1. Muscular System

2. The Muscular System Three basic muscle types are found in the body
Skeletal muscle
Cardiac muscle
Smooth muscle

3. Characteristics of Muscles
(muscle cell = muscle fiber)
Excitability (responsiveness or irritability):
Contractility:
Extensibility:
Elasticity:

4. Skeletal Muscle Characteristics
Most are attached by tendons to bones
Cells are multinucleate
Striated
Voluntary
surrounded and bundled by connective tissue

5. Smooth Muscle Characteristics
Has no striations
Single nucleus
Involuntary
Found mainly in the walls of hollow organs

6. Cardiac Muscle Characteristics
Has striations
Usually has a single nucleus
Joined to another muscle cell at an intercalated disc
Involuntary
Found only in the heart

7. Table 9.3

8. Connective Tissue Wrappings of Skeletal Muscle
Endomysium
Dense regular
Perimysium
fibrous
Epimysium
loose
Fascia

9. (wrapped by perimysium)
Epimysium
Epimysium
Bone
Perimysium
Tendon
Endomysium
Muscle fiber
in middle of
a fascicle
(b)
Blood vessel
Fascicle
(wrapped by perimysium)
Endomysium
(between individual
muscle fibers)
Perimysium
Fascicle
Muscle fiber
(a)
Figure 9.1

10. Table 9.1

11. Skeletal Muscle Attachments
Sites of muscle attachment
Bones
Cartilages
Connective tissue coverings

12. Function of Muscles Produce movement Maintain posture Stabilize joints
Generate heat

13. Microscopic Anatomy of a Skeletal Muscle Fiber
Cylindrical cell 10 to 100 m in diameter, up to 30 cm long
Multiple peripheral nuclei
Many mitochondria
Glycosomes for glycogen storage, myoglobin for O2 storage
Also contain myofibrils, sarcoplasmic reticulum, and T tubules

14. Microscopic Anatomy of Skeletal Muscle

15. Microscopic Anatomy of Skeletal Muscle
Myofibril
Bundles of myofilaments

16. Features of a Sarcomere
Contractile unit
Thick filaments: run the entire length of an A band
Thin filaments: run the length of the I band and partway into the A band
Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
H zone: lighter midregion where filaments do not overlap
M line: line of protein myomesin that holds adjacent thick filaments together

17. Thin (actin)
filament
Z disc
H zone
Z disc
Thick (myosin)
filament
I band
A band
Sarcomere
I band
M line
(c)
Small part of one myofibril enlarged to show the myofilaments
responsible for the banding pattern. Each sarcomere extends from
one Z disc to the next.
Sarcomere
Z disc
M line
Z disc
Thin (actin)
filament
Elastic (titin)
filaments
Thick
(myosin)
filament
(d)
Enlargement of one sarcomere (sectioned lengthwise). Notice the
myosin heads on the thick filaments.
Figure 9.2c, d

18. Sarcomere

19. Longitudinal section of filaments within one sarcomere of a myofibril
Thick filament
Thin filament
In the center of the sarcomere, the thick
filaments lack myosin heads. Myosin heads
are present only in areas of myosin-actin overlap.
Thick filament
Thin filament
Each thick filament consists of many
myosin molecules whose heads protrude
at opposite ends of the filament.
A thin filament consists of two strands
of actin subunits twisted into a helix
plus two types of regulatory proteins
(troponin and tropomyosin).
Portion of a thick filament
Portion of a thin filament
Myosin head
Tropomyosin
Troponin
Actin
Actin-binding sites
Heads
Tail
Active sites
for myosin
attachment
ATP-
binding
site
Actin
subunits
Flexible hinge region
Myosin molecule
Actin subunits
Figure 9.3

20. Microscopic Anatomy of Skeletal Muscle

21. Microscopic Anatomy of Skeletal Muscle
Sarcolemma – (carry electrical current and connect to tendons
Sarcoplasmic reticulum- store Ca.

