1. PowerLecture: Chapter 6
The Muscular System

2. Learning Objectives
Explain the structure of muscles, from the molecular level to the organ systems level.
Explain how biochemical events occur in muscle contractions and how antagonistic muscle action refines movements.
Explain the differences in stimulation required for each type of muscle and how each responds.
Demonstrate how muscle disorders impact the function of both the muscular and skeletal systems.

3. Impacts/Issues
Pumping Up Muscles

4. Pumping Up Muscles
Can you bulk up your muscles by using a pill and do so safely?
Androstenedione and THG are available as supplements but are really steroid-like drugs unapproved by the FDA.
Another unapproved performance enhancer is creatine, a substance normally produced by the body during muscle activity.

5. Pumping Up Muscles The intricate movements of the human body are the
result of interactions between
the skeleton and muscle.

6. How Would You Vote?
To conduct an instant in-class survey using a classroom response system, access “JoinIn Clicker Content” from the PowerLecture main menu.
Dietary supplements are largely unregulated. Should they be subject to more stringent testing for effectiveness and safety?
a. Yes, or companies could make any claim about their product.
b. No, so long as the extract does no harm, let the buyer decide what to buy.

7. The Body’s Three Kinds of Muscle
Section 1
The Body’s Three Kinds of Muscle

8. The Body’s Three Kinds of Muscle
The three kinds of muscle are built and function in different ways.
Skeletal muscle, composed of long thin cells called muscle “fibers,” allows the body to move.
Smooth muscle is found in the walls of hollow organs and tubes; the cells are smaller than those of skeletal muscle and are not striated.
The heart is the only place where cardiac muscle is found.

9. Skeletal muscle One skeletal muscle fiber Fig. 6.1, p.104
Figure 6.1: The three kinds of muscle in the body and where each type is found.
One skeletal muscle fiber
Skeletal muscle
Fig. 6.1, p.104

10. Smooth muscle Smooth muscle fibers Fig. 6.1, p.104
Figure 6.1: The three kinds of muscle in the body and where each type is found.
Smooth muscle fibers
Smooth muscle
Fig. 6.1, p.104

11. Cardiac muscle Cardiac muscle fibers Fig. 6.1, p.104
Figure 6.1: The three kinds of muscle in the body and where each type is found.
Cardiac muscle fibers
Cardiac muscle
Fig. 6.1, p.104

12. The Body’s Three Kinds of Muscle
Cardiac muscle and smooth muscle are considered involuntary muscles because we cannot consciously control their contraction; skeletal muscles are voluntary muscles.
Skeletal muscle comprises the body’s muscular system.

13. © 2007 - Thomson Higher Education
© 2001 Brooks/Cole . Thomson
©2005 Brooks/Cole . Thomson
© Thomson Higher Education
TRICEPS BRACHII
BICEPS BRACHII
PECTORALIS MAJOR
DELTOID
SERRATUS ANTERIOR
TRAPIZIUS
EXTERNAL OBLIQUE
LATISSIMUS DORSI
RECTUS ABDOMINUS
GLUTEUS MAXIMUS
ADDUCTOR LONGUS
SARTORIUS
BICEPS FEMORIS
Figure 6.2: Some of the major muscles of the muscular system.
QUADRICEPS FEMORIS
GASTROCNEMIUS
TIBIALIS ANTERIOR
Fig. 6.2, p.105

14. flexor digitorum superficialis Fig. 6.20a, p.118
FigureSomemusclestoexplore
Fig. 6.20a, p.118

15. zygomaticus major Fig. 6.20b, p.118
FigureSomemusclestoexplore
Fig. 6.20b, p.118

16. The Structure and Function of Skeletal Muscles
Section 2
The Structure and Function of Skeletal Muscles

17. The Structure and Function of Skeletal Muscles
A skeletal muscle is built of bundled muscle cells.
Inside each cell are threadlike myofibrils, which are critical to muscle contraction.
The cells are bundled together with connective tissue that extends past the muscle to form tendons, which attach the muscle to bones.

