Muscle is one of the four primary tissue types of the body, and the body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.

1. THE MUSCULAR SYSTEM

2. Functions
1. Locomotion
2. Vasoconstriction & vasodilatation
3. Peristalsis
4. Cardiac motion
5. Posture maintenance
6. Heat generation

3. Properties
 Striations: only present in skeletal and cardiac
muscles. Absent in smooth muscle.
 Nucleus: smooth and cardiac muscles are
uninucleated; skeletal muscle is multinucleated.
 Transverse tubule (T tubule): well developed
in skeletal and cardiac muscles to transport
calcium. Absent in smooth muscle.

4. Properties
 Intercalated disk: specialized intercellular
junction that only occurs in cardiac muscle.
 Control: skeletal muscle is always under voluntary
control; smooth and cardiac muscles are under
involuntary control.

5. Characteristics
1. Excitability
2. Contractility
3. Elasticity
4. Extensibility

7. THE SKELETAL MUSCLE

9. Skeletal muscle fiber
 Each skeletal muscle fiber is a single muscle cell.
 Muscle fibers are cylindrical cells with many nuclei .
 The cell membrane is called sarcolemma; the
cytoplasm is called sarcoplasm.
 The sarcoplasm contains abundant , parallel thread
like myofibrils, that run in parallel fashion.

16. Neuromuscular junction
 The site where a motor neuron’s terminal meets the
muscle fiber.
 This is where the muscle fiber first responds to
signaling by the motor neuron.
 Every skeletal muscle fiber in every skeletal muscle
is innervated by a motor neuron at the NMJ.

19. T Tubules
 Carry the action potential into the interior of the
cell – triggers the opening of calcium channels in
the membrane of the adjacent SR, causing Ca++ to
diffuse out of the SR and into the sarcoplasm.

21. Muscle fiber contraction
 The Ca++ ions combine with troponin molecules of
the myofibrils*
 Binding of Ca++ to troponin shifts tropomyosin to
expose active sites on the actin molecules.

22. Mechanisms of Muscle
Contraction

27. POWER STROKE

30. VIDEO ON THE MECHANISM OF CROSS
BRIDGE

31. Relaxation of skeletal muscle
1. After the impulse is over, the SR begins actively
pumping Ca++ back into its sacs.
2. As Ca++ is stripped from troponin molecules,
tropomyosin returns to its position, blocking actin’s
active sites.
3. Myosin cross bridges are prevented from binding to
actin & can no longer sustain the contraction.
4. The muscle fiber return to its longer, resting length.

32. ATP as energy source
 ATP supplies the energy for muscle contraction –
cross bridge cycle*
 ATP also provides the energy for the active-
transport Ca++ pumps in the SR.
 The amount of ATP stored in muscle is very low;
ATP must therefore be regenerated and replaced
quickly to allow for sustained contraction.

33. ATP as Energy Source
 There are three mechanisms by which ATP can be
regenerated:
1. Creatine phosphate metabolism
2. Anaerobic glycolysis
3. Fermentation & aerobic respiration

34. Creatine phosphate metabolism

37. Muscle tension
 The force generated by the contraction of the
muscle (or shortening of the sarcomeres).
 Generated when moving a load or when the muscle
contract against a load that does not move.
 2 types of skeletal muscle contractions:
Isotonic contractions & isometric contractions

38. Concentric
Eccentric

40. Motor unit
 The actual group of muscle fibers in a muscle
innervated by a single motor neuron*
 Small motor unit is an arrangement where a single
motor neuron supplies a small number of muscle fibers
in a muscle**
 Large motor unit is an arrangement where a single
motor neuron supplies a large number of muscle fibers
in a muscle***

41. Motor unit. A motor unit consists of one somatic motor neuron and the
muscle fibers sup-plied by its branches.
Motor neuron
Myelin sheath
NMJ
Muscle fibers

42. Twitch
 An single (isolated) contraction in the muscle fibers
produced by the activation when a single action
potential from a motor neuron.
 A twitch can last for a few milliseconds or 100
milliseconds, depending on the muscle type.

43. A Myogram of a Muscle Twitch

44. Twitch
 Significance:
 A single twitch does not produce any significant muscle
activity in a living body.
 A series of action potentials to the muscle fibers is
necessary to produce a muscle contraction that can
produce work.

45. The Frequency of Motor Neuron Stimulation

46. Muscle tone
 Constant low-level contractions that allow for posture and
stability (joints)*
 Hypotonia
 The absence of low-level contractions that lead to muscle
tone**
 Hypertonia
 Excessive muscle tone; present with muscle rigidity or
spasticity***

47. Types of muscle fibers
1. Slow oxidative (SO) fibers
2. Fast oxidative (FO) fibers
3. Fast glycolytic (FG) fibers

48. Fast fibers vs. Slow fibers
 Fast & Slow – speed of contraction: dependent on how
quickly myosin’s ATPase hydrolyzes ATP to produce
cross-bridge.
 Fast fibers hydrolyze ATP approximately twice as
quickly as slow fibers, resulting in much quicker cross-
bridge cycling.

