MUSCULAR SYSTEM

❖ Produce movement

❖ Maintain posture

❖ Stabilize joints

❖ Generate heat

Muscles and bones work like a lever system

• When skeletal muscles contract, they transmit force to a tendon that makes the bones move

Muscles can only pull on bones, they cannot push them

Skeletal muscles often work as opposing pairs

Action of one reverses the action of the other

• Example: biceps and triceps

Stapedius

• smallest muscle; middle ear

• controls the tiniest bone in the body; stapes or stirrup bone

Skeletal Muscles

• 640 individual muscles

Gluteus Maximus

• biggest muscle in the body

Sartorius

• longest muscle; strap like narrow muscle runs hip to knee

You use 200 muscles just to take a single step. The hardest working muscle is in the eye.

Organization of the Muscle

• Blood Vessel

• Tendon

• Bone

• Muscle fiber (cell)

• Fascicle (wrapped by perimysium)

• Epimysium (wraps entire muscle)

• Endomysium (between fibers)

Microscopic Anatomy

• Many myofibrils make up a skeletal muscle fiber

• A myofibril consists of units of sarcomeres, lined up along its length

• Each sarcomere has parallel arrays of actin and myosin filaments

• Sarcomere (segment of a myofibril)

  
  

• Skeletal Muscle

  • Myofibrils are aligned to give distinct bands

    • I band = light band

      • contains only thin filaments

    • A band = dark band

      • contains the entire length of the thick filaments

  • Myofibril or fibril (complex organelle composed of bundles of myofilaments)

  • Thick filaments = myosin filaments

    • as ATPase enzymes

    • Myosin filaments have heads (extensions, or cross bridges)

    • Myosin and actin overlap somewhat

  • Thin filaments = actin filaments

    • anchored to the Z disc

  • At rest, within the A band, the H zone or bare zone lacks actin

    filaments

  • Sarcoplasmic reticulum (SR)

    • Stores and releases calcium

    • Surrounds the myofibril

Nerve Stimulus

Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract

• Motor unit - one motor neuron and all the skeletal muscle cells stimulated by that neuron

Stimulation and Contraction of Single Skeletal Muscle Cell

Excitability

• also called responsiveness or irritability

• ability to receive and respond to a stimulus

Contractility

• ability to shorten when an adequate stimulus is received

Extensibility

• ability of muscle cells to be stretched

Elasticity

• ability to recoil and resume resting length after stretching

Action Potential

Production of muscle action potential

When Na+ flows into the muscle fiber, it makes the inside more positively charged, triggering a muscle action potential. This action potential causes the sarcoplasmic reticulum to release stored Ca2+ into the sarcoplasm, leading to muscle contraction.

The Nerve Stimulus and Action Potential

• Synaptic cleft

  • Gap between nerve and muscle

  • Nerve and muscle do not make contact

  • Area between nerve and muscle is filled with interstitial fluid

• Action potential reaches the axon terminal of the motor neuron

• Calcium channels open and calcium ions enter the axon terminal

Transmission of Nerve Impulse to Muscle

• Calcium ion entry causes some synaptic vesicles to release their contents (acetylcholine, a neurotransmitter) by exocytosis

• Neurotransmitter

  • chemical released by nerve upon arrival of nerve impulse in the axon terminal

  • The neurotransmitter for skeletal muscle is acetylcholine (ACh)

• Acetylcholine attaches to receptors on the sarcolemma of the muscle cell

• In response to the binding of ACh to a receptor, the sarcolemma becomes permeable to sodium (Na+)

• Sodium rushes into the cell generating an action potential and potassium leaves the cell

• Once started, muscle contraction cannot be stopped

1. Action potential reaches axon terminal of motor neuron

2. Calcium channels open and Ca enters the axon terminal

3. Ca2 entry causes some synaptic vesicles to release their contents by exocytosis

4. Acetylcholine diffuses across synaptic cleft and binds to receptors in the sarcolemma

5. ACh binds and channels open that allow simultaneous passes of Na into the msucle fiber and K out of the muscle fiber. More Na ions enter the K ions leaving and

6. This produces a local change in the electrical conditions of the membrane (depolarization), which eventually leads to an action potential

7. ACh effects are ended by its breakdown in the synaptic cleft by the enzyme acetylcholinesterase

Transmission of Nerve Impulse to Muscle

1. The sodium-potassium pump binds three sodium ions (Na+) and one ATP molecule.

2. Splitting ATP provides energy to change the pump’s shape, pushing the sodium ions outside the cell.

3. The pump then binds two potassium ions (K+) in its new shape.

4. Releasing the phosphate allows the pump to return to its original shape, releasing the potassium ions inside the cell.

1. Na diffuses into the cell

2. Action potential spreads rapidly along the sarcolemma

Muscle Contraction

Sliding Filament Mechanism

Muscle contraction happens when myosin heads bind to thin filaments at each end of a sarcomere and pull them toward the M line. This causes the thin filaments to slide inward and meet in the center of the sarcomere.

