1. Skeletal Muscle Physiology

2. Muscular System Functions
Body movement (Locomotion)
Maintenance of posture
Respiration
Diaphragm and intercostal contractions
Communication (Verbal and Facial)
Constriction of organs and vessels
Peristalsis of intestinal tract
Vasoconstriction of b.v. and other structures (pupils)
Heart beat
Production of body heat (Thermogenesis)

3. Properties of Muscle
Excitability: capacity of muscle to respond to a stimulus
Contractility: ability of a muscle to shorten and generate pulling force
Extensibility: muscle can be stretched back to its original length
Elasticity: ability of muscle to recoil to original resting length after stretched

4. Types of Muscle Skeletal Smooth Cardiac Attached to bones
Makes up 40% of body weight
Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement
Voluntary in action; controlled by somatic motor neurons
Smooth
In the walls of hollow organs, blood vessels, eye, glands, uterus, skin
Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow,
In some locations, autorhythmic
Controlled involuntarily by endocrine and autonomic nervous systems
Cardiac
Heart: major source of movement of blood
Autorhythmic

5. Connective Tissue Sheaths
Connective Tissue of a Muscle
Epimysium. Dense regular c.t. surrounding entire muscle
Separates muscle from surrounding tissues and organs
Connected to the deep fascia
Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle
Contains b.v and nerves
Endomysium. Loose connective tissue that surrounds individual muscle fibers
Also contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissue
Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

6. Nerve and Blood Vessel Supply
Motor neurons
stimulate muscle fibers to contract
Neuron axons branch so that each muscle fiber (muscle cell) is innervated
Form a neuromuscular junction (= myoneural junction)
Capillary beds surround muscle fibers
Muscles require large amts of energy
Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

7. Muscle Tissue Types

8. Skeletal Muscle Long cylindrical cells Many nuclei per cell Striated
Voluntary
Rapid contractions
Skeletal muscle attaches to our skeleton. *The muscle cells a long and cylindrical. *Each muscle cell has many nuclei. *Skeletal muscle tissue is striated. It has tiny bands that run across the muscle cells. *Skeletal muscle is voluntary. We can move them when we want to. *Skeletal muscle is capable of rapid contractions. It is the most rapid of the muscle types.

9. Basic Features of a Skeletal Muscle
Muscle attachments
Most skeletal muscles run from one bone to another
One bone will move – other bone remains fixed
Origin – less movable attach- ment
Insertion – more movable attach- ment

10. Basic Features of a Skeletal Muscle
Muscle attachments (continued)
Muscles attach to origins and insertions by connective tissue
Fleshy attachments – connective tissue fibers are short
Indirect attachments – connective tissue forms a tendon or aponeurosis
Bone markings present where tendons meet bones
Tubercles, trochanters, and crests

11. Skeletal Muscle Structure
Composed of muscle cells (fibers), connective tissue, blood vessels, nerves
Fibers are long, cylindrical, and multinucleated
Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length
Develop from myoblasts; numbers remain constant
Striated appearance
Nuclei are peripherally located

12. Muscle Attachments

13. Antagonistic Muscles

14. Microanatomy of Skeletal Muscle
In this unit we will primarily study skeletal muscle. Each muscle cell is called a muscle fiber. Within each muscle fiber are many myofibrils.

15. Muscle Fiber Anatomy Sarcolemma - cell membrane
Surrounds the sarcoplasm (cytoplasm of fiber)
Contains many of the same organelles seen in other cells
An abundance of the oxygen-binding protein myoglobin
Punctuated by openings called the transverse tubules (T-tubules)
Narrow tubes that extend into the sarcoplasm at right angles to the surface
Filled with extracellular fluid
Myofibrils -cylindrical structures within muscle fiber
Are bundles of protein filaments (=myofilaments)
Two types of myofilaments
Actin filaments (thin filaments)
Myosin filaments (thick filaments)
At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemma
When myofibril shortens, muscle shortens (contracts)

16. Sarcoplasmic Reticulum (SR)
SR is an elaborate, smooth endoplasmic reticulum
runs longitudinally and surrounds each myofibril
Form chambers called terminal cisternae on either side of the T-tubules
A single T-tubule and the 2 terminal cisternae form a triad
SR stores Ca++ when muscle not contracting
When stimulated, calcium released into sarcoplasm
SR membrane has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR after contraction