49. Oxidative vs. Glycolytic
 Oxidative fibers
 Produce more ATP during each metabolic cycle, making
it more resistant to fatigue.
 Contain more mitochondria*
 Glycolytic fibers
 Produce less ATP per cycle; fatigue at a quicker rate.

50. Slow oxidative (SO) fibers
 Possess a large number of mitochondria
 Capable of contracting for longer period*
 Extensively supplied with blood capillaries**
 Possess myoglobin (O2-carrying molecule)***
 Useful to maintain posture, stabilize bones and joints,
and make small movements that do not require large
amounts of energy.

51. Fast oxidative (FO) fibers
 Produce ATP relatively quickly – produce higher
amount of tension than SO fibers.
 Do not possess significant myoglobin*
 They produce more tension than SO fibers but they are
more fatigue-resistant than FG fibers.
 Used primarily: walking (require more energy than
postural control but less energy than an explosive
movement, such as sprinting)

52. Fast glycolytic (FG) fibers
 Use glycolysis as ATP source
 Produce high levels of tension*
 Produce rapid, forceful contractions to make quick,
powerful movements.
 Fatigue quickly, permitting them to only be used for
short periods.
The predominant fiber type in a muscle is determined by the
primary function of the muscle.

53. Terms (skeletal muscle)
 Hypertrophy
 An increase in muscle mass due to the additions of
structural proteins*
 Atrophy
 The loss of muscle mass due to breakdown of structural
proteins.
 Sarcopenia
 Age-related muscle atrophy.

54. Marathoners. Long-distance runners have a large number of SO fibers and relatively few FO and
FG fibers. (credit: “Tseo2”/Wikimedia Commons). SO fibers are capable to sustain low levels of
muscle contractions for greater periods without fatiguing.
Endurance Exercise

55. Body builders have a large number of FG fibers and
relatively few FO and SO fibers. (credit: Lin Mei/flickr)
Resistance Exercise

57. SMOOTH MUSCLE
 Present in the walls of hollow organs like the
urinary bladder, uterus, stomach, intestines, and in
the walls of passageways, such as the arteries and
veins and the tracts of the respiratory, urinary,
and reproductive systems.

58. Smooth muscle tissue is found around organs in the digestive, respiratory,
reproductive tracts and the iris of the eye. LM × 1600. (Micrograph provided by the
Regents of University of Michigan Medical School © 2012)

59. Smooth muscle
 Do not have striations, sarcomere & T tubules*
 Dense body – analogous to the Z-discs of skeletal
and cardiac muscle fibers (fastened to the
sarcolemma); anchor the thin filaments.
 Calveoli – membrane indentations that supplies
calcium ions from ECF.
 Limited SR**

60. The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause
the muscle fiber to contract.

61. Smooth muscle
 Cross-bridge formation is regulated by the the regulatory
protein calmodulin.
 Ca++ bind to calmodulin.
 Ca++-calmodulin complex activates myosin kinase, which,
in turn, activates the myosin heads by phosphorylating
them*
 The heads can then attach to actin-binding sites and pull
on the thin filaments.

62. Relaxed
Contracted
Plasma
membrane
Thin myofilament
Thick myofilament
A
B

63. Latch-bridges
 Subset of cross-bridges between myosin heads and actin
that keep the thick and thin filaments linked together for a
prolonged period without the need for ATP.
 Allows for maintenance of muscle “tone” in SM with very
little energy expenditure.

64. Varicosity
 Series of neurotransmitter-filled bulges of axons
through smooth muscle; loosely forming motor units.
 Releases neurotransmitters into the synaptic cleft.
 Corresponds to the NMJ of the skeletal muscles.

65. A series of axon-like swelling, called varicosities or “boutons,” from autonomic neurons form motor
units through the smooth muscle.

66. Types of smooth muscle
1. Single-unit or visceral
2. Multiunit

67. Single unit muscle
 Has its muscle fibers joined by gap junctions so that the
muscle contracts as a single unit.
 Found in the walls of all visceral organs except the heart,
and so it is commonly called visceral muscle.
 Exhibit stress-relaxation response*
 Produces slow, steady contractions that allow substances,
such as food, to move through the body (peristalsis)

68. Multiunit muscle
 Does not act as a single unit but instead is
composed of may independent single-cell units.
 Stimuli come from autonomic nerves or hormones
but not from stretching.
 Found around large blood vessels, in the respiratory
airways, and in the eyes.

69. Hyperplasia
 Unlike other muscle, SM can divide to produce
more cells, a process called hyperplasia.
 Observed in the uterus at puberty, which responds
to increased estrogen levels by producing more
uterine smooth muscle fibers, and greatly increases
the size of the myometrium.