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.