Neuromuscular System

 Sliding Filament Theory

 What four characteristics do all muscle tissues have?  

• Excitability (responding to stimuli)

• Contractility (ability to shorten forcefully

•  Extensibility (ability to be stretched)

• Elasticity (ability to return to resting length after stretching)

• Excitability in Muscle Tissue
  The characteristic that allows muscle tissue to be stimulated and carry an action potential is excitability. This is the ability of muscle cells to respond to electrical or chemical stimuli by changing their membrane potential and generating an electrical signal (action potential) that travels along the cell membrane.

• Prime Mover of a Joint
  Another name for the prime mover of a joint is the agonist, which is the main muscle responsible for producing a specific movement.

• Muscle Opposing the Prime Mover
  The muscle that opposes the prime mover is called the antagonist.

  • Filaments Making Up the Sarcomere
    The two filaments making up a sarcomere are:

• Myosin: This is the thick filament, forming the core.

• Actin: This is the thin filament, forming thinner strands.
  These filaments slide past each other during muscle contraction.

  • Attachment of Filaments in the Sarcomere
    In a sarcomere, the filaments are attached as follows:

• Thin (actin) filaments attach to the Z-discs at the ends.

• Thick (myosin) filaments are anchored at the M-line in the center.
  Both filaments are linked by the elastic titin protein, which allows them to slide past each other for muscle contraction.

• Molecule Aiding Myosin-Actin Interaction
  The molecule that helps the myosin head attach and detach from actin is ATP (adenosine triphosphate). ATP binds to the myosin head to cause detachment, and its hydrolysis provides the energy for the next attachment and power stroke in the muscle contraction cycle.

• Rigor Mortis
  Rigor mortis is the temporary stiffening of muscles after death due to a lack of adenosine triphosphate (ATP), which is necessary to relax the muscles.

  • Three Steps in Muscle Contraction
    According to the video, the three phases of muscle contraction are:

• Excitation-Contraction Coupling: This phase begins with a nerve signal, leading to the release of calcium ions ($ext{Ca}^{2+}$).

• Contraction: During this phase, the muscle fibers shorten as the filaments slide.

• Relaxation: In this phase, calcium is pumped back into storage, and the muscle fibers lengthen and return to their resting state.

  • Proteins Regulating Actin Binding Sites
    The primary proteins regulating actin's myosin-binding sites, especially in muscle, are:

• Tropomyosin: This protein physically blocks the binding sites on actin in resting muscle.

• Troponin: This protein binds calcium ($ext{Ca}^{2+}$) and then moves tropomyosin to expose the sites for contraction. Together, they form the troponin-tropomyosin complex.

• Ion Opening Actin Binding Sites
  The key ion that helps open the binding sites on actin for muscle contraction is the calcium ion ($ext{Ca}^{2+}$). It binds to the troponin protein, causing it to move tropomyosin and uncover the myosin-binding sites on actin filaments, allowing the muscle to contract.

Summary:

Muscle tissue exhibits excitability, responding to stimuli by generating action potentials. In movement, the agonist is the prime mover, while the antagonist opposes it. The sarcomere, the basic unit of muscle contraction, is composed of myosin (thick) and actin (thin) filaments. Actin filaments attach to Z-discs, and myosin filaments are anchored at the M-line, with titin protein linking them, allowing them to slide past each other. ATP is crucial for the myosin head to attach to and detach from actin, powering the contraction cycle. A lack of ATP after death leads to rigor mortis, the temporary stiffening of muscles. Muscle contraction unfolds in three phases: Excitation-Contraction Coupling, where a nerve signal triggers the release of calcium ions ($ext{Ca}^{2+}$); Contraction, involving the sliding of filaments; and Relaxation, where calcium is pumped back, and muscle fibers return to rest. Key proteins, tropomyosin and troponin, regulate actin's myosin-binding sites. Tropomyosin physically blocks these sites, while troponin binds to $ext{Ca}^{2+}$, causing tropomyosin to move and expose the sites for muscle contraction.