17. Sarcoplasmic Reticulum (SR)

18. Parts of a Muscle

19. Sarcomeres: Z Disk to Z Disk
Sarcomere - repeating functional units of a myofibril
About 10,000 sarcomeres per myofibril, end to end
Each is about 2 µm long
Differences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance.
A bands: a dark band; full length of thick (myosin) filament
M line - protein to which myosins attach
H zone - thick but NO thin filaments
I bands: a light band; from Z disks to ends of thick filaments
Thin but NO thick filaments
Extends from A band of one sarcomere to A band of the next sarcomere
Z disk: filamentous network of protein. Serves as attachment for actin myofilaments
Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment
Sarcomeres: Z Disk to Z Disk

20. Structure of Actin and Myosin

21. Myosin (Thick) Myofilament
Many elongated myosin molecules shaped like golf clubs.
Single filament contains roughly 300 myosin molecules
Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally.
Myosin heads
Can bind to active sites on the actin molecules to form cross-bridges. (Actin binding site)
Attached to the rod portion by a hinge region that can bend and straighten during contraction.
Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction
Myosin (Thick) Myofilament

22. Actin (Thin) Myofilaments
Thin Filament: composed of 3 major proteins
F (fibrous) actin
Tropomyosin
Troponin
Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere.
Composed of G actin monomers each of which has a myosin-binding site (see yellow dot)
Actin site can bind myosin during muscle contraction.
Tropomyosin: an elongated protein winds along the groove of the F actin double helix.
Troponin is composed of three subunits:
Tn-A : binds to actin
Tn-T :binds to tropomyosin,
Tn-C :binds to calcium ions.
Actin (Thin) Myofilaments

23. Now, putting it all together to perform the function of muscle: Contraction
Dark and light bands can be seen in the muscle fiber and also in the smaller myofibrils. An enlargement of the myofibril reveals that they are made of smaller filaments or myofilaments. *There is a thick filament called myosin and *a thin filament called actin. Note the I band, A band H zone or band and Z disc or line. These will be discussed shortly.

24. Z line
A small section of a myofibril is illustrated here. Note the thick myosin filaments are arranged between overlapping actin filaments. *The two Z lines mark the boundary of a sarcomere. The sarcomere is the functional unit of a muscle cell .We will examine how sarcomeres function to help us better understand how muscles work.

25. A myosin molecule is elongated with an enlarged head at the end.

26. Many myosin molecules form the thick myosin filament
Many myosin molecules form the thick myosin filament. It has many heads projecting away from the main molecule.

27. The thinner actin filament is composed of three parts: actin, tropomyosin and troponin.

28. H Band
Here is a sarcomere illustrating the thin actin and thick myosin filaments. The area of the sarcomere has only myosin is called the H band.

29. Sarcomere Relaxed
Here is another diagram of a sarcomere. Note the A band. It is formed by both myosin and actin filaments. The part of the sarcomere with only actin filaments is called the I band. This is a sarcomere that is relaxed.

30. Sarcomere Partially Contracted
This sarcomere is partially contracted. Notice than the I bands are getting shorter.

31. Sarcomere Completely Contracted
The sarcomere is completely contracted in this slide. The I and H bands have almost disappeared.

32. Which filament has moved as the sarcomere contracted
Which filament has moved as the sarcomere contracted? Note the thick myosin filaments have not changed, but the thin actin filaments have moved closer together.

33. The actin filaments are moved by the heads of the myosin filaments
The actin filaments are moved by the heads of the myosin filaments. In step one the myosin head attaches to an actin filament to create a cross bridge. Step two shows that the attached myosin head bends to move the actin filament. The myosin head as expended energy to create this movement. This is a power stroke or working stroke. Step three shows that energy in the form of ATP will unhook the myosin head. In step 4 the myosin head is cocked and ready to attach to an actin filament to start another power stroke.

34. Binding Site Tropomyosin Troponin Ca2+
The string of green circles represents an actin filament. There are binding sites in the filament for the attachment of myosin heads. *In a relaxed muscle the binding sites are covered by tropomyosin. The tropomyosin has molecules of troponin attached to it. *Calcium, shown in yellow, will attach to troponin. *Calcium will change the position of the troponin, tropomyosin complex. *The troponin, tropomyosin complex has now moved so that the binding sites are longer covered by the troponin, tropomyosin complex.

