1. Muscles and Muscle Tissue
Human Anatomy & Physiology
Chapter 9 (Pages 275 – 319)

2. Three Types of Muscle Tissue
1. Skeletal muscle tissue:
Attached to bones and skin
Striated
Voluntary (conscious control)
Powerful
2. Cardiac muscle tissue:
Only in the heart
Involuntary
3. Smooth muscle tissue:
In the walls of hollow organs (stomach, urinary bladder, airways)
Not striated
Skeletal muscle is the primary topic in this unit
Longest muscle cells
Skeletal, striated, voluntary
Responsible for mobility, tires easily
Cardiac Muscle
Makes up heart walls
Controlled by nervous system
Cardiac, striated, involuntary
Smooth Muscle
Function = force fluids and other substances through internal body channels
Visceral, nonstriated, involuntary
Walls of hollow organs (digestive, urinary, reproductive tracts)

3. Skeletal – attached to skeleton
Cardiac – walls of the heart
Smooth – walls of hollow organs, airway
Table 9.3

4. Muscle Functions
1. Movement of bones or fluids (blood) 2. Maintaining posture and body position 3. Stabilizing joints 4. Heat generation (especially skeletal muscle)

5. Skeletal Muscle
Each muscle is served by one artery, one nerve, and one or more veins
Connective tissue sheaths of skeletal muscle:
Epimysium: surrounds entire muscle
Perimysium: surrounds fascicles (groups of muscle fibers)
Endomysium: surrounds each muscle fiber

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

7. Table 9.1

8. Microscopic Anatomy of Skeletal Muscle Fiber
Cylindrical cell 10 to 100 m in diameter, up to 30 cm long
Multiple nuclei
Many mitochondria
Glycosomes for glycogen storage, myoglobin for O2 storage
Myofibrils – contractile filaments
Sarcoplasmic reticulum – stores and releases Calcium when needed for contraction
T tubules – conduct impulses deep into muscle

9. Diagram of part of a muscle fiber showing the myofibrils
Sarcolemma
Mitochondrion
Myofibril
Nucleus
Diagram of part of a muscle fiber showing the myofibrils
(One myofibril is extended from the cut end of the fiber)
Table 9.1

10. Sarcomere
Smallest contractile unit (functional unit) of a muscle fiber
Composed of thick and thin myofilaments made of contractile proteins

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

12. Longitudinal section of filaments within one sarcomere of a myofibril
Thick filament
Thin filament
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
Myosin molecule
Actin subunits
Figure 9.3

13. 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
Sarcomeres shorten, muscle fibers shorten, whole muscle shortens

14. Fully relaxed sarcomere of a muscle fiber1
Fully relaxed sarcomere of a muscle fiber 2
Fully contracted sarcomere of a muscle fiber
Figure 9.6

15. Requirements for Skeletal Muscle Contraction
1. Activation: neural stimulation at neuromuscular junction 2. Excitation-contraction coupling: a. Generation and propagation of an action potential along the sarcolemma b. Final trigger: a brief rise in intracellular Ca2+ levels (Ca2+ released from the sarcoplasmic reticulum)

16. 2. Excitation-Contraction (E-C) Coupling
Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments
Latent period:
Time when E-C coupling events occur
Time between AP initiation and the beginning of contraction
AP is propagated along sarcomere to T tubules
Voltage-sensitive proteins stimulate Ca2+ release from SR
Ca2+ is necessary for contraction

17. Terminal cisterna of SR
Setting the stage
Axon terminal
of motor neuron
Synaptic cleft
Action potential
is generated
ACh
Sarcolemma
Terminal cisterna of SR
Muscle fiber
Ca2+
One sarcomere
Figure 9.11, step 1