18. muscle tendon (attached to bone) fluid tendon sheath bone
Figure 6.4: A tendon sheath. Notice the lubricating fluid inside each of the sheaths sketched here.
bone
Fig. 6.4, p.106

19. cells (each has its own connective tissue sheath)
muscle’s outer sheath
(connective tissue)
two bundles of muscle
cells (each has its own connective tissue sheath)
Figure 6.3: Structure of a skeletal muscle. The muscle’s cells bundled together inside a wrapping of connective tissue.
one muscle cell
one myofibril
Fig. 6.3, p.106

20. The Structure and Function of Skeletal Muscles
Bones and skeletal muscles work like a system of levers.
The human body’s skeletal muscles number more than 600.
The origin end of each muscle is designated as being attached to the bone that moves relatively little; whereas the insertion is attached to the bone that moves the most.
Because most muscle attachments are located close to joints, only a small contraction is needed to produce considerable movement of some body parts (leverage advantage).

21. The Structure and Function of Skeletal Muscles
Many muscles are arranged as pairs or in groups.
Many muscles are arranged as pairs or grouped for related function.
Some work antagonistically (in opposition) so that one reverses the action of the other.
Others work synergistically, the contraction of one stabilizes the contraction of another.
Reciprocal innervation dictates that only one muscle of an antagonistic pair (e.g. biceps and triceps) can be stimulated at a time.

22. at the same time, biceps relaxes
triceps relaxes
origin
triceps contracts,
pulls the forelimb
down
biceps contracts at
the same time, and
pulls forelimb up
at the same time, biceps relaxes
FigureAnimatedTwoopposingmusclegroupsinhumanarmsaWhenthetricepsrelaxesanditsopposingpartnerbicepscontractstheelbowjointflexesandtheforearmbendsupbWhenthetricepscontractsandthebicepsrelaxestheforearmisextendeddown
insertion
Fig. 6.5, p.107

23. The Structure and Function of Skeletal Muscles
“Fast” and “slow” muscle.
Humans have two general types of skeletal muscles:
“Slow” muscle is red in color due to myoglobin and blood capillaries; its contractions are slower but more sustained.
“Fast” or “white” muscle cells contain fewer mitochondria and less myoglobin but can contract rapidly and powerfully for short periods.

24. FigureFastandslowskeletalmuscleaThismicrographshowsacrosssectionofthedifferentkindsofcellsinaskeletalmuscleThelighter“whitefibers”arefastmuscleTheyhavelittlemyoglobinandfewermitochondriathanthedarkredfiberswhichareslowmusclebAdistanceswimmercanworkhershouldermusclesforextendedperiodsduetothemanywell-developedslowmusclecellstheycontain
Fig. 6.6, p.107

25. The Structure and Function of Skeletal Muscles
When athletes train, one goal is to increase the relative size and contractile strength of fast (sprinters) and slow (distance swimmers) muscle fibers.
Figure 6.6b

26. Section 3
How Muscles Contract

27. How Muscles Contract A muscle contracts when its cells shorten.
Muscles are divided into contractile units called sarcomeres.
Each muscle cell contains myofibrils composed of thin (actin) and thick (myosin) filaments.
Each actin filament is actually two beaded strands of protein twisted together.
Each myosin filament is a protein with a double head (projecting outward) and a long tail (which is bound together with others).
The arrangement of actin and myosin filaments gives skeletal muscles their characteristic striped appearance.