35. Myosin
The binding sites are now exposed and myosin heads are able to attach to form cross bridges.*

36. This diagram shows the microanatomy of skeletal muscle tissue again
This diagram shows the microanatomy of skeletal muscle tissue again. *The blue sarcoplasmic reticulum is actually the endoplasmic reticulum. It stores calcium. *The mitochondria are illustrated in orange. They generate ATP, which provides the energy for muscle contractions.

37. Excitation-Contraction Coupling
Muscle contraction
Alpha motor neurons release Ach
ACh produces large EPSP in muscle fibers (via nicotinic Ach receptors
EPSP evokes action potential
Action potential (excitation) triggers Ca2+ release, leads to fiber contraction
Relaxation, Ca2+ levels lowered by organelle reuptake

38. Excitation-Contraction Coupling

39. Excitation-Contraction Coupling

40. Sliding Filament Model of Contraction
Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree
In the relaxed state, thin and thick filaments overlap only slightly
Upon stimulation, myosin heads bind to actin and sliding begins

41. How striated muscle works: The Sliding Filament Model
The lever movement drives displacement of the actin filament relative to the myosin
head (~5 nm), and by deforming internal elastic structures, produces force (~5 pN).
Thick and thin filaments interdigitate and “slide” relative to each other.

42. Neuromuscular Junction
The next few slides will summarize the events of a muscle contraction.
The nerve impulse reaches the neuromuscular junction (myoneural junction).

43. Neuromuscular Junction
Region where the motor neuron stimulates the muscle fiber
The neuromuscular junction is formed by :
1. End of motor neuron axon (axon terminal)
Terminals have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)
2. The motor end plate of a muscle
A specific part of the sarcolemma that contains ACh receptors
Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

44. Neuromuscular Junction

45. Motor Unit: The Nerve-Muscle Functional Unit
A motor unit is a motor neuron and all the muscle fibers it supplies
The number of muscle fibers per motor unit can vary from a few (4-6) to hundreds ( )
Muscles that control fine movements (fingers, eyes) have small motor units
Large weight-bearing muscles (thighs, hips) have large motor units

46. Motor Unit: The Nerve-Muscle Functional Unit
Muscle fibers from a motor unit are spread throughout the muscle
Not confined to one fascicle
Therefore, contraction of a single motor unit causes weak contraction of the entire muscle
Stronger and stronger contractions of a muscle require more and more motor units being stimulated (recruited)

47. Motor Unit All the muscle cells controlled by one nerve cell
A motor unit is all the muscle cells controlled by one nerve cell. This diagram represents two motor units. Motor unit one illustrates two muscle cells controlled by one nerve cell. When the nerve sends a message it will cause both muscle cells to contract. Motor unit two has three muscle cells innervated by one nerve cell.

48. Acetylcholine is released from the motor neuron.

49. Acetylcholine Opens Na+ Channel
Acetylcholine binds with receptors in the muscle membrane to allow sodium ions to enter the muscle.

50. The influx of sodium will create an action potential in the sarcolemma
The influx of sodium will create an action potential in the sarcolemma. Note: This is the same mechanism for generating action potentials for the nerve impulse. The action potential travels down a T tubule. As the action potential passes through the sarcoplamic reticulum it stimulates the release of calcium ions. Calcium binds with troponin to move tropomyosin and expose the binding sites. Myosin heads attach to the binding sites of the actin filament and create a power stroke. ATP detaches the myosin heads and energizes them for another contraction. The process will continue until the action potentials cease. Without action potentials the calcium ions will return to the sarcoplasmic reticulum.

51. Muscle Contraction Summary
Nerve impulse reaches myoneural junction
Acetylcholine is released from motor neuron
Ach binds with receptors in the muscle membrane to allow sodium to enter
Sodium influx will generate an action potential in the sarcolemma

52. Muscle Contraction (Cont’d)
Action potential travels down T tubule
Sarcoplamic reticulum releases calcium
Calcium binds with troponin to move the troponin, tropomyosin complex
Binding sites in the actin filament are exposed

53. Muscle Contraction (cont’d)
Myosin head attach to binding sites and create a power stroke
ATP detaches myosin heads and energizes them for another contaction
When action potentials cease the muscle stop contracting

