Pt3

Muscle Contraction Overview

• Muscle fibers are excitable, meaning they respond to chemical signals.

• The site of muscle fiber stimulation is the neuromuscular junction (NMJ), where the motor neuron connects with the muscle fiber.

Components of the Neuromuscular Junction

• Motor end plate: This is a specialized area of the muscle cell's membrane that interacts with the motor neuron.

• Axon terminal: The end of a motor neuron where neurotransmitters are stored.

• Synaptic cleft: The narrow space that separates the axon terminal from the muscle fiber.

• Neurotransmitter: Acetylcholine is the chemical responsible for transmitting the signal from the neuron to the muscle fiber.

Process of Muscle Contraction

1. Excitation Phase

   • An action potential from the brain travels down the motor nerve to the axon terminal.

   • This action potential opens calcium channels at the terminal, allowing calcium to enter.

   • Calcium triggers exocytosis of synaptic vesicles, releasing acetylcholine into the synaptic cleft.

   • Acetylcholine binds to receptors on the motor end plate, specifically ligand-gated sodium channels.

   • Binding opens these channels, allowing sodium ions to flow into the muscle fiber and causing depolarization (localized depolarization is called end plate potential).

   • Multiple end plate potentials are required to generate a full muscle contraction.

2. Action of Acetylcholine

   • Acetylcholine must be cleared from the synaptic cleft to stop its effects.

   • Acetylcholine esterase is the enzyme that breaks down acetylcholine, ending the stimulation of the muscle.

3. Excitation-Contraction Coupling Phase

   • The depolarization of the motor end plate spreads along the sarcolemma (muscle membrane) and down the T-tubules, leading to the opening of voltage-gated sodium channels.

   • This results in a larger action potential propagation in the muscle fiber.

   • Depolarization activates calcium channels in the sarcoplasmic reticulum, releasing stored calcium into the cytoplasm.

4. Contraction Phase

   • Calcium binds to troponin, leading to a change in the conformation of tropomyosin.

   • The move of tropomyosin exposes the binding sites on actin molecules.

   • Myosin heads bind to the exposed active sites on actin, initiating the crossbridge cycle and leading to muscle contraction.

   • The contraction mechanism continues as long as calcium remains present in the cytoplasm and the ATP is available.