28. a Arrangement of actin molecules in the thin filaments
one actin molecule
part of a thin filament
a Arrangement of actin molecules in the thin filaments
part of a myosin molecule
part of a thick filament
FigureActinandmyosinfilaments
b Arrangement of myosin molecules in the thick filaments
Fig. 6.8, p.108

29. One myofibril inside cell:
FigureAnimatedZoomingdownthroughskeletalmusclefromabicepstofilamentsoftheproteinsactinandmyosinTheseproteinscancontract
a. Skeletal muscle cell, longitudinal section. All bands of its myofibrils are in register and give the cell a striped appearance.
Fig. 6.7a, p.108

30. thick and thin filaments overlap in an A band. Only thick filaments
sarcomere
sarcomere
Z band
H zone
Z band
Z band
b. Sarcomeres. Many
thick and thin filaments
overlap in an A band.
Only thick filaments
extend across the H
zone. Only thin filaments
extend across I bands
to the Z bands.
FigureAnimatedZoomingdownthroughskeletalmusclefromabicepstofilamentsoftheproteinsactinandmyosinTheseproteinscancontract
I band
A band
I band
Fig. 6.7b, p.108

31. How Muscles Contract
Muscle cells shorten when actin filaments slide over myosin.
Within each sarcomere there are two sets of actin filaments, which are attached on opposite sides of the sarcomere; myosin filaments lie suspended between the actin filaments.

32. a b actin myosin actin Sarcomere when muscle cell is relaxed
FigureAnimatedaActinandmyosinfilamentsinasarcomereInteractionsbetweenthetwokindsoffilamentsshortenthesarcomereb–eSlidingfilamentmodelofcontractioninthesarcomeresofmusclecells b
Same sarcomere, contracted
Fig. 6.9, p.109

33. myosin
actin
actin
p.116

34. How Muscles Contract
During contraction, the myosin filaments physically slide along and pull the two sets of actin filaments toward each other at the center of the sarcomere; this is called the sliding-filament model of contraction.
When a myosin head is energized, it forms cross-bridges with an adjacent actin filament and tilts in a power stroke toward the sarcomere’s center.
Energy from ATP drives the power stroke as the heads pull the actin filaments along.
After the power stroke the myosin heads detach and prepare for another attachment (power stroke).

35. one of many myosin binding sites on actin
myosin head
one of many myosin binding sites on actin
FigureAnimatedaActinandmyosinfilamentsinasarcomereInteractionsbetweenthetwokindsoffilamentsshortenthesarcomereb–eSlidingfilamentmodelofcontractioninthesarcomeresofmusclecells
cross bridge
cross bridge
Fig. 6.9cd, p. 109

36. FigureAnimatedaActinandmyosinfilamentsinasarcomereInteractionsbetweenthetwokindsoffilamentsshortenthesarcomereb–eSlidingfilamentmodelofcontractioninthesarcomeresofmusclecells
Fig. 6.9ef, p. 109

37. ATP
ATP
FigureAnimatedaActinandmyosinfilamentsinasarcomereInteractionsbetweenthetwokindsoffilamentsshortenthesarcomereb–eSlidingfilamentmodelofcontractioninthesarcomeresofmusclecells
Fig. 6.9g, p. 109

38. How Muscles Contract
When a person dies, ATP production stops, myosin heads become stuck to actin, and rigor mortis sets in, making the body stiff.

39. How the Nervous System Controls Muscle Contraction
Section 4
How the Nervous System Controls Muscle Contraction

40. How the Nervous System Controls Muscle Contraction
Calcium ions are the key to contraction.
Skeletal muscles contract in response to signals from motor neurons of the nervous system.
Signals arrive at the T tubules of the sarcoplasmic reticulum (SR), which wraps around the myofibrils.
The SR responds by releasing stored calcium ions; calcium binds to the protein troponin, changing the conformation of actin and allowing myosin cross-bridges to form.
Another protein, tropomyosin, is also associated with actin filaments.

41. How the Nervous System Controls Muscle Contraction
When nervous stimulation stops, calcium ions are actively taken up by the sarcoplasmic reticulum and the changes in filament conformation are reversed; the muscle relaxes.