54. Contraction Speed

55. Refresher Course in Muscle Physiology
4/11/2003
Myosin is a hexamer:
2 myosin heavy chains
4 myosin light chains
C terminus
2 nm
Coiled coil of two a helices
Myosin is a Molecular Motor
Myosin head: retains all of the motor functions of myosin,
i.e. the ability to produce movement and force.
Myosin S1 fragment
crystal structure
Ruegg et al., (2002)
News Physiol Sci 17:
NH2-terminal catalytic
(motor) domain
neck region/lever arm
Nucleotide
binding site
Catalytic domain responsible for binding & hydrolysis of ATP, and binding actin.
Neck region responsible for transport of the load.
Converter responsible for energy transduction.
EB2003--Susan Brooks

56. Chemomechanical coupling – conversion of chemical energy
(ATP about 7 kcal x mole-1) into force/movement.
ATP is unstable thermodynamically
Two most energetically favorable steps:
1. ATP binding to myosin
2. Phosphate release from myosin
Rate of cycling determined by M·ATPase activity and external load
Adapted from Goldman & Brenner (1987) Ann Rev Physiol 49:

57. Shortening Velocity Vependent on ATPase Activity
Different myosin heavy chains (MHCs) have different ATPase activities.
There are at least 7 separate skeletal muscle MHC genes…arranged in series
on chromosome 17.
Two cardiac MHC genes located in tandem on chromosome 14.
The slow b cardiac MHC is the predominant gene expressed in slow fibers
of mammals.
Goldspink (1999) J Anat 194:

58. Power Output: The Most Physiologically Relevant
Marker of Performance
Power = work / time
= force x distance / time
= force x velocity
Peak power obtained at intermediate loads and intermediate
velocities.
Figure from Berne and Levy, Physiology
Mosby—Year Book, Inc., 1993.

59. Three Potential Actions During Muscle Contraction:
Biceps muscle shortens
during contraction
shortening
(Isotonic: shortening against fixed load, speed dependent on M·ATPase activity and load)
isometric
Most likely to cause
muscle injury
lengthening
Biceps muscle lengthens
during contraction

60. Motor Unit Ratios Back muscles Finger muscles Eye muscles 1:100 1:10
Motor units come indifferent sizes. *The ratio is about one nerve cell to 100 muscle cells in the back. *Finger muscles have a much smaller ratio of 1:10. *Eye muscles have a 1:1 ratio because of the precise control needed in vision.

61. Recall The Motor Unit:
motor neuron and the muscle fibers it innervates
Spinal
cord
The smallest amount of
muscle that can be activated
voluntarily.
Gradation of force in skeletal
muscle is coordinated largely
by the nervous system.
Recruitment of motor units
is the most important means
of controlling muscle tension.
Since all fibers in the motor
unit contract simultaneously,
pressures for gene expression
(e.g. frequency of stimulation,
load) are identical in all fibers
of a motor unit.
To increase force:
Recruit more M.U.s
Increase freq.
(force –frequency)

62. Physiological profiles of motor units:
all fibers in a motor unit are of the same fiber type
Slow motor units contain slow fibers:
Myosin with long cycle time and therefore uses ATP at a slow rate.
Many mitochondria, so large capacity to replenish ATP.
Economical maintenance of force during isometric contractions and efficient performance of repetitive slow isotonic contractions.
Fast motor units contain fast fibers:
Myosin with rapid cycling rates.
For higher power or when isometric force produced by slow motor units is insufficient.
Type 2A fibers are fast and adapted for producing sustained power.
Type 2X fibers are faster, but non-oxidative and fatigue rapidly.
2X/2D not 2B.
Modified from Burke and Tsairis, Ann NY Acad Sci 228: , 1974.

63. Increased use: strength training
Early gains in strength appear to be predominantly due to neural factors…optimizing recruitment patterns.
Long term gains almost solely the result of hypertrophy i.e. increased size.

64. The PI(3)K/Akt(PKB)/mTOR pathway is a
crucial regulator of skeletal muscle
hypertrophy/atrophy.
Application of IGF-I to C2C12 myotube cultures induced both increased width and phosphor-ylation of downstream targets of Akt (p70S6 kinase, p70S6K; PHAS-1/4E-BP1; GSK3) but did NOT activate the calcineurin pathway.
Treatment with rapamycin almost completely prevented increase in width of C2C12 myotubes.
Treatment with cyclosporin or FK506 does not prevent myotube growth in vitro or compensatory hypertrophy in vivo
Recovery of muscle weight after following reloading is blocked by rapamycin but not cyclosporin.
Rommel et al. (2001) Nature Cell Biology 3, 1009.

