Muscle Contraction

Muscle Contraction Overview

  • In this section, we will explore the process of muscle contraction, focusing particularly on skeletal muscle contraction, the cellular mechanisms involved, and the role of the nervous system in facilitating this process.

Muscle Cell versus Muscle Organ

  • Distinction: It is important to clarify when referring to the muscle cell (fiber) versus the entire muscle organ.

  • Characteristics of Muscle Contraction:

    • All sarcomeres within a single muscle fiber contract simultaneously.

    • The contraction of muscle fibers within a muscle organ occurs in a graded manner based on the force required; not all fibers activate at once.

    • The active muscle fibers depend on the needed force generated by the exercise or movement.

Connective Tissue in Muscle

  • Muscle fibers are surrounded by connective tissue components:

    • Endomysium: Surrounds individual muscle fibers.

    • Perimysium: Groups muscle fibers into fascicles.

    • Epimysium: Surrounds the entire muscle.

  • Tendon Formation: The connective tissues fuses at both ends of the muscle, forming tendons, which allows for a uniform pull on the tendon during muscle contraction.

Sliding Filament Theory

  • Definition: The mechanism that describes muscle contraction.

  • Four Steps of Muscle Contraction:

    1. Excitation

    2. Excitation-Contraction Coupling

    3. Contraction Phase

    4. Relaxation

Nervous System Communication with Muscle Cells

  • Central Nervous System (CNS): Comprised of the brain and spinal cord, from which motor neurons extend.

    • Spinal Nerves: These nerves emerge between vertebrae and consist of axons coming from neurons located in the spinal cord.

  • Motor Neuron Structure:

    • Soma: Located in the spinal cord.

    • Axon: Propagates action potentials to muscle fibers and connects via axon terminals.

    • Telo Dendria: Branches that end in synaptic terminals, each connecting to individual muscle fibers, forming a motor unit.

  • Motor Units: Combination of a single motor neuron and all muscle fibers it innervates.

    • Large Motor Units: Innervate hundreds to thousands (up to 5000) of muscle fibers, generating (lots of tension) strong contractions (e.g., postural muscles, quads, gluteus muscles).

    • Small Motor Units: Innervate fewer muscle fibers (e.g., muscle fibers in the eye have about 3 fibers per neuron), allowing for precise control and dexterity.

Phases of Muscle Contraction

1. Excitation Phase

  • Neuromuscular Junction (NMJ): The synapse between a motor neuron and a muscle fiber, critical for muscle activation.

    • Components of NMJ:

      • Axon terminal (presynaptic membrane)

      • Motor end plate (postsynaptic membrane)

      • Synaptic cleft (space between)

      • Vesicles containing Acetylcholine (ACh), the neurotransmitter for muscle contraction.

  • Mechanism of Action:

    • An action potential travels down the neuron, leading to the opening of voltage-gated calcium channels in the axon terminal, allowing calcium to flow in.

    • This influx triggers vesicles to fuse with the presynaptic membrane, releasing the neurotransmitters (Acetylcholine) into the synaptic cleft.

    • ACh binds to chemically gated sodium channels on the motor end plate, causing sodium influx which depolarizes the membrane (• Motor End Plate Potential).

    • If depolarization reaches the threshold, voltage-gated sodium channels open, leading to an action potential in the muscle fiber. (• End of Excitation Phase)

2. Excitation-Contraction Coupling

  • This phase translates the electrical action potential into a chemical signal for muscle contraction.

  • Process:

    • The action potential propagates along the sarcolemma and down T-tubules, resulting in calcium release from the sarcoplasmic reticulum (SR).

    • Triad Structure: Each T-tubule section is flanked by terminal cisternae of the SR, forming a triad, which is key for excitation-contraction coupling.

      • two terminal cisternae with a T-tubule in between

  • Voltage-Sensitive Proteins: As action potentials travel down T-tubules, it activates voltage-sensitive proteins in the terminal cisternae, causing calcium channels to open and release calcium into the cytoplasm, crucial for muscle contraction.

3. Contraction Phase

  • Rest State: At rest, myofibrils consist of actin and myosin where the actin binding sites are blocked by tropomyosin.

  • Role of Calcium: Upon calcium release from SR, calcium binds to troponin, causing tropomyosin to shift, exposing actin's binding sites for myosin.

  • Cross-bridge Formation: Myosin heads bind to actin forming cross-bridges, which are locked in place by the release of phosphate from myosin after ATP hydrolysis.

  • Power Stroke: The conformational change in myosin heads pulls actin filaments toward the sarcomere's center, resulting in sarcomere shortening (muscle contraction).

  • Cycle Continuation: Myosin now binds fresh ATP, causing the release of the actin filament and re-cocking of the myosin head to bind to the next actin molecule, continuing cross-bridge cycling until ATP or calcium is depleted.

4. Relaxation Phase

  • Relaxation begins immediately after contraction ends when the action potential ceases.

  • Key Processes for Relaxation:

    • ACh Breakdown: Acetylcholine in the synaptic cleft is degraded by acetylcholine esterase, preventing continuous stimulation of muscle fibers.

    • Repolarization: The sarcolemma's membrane potential returns to resting state.

    • Calcium Reuptake: Calcium ions are actively pumped back into the sarcoplasmic reticulum via ATP-dependent calcium pumps, and as calcium levels drop, troponin returns to its original shape, covering binding sites on actin, ceasing contraction.

Conclusion

  • Understanding the processes involved in muscle contraction is essential for appreciating how muscles function and how they are controlled by the nervous system. As we transition into muscle metabolism and performance, these contraction mechanics play a vital role in understanding muscle physiology and activities.