AP 151 Lab: Skeletal Muscle Physiology

Skeletal Muscle Physiology

Lab Information

• AP 151 Lab focuses on skeletal muscle physiology.
• A 10-point worksheet (pages 157-163) is due next week.

Neural Control of Skeletal Muscle Contraction

• The process involves the nervous system's control over skeletal muscle contraction.
• Key components:
  • Cell body.
  • Upper motor neuron (posterior).
  • Anterior.
  • Dorsal root (sensory).
  • Ventral root (motor).
  • Alpha motor neuron (lower motor neuron).
  • Myelin sheath.
  • Blood vessels.
  • Muscle fibers within a fascicle.

Neuromuscular Junction

• The motor neuron releases acetylcholine (Ach) at the neuromuscular junction.
• Key structures:
  • Muscle fiber with nucleus, sarcolemma, sarcoplasmic reticulum, T tubule, and mitochondria.
  • Axon of motor neuron forming the synaptic terminal and synaptic vesicles.
  • Synaptic cleft.
  • Motor end plate on the muscle fiber.
  • Myofibrils containing thick (myosin) and thin (actin) filaments.

Muscle Contraction Mechanism

• Myosin heads attach to actin filaments.
• Filaments slide toward the center, shortening the sarcomere.
• Calcium is essential as an activating signal for this sliding filament mechanism.

Calcium's Role in Muscle Contraction and Relaxation

• Action potential on the muscle cell's plasma membrane triggers calcium release from the sarcoplasmic reticulum (SR), which is the muscle cell's endoplasmic reticulum (ER).
• Muscle contracts when calcium is released from SR.
• Muscle relaxes when calcium is pumped back into SR.

Review Questions: Calcium Transport

1. Calcium ions moving out of SR into the sarcoplasm is an example of passive transport. Specifically, calcium ions use the calcium channels to move out of SR.
2. Calcium ions moving from the sarcoplasm back into SR is an example of active transport. Specifically, calcium ions use the calcium pumps to move into SR.

Excitation-Contraction Coupling

• Skeletal muscle requires stimulation by an electrical signal, either from a motor neuron or an electric shock, to contract.
• Calcium released from the sarcoplasmic reticulum (SR) serves as the signal for muscle contraction.

Historical Context

• Luigi Galvani's frog leg experiment (1780) demonstrated the electrical basis of muscle contraction.

Experiment: Muscle Stimulation

• Experiment involves using electric shock to stimulate the palmaris longus muscle (flexion of the middle finger).
• Setup includes a stimulator with isolated inputs, ground (Gnd), and channels (CH1, CH2, CH3, CH4).

Muscle Twitch

• Twitch: A muscle quickly contracts and relaxes after a single electrical shock of sufficient voltage.
• Increasing the voltage increases the strength of the twitch up to a maximum.

Phases of a Muscle Twitch

• Lag (latent) phase: Time between the stimulus and the contraction.
• Contraction phase: Ca^{2+} is released from SR.
• Relaxation phase: Ca^{2+} is pumped back into SR.

Innervation of Skeletal Muscle Fibers

• Motor unit: A motor neuron and all the muscle fibers it controls.
• Muscle fibers under control of a single motor neuron contract together when stimulated.
• The muscle fibers in a single motor unit are spread out within the muscle.
• Stimulation of a motor unit results in weak contraction of the entire muscle.

Motor Unit Details

• The number of muscle fibers a neuron innervates varies.
• Small motor units have less than five muscle fibers.
  • Allow for precise control of force output.
• Large motor units have thousands of muscle fibers.
  • Allow for production of a large amount of force but not precise control.

Motor Unit Recruitment

• A measure of how many motor neurons are activated in a particular muscle.
• The higher the recruitment, the stronger the muscle contraction will be.
  • Recruit few motor units to lift a pencil vs. many to lift a suitcase.
• Recruitment order is based on the size of motor units: small first, large last.

Visual Representation of Motor Units

• Motor unit X (7 fibers) and motor unit Y (12 fibers).
• Tension increases as more motor units are stimulated.

Fiber Size and Strength

• Small, weak fibers in small motor units.
• Large, strong fibers in large motor units.

Stimulus Intensity and Muscle Force

• Increase in voltage can increase the amount of force generated by the muscle (to a certain extent).
• Threshold stimulus: The minimum voltage that results in a contraction.
  • Stimulates the smallest motor unit and results in a twitch.
• Maximal stimulus: The minimum voltage that triggers the maximal contraction.
  • Any further increase in stimulus does not result in a stronger contraction because all motor units of a muscle have been recruited.

Wave Summation

• Muscle fibers developing tension are stimulated again before the fibers have relaxed.
• Identical stimuli can trigger stronger muscle contractions if applied frequently enough.
• Rationale: More Ca^{2+} is released from SR (as opposed to being pumped back to SR), resulting in a stronger contraction.

Methods to Increase Muscle Contraction Strength

• (A) Motor unit recruitment (increase in stimulus intensity).
• (B) Wave summation (increase in stimulus frequency).

Tetanus

• Increasing the frequency of electrical shocks decreases the relaxation time between twitches.
• Unfused (incomplete) tetanus: some relaxation between twitches
• Fused (complete) tetanus: No relaxation (smooth, sustained contraction).

Frequency vs. Intensity

• Increase in stimulus frequency increases contraction strength due to wave summation (i.e., increase in quality of muscle fibers).
• Increase in stimulus intensity increases contraction strength due to motor unit recruitment (i.e., increase in quantity of muscle fibers).

Real-World Application

• We use both motor unit recruitment and wave summation when lifting a heavy object.