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 protein filament = light band
Myosin = thick protein 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)

11. The Sarcomere Functional unit of skeletal muscle
Sarcomeres arranged end to end along a myofibril
Arrangement of actin and myosin within the sarcomere produces a striped appearance
Actin = light
Myosin = dark
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
nimation

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

17. Changes to the sarcomere during contraction
I band gets smaller
Z lines move closer together
H zone decreases
Length of A bands doesn’t change
Length of filaments doesn’t change
animation

18. Sliding Filament Theory
Model of muscle contraction in which actin “slides” past myosin to cause sarcomere shortening
Involves 5 different molecules and calcium
Myosin
Actin
Tropomyosin
Troponin
ATP
myosin

19. 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

20. Click the image to view the infographic

21. At Rest

22. Sliding Filament Theory
1. The brain or spinal cord sends an impulse to the muscle; acetylcholine released at the neuromuscular junction

23. Sliding Filament Theory
2. Impulse triggered in the sarcolemma of the muscle fiber; impulse travels along T-tubules

24. Sliding Filament Theory
3. Calcium release: sarcoplasmic reticulum releases calcium; calcium diffuses through muscle fiber

25. Sliding Filament Theory
4. Active Site exposed: Ca2+ binds to troponin; tropomoyosin shifts; active sites for myosin are exposed

26. Sliding Filament Theory
5. Cross-bridge formation: Myosin head binds to actin

27. Sliding Filament Theory
6. Powerstroke: ADP and Pi are released from myosin; myosin moves, pulling the actin with it; sarcomere shortens

28. Sliding Filament Theory
7. Release: ATP binds to myosin; myosin released from actin; ATP reverts into ADP and Pi.
Myosin is ready to form another crossbridge

29. Sliding Filament Theory
8. Relaxation: Impulse stops, calcium is released from troponin; tropomyosin covers the active site
Ca+ is transported back into the SR

30. Contraction animation
Muscle Contraction
Contraction ends when nerve impulses stop AND calcium ion concentration returns to normal
Contraction animation

31. Muscle stimulation Muscles shorten to produce tension (force; pull)
Movement occurs when tension overcomes resistance
Amount of tension produced depends on
Frequency of fiber stimulation
Number of fibers activates

32. Frequency of muscle fiber stimulation
single stimulus-contraction-relaxation sequence (twitch) not enough to produce movement
successive stimuli  rise in tension (summation)
Incomplete tetanus = tension peaks; relaxation is decreased
Complete tetanus = relaxation phase completely eliminated

33. Frequency of muscle fiber stimulation

34. Tetanus shot? Lockjaw?
Protects against infection by bacterium Clostridium tetani which can cause tetanus

35. Number of muscle fibers activated
Motor unit = all the muscle fibers controlled by a single neuron
The more precise the motor movement, the smaller the motor unit ( 1 nerve – 2 or 3 fibers)
over time, more motor units are involved to produce smooth movements (recruitment)

36. Muscle Tone Muscle tone = resting tension
Stabilizes the position of bones
Low muscle tone = limp

37. atrophy Occurs when a muscle is not stimulated
Muscle fibers become smaller and weaker
Initially reversible
Dying muscle fibers are not replaced
Why physical therapy is important

38. Hypertrophy
Excessive muscle stimulation
Result = big muscles

39. Isotonic and Isometric Contraction
Depends on pattern of tension production and overall change in length of muscle
Isotonic = tension rises and muscle length changes
EX: lifting something
Isometric = tension rises but muscle length doesn’t change
EX: postural muscles

40. Slow vs fast skeletal muscle fibers
Slow fibers
Endurance
Contract slowly but for long periods of time
Large amounts of mitochondria
Fast fibers
Speed
Large in diameter
Densely packed myofibrils
Science of Marathon Running

41. Energy Muscle movements require HUGE amounts of ATP
~600 trillion molecules per second
ATP in an energy transfer molecule not an energy storage molecules so not much ATP is stored
Creatine phosphate (CP) can release stored energy to produce ATP; large amounts of CP stored

42. Click the image to view the infographic

45. Muscle Sketches 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)
Contracted sarcomere should be shorter than the relaxed