Overview
This lecture explains the sliding filament theory of muscle contraction, focusing on excitation-contraction coupling and the role of synapses, neurotransmitters, and calcium in triggering muscle movement.
Sliding Filament Theory
- The sliding filament theory states muscles contract when filaments slide past each other within muscle fibers.
- It is a well-supported scientific model with extensive evidence.
Excitation-Contraction Coupling
- Excitation refers to a nervous system signal (action potential) that initiates muscle contraction.
- Contraction is the shortening of muscle fibers due to filament sliding.
- The process begins at the neuromuscular junction, where a neuron communicates with a muscle fiber.
Neuromuscular Junction and Synaptic Transmission
- The neuromuscular junction is where a neuron meets a muscle fiber, separated by a small gap called the synapse or synaptic cleft.
- When you decide to move, an action potential travels down a neuron to its axon terminal, reaching the muscle.
- Inside the axon terminal, synaptic vesicles containing the neurotransmitter acetylcholine (ACh) fuse with the neuron’s membrane and release ACh into the synaptic cleft.
- Acetylcholine binds to receptors on the muscle fiber’s membrane without entering the cell, triggering ion channels to open.
- This generates a new action potential in the muscle fiber.
T Tubules and Sarcoplasmic Reticulum
- The new action potential travels along the muscle membrane and down into the cell via T tubules (transverse tubules).
- The T tubules allow the signal to reach deep inside the muscle fiber.
- At the end of T tubules are terminal cisternae, part of the sarcoplasmic reticulum (specialized endoplasmic reticulum in muscle cells).
- The sarcoplasmic reticulum stores calcium ions, which are released in response to the action potential.
- Calcium release is crucial for initiating muscle contraction by enabling filament interaction.
Importance of Synaptic Cleft
- The synaptic cleft is where neurotransmitters like acetylcholine act and are removed after signaling by specific enzymes.
- Disruption in neurotransmitter breakdown (e.g., by nerve agents) can cause continuous muscle contraction, leading to spasms and possible suffocation.
Key Terms & Definitions
- Sliding Filament Theory — Muscles contract when actin and myosin filaments slide past one another.
- Excitation-Contraction Coupling — The process linking nerve stimulation to muscle contraction.
- Neuromuscular Junction — The synapse where a neuron meets a muscle fiber.
- Synaptic Cleft — The small gap between neuron and muscle fiber at the neuromuscular junction.
- Acetylcholine (ACh) — A neurotransmitter that carries the action potential across the synaptic cleft.
- T Tubules — Tube-like extensions of the muscle cell membrane that carry action potentials into the cell.
- Sarcoplasmic Reticulum — Specialized organelle in muscle fibers that stores and releases calcium ions.
Action Items / Next Steps
- Review the structure and function of striations in muscle tissue for deeper understanding in the next lecture.
- Study the steps of excitation-contraction coupling and key terminology.