Neuromuscular Junction Muscle Contraction Part 2 Excitation Contraction Coupling Anatomy

Motor Neurons and Muscle Fibers

• Single Motor Neuron:

  • Originates in the brain or spinal cord.

  • Conducts action potentials to hundreds of skeletal muscle fibers within a muscle.

Excitation-Contraction Coupling

• Definition:

  • Sequence of events converting action potentials in a muscle fiber into a contraction.

Muscle Fiber Action Potential

• Sarcolemma:

  • Action potential travels across the entire sarcolemma of a muscle fiber.

  • Quickly conducted into the interior via transverse tubules (T-tubules).

• Transverse Tubules (T-tubules):

  • Infoldings of the sarcolemma.

  • Branch extensively throughout the muscle fiber.

Sarcoplasmic Reticulum (SR)

• Calcium-Storing Network:

  • T-tubules make contact with SR, which stores calcium.

• Terminal Cisternae:

  • Sack-like bulges formed by the SR at junctions with T-tubules.

• Triad Formation:

  • Structure consists of one T-tubule plus two adjacent terminal cisternae.

  • Linked by proteins that control calcium release.

Calcium Release Mechanism

• Action Potential Propagation:

  • Action potential travels down the T-tubule causing shape change in a voltage-sensitive protein.

• Calcium Release:

  • Shape change opens calcium release channels in the SR.

  • Calcium ions flood the cytoplasm rapidly.

Muscle Contraction Trigger

• Role of Calcium Ions:

  • Rapid influx of calcium ions triggers skeletal muscle fiber contraction.

  • Calcium ions are essential for coupling excitation to contraction.