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

19. 3. Role of Calcium (Ca2+) in Contraction
3a. At low intracellular Ca2+ concentration:
Tropomyosin blocks the active sites on actin
Myosin heads cannot attach to actin
Muscle fiber relaxes
3b. At higher intracellular Ca2+ concentrations:
Ca2+ binds to troponin
Troponin changes shape and moves tropomyosin away from active sites
Events of the cross bridge cycle occur
When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends

20. 4. Cross Bridge Cycle
Continues as long as the Ca2+ signal and adequate ATP are present
Cross bridge formation—high-energy myosin head attaches to thin filament
Working (power) stroke—myosin head pivots and pulls thin filament
Cross bridge detachment —ATP attaches to myosin head and the cross bridge detaches
“Cocking” of the myosin head—energy from hydrolysis of ATP cocks the myosin head into the high-energy state

21. Figure 9.12 Thin filament Actin Ca2+ Myosin cross bridge Thick
ADP Pi
Thick
filament
Myosin 1
Cross bridge formation.
ADP
ADP Pi
ATP
hydrolysis Pi 4
Cocking of myosin head. 2
The power (working) stroke.
ATP
ATP 3
Cross bridge detachment.
Figure 9.12

22. Motor Unit: The Nerve-Muscle Functional Unit
Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies
Small motor units in muscles that control fine movements (fingers, eyes)
Large motor units in large weight-bearing muscles (thighs, hips)

23. neuromuscular junctions
Spinal cord
Motor neuron
cell body
Muscle
Nerve
Motor
unit 1
unit 2
fibers
neuron
axon
Axon terminals at
neuromuscular junctions
Axons of motor neurons extend from the spinal cord to the
muscle. There each axon divides into a number of axon
terminals that form neuromuscular junctions with muscle
fibers scattered throughout the muscle.
Figure 9.13a

24. Muscle Twitch
Response of a muscle to a single, brief threshold stimulus
Three phases of a twitch:
Latent period: events of excitation-contraction coupling
Period of contraction: cross bridge formation; tension increases
Period of relaxation: Ca2+ reentry into the SR; tension declines to zero

25. Myogram showing the three phases of an isometric twitch
Latent
period
Single
stimulus
Period of
contraction
relaxation
Myogram showing the three phases of an isometric twitch
Figure 9.14a

26. Graded Muscle Responses
Variations in the degree of muscle contraction
Required for proper control of skeletal movement
Responses are graded by:
Changing the frequency of stimulation
A single stimulus results in a single contractile response—a muscle twitch
If stimuli are given quickly enough, fused (complete) tetany results
Changing the strength of the stimulus

27. A single stimulus is delivered. The muscle contracts and relaxes
Contraction
Relaxation
Stimulus
Single stimulus
single twitch
A single stimulus is delivered. The muscle contracts and relaxes
Figure 9.15a

28. Response to Change in Stimulus Strength
Threshold stimulus: stimulus strength at which the first observable muscle contraction occurs
Muscle contracts more vigorously as stimulus strength is increased above threshold
Contraction force is precisely controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into action

29. Maximal stimulus Threshold
Stimulus strength
Proportion of motor units excited
Strength of muscle contraction
Maximal contraction
Maximal
stimulus
Threshold
Figure 9.16

30. Response to Change in Stimulus Strength
Size principle: motor units with larger and larger fibers are recruited as stimulus intensity increases

31. Figure 9.18a

32. Figure 9.18b

33. Muscle Metabolism: Energy for Contraction
ATP is the only source used directly for contractile activities
Available stores of ATP are depleted in 4–6 seconds
ATP is regenerated by:
Direct phosphorylation of ADP by creatine phosphate (CP)
Anaerobic pathway (glycolysis)
Aerobic respiration

34. Glucose (from glycogen breakdown or delivered from blood)
Energy source: glucose
Glycolysis and lactic acid formation
(b) Anaerobic pathway
Oxygen use: None
Products: 2 ATP per glucose, lactic acid
Duration of energy provision:
60 seconds, or slightly more
Glucose (from
glycogen breakdown or
delivered from blood)
Glycolysis
in cytosol
Pyruvic acid
Released
to blood
net gain 2
Lactic acid O2
ATP
Figure 9.19b

