Main points lecture 9 part 3
• Briefly describe the events involved in initiating muscle contraction (starting at the motor
neuron up to the release of Ca from the SR. Nerve impulse reaches axon terminal voltage-
gated calcium channels open ACh released to synaptic cleft. 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
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.
o Why is this? Tropomyosin blocks active sites and myosin heads of thick filaments cannot
attach.
• When sarcoplasmic calcium increases from its release from the SR (sarcoplasmic reticulum),
what specifically does it eventually bind to in the sarcomere? Troponin-C
o What change in the thin filament occurs? Troponin changes shape and moves
tropomyosin away from myosin-binding sites.
o What can the thick filament now do? Myosin heads bind to actin and undergo power
stroke causing sarcomere shortening and contraction.
• What happens when nervous system (motor neuron) stimulation of the skeletal muscle cell
ceases? The muscle relaxes.
o What produces the decline in sarcoplasmic calcium concentration?
▪ SR pump name? SERCA pumps.
▪ Sarcoplemmal? Sarcolemnal calcium pumps.
• How is the position of the troponin-tropomyosin complex affected by:
o High sarcoplasmic calcium? Tropomyosin blocks active sites on actin of thin filaments.
o Low sarcoplasmic calcium? Troponin changes shape and moves tropomyosin away from
myosin-binding site..
• Define cross-bridge formation. High-energy myosin attaches to thin filaments.
o Briefly describe the process of cross-bridge cycling and the change in location of the z-
discs relative to the m-line. Energized myosin head attaches to an actin myofilament,
forming a cross bridge. Power stoke is the bending and twisting of the myosin head
which pulls the thin actin filament toward the M-line of the sarcomere. Pi is released
from the myosin head initiating power stroke. ADP is released toward the middle/end.
After ATP attaches to myosin, the link between myosin and actin weakens, and the
myosin head detaches. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to
its prestroke high-energy, or “cocked,” position.
o What zones/bands are affected by the shortening of the sarcomeres?
o What zones/bands are Not affected by shortening of the sarcomeres?
• What sarcoplasmic ion concentration allows for cross-bridge cycling to continue in the
contracting muscle cell? The cycle continues as long as ATP is available and Calcium is bound to
troponin.
• Briefly describe the events involved in initiating muscle contraction (starting at the motor
neuron up to the release of Ca from the SR. Nerve impulse reaches axon terminal voