Muscle Contraction

  • Neuromuscular Junction

    • Key focus of the assignment due Wednesday.

    • Ensure to start the answer at the neuromuscular junction, where the axon terminal meets the sarcolemma (motor end plate).

    • The junction includes a synapse that facilitates communication between a neuron and muscle cell.

  • Skeletal Muscle Contraction

    • Mechanical contraction of skeletal muscle must be initiated by a neuron.

    • At the neuromuscular junction, the neurotransmitter acetylcholine (ACH) is released, which triggers an electrical signal in the muscle fiber membrane (sarcolemma).

    • An enzyme called acetylcholinesterase (ACHE) dismantles ACH from receptors to stop muscle fiber stimulation.

  • Sliding Filament Theory

    • Theory stating that the sarcomere (structural unit of muscle) shortens during contraction.

    • Myosin proteins grab actin proteins and pull them towards the middle of the sarcomere.

    • Myosin heads pivot and pull actin using energy from ATP.

  • Calcium's Role

    • When an electrical signal reaches the sarcoplasmic reticulum, calcium ions are released.

    • Calcium binds to troponin, which then changes shape and moves tropomyosin, exposing active sites on actin for myosin binding.

  • Muscle Fiber Contraction Process:

    1. ACH is released at the neuromuscular junction.

    2. Action potential travels down sarcolemma.

    3. Calcium is released from the sarcoplasmic reticulum.

    4. Calcium binds to troponin, allowing myosin-actin crossbridging.

    5. Myosin pulls actin toward the center of the sarcomere, resulting in contraction.

  • Factors Influencing Tension in Muscle Fibers:

    • Resting length of the fiber (optimal length allows more tension).

    • Frequency of stimulation from motor neurons determines if the muscle fibers are in a twitch (single contraction) or summation (multiple contractions).

  • Types of Contractions:

    • Isometric: Muscle tension without shortening (e.g., pushing against a wall).

    • Isotonic: Muscle tension with shortening (e.g., lifting weights).

  • Muscle Tone:

    • Represents the constant partial contraction of skeletal muscles even at rest, helping maintain posture.

    • Muscle tone contributes to energy expenditure by maintaining a slight state of contraction.

  • Energy Production for Muscle Contraction:

    • ATP is the energy currency of muscle cells, produced through breakdown of glucose—requires oxygen for aerobic respiration.

    • Without enough oxygen, muscle cells can rely on anaerobic glycolysis, moving quickly but producing only 2 ATP per glucose.

    • Creatine Phosphate can be used to store and transfer phosphate to regenerate ATP quickly during contraction.

  • Fatigue:

    • Caused by a combination of low ATP availability and lactic acid buildup from anaerobic respiration, leading to impaired contraction ability.

  • Muscle Fiber Recruitment:

    • Muscles use small motor units for precision tasks.

    • Larger motor units will be recruited for heavier loads.

    • Recruitment leads to increased tension in the whole muscle, allowing for smooth transitions between contraction and relaxation.

  • Return to Resting Length:

    • A passive process aided by elastic forces, opposing muscle contractions, and gravity.