3D

Muscle Physiology Lecture Notes

Lecture Participation and Extra Credit

  • Extra Credit: 10 points available in lab if mid-semester survey is completed.

  • Worth 10 points, equivalent to roughly 2.5 questions on the exam.

Class Structure and Objectives

  • Today's Focus: Transition from skeletal muscle physiology to neuromuscular junction details.

  • Learning Objectives:

    • Identify the components of the neuromuscular junction.

    • Explain the flow of acetylcholine through the neuromuscular junction.

    • Describe the function of:

    • Voltage-gated ion channels

    • Chemically gated acetylcholine channels

    • Explain excitation-contraction coupling.

    • Describe the process of cross-bridging.

Neuromuscular Junction (NMJ)

  • Components of the NMJ:

    • Presynaptic Terminal: Contains acetylcholine vesicles.

    • Synaptic Cleft: Space between the motor neuron and muscle fiber.

    • Postsynaptic Membrane (Motor End Plate): Region of the muscle fiber membrane with acetylcholine receptors.

Flow of Acetylcholine at the NMJ

  • An action potential arrives at the presynaptic terminal.

  • Voltage-gated Ca^{2+} channels open, allowing Ca^{2+} to enter the terminal.

  • Ca^{2+} influx triggers the release of acetylcholine into the synaptic cleft.

  • Acetylcholine binds to chemically gated acetylcholine channels on the motor end plate.

Function of Ion Channels

  • Voltage-gated ion channels:

    • Primarily found in the axon and presynaptic terminal.

    • Open or close in response to changes in membrane potential.

    • Essential for action potential propagation and neurotransmitter release (e.g., Ca^{2+} channels at NMJ).

  • Chemically gated acetylcholine channels:

    • Located on the motor end plate.

    • Open when acetylcholine binds to them, allowing Na^{+} to enter and K^{+} to exit.

    • This specific ion movement generates an end-plate potential (EPP).

Excitation-Contraction Coupling

  • The EPP, if strong enough, triggers an action potential on the muscle fiber membrane.

  • The muscle action potential propagates along the sarcolemma and down the T-tubules.

  • This leads to the release of Ca^{2+} from the sarcoplasmic reticulum (SR) into the sarcoplasm.

  • Ca^{2+} binds to troponin, causing a conformational change that moves tropomyosin away from actin binding sites.

Cross-Bridging Cycle

  • Once actin binding sites are exposed, the myosin head (which is already energized with ADP and P_i) binds to actin, forming a cross-bridge.

  • Power Stroke: ADP and P_i are released, and the myosin head pivots, pulling the actin filament towards the M-line.

  • Detachment: A new ATP molecule binds to the myosin head, causing it to detach from actin.

  • Reactivation: ATP is hydrolyzed into ADP and P_i, re-energizing the myosin head and preparing it for the next cycle.