, you could not sit upright without collapsing.

4. Organization of Skeletal Muscle
Organ-level structure
“Muscle” = many bundles of muscle fibers
Muscle fiber = single muscle cell (elongated)
Fascicle = bundle of fibers
fiber
fascicle
muscle

5. Organization of Skeletal Muscle
Organ-level structure
Connective Tissue layers:
Endomysium (around a fiber)
Perimysium (around a fascicle)
Epimysium (outer layer)
Connective tissue layers converge to become the tendon

6. Cellular Level Anatomy
Sarcolemma = muscle fiber membrane
Sarcoplasm = cytoplasm of muscle cell
Sarcoplasmic reticulum = stores calcium for contraction
Transverse (T) tubules = network of narrow tubes that form passageways through a muscle fiber
Each muscle fiber contains hundreds to thousands of myofibrils. Myofibrils are responsible for muscle contraction. Because they are attached to the sarcolemma at each end of the cell, their contraction shortens the entire cell. Scattered among the myofibrils are mitochondria and granules of glycogen, a source of glucose. The breakdown of glucose and the activity of the mitochondria provide the ATP needed to power muscluar contractions

7. Organization of Skeletal Muscle
Myofibrils = cytoskeletal proteins; composed of myofilaments (actin and myosin)
Alternating arrangement of myofilaments gives skeletal muscle a striated appearance
Actin = thin filament = light band
Myosin = thick filament = dark band

8. Organization of Skeletal Muscle
Nuclei (many)
sarcolemma
mitochondria
sarcoplasm
Myofibril; myofilaments

9. Organization of Skeletal Muscle
Hierarchy of organization:
Muscle
fascicle 
fiber 
myofibrils 
myofilaments (actin/myosin)

10. The Sarcomere Functional unit of skeletal muscle
Each myfobril consists of ~10,000 sarcomeres arranged end to end
Arrangement of actin and myosin within the sarcomere produce a striped appearance
Each myofibril consists of 10,000 sarcomeres arranged end to end. The sarcomere is the smallest functional unit of the muscle fiber. Interactions between the thick and thin filaments of sarcomeres are responsible for muscle contraction.

12. Sarcoplasmic reticulum
Sarcomere Structure
Z line = marks the boundaries of each sarcomere
M line = middle of the sarcomere
Sarcomere
Sarcoplasmic reticulum
Z line
Z line
M line

13. Sarcoplasmic reticulum
Sarcomere Structure
A band = Myosin (thick filament; dark band)
I band = Actin (thin filament; light band)
Sarcomere
Sarcoplasmic reticulum
Myosin
Actin
Z line
Z line
Neither type of filament spans the entire length of an individual sarcomere. The thick filaments lie in the center of the sarcomere, thin filaments at either end of the sarcomere.
Differences in densities between actin and myosin account for the banded appearance of the sarcomere.
M line
I band
A band
I band

14. Sarcoplasmic reticulum
Sarcomere Structure
Animation
H zone = no overlap between thick and thin filaments
Sarcomere
Sarcoplasmic reticulum
Myosin
Actin
Z line
H zone
Z line
M line
I band
A band
I band

15. Label…A, I, M, Z, H
animation

16. Actin Most abundant protein on earth
Active site = location of interaction with myosin
At rest, proteins (troponin and tropomyosin) block the active site on actin, preventing actin and myosin from interacting

17. Myosin
Motor protein

18. Muscle at Rest Actin and myosin lie side-by side
Myosin heads are “primed” for contraction
H zone and I band are at maximum width

19. Changes to the sarcomere during contraction
I band gets smaller
Z lines move closer together
H zone decrease
Width of A bands doesn’t change
animation

20. Sliding Filament Theory
Model of muscle contraction
Thin filament “slides” past the thick filament
Contraction = sarcomere shortening
Involves 5 different molecules and calcium
Myosin
Actin
Tropomyosin
Troponin
ATP
myosin

21. Role of Nervous System
Brain/spinal cord sends an impulse to the muscle
Impulse = electrical signal
Neuromuscular junction = nerve + muscle fiber
Acteylcholine = neurotransmitter, activates the muscle
Impulse travels through T-tubules triggering release of calcium from the sarcoplasmic reticulum

24. Muscle Contraction
The brain or spinal cord sends an impulse to the muscle

25. impulse travels through the plasma membrane (sarcolemma) and down T tubules surrounding the myofibrils

26. 4. As the impulse passes through the T tubules, it causes the sarcoplasmic reticulum to release calcium ions
Calcium ions diffuse and reach the sarcomere

27. The Ca2+ binds to troponin located on the actin filament, causing tropomoyosin to move and expose active sites for myosin

28. Myosin head binds to actin and forms a crossbridge

29. ADP and Pi are released from myosin, which causes the myosin to move
ADP and Pi are released from myosin, which causes the myosin to move. This is called the power stroke

30. ATP binds to myosin causing it to release the actin and reverting ATP into ADP and Pi. Myosin is ready to form another crossbridge and the cycle of contraction will continue until the impulse stops

31. Once the impulse stops, calcium is released from troponin causing tropomyosin to cover the active sire to prevent contraction. Ca is transported back into the SR and waits for another impulse
Rellaxation

32. Muscle Contraction
Muscle at rest

33. Muscle Contraction
Calcium binds to troponin; tropomyosin shifts, exposing active sites on actin

34. Muscle Contraction
Myosin binds to actin (cross-bridge is formed); ADP released from myosin

35. Muscle Contraction
Myosin head pivots, pulling actin toward the center of the sarcomere; ADP + P released
POWER STROKE

36. Muscle Contraction
ATP binds myosin; actin released; cross bridge is broken

37. Muscle Contraction Myosin re-extends into “ready” position
ATP ADP + P
As long as calcium is still around, another contraction cycle will occur

38. Contraction animation
Muscle Contraction
Contraction ends when
Impulses stop
Calcium ion concentration returns to normal
Contraction animation

40. Muscle Sketches
Skeletal muscle (see page 113; compare with what you see on a microscope)
Label nuclei, muscle fiber
Actin filament (see page 198)
Label actin, troponin, tropomyosin, active site
Myosin filament (see page 198)
Label head
Sarcomere (at rest and during contraction)
Label all parts (A, I, actin, myosin, H, M, Z)