35. Anaerobic Pathway No O2 used! At 70% of maximum contractile activity:
Bulging muscles compress blood vessels
Oxygen delivery is impaired
Pyruvic acid is converted into lactic acid
Lactic acid:
Diffuses into the bloodstream
Used as fuel by the liver, kidneys, and heart
Converted back into pyruvic acid by the liver

36. Glucose (from glycogen breakdown or delivered from blood)
Energy source: glucose; pyruvic acid;
free fatty acids from adipose tissue;
amino acids from protein catabolism
(c) Aerobic pathway
Aerobic cellular respiration
Oxygen use: Required
Products: 32 ATP per glucose, CO2, H2O
Duration of energy provision: Hours
Glucose (from
glycogen breakdown or
delivered from blood) 32 O2
H2O
CO2
Pyruvic acid
Fatty
acids
Amino
Aerobic respiration
in mitochondria
ATP
net gain per
glucose
Figure 9.19c

37. Aerobic Pathway Requires O2
Produces 95% of ATP during rest and light to moderate exercise
Fuels: stored glycogen, then blood borne glucose, pyruvic acid from glycolysis, and free fatty acids
Glucose + O2  CO2 + H2O + ATP

38. Short-duration exercise
Prolonged-duration
exercise
ATP stored in
muscles is
used first.
ATP is formed
from creatine
Phosphate
and ADP.
Glycogen stored in muscles is broken
down to glucose, which is oxidized to
generate ATP.
ATP is generated by
breakdown of several
nutrient energy fuels by
aerobic pathway. This
pathway uses oxygen
released from myoglobin
or delivered in the blood
by hemoglobin. When it
ends, the oxygen deficit is
paid back.
Figure 9.20

39. Muscle Fatigue Physiological inability to contract Occurs when:
Ionic imbalances (K+, Ca2+, Pi) interfere with E-C coupling
Prolonged exercise damages the SR and interferes with Ca2+ regulation and release
Total lack of ATP occurs rarely, during states of continuous contraction, and causes contractures (continuous contractions)

40. Force of Muscle Contraction
The force of contraction is affected by:
Number of muscle fibers stimulated (recruitment)
Relative size of the fibers—hypertrophy of cells increases strength
Frequency of stimulation— frequency allows time for more effective transfer of tension to noncontractile components
Length-tension relationship—muscles contract most strongly when muscle fibers are 80–120% of their normal resting length

41. 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

42. Sarcomeres excessively
greatly
shortened
Sarcomeres at
resting length
Sarcomeres excessively
stretched
75%
100%
170%
Optimal sarcomere
operating length
(80%–120% of
resting length)
Figure 9.22

43. 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
Metabolic pathways for ATP synthesis:
Oxidative fibers—use aerobic pathways
Glycolytic fibers—use anaerobic glycolysis
Three fiber types:
Slow oxidative fibers (Type I)
Fast oxidative fibers (Type IIa)
Fast glycolytic fibers (Type IIb)

44. Maximum duration of use Mitochondrial density
Fiber Type
Type I fibers
Type II a fibers
Type II x fibers
Type II b fibers
Contraction time
Slow
Moderately Fast
Fast
Very fast
Size of motor neuron
Small
Medium
Large
Very large
Resistance to fatigue
High
Fairly high
Intermediate
Low
Activity Used for
Aerobic
Long-term anaerobic
Short-term anaerobic
Maximum duration of use
Hours
<30 minutes
<5 minutes
<1 minute
Power produced
Very high
Mitochondrial density
Capillary density
Oxidative capacity
Glycolytic capacity
Major storage fuel
Triglycerides
Creatine phosphate, glycogen