Muscle Contraction Cycle

1. ATP Hydrolysis: Myosin heads hydrolyze ATP and become reoriented and energized

2. Attachment of Myosin to Actin: Myosin heads bind to actin forming cross-bridges by releasing the phosphate group.

3. Power Stroke: Myosin cross-bridges rotate toward center of sarcomere (power stroke)

4. Detachment of Myosin from Actin. As myosin heads bind ATP, the cross-bridges detach from actin

Contraction cycle continues if ATP is available and Ca level in sarcoplasm is high

Role of Calcium Ions in Muscular Contraction

❑ Muscle contraction is initiated by a nerve impulse

❑ Nerve fibers are embedded in the surface of the muscle fiber forming a neuromuscular junction

• When a signal reaches the end of a neuron

• The neuron releases acetylcholine into the gap between neuron and muscle

• This causes depolarization of the muscle cell

❑ When a muscle is relaxed, attachment sites for myosin heads are blocked by tropomyosin.

❑ For the muscle to contract, tropomyosin must be moved by another protein called troponin.

❑ The troponin-tropomyosin complex is regulated by calcium ion concentrations in the muscle cell.

How muscles produce movement

Muscles produce movement by exerting force on tendons, which then pull on bones or other structures, such as the skin.

• When a muscle contracts, it pulls one bone toward another.

  • The origin is the tendon attachment to a stationary bone.

  • The insertion is the tendon attachment to a movable bone.

Muscle movement operates like a lever, where the arrangement of the fulcrum (pivot point), load (resistance), and effort (force applied) determines the mechanical advantage or disadvantage:

• Mechanical Advantage: If the load is closer to the fulcrum and the effort is farther away, a smaller effort can move a large load over a short distance. CLOSER→EFFORT FARTHER→SMALL EFFORT

• Mechanical Disadvantage: If the load is farther from the fulcrum and the effort is closer, a larger effort is needed to move a small load (but at greater speed) FARTHER→EFFORT CLOSER→LARGER EFFORT

fulcrum (the jaw or temporomandibular joint) than are the front teeth.

In creating movement:

• Bones act as levers, and joints function as fulcrums (F).

• A lever has two forces acting on it:

  • The effort (E), which drives movement.

  • The load (L), or resistance, which opposes movement.

Motion happens when the effort applied at the bone’s insertion is greater than the load.

Levers are classified into three types based on the positions of the fulcrum, effort, and load:

1. First-Class Levers: The fulcrum is between the effort and the load (EFL).

2. Second-Class Levers: The load is between the fulcrum and the effort (ELF).

3. Third-Class Levers: The effort is between the fulcrum and the load (FEL).

Contraction of Skeletal Muscle

• Muscle fiber contraction is “all or none.”

• Within a skeletal muscle, not all fibers may be stimulated during the same interval.

• Different combinations of muscle fiber contractions may give differing responses.

• Graded responses - different degrees of skeletal muscle shortening.

• Graded responses can be produced by changing:

  • The frequency of muscle stimulation.

  • The number of muscle cells being stimulated at one time.

Types of Graded Responses:

• Twitch

  • Single, brief contraction.

  • Not a normal muscle function, only done in the laboratory.

• Summing of contractions (Wave Summation)

  • One contraction is immediately followed by another.

  • The muscle does not completely return to a resting state due to more frequent stimulations.

  • The effects are added.

• Unfused (incomplete) tetanus

  • Some relaxation occurs between contractions but nerve stimuli arrive at an even faster rate than during summing of contractions.

  • Unless the muscle contraction is smooth and sustained, it is said to be in unfused tetanus.

• Fused (complete) tetanus

  • No evidence of relaxation before the following contractions.

  • Frequency of stimulations does not allow for relaxation between contractions.

  • The result is a smooth and sustained muscle contraction.

Muscle Response to Strong Stimuli:

• Muscle force depends upon the number of fibers stimulated.

• More fibers contracting results in greater muscle tension.

• Muscles can continue to contract unless they run out of energy.

Energy for Muscle Contraction

Initially, muscles use stored ATP for energy

• ATP bonds are broken to release energy

• Only 4–6 seconds worth of ATP is stored by muscles

• After this initial time, other pathways must be utilized to produce ATP

Muscle fibers have three ways to produce ATP:

1. Direct phosphorylation of ADP from creatine phosphate

   Muscle cells store CP

   • CP is a high-energy molecule

   • After ATP is depleted, ADP is left

   • CP transfers a phosphate group to ADP, to regenerate ATP

   • CP supplies are exhausted in less than 15 seconds

   • About 1 ATP is created per CP molecule

2. by Anaerobic cellular respiration, and

3. by Aerobic cellular respiration

Creatine

• small, amino acid - like molecule that is synthesized in the liver, kidneys, and pancreas and then transported to muscle fibers.

• Creatine phosphate is three to six times more plentiful than ATP in the sarcoplasm of a relaxed muscle fiber. When contraction begins and the ADP level starts to rise providing extra energy.

• The enzyme creatine kinase (CK) catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine, forming creatine phosphate and ADP.

  
  

• Anaerobic glycolysis and lactic acid formation

  • Reaction that breaks down glucose without oxygen

    • Glucose is broken down to pyruvic acid to produce about 2 ATP

    • 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

• Aerobic respiration

  • Glucose is broken down to carbon dioxide and water,

    releasing energy (about 32 ATP)

  • A series of metabolic pathways occur in the mitochondria

  • This is a slower reaction that requires continuous oxygen

  • Carbon dioxide and water are produced

Muscle Fatigue and Oxygen Deficit

• When a muscle is fatigued, it is unable to contract even with a stimulus

• Common cause for muscle fatigue is oxygen debt

• Oxygen must be “repaid” to tissue to remove oxygen deficit

• 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