65. Performance Declines with Aging
--despite maintenance of physical activity
100 80
Performance (% of peak) 60 40
Shotput/Discus
Marathon 20
Basketball (rebounds/game) 10 20 30 40 50 60
Age (years)
D.H. Moore (1975) Nature 253:
NBA Register, Edition

66. Number of motor units declines during aging
- extensor digitorum brevis muscle of humans
AGE-ASSOCIATED
ATROPHY DUE TO BOTH…
Individual fiber atrophy
(which may be at least
partially preventable and
reversible through exercise).
Loss of fibers
(which as yet appears
irreversible).
Campbell et al., (1973) J Neurol Neurosurg Psych 36:

67. Motor unit remodeling with aging
Central
nervous
system
Muscle
Motor
neuron
loss
AGING
Fewer motor units
More fibers/motor unit

68. Mean Motor Unit Forces:
FF motor units get smaller in old age and decrease in number
S motor units get bigger with no change in number
Decreased rate of force generation and POWER!!
225
200
175
Adult
Old
150
125
Maximum Isometric Force (mN)
100 75 50 25 FF FI FR S
Motor Unit Classification
Kadhiresan et al., (1996)
J Physiol 493:

69. Refresher Course in Muscle Physiology
4/11/2003
Muscle injury may play a role in the development of
atrophy with aging.
Muscles in old animals are more susceptible to contraction-
induced injury than those in young or adult animals.
Muscles in old animals show delayed and impaired recovery
following contraction-induced injury.
By way of introduction, I’ll briefly reiterate some points that John made in the previous talk…
Increased susceptibility
Decreased ability to recover and prolonged deficits
Because muscles are injured repeatedly throughout life, these two observations, along with others provide circumstantial evidence that muscle injury plays a role in the development of atrophy and weakness with aging.
Muscles of animals of all ages, except perhaps the oldest-old, can continue to adapt to the habitual level of activity.
Following severe injury, muscles in old animals display
prolonged, possibly irreversible, structural and functional
deficits.
EB2003--Susan Brooks

70. Disorders of Muscle Tissue
Muscle tissues experience few disorders
Heart muscle is the exception
Skeletal muscle – remarkably resistant to infection
Smooth muscle – problems stem from external irritants

71. Disorders of Muscle Tissue
Muscular dystrophy – a group of inherited muscle destroying disease
Affected muscles enlarge with fat and connective tissue
Muscles degenerate
Types of muscular dystrophy
Duchenne muscular dystrophy
Myotonic dystrophy

72. Disorders of Muscle Tissue
Myofascial pain syndrome – pain is caused by tightened bands of muscle fibers
Fibromyalgia – a mysterious chronic-pain syndrome
Affects mostly women
Symptoms – fatigue, sleep abnormalities, severe musculoskeletal pain, and headache

73. Muscular Dystrophy: A frequently fatal disease of muscle deterioration
Muscular dystrophies have in the past been classified based on subjective and sometimes
subtle differences in clinical presentation, such as age of onset, involvement of particular
muscles, rate of progression of pathology, mode of inheritance.
Since the discovery of dystrophin, numerous genetic disease loci have been linked to protein
products and to cellular phenotypes, generating models for studying the pathogenesis of the
dystrophies.
Proteins localized in the nucleus, cytosol, cytoskeleton, sarcolemma, and ECM.
Cohn and Campbell (2000) Muscle Nerve 23:

74. transmission of force to extracellular matrix
Dystrophin function:
transmission of force to extracellular matrix
DGC
dystrophin
dystroglycan (a and b)
sarcoglycans (a, b, g, d)
syntrophins (a, b1)
dystrobrevins (a, b)
sarcospan
laminin-a2 (merosin)
(Some components of
the dystrophin glycoprotein
complex are relatively
recent discoveries, so one
cannot assume that all
players are yet known.)
Cohn and Campbell (2000) Muscle Nerve 23:

75. Oxidative and Glycolytic Fibers

76. ATP
ATP or adenosine triphosphate is the form of energy that muscles and all cells of the body use. *The chemical bond between the last two phosphates has just the right amount of energy to unhook myosin heads and energize them for another contraction. Pulling of the end phosphate from ATP will release the energy. ADP and a single phosphate will be left over. New ATP can be regenerated by reconnecting the phosphate with the ADP with energy from our food.

