neuromuscular junction
Ion Movement During Action Potential Generation
The medical illustration uses arrows to depict ion movement.
Sodium Ions (Na⁺): Shown with a thick arrow moving into the cell.
Potassium Ions (K⁺): Shown with a narrow arrow indicating outflow from the cell.
Direction: Both ions move in opposite directions when ion channels open.
Sodium Influx vs. Potassium Efflux:
Sodium moves into the cell at a higher rate than potassium moves out.
This causes an increase in positive charge inside the cell, thereby making the inside progressively less negative.
Membrane Potential Changes:
Resting potential: -90 mV.
As sodium enters, the potential progresses to -85 mV, -75 mV, -77 mV.
End Plate Potential and Threshold
End Plate Potential (EPP):
Defined as the process of the cell becoming progressively less negative from the resting potential.
The threshold for generating an action potential in muscle cells is -65 mV.
At this threshold, significant changes in membrane potential occur.
At EPP, the membrane reaches -65 mV, ceasing to be a resting potential and therefore initiating action potential generation.
Voltage-Gated Channels and Depolarization
Voltage-Gated Channels:
Purple channels represent voltage-gated sodium channels.
Blue channels represent voltage-gated potassium channels.
These channels are stimulated to open based on changes in electrical potential across the membrane.
Significance of -65 mV:
This value triggers the opening of voltage-gated sodium channels, leading to a rapid influx of sodium ions, resulting in depolarization.
The cell's potential changes from -90 mV to +30 mV during this depolarization phase.
Reversal of Polarity and Repolarization
Depolarization:
Defined as the reversal of polarity across the membrane.
The inside of the cell becomes positive due to the influx of sodium ions.
Potassium Outflow:
After depolarization, sodium channels close, and potassium channels open, resulting in potassium exiting the cell.
This leads to repolarization, wherein the inside of the cell becomes negative again, going back down to establish -90 mV.
Action Potential Propagation
The action potential travels as a wave of depolarization and repolarization along the cell membrane.
The involvement of t-tubules, which carry the depolarization signal deep into the muscle cell, allows for coordinated muscle contraction.
Relationship with Sarcoplasmic Reticulum
As the action potential reaches the t-tubules, it stimulates the opening of voltage-gated calcium channels in the sarcoplasmic reticulum (SR).
Calcium stored within the SR is released into the cytoplasm, which is essential for muscle contraction.
Events of Muscle Contraction
Excitation-Contraction Coupling: The sequence of events that occurs from action potential generation to muscle contraction includes:
End Plate Potential caused by neurotransmitter binding (acetylcholine) and opening of chemically gated ion channels.
Reaching threshold potential and opening sodium channels leading to depolarization.
Closing of sodium channels and opening potassium channels resulting in repolarization.
The presence of calcium ions activates contraction cycles within muscle fibers.
Action Potential Graph Explanation
Graph Details:
The y-axis represents the electric potential across the membrane in millivolts, and the x-axis represents time in milliseconds ().
Starting point illustrated at -90 mV (resting potential).
Initial flat period represents time without voltage change during the initiation of a nerve impulse.
Once acetylcholine opens channels and sodium influx surpasses potassium outflux, a progressive depolarization is observed leading to reaching the threshold of -65 mV.
Crossbridge Cycle in Muscle Contraction
Calcium Functionality in Muscle Contraction:
Calcium binds to troponin, causing a conformational change in tropomyosin, exposing myosin-binding sites on actin.
Power Stroke Mechanism:
Myosin heads attach to actin, triggering a power stroke that slides actin filaments toward the center of the sarcomere, thereby shortening the muscle.
Energy Use in Mechanism:
Involves breaking down ATP to ADP and inorganic phosphate to provide energy for the power stroke and reset myosin heads.
Process of Muscle Relaxation:
Cessation of nerve impulses leads to breakdown of acetylcholine and closure of calcium channels. Muscle relaxation is achieved as calcium is pumped back into the sarcoplasmic reticulum.
ATP Generation During Muscle Activity
Muscle cells rely on three processes for ATP generation based on activity intensity and duration:
Phosphate Transfer (Immediate ATP supply for about 10 seconds)
Transfer of phosphate from ADP to regenerate ATP.
Glycolysis (Short-term supply for about 1 minute)
Breakdown of glucose yielding a net of 2 ATP.
Cellular Respiration (Long-term supply)
Involves citric acid cycle and oxidative phosphorylation for higher ATP yields.
The process for different types of running races demonstrates the demand for ATP generation methods according to the length and intensity of the activity, e.g., sprints versus longer distances requiring different ATP sources.