22. Sarcoplasmic Reticulum (SR)
Network of smooth endoplasmic reticulum surrounding each myofibril
Pairs of terminal cisternae form perpendicular cross channels
Functions in the regulation of intracellular Ca2+ levels

23. T Tubules Continuous with the sarcolemma
Penetrate the cell’s interior at each A band–I band junction
Associate with the paired terminal cisternae to form triads that encircle each sarcomere

24. Part of a skeletal muscle fiber (cell) I band A band I band Z disc
H zone
Z disc
Myofibril
M line
Sarcolemma
Triad: •
T tubule •
Terminal
cisternae
of the SR (2)
Sarcolemma
Tubules of
the SR
Myofibrils
Mitochondria
Figure 9.5

25. Nerve Stimulus to Muscles
Skeletal muscles must be stimulated by a nerve to contract

26. Nerve Stimulus to Muscles
Neuromuscular junctions
Synaptic cleft

27. Transmission of Nerve Impulse to Muscle
Neurotransmitter
for skeletal muscle is acetylcholine
Neurotransmitter attaches to receptors on the sarcolemma
Sarcolemma becomes permeable to sodium (Na+)
Sodium rushing into the cell generates an action potential

28. Figure 9.8 1 2 3 4 5 6 Myelinated axon of motor neuron Action
potential (AP)
Axon terminal of
neuromuscular
junction
Nucleus
Sarcolemma of
the muscle fiber
Action potential arrives at
axon terminal of motor neuron. 1
Ca2+
Synaptic vesicle
containing ACh
Ca2+
Voltage-gated Ca2+ channels
open and Ca2+ enters the axon
terminal. 2
Mitochondrion
Synaptic
cleft
Axon terminal
of motor neuron
Ca2+ entry causes some
synaptic vesicles to release
their contents (acetylcholine)
by exocytosis. 3
Fusing synaptic
vesicles
Junctional
folds of
sarcolemma
ACh
Acetylcholine, a
neurotransmitter, diffuses across
the synaptic cleft and binds to
receptors in the sarcolemma. 4
Sarcoplasm of
muscle fiber
ACh binding opens ion
channels that allow simultaneous
passage of Na+ into the muscle
fiber and K+ out of the muscle
fiber. 5
Na+ K+
Postsynaptic membrane
ion channel opens;
ions pass.
ACh effects are terminated
by its enzymatic breakdown in
the synaptic cleft by
acetylcholinesterase. 6
Ach–
Degraded ACh
Postsynaptic membrane
ion channel closed;
ions cannot pass.
Na+
Acetyl-
cholinesterase K+
Figure 9.8

29. 2 1 1 Axon terminal Open Na+ Channel Closed K+ Channel Synaptic cleft
Action potential + + + o + +
Na+ K+ a t i z r i 2 a
Generation and propagation of the
action potential (AP) o l p d e o f v e W a 1 1
Local depolarization: generation of the
end plate potential on the sarcolemma
Sarcoplasm of muscle fiber
Figure 9.9, step 2

30. 1 2 Action potential is propagated along the sarcolemma and down
the T tubules. 1
Steps in
E-C Coupling:
Sarcolemma
Voltage-sensitive
tubule protein
T tubule
Ca2+
release
channel 2
Calcium
ions are
released.
Terminal
cisterna
of SR
Ca2+
Figure 9.11, step 4

31. 3 4 Actin Troponin Tropomyosin blocking active sites Ca2+ Myosin
Calcium binds to
troponin and removes
the blocking action of
tropomyosin. 3
Active sites exposed and
ready for myosin binding
Contraction begins 4
Myosin
cross
bridge
The aftermath
Figure 9.11, step 7

32. Cross bridge formation.
Actin
Ca2+
Thin filament
ADP
Myosin
cross bridge Pi
Thick filament
Myosin 1
Cross bridge formation.
Figure 9.12, step 1

33. The power (working) stroke.
ADP Pi 2
The power (working) stroke.
Figure 9.12, step 3

34. Cross bridge detachment.
ATP 3
Cross bridge detachment.
Figure 9.12, step 4

35. Sliding Filament Model of Contraction
In the relaxed state, thin and thick filaments overlap only slightly
During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line
As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens

