Ch 10

Functions of Skeletal Muscle

• Produces body movement

• Maintains posture and body position

• Stabilizes joints

• Generates heat (thermogenesis)

• Protects soft tissues

• Guards body openings with sphincters

• Stores nutrient reserves (glycogen and proteins)

Characteristics of Muscle Tissue

contractility, conductivity, excitability, extensibility, elasticity.

Contractility

Ability of muscle cells to shorten forcefully and generate tension.

Conductivity

Ability to transmit electrical signals (action potentials) along the muscle fiber.

Excitability

Ability to respond to a stimulus by generating an electrical signal.

Extensibility

Ability to be stretched without being damaged.

Elasticity

Ability to return to its original resting length after being stretched or contracted.

Skeletal Muscle Organization

• skeletal muscle fiber - cell

• fascicle

• entire muscle

Entire Muscle

Organ composed of many fascicles surrounded by epimysium.

Fascicle

• Bundle of skeletal muscle fibers surrounded by perimysium.

Skeletal Muscle Fiber (Muscle Cell)

• Long, cylindrical, multinucleated muscle cell surrounded by endomysium.

Connective Tissue Layers

• Endomysium

• Perimysium

• Epimysium

• Fascia

Endomysium

Thin connective tissue surrounding each individual muscle fiber.

• Contains capillaries and nerve fibers.

Perimysium

Connective tissue surrounding each fascicle.

• Houses larger blood vessels and nerves.

Epimysium

Dense connective tissue surrounding the entire muscle.

• Separates muscles from surrounding tissues.

Fascia

Connective tissue surrounding and separating groups of muscles.

• Supports muscles and provides pathways for blood vessels and nerves.

Components of a Skeletal Muscle Fiber

sarcomere, myofibrils, sarcoplasmic reticulum, T-tubule, motor end plate, myoglobin, glycosomes.

Sarcomere

Functional contractile unit of muscle located between two Z discs.

• Shortens during contraction.

Myofibrils

Long bundles inside muscle fibers made of repeating sarcomeres.

• Responsible for muscle contraction.

Sarcoplasmic Reticulum (SR)

Specialized smooth ER.

• Stores and releases calcium needed for muscle contraction.

T-Tubules

Extensions of the plasma membrane that carry action potentials deep into the muscle fiber.

• Trigger calcium release from the SR.

Motor End Plate

Specialized region of the muscle fiber membrane where the motor neuron forms the neuromuscular junction.

• Contains acetylcholine receptors.

Myoglobin

Oxygen-binding protein found inside muscle fibers.

• Stores oxygen for aerobic respiration.

Glycosomes

• Storage granules containing glycogen.

• Provide glucose for ATP production.

Neuromuscular Junction (NMJ)

Synapse between a motor neuron and skeletal muscle fiber.

Steps at the Neuromuscular Junction

1. Action potential arrives at axon terminal.

2. Voltage-gated calcium channels open.

3. Calcium enters the neuron.

4. Synaptic vesicles release acetylcholine (ACh).

5. ACh binds ligand-gated receptors on the motor end plate.

6. Sodium enters the muscle fiber.

7. End-plate potential is generated.

8. If threshold is reached, voltage-gated sodium channels open.

9. Action potential spreads across the sarcolemma and down T-tubules.

10. Acetylcholine is broken down by acetylcholinesterase.

Excitation-Contraction Coupling

Process linking the muscle action potential to contraction.

Excitation-Contraction Coupling Steps

1. Action potential travels along sarcolemma.

2. Action potential enters T-tubules.

3. T-tubules stimulate the sarcoplasmic reticulum.

4. Calcium is released into the sarcoplasm.

5. Calcium binds troponin.

6. Tropomyosin moves away from actin binding sites.

7. Cross-bridge cycling begins.

Sliding Filament Model

Thin filaments slide past thick filaments, shortening the sarcomere without changing filament length.

Sliding Filament steps

1. Calcium exposes binding sites on actin.

2. Myosin head binds actin (cross-bridge forms).

3. Power stroke pulls actin toward the center.

4. ATP binds myosin causing detachment.

5. ATP is split, recocking the myosin head.

6. Cycle repeats while calcium and ATP are available.

Ligand (Chemically)-Gated Ion Channels

• Open when acetylcholine binds.

• Located on the motor end plate.

• Allow sodium to enter, creating the end-plate potential.

Leakage Channels

• Always open.

• Maintain resting membrane potential by allowing passive ion movement.

Voltage-Gated Ion Channels

• Open when threshold is reached.

• Generate and propagate the action potential across the sarcolemma and T-tubules.

Isotonic Contraction

• Muscle changes length while producing movement.

• Tension exceeds the load.

Concentric Contraction

• Muscle shortens while producing force.

Eccentric Contraction

• Muscle lengthens while still producing force.

Isometric Contraction

• Muscle develops tension without changing length.

• No movement occurs.

Latent Period (Twitch)

• Time between stimulation and contraction.

• Action potential occurs and calcium is released.

Contraction Phase (Twitch)

• Cross-bridge cycling occurs.

• Muscle develops tension.

Relaxation Phase (Twitch)

• Calcium is pumped back into the SR.

• Cross-bridges stop forming.

• Muscle returns to resting state.

Frequency of Stimulation

• More frequent stimulation increases muscle tension because calcium remains elevated.

Summation

• Successive twitches add together, increasing force.

• muscle tension

Incomplete (Unfused) Tetanus

• Partial relaxation occurs between contractions.

• muscle tension

Complete (Fused) Tetanus

• No relaxation between contractions.

• Produces maximum sustained tension.

Motor Unit

• One motor neuron and all the muscle fibers it controls.

• muscle tension

Motor Unit Recruitment

• Increasing the number of active motor units increases muscle tension.

Direct Phosphorylation (Creatine Phosphate)

• Creatine phosphate donates a phosphate to ADP.

• Produces 1 ATP per creatine phosphate.

• Provides energy for about 10–15 seconds of intense activity.

Aerobic Respiration

• Requires oxygen.

• Occurs in mitochondria.

• Produces approximately 30–32 ATP per glucose (some courses may list 36–38 ATP).

• Supports long-duration activity.

Anaerobic Pathway (Glycolysis)

• Does not require oxygen.

• Occurs in cytoplasm.

• Produces 2 ATP per glucose.

• Generates lactic acid.

• Used during short, high-intensity exercise.

Skeletal muscle

• attached to bones

• striated

• multinucleate

• voluntary

• Calcium binds Troponin

• Sarcoplasmic reticulum: calcium source

• NO gap junctions

Smooth Muscle

• walls of hollow organs

• non-striated

• involuntary

• uninucleate

• extracellular fluid: calcium source

• Calcium binds calmodulin

• gap junctions in single unit muscle coordinates contractions

Smooth muscle innervation

Autonomic nervous system uses varicosities that release neurotransmitters over a wide area

Skeletal muscle innervation

Neuromuscular junction with one motor end plate