CH 10 Elemen. HA Notes

Muscle Types and Functions

• Three types of muscle:

  1. Skeletal Muscle

  2. Cardiac Muscle

  3. Smooth Muscle

• Muscular system: Refers only to skeletal muscles.

• Myology: The study of skeletal muscles.

Functions of a Muscle

1. Movement

   • Body parts and contents movement

   • Communication

2. Stability

   • Maintains posture

   • Resists pull of gravity

3. Control

   • Regulates body openings and passages

4. Heat Production

   • Necessary for enzyme function

   • Involves contraction of skeletal muscles

5. Glycemic Control

   • Muscles absorb, store, and use glucose

Properties of a Muscle

1. Excitability (Responsiveness)

   • Reacts to stimuli

2. Conductivity

   • Spreads electrical impulses through the whole muscle cell

3. Contractility

   • Ability to shorten when stimulated

4. Extensibility

   • Can stretch without harm

5. Elasticity

   • Can recoil from stretches

Types of Muscle Tissue

1. Skeletal Muscle:

   • Voluntary: Under conscious control

   • Striated: Alternating light and dark bands

   • Attached to bones

   • Muscle cells = muscle fibers or myofibers

2. Cardiac Muscle:

   • Striated

   • Involuntary: Cannot be controlled consciously

   • Muscle cells = cardiomyocytes

Smooth Muscle

• Involuntary

• Located in the walls of viscera (organs)

• Nonstriated

• Fusiform cell shape, tapered at ends, known as myocytes

Ultrastructure of Muscle Fibers

• Sarcolemma: The muscle cell membrane

• Transverse (T) Tubules

• Sarcoplasm

  • Myofibrils: Smaller versions of fibers

  • Glycogen: Energy storage

  • Myoglobin: Oxygen binding protein

  • Sarcoplasmic Reticulum: Storage of calcium with terminal cisterns

Myofilaments

• Thick Myofilaments:

  • Myosin: Heads project from bundled tails of molecules

• Thin Myofilaments:

  • Actin: Contains binding area for myosin

  • Tropomyosin: Blocks active sites when muscle is relaxed

  • Troponin: Binds to tropomyosin and calcium when excited

• Elastic Filaments: Anchors thick myofilaments

• Protein Types in Myofilaments:

  • Contractile Proteins: Myosin and Actin

  • Regulatory Proteins: Tropomyosin and Troponin

Striations and Sarcomeres

• Striations: Narrow stripes of light and dark that run perpendicular to cell length

• Sarcomere: Functional contractile unit of muscle

  • A Band: Dark stripe of thick filaments partially overlapping thin filaments

  • H Band: Lighter central region of A band (lacks thin filaments)

  • M Line: Midline of A band anchoring thick filaments

  • I Band: Light stripe with only thin filaments

  • Z Disc (Line): Midline of I band, protein anchoring thin and elastic filaments; defines sarcomere boundaries

Blood Supply to Muscle

• At rest, muscles receive about one-quarter of heart’s output

• During exercise, that increases to over three-quarters

• Muscle tissue is well vascularized

Nerve and Muscle Relationship

• Neuromuscular Junction: Synapse between motor neuron and muscle

  • Synaptic Knob: Contains vesicles with acetylcholine (ACh)

  • Synaptic Clef: Gap between neuron and muscle

  • Postsynaptic Membrane: Contains ACh receptors

  • Axon Terminal: Filled with synaptic vesicles

  • Neurotransmitter: Transmits signals to muscle

  • Calcium: Essential for muscle contraction

Motor Unit

• Comprises one motor neuron and multiple muscle fibers it innervates

• Behaves as a single functional unit; fibers dispersed within muscle

• Two types:

  • Large Motor Units

  • Small Motor Units

Contraction and Relaxation

Excitation

1. Nerve signal arrives at the knob.

2. Knob releases ACh, which binds to muscle receptors, exciting it.

3. Excitation spreads throughout the muscle cell.

Excitation-Contraction Coupling

4. Electrical impulse triggers calcium release from the SR.

5. Calcium binds troponin, moving tropomyosin to expose actin.

6. Myosin breaks down ATP and extends its head.

7. Myosin forms cross-bridge with actin.

8. Myosin pulls actin during power stroke, then detaches and reattaches.

Relaxation

9. Nerve signal stops.

10. SR reabsorbs calcium, troponin blockade resumes, and tension subsides.

Classes of Muscle Fibers

1. Slow Oxidative (SO):

   • Slow-twitch, red, type I fibers

   • Small diameter

   • Aerobic ATP production

   • Abundant in mitochondria, myoglobin, capillaries

   • Low glycogen levels

   • Resistant to fatigue

   • Predominant in postural muscles (e.g., soleus)

2. Fast Glycolytic (FG):

   • Fast-twitch, white, type II fibers

   • Large diameter

   • Anaerobic ATP production

   • Modest mitochondria, myoglobin, capillaries

   • High glycogen levels

   • Fatigues quickly

   • Predominant in quick response muscles (e.g., gastrocnemius)

Muscle Utilization of Glucose

1. Aerobic Respiration:

   • Utilizes oxygen, occurring in mitochondria

   • Converts one glucose molecule to CO2 and H2O, releases 26-30 ATPs

2. Anaerobic Fermentation:

   • No oxygen used, results in lactate and only 2 ATPs per glucose molecule