42. a b c d section from spinal cord motor neuron
© 2007 Thomson Higher Education
motor neuron a
Signals from the nervous system
Endings of motor neuron b
sarcoplasmic reticulum (calcium in storage)
T tubule
plasma membrane of skeletal muscle fiber
section from a skeletal muscle
FigureAnimatedPathwayforsignalsfromthenervoussystemthatstimulatecontractionofskeletalmuscle
one of the myofibrils inside the muscle fiber
part of one muscle cell
Signals travel along muscle cell’s plasma membrane to sarcoplasmic reticulum around myofibrils. c
Z line
Z line d
Signals trigger the release of calcium ions from sarcoplasmic reticulum threading among the myofibrils.
Fig. 6.10, p. 110

43. myosin binding site blocked
troponin
a Actin molecule
myosin binding site blocked
b Cross-section of (a). Red dots are calcium ions bound to troponin (green).
c Calcium ions flow in; troponin binds additional calcium.
d Troponin changes shape,moving away from the myosin binding site.
myosin head
FigureAnimatedTheinteractionsofactintropomyosinandtroponinsinaskeletalmusclecell
e The binding site is now exposed; actin can bind the myosin head.
myosin head
cross-bridge
f Cross-bridge forms between
actin and myosin.
Fig. 6.11, p. 111

44. troponin a Actin molecule myosin binding site blocked
FigureAnimatedTheinteractionsofactintropomyosinandtroponinsinaskeletalmusclecell
b Cross-section of (a). Red dots are calcium ions bound to troponin (green).
c Calcium ions flow in; troponin
binds additional calcium.
Fig. 6.11a.c, p.111

45. myosin head myosin head
d Troponin changes shape,moving away from the myosin binding site.
e The binding site is now exposed; actin can bind the myosin head.
FigureAnimatedTheinteractionsofactintropomyosinandtroponinsinaskeletalmusclecell
myosin head
f Cross-bridge forms between
actin and myosin.
Fig. 6.11d.f, p.111

46. How the Nervous System Controls Muscle Contraction
Neurons act on muscle cells at neuromuscular junctions.
At neuromuscular junctions, impulses from the branched endings (axons) of motor neurons pass to the muscle cell membranes.
Between the axons and the muscle cell is a gap called a synapse.
Signals are transmitted across the gap by a neurotransmitter called acetylcholine (ACh).
When the neuron is stimulated, calcium channels open to allow calcium ions to flow inward, causing a release of acetylcholine into the synapse.

47. Vesicles containing ACh molecules
Axon ending of
motor neuron
Synapse
Muscle cell
FigureHowachemicalmessengercalledaneurotransmittercarriesasignalacrossaneuromuscularjunction
Muscle cell receptor for ACh
Fig. 6.12, p.111

48. How Muscle Cells Get Energy
Section 5
How Muscle Cells Get Energy

49. How Muscle Cells Get Energy
ATP supplies the energy for muscle contraction.
Initiation of muscle contraction requires much ATP; this will initially be provided by creatine phosphate, which gives up a phosphate to ADP to make ATP.
Cellular respiration provides most of the ATP needed for muscle contraction after this, even during the first 5-10 minutes of moderate exercise.

50. How Muscle Cells Get Energy
During prolonged muscle action, glycolysis alone produces low amounts of ATP; lactic acid is also produced, which hinders further contraction.
Muscle fatigue is due to the oxygen debt that results when muscles use more ATP than cellular respiration can deliver.

51. Phosphate Transferred from
ADP + Pi
Pathway 1
Phosphate Transferred from
Creatine Phosphate
Relaxation
Contraction
creatine
ATP
Pathway 2
Aerobic
Respiration
Pathway 3
Glycolysis
Alone
FigureAnimatedThreemetabolicpathwaysbywhichATPformsinmusclesinresponsetothedemandsofphysicalexercise
oxygen
glucose from bloodstream and from glycogen breakdown in cells
Fig. 6.13, p. 112

52. Properties of Whole Muscles
Section 6
Properties of Whole Muscles

53. Properties of Whole Muscles
Several factors determine the characteristics of a muscle contraction.
A motor neuron and the muscle cells under its control are a motor unit; the number of cells in a motor unit depends on the precision of the muscle control needed.
A single, brief stimulus to a motor unit causes a brief contraction called a muscle twitch.
Repeated stimulation makes the twitches run together in a sustained contraction called tetanus (tetany).