36. The Sliding Filament Theory of Muscle Contraction

37. The Sliding Filament Theory

38. Contraction of a Skeletal Muscle
Muscle fiber contraction is “all or none”
not all fibers may be stimulated during the same interval
Graded responses

39. Types of Graded Responses
Twitch
Not a normal muscle function

40. Types of Graded Responses
Tetanus (summing of contractions)
One contraction is immediately followed by another
The muscle does not completely return to a resting state
The effects are added

41. Types of Graded Responses
Unfused (incomplete) tetanus
Fused (complete) tetanus
Figure 6.9a, b

42. Muscle Response to Strong Stimuli
force depends upon the number of fibers stimulated
can continue to contract unless they run out of energy (ATP)

43. Energy for Muscle Contraction
Initially, muscles used stored ATP for energy
Only 4-6 seconds worth of ATP is stored by muscles
After this initial time, other pathways must be utilized to produce ATP

44. Energy for Muscle Contraction
Direct phosphorylation
Muscle cells contain creatine phosphate (CP)
After ATP is depleted, ADP is left
CP transfers energy to ADP, to regenerate ATP
CP supplies are exhausted in about 20 seconds

45. Energy for Muscle Contraction
Aerobic Respiration
This is a slower reaction that requires continuous oxygen

46. Energy for Muscle Contraction
Anaerobic glycolysis
Pyruvic acid is converted to lactic acid
This reaction is not as efficient, but is fast
Huge amounts of glucose are needed
Lactic acid produces muscle fatigue
Figure 6.10b
Slide 6.26a

47. Muscle Fatigue and Oxygen Debt
When a muscle is fatigued, it is unable to contract
The common reason for muscle fatigue is oxygen debt
Oxygen is required to get rid of accumulated lactic acid
Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

48. Types of Muscle Contractions
Isotonic contractions
The muscle shortens
Isometric contractions
The muscle is unable to shorten

49. Muscle Tone Some fibers are contracted even in a relaxed muscle
under involuntary control
The more muscle is used the more tone it becomes.

50. Muscles and Body Movements

51. Effects of Exercise on Muscle
Results of increased muscle use
Increase in muscle size
Increase in muscle strength
Increase in muscle efficiency
Muscle becomes more fatigue resistant

52. Large number of muscle fibers activated Muscle and sarcomere
stretched to
slightly over 100%
of resting length
Large
muscle
fibers
High
frequency of
stimulation
Contractile force
Figure 9.21

53. Muscle Fiber Type Classified according to two characteristics:
Speed of contraction: slow or fast, according to:
Speed at which myosin ATPases split ATP
Pattern of electrical activity of the motor neurons

54. Muscle Fiber Type Metabolic pathways for ATP synthesis:
Oxidative fibers—use aerobic pathways
Glycolytic fibers—use anaerobic glycolysis

55. Muscle Fiber Type Three types: Slow oxidative fibers
Fast oxidative fibers
Fast glycolytic fibers

56. Table 9.2

57. Longitudinal layer of smooth muscle (shows smooth muscle fibers in
cross section)
Small
intestine
Mucosa
(a)
(b) Cross section of the intestine showing the smooth muscle layers (one circular and the other longitudinal) running at right angles to each other.
Circular layer of
smooth muscle
(shows longitudinal
views of smooth
muscle fibers)
Figure 9.26

58. Peristalsis
Alternating contractions and relaxations of smooth muscle layers that mix and squeeze substances through the lumen of hollow organs
Longitudinal layer contracts; organ dilates and shortens
Circular layer contracts; organ constricts and elongates

59. Microscopic Structure
Spindle-shaped fibers: thin and short compared with skeletal muscle fibers
Connective tissue: endomysium only
SR: less developed than in skeletal muscle
Pouchlike infoldings (caveolae) of sarcolemma sequester Ca2+
No sarcomeres, myofibrils, or T tubules

60. Myofilaments in Smooth Muscle
Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2)
Thick filaments have heads along their entire length
No troponin complex; protein calmodulin binds Ca2+

61. Myofilaments in Smooth Muscle
Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner
Dense bodies: proteins that anchor noncontractile intermediate filaments to sarcolemma at regular intervals

62. Figure 9.28a

63. Figure 9.28b