Module 10.1 – Overview of Muscle Tissue

Properties of Muscle Cells

• Contractility: The ability of muscle fibers to contract, pulling structures together. Skeletal muscle contraction is voluntary and controlled by the brain.

• Excitability: The capacity of muscle cells to respond to a stimulus, which may be chemical or hormonal in nature.

• Conductivity: The ability of a stimulus to be transmitted across the entire muscle cell, ensuring coordinated contraction.

• Distensibility: The ability of muscle fibers to stretch or lengthen without tearing.

• Elasticity: The property that allows muscle to return to its original shape after being stretched or contracted.

Module 10.2 – Structure and Function of Skeletal Muscle Fibers

Sliding Filament Theory

• Describes how muscle contraction occurs at the molecular level.

• Thin filaments (actin) slide past thick filaments (myosin), increasing the overlap between them.

• The sarcomere shortens as Z discs move closer together.

• Relaxed muscle: Sarcomere is longer; Contracted muscle: Sarcomere is shorter.

Module 10.3 – Electrically Excitable Cells

Membrane Potentials

• Resting Potential: For skeletal muscle fibers, the resting membrane potential is about -90 mV. The inside of the cell is more negative than the outside due to the distribution of ions.

• This potential is maintained by the sodium-potassium pump and selective permeability of the membrane.

Ion Channels and Gradients

• Gated Channels: Closed at rest; open in response to specific stimuli, allowing ions to move across the membrane.

• Electrochemical Gradient: The combined effect of concentration (chemical) and electrical gradients that drive ion movement.

• Generating a Resting Potential: Sodium ions (Na+) are more concentrated outside, potassium ions (K+) inside. The gradients and selective permeability result in a negative resting potential.

Action Potentials (AP)

• Brief, rapid changes in membrane potential from negative to positive and back.

• Generated by opening and closing of voltage-gated sodium (Na+) and potassium (K+) channels.

• Depolarization: Na+ channels open, Na+ enters, making the inside less negative.

• Repolarization: K+ channels open, K+ leaves, restoring negativity.

• Propagation: The action potential travels along the muscle fiber until it reaches the end.

Module 10.4 – Process of Skeletal Muscle Contraction and Relaxation

The Neuromuscular Junction (NMJ)

• A synapse where a single motor neuron communicates with many muscle fibers.

• Motor neuron: Nerve cell that transmits signals to muscle fibers.

• Axon terminal: Swollen end of the neuron that comes close to the muscle cell.

• Synaptic cleft: Small gap between axon terminal and muscle fiber.

• Motor end plate: Specialized region of the muscle cell membrane with receptors for acetylcholine (ACh).

Skeletal Muscle Contraction

1. Excitation Phase: Action potential arrives at the axon terminal, opening calcium channels. Calcium influx triggers release of ACh into the synaptic cleft. ACh binds to receptors on the motor end plate, opening sodium channels and depolarizing the sarcolemma.

2. Excitation-Contraction Coupling: The end-plate potential triggers an action potential in the muscle fiber, which travels down T-tubules. This causes calcium release from the sarcoplasmic reticulum. Calcium binds to troponin, causing tropomyosin to move and expose active sites on actin.

3. Contraction Phase:

   • ATP hydrolysis "cocks" the myosin head.

   • Myosin head binds to actin, forming a crossbridge.

   • Power stroke: Myosin pulls actin toward the center of the sarcomere, releasing ADP.

   • ATP binds to myosin, causing it to detach from actin and reset.

4. Relaxation Phase:

   • Acetylcholinesterase degrades ACh, ending stimulation.

   • Sarcolemma repolarizes; calcium is pumped back into the SR.

   • Troponin and tropomyosin return to their resting positions, blocking actin's active sites.

   • Muscle relaxes; this is a passive process.

Spasm: Involuntary, forceful contraction when muscle cannot relax.

Module 10.5 – Energy Sources for Muscle Contraction

Immediate Energy

• ATP: Stored ATP provides immediate energy for contraction.

• Creatine Phosphate: Can rapidly donate a phosphate to ADP to regenerate ATP. The reaction is catalyzed by creatine kinase:

• Provides energy for short, intense bursts of activity.

Anaerobic Catabolism (Glycolysis)

• Breaks down glucose into two pyruvate molecules, producing 2 ATP per glucose.

• Occurs in the cytosol; does not require oxygen.

• Glucose sources: Diet or glycogen stored in muscle and liver.

Aerobic Catabolism (Oxidative Phosphorylation)

• Requires oxygen; occurs in mitochondria.

• Breaks down fuel molecules to generate many ATP via oxidative reactions.

• Myoglobin: Protein that binds and stores oxygen in muscle cells.

• Provides long-lasting energy for sustained activity.

Module 10.6 – Muscle Tension at the Fiber Level

Muscle Twitch

• The smallest muscle contraction in response to a single action potential.

• Measured in laboratory settings, not in whole muscles.

• Phases of a twitch (as shown on a myogram):

  • Latent period: Time for action potential to propagate and calcium to be released.

  • Contraction period: Crossbridge cycling generates tension.

  • Relaxation period: Calcium is reduced in the cytosol, tension diminishes.

  • Refractory period: Time after contraction when the muscle cannot respond to another stimulus (about 5 ms).

Wave Summation and Tetanus

• Wave Summation: Repeated stimuli before the muscle fully relaxes cause greater tension.

• Unfused (incomplete) tetanus: Muscle fiber is not allowed to relax completely; tension fluctuates.

• Fused (complete) tetanus: Muscle fiber is stimulated so rapidly that it cannot relax at all, producing maximum tension.

Length-Tension Relationship

• The number of crossbridges formed depends on the length of the sarcomere prior to contraction.

• Optimal overlap of actin and myosin allows for maximal tension generation.

• Overly shortened or stretched sarcomeres produce less tension.

Classes of Skeletal Muscle Fibers

• Type I fibers (slow-twitch): Small diameter, contract slowly, produce less force, rely on oxidative metabolism, and are fatigue-resistant. Predominate in postural muscles.

• Type II fibers (fast-twitch): Larger diameter, contract quickly, produce more force, fatigue rapidly, and rely more on glycolytic metabolism. Used for rapid, powerful movements.