54. neuromuscular junction
motor unit
slice from spinal cord
motor neuron leading from spinal cord to muscle fibers
neuromuscular junction
FigureaExampleofmotorunitspresentinmusclesbThemicrographshowstheaxonendingsofamotorneuronthatactsonindividualmusclecellsinthemuscle
Fig. 6.14a, p. 112

55. tetanic contraction relaxation starts Force stimulus contraction Force
six stimulations per second
FigureAnimatedRecordingsoftwitchesinmusclesartificiallystimulatedindifferentwaysaAsingletwitchbSixpersecondcauseasummationoftwitchesandcaboutpersecondcausetetaniccontractiondPaintingofasoldierdyingofthediseasetetanusinamilitaryhospitalinthesafterthebacteriumClostridiumtetaniinfectedabattlefieldwound
tetanic contraction
Force
repeated stimulation
twitch
Time
Fig. 6.15ac, p.113

56. Properties of Whole Muscles
Not all muscle cells in a muscle contract at the same time.
The number of motor units that are activated determines the strength of the contraction: Small number of units = weak contraction; large number of units at greater frequency = stronger contraction.
Muscle tone is the continued steady, low level of contraction that stabilizes joints and maintains general muscle health.

57. Properties of Whole Muscles
Muscle tension is the force a contracting muscle exerts on an object; to contract, a muscle’s tension must exceed the load opposing it.
An isotonically contracting muscle shortens and moves a load
An isometrically contracting muscle develops tension but does not shorten.

58. contracted muscle can shorten contracted muscle can’t shorten
FigureaAnisotoniccontractionTheloadislessthanamuscle’speakcapacitytocontractsothemusclecancontractshortenandlifttheloadbInanisometriccontractiontheloadexceedsthemuscle’speakcapacityItcontractsbutcan’tshorten
Fig. 6.16, p. 113

59. Properties of Whole Muscles
Tired muscles can’t generate much force.
Muscles fatigue when strong stimulation keeps a muscle in a state of tetanus too long.
After resting, muscles will be able to contract again; muscles may need to rest for minutes up to a day to fully recover.

60. Section 7
Muscle Disorders

61. Muscle Disorders Strains and tears are muscle injuries.
Muscle strains come from movement that stretches or tears muscle fibers; ice, rest and anti-inflammatory drugs (ibuprofen) allow damage to repair.
If the whole muscle is torn,
scar tissue may develop,
shortening the muscle and
making it function less
effectively.
Figure 6.17

62. Muscle Disorders Sometimes a skeletal muscle will contract abnormally.
A muscle spasm is a sudden, involuntary contraction that rapidly releases, while cramps are spasms that don’t immediately release; cramps usually occur in calf and thigh muscles.
Tics are minor, involuntary twitches of muscles in the face and eyelids.

63. Muscle Disorders Muscular dystrophies destroy muscle fibers.
Muscular dystrophies are genetic diseases leading to breakdown of muscle fibers over time.
Duchenne muscular
dystrophy (DMD) is
common in children; a
single mutant gene
interferes with
sarcomere contraction.
Figure 6.18

64. Muscle Disorders Exercise makes the most of muscles.
Myotonic muscular dystrophy is usually found in adults; muscles of the hands and feet contract strongly but fail to relax normally.
In these diseases, muscles progressively weaken and shrivel.
Exercise makes the most of muscles.
Muscles that are damaged or which go unused for prolonged periods of time will atrophy (waste away).

65. Muscle Disorders
Aerobic exercise improves the capacity of muscles to do work.
Walking, biking, and jogging are examples of exercises that increase endurance.
Regular aerobic exercise increases the number and size of mitochondria, the number of blood capillaries, and the amount of myoglobin in the muscle tissue.
Figure 6.19a

66. Muscle Disorders
Strength training improves function of fast muscle but does not increase endurance.
Even modest activity slows the loss of muscle strength that comes with aging.
Figure 6.19b