45. Table 9.2

46. Predominance of fast glycolytic (fatigable) fibers Small load
of slow oxidative
(fatigue-resistant)
fibers
Contractile
velocity
Contractile
duration
Figure 9.23

47. FO SO FG
Figure 9.24

49. The greater the load, the less the muscle contraction
Light load
Intermediate load
Heavy load
Stimulus
The greater the load, the less the muscle
shortens and the shorter the duration of
contraction
The greater the load, the
slower the contraction
Figure 9.25

50. Effects of Exercise Aerobic (endurance) exercise: Leads to increased:
Muscle capillaries
Number of mitochondria
Myoglobin synthesis
Results in greater endurance, strength, and resistance to fatigue
May convert fast glycolytic fibers into fast oxidative fibers

51. Effects of Resistance Exercise
Resistance exercise (typically anaerobic) results in:
Muscle hypertrophy (due to increase in fiber size)
Increased mitochondria, myofilaments, glycogen stores, and connective tissue

52. The Overload Principle
Forcing a muscle to work hard promotes increased muscle strength and endurance
Muscles adapt to increased demands
Muscles must be overloaded to produce further gains

53. Why Warm up? Treppe - muscles increase strength of contraction due to:
increased availability of Ca2+
Increased warmth due to activity causes an increase in the efficiency of muscle enzymes

54. Smooth Muscle Longitudinal layer of smooth muscle (shows smooth
muscle fibers in
cross section)
Small
intestine
Mucosa
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

55. 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

56. Table 9.3

57. Table 9.3

58. 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+
Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner

59. Figure 9.28a

60. Figure 9.28b

61. Contraction of Smooth Muscle
Slow, synchronized contractions
Cells are electrically coupled by gap junctions
Some cells are self-excitatory (depolarize without external stimuli); act as pacemakers for sheets of muscle
Rate and intensity of contraction may be modified by neural and chemical stimuli
Sliding filament mechanism
Final trigger is  intracellular Ca2+
Ca2+ is obtained from the SR and extracellular space

62. Table 9.3

63. Special Features of Smooth Muscle Contraction
Stress-relaxation response:
Responds to stretch only briefly, then adapts to new length
Retains ability to contract on demand
Enables organs such as the stomach and bladder to temporarily store contents
Length and tension changes:
Can contract when between half and twice its resting length
Hyperplasia
Muscle cells divide to multiply numbers
EX: response of uterus to estrogen

64. Table 9.3

65. Types of Smooth Muscle Single-unit (visceral) smooth muscle:
Sheets contract rhythmically as a unit (gap junctions)
Arranged in opposing sheets and exhibit stress-relaxation response
Multiunit smooth muscle:
Located in large airways, large arteries, arrector pili muscles, and iris of eye
Gap junctions are rare
Arranged in motor units
Graded contractions occur in response to neural stimuli

66. Developmental Aspects
Cardiac and skeletal muscle become amitotic, but can lengthen and thicken
Skeletal muscle cells have limited regenerative ability
Injured heart muscle is mostly replaced by connective tissue
Smooth muscle regenerates throughout life
Athletics and training can improve neuromuscular control

67. Developmental Aspects
Female skeletal muscle makes up 36% of body mass
Male skeletal muscle makes up 42% of body mass, primarily due to testosterone
Body strength per unit muscle mass is the same in both sexes
With age, connective tissue increases and muscle fibers decrease
By age 30, loss of muscle mass (sarcopenia) begins
Regular exercise reverses sarcopenia

68. Muscular Dystrophy Group of inherited muscle-destroying diseases
Muscles enlarge due to fat and connective tissue deposits
Muscle fibers atrophy
Duchenne muscular dystrophy (DMD):
Most common and severe type
Inherited, sex-linked, carried by females and expressed in males (1/3500) as lack of dystrophin
Victims become clumsy and fall frequently; usually die of respiratory failure in their 20s
No cure, but viral gene therapy or infusion of stem cells with correct dystrophin genes show promise