77. Creatine Molecule capable of storing ATP energy Creatine + ATP
Creatine phosphate + ADP
Creatine is a molecule capable of storing ATP energy. It can combine with ATP to produce creatine phosphate and ADP. The third phosphate and the energy from ATP attaches to creatine to form creatine phosphate.

78. Creatine Phosphate Molecule with stored ATP energy
Creatine phosphate + ADP
Creatine + ATP
Creatine phosphate is an important chemical to muscles. *It is a molecule that is able to store ATP energy. *Creatine phosphate can combine with an ADP * to produce creatine and ATP. This process occurs faster than the synthesis of ATP from food.

79. Muscle Fatigue Lack of oxygen causes ATP deficit
Lactic acid builds up from anaerobic respiration
Muscle fatigue is often due to a lack of oxygen that causes ATP deficit. Lactic acid builds up from anaerobic respiration in the absence of oxygen. Lactic acid fatigues the muscle.

80. Muscle Fatigue

81. Muscle Atrophy Weakening and shrinking of a muscle May be caused
Immobilization
Loss of neural stimulation
Muscle atrophy is a weakening and shrinking of a muscle. It can be caused by immobilization or loss of neural stimulation.

82. Muscle Hypertrophy Enlargement of a muscle More capillaries
More mitochondria
Caused by
Strenuous exercise
Steroid hormones
Hypertrophy is the enlargement of a muscle. Hypertrophied muscles have more capillaries and more mitochondria to help them generate more energy. Strenuous exercise and steroid hormones can induce muscle hypertrophy. Since men produce more steroid hormones than women, they usually have more hypertrophied muscles.

83. Steroid Hormones Stimulate muscle growth and hypertrophy
Steroid hormones such as testosterone stimulate muscle growth and hypertrophy.

84. Muscle Tonus Tightness of a muscle Some fibers always contracted
Muscle tonus or muscle tone refers to the tightness of a muscle. In a muscle some fibers are always contracted to add tension or tone to the muscle.

85. Tetany Sustained contraction of a muscle
Result of a rapid succession of nerve impulses
Tetany is a sustained contraction of a muscle. It results from a rapid succession of nerve impulses delivered to the muscle.

86. Tetanus
This slide illustrates how a muscle can go into a sustained contraction by rapid neural stimulation. In number four the muscle is in a complete sustained contraction or tetanus.

87. Refractory Period
Brief period of time in which muscle cells will not respond to a stimulus
The refractory period is a brief time in which muscle cells will not respond to stimulus.

88. Refractory
The area to the left of the red line is the refractory period for the muscle contraction. If the muscle is stimulated at any time to the left of the line, it will not respond. However, stimulating the muscle to the right of the red line will produce a second contraction on top of the first contraction. Repeated stimulations can result in tetany.

89. Refractory Periods Skeletal Muscle Cardiac Muscle
Cardiac muscle tissue has a longer refractory period than skeletal muscle. This prevents the heart from going into tetany.
Skeletal Muscle
Cardiac Muscle

90. Isometric Contraction
Produces no movement
Used in
Standing
Sitting
Posture
Isometric contractions produce no movement. They are used in standing, sitting and maintaining our posture. For example, when you are standing muscles in your back and abdomen pull against each other to keep you upright. They do not produce movement, but enable you to stand.

91. Isotonic Contraction Produces movement Used in Walking
Moving any part of the body
Isotonic contractions are the types that produce movement. Isotonic contractions are used in walking and moving any part of the body.

92. Muscle Spindle

93. Muscle Spindle Responses

94. Alpha / Gamma Coactivation

95. Golgi Tendon Organs

96. Developmental Aspects: Regeneration
Cardiac and skeletal muscle become amitotic, but can lengthen and thicken
Myoblast-like satellite cells show very limited regenerative ability
Cardiac cells lack satellite cells
Smooth muscle has good regenerative ability
There is a biological basis for greater strength in men than in women
Women’s skeletal muscle makes up 36% of their body mass
Men’s skeletal muscle makes up 42% of their body mass

97. Developmental Aspects: Male and Female
These differences are due primarily to the male sex hormone testosterone
With more muscle mass, men are generally stronger than women
Body strength per unit muscle mass, however, is the same in both sexes

98. Developmental Aspects: Age Related
With age, connective tissue increases and muscle fibers decrease
Muscles become stringier and more sinewy
By age 80, 50% of muscle mass is lost (sarcopenia)
Decreased density of capillaries in muscle
Reduced stamina
Increased recovery time
Regular exercise reverses sarcopenia