A&P lecture 9 part 3 main points

Briefly describe the events involved in initiating muscle contraction (starting at the motor neuron up to the release of Ca from the SR)
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Briefly describe the events involved in initiating muscle contraction (starting at the motor neuron up to the release of Ca from the SR)
(1)Nerve impulse reaches axon terminal → voltage-gated calcium channels open → ACh released to synaptic cleft
(2)ACh binds to its receptors on sarcolemma → opens ligand-gated Na+ and K+ channels → end plate potential → Opens voltage-gated Na+ channels → AP and AP propagation across the sarcolemma down into the t-tubules
(3)Voltage-sensitive proteins in T tubules (DHP receptors) change shape in response to AP and activate the Ryanodine receptors→ SR releases Ca2+ to cytosol
The muscle fiber is in a relaxed state when intracellular (sarcoplasmic) calcium concentrations are low. Why is this?
Tropomyosin blocks active sites (myosin binding sites) on actin of thin filaments; Myosin heads of thick filaments cannot attach when binding sites of thin filament are blocked
When sarcoplasmic calcium increases from its release from the SR (sarcoplasmic reticulum), what specifically does it eventually bind to in the sarcomere?
troponin C of the troponin-tropomyosin complex
What change in the thin filament occurs when calcium binds to troponin C?
Troponin changes shape and moves tropomyosin away from myosin-binding sites
What can the thick filament now do that troponin changed shape?
Myosin heads bind to actin and undergo power stroke causing sarcomere shortening and muscle contraction
What happens when nervous system (motor neuron) stimulation of the skeletal muscle cell ceases?
the majority of Ca2+ in the sarcoplasm is pumped back into SR by SERCA pumps (active transport)
What produces the decline in sarcoplasmic calcium concentration?
SERCA pumps and Ca pumps in the sarcolemma (both active transport)
How is the position of the troponin-tropomyosin complex affected by: High sarcoplasmic calcium? Low sarcoplasmic calcium?
high sarcoplasmic calcium: troponin-tropomyosin complex shifts to uncover myosin-binding sites.
low sarcoplasmic calcium: troponin-tropomyosin complex shifts back to cover myosin-binding sites
Define cross-bridge formation
Myosin heads bind to actin
Briefly describe the process of cross-bridge cycling and the change in location of the z-discs relative to the m-line
Sliding of the thin filaments with respect to the thick begins as power stroke of the myosin heads take place; cross bridges form and break several times, ratcheting thin filaments toward center of sarcomere; Causes shortening of muscle fiber; Pulls Z discs toward M line