Skeletal Muscle Flashcards
Outline the major electrical events that occur at the neuromuscular junction (steps 1 and 2).
- An electrical signal (action potential) travels down the axon of the motor neuron.
- When the action potential reaches the synaptic terminal of the motor neuron, voltage-gated calcium channels open, allowing calcium ions (Ca²⁺) to enter the neuron.
Outline the major chemical events that occur at the neuromuscular junction (steps 3, 4 and 5).
- ACh is released to influx of calcium and binds to nicotinic receptors on the motor end plate.
- The binding of ACh causes ligand-gated sodium channels to open, leading to an influx of Na⁺ into the muscle cell, causing depolarization.
- Acetylcholinesterase (AChE) breaks down ACh in the synaptic cleft.
What are the key events responsible for excitation-contraction coupling in skeletal muscle? (4)
- Action potential propagation
- Calcium Release from Sarcoplasmic Reticulum (SR)
- Calcium Binding to Troponin
- Cross-Bridge Formation
Describe what is meant by action potential propagation.
The action potential propagates along the sarcolemma and down the T-tubules (transverse tubules).
Explain calcium binding to troponin.
Released calcium binds to troponin-C, a component of the troponin complex on the actin filaments, causing a conformational change that moves tropomyosin away from actin’s myosin-binding sites.
Explain cross-bridge formation.
Myosin heads bind to exposed binding sites on actin, initiating the contraction cycle (sliding filament mechanism).
Describe the sliding filament hypothesis.
- During contraction, the thin (actin) filaments slide past the thick (myosin) filaments, resulting in the shortening of the sarcomere.
- The Z-lines are drawn closer together, the I-band shortens, and the H-zone disappears as the A-band remains constant.
Describe the different zones in a sarcomere.
Z-lines: The boundaries of each sarcomere. Sarcomeres are the functional contractile units.
I-band: The region containing only thin filaments (actin). This band shortens during contraction.
A-band: The length of the thick filaments (myosin), which does not change during contraction.
H-zone: The central region of the A-band where only thick filaments are present (no overlap with actin). The H-zone narrows during contraction.
M-line: The centre of the sarcomere where thick filaments are anchored.
Describe the four stages of the cross-bridge cycle (with reference to ATP).
- Myosin heads, energized by ATP hydrolysis, bind to actin, forming cross-bridges.
- The myosin head pivots, pulling the actin filament toward the center of the sarcomere (the power stroke). This movement shortens the sarcomere.
- A new ATP molecule binds to the myosin head, causing it to detach from the actin filament.
- ATP is hydrolyzed by myosin ATPase into ADP and Pi, re-cocking the myosin head into its high-energy state, ready for another cycle.
What factors affect the development of force in skeletal muscle?
- Muscle fibre length (length-tension relationship)
- Frequency of stimulation (summation and tetanus)
- Number of motor units recruited
What are the three ways ATP can be produced in skeletal muscle fibres?
- Phospocreatine system
- Anaerobic glycolysis
- Aerobic respiration
What is the phosphocreatine system?
Creatine phosphate (CP) donates a phosphate group to ADP to quickly regenerate ATP.
Describe the two functional classifications of muscle fibres.
- Type 1 (slow twitch, oxidative): aerobic respiration, high myoglobin so red appearance, high endurance.
- Type 3 (fast-twitch, glycolytic): anaerobic respiration, white in appearance due to low myoglobin content, fatigue quickly.
Describe and explain the shape of the myoglobin oxygen dissociation curve.
- Hyperbolic
- Has higher affinity for oxygen than haemoglobin
- Allows it to store oxygen effectively in muscle tissues
What is the function of myoglobin?
Releases O₂ during periods of low oxygen availability, such as during intense muscle activity, to maintain ATP production via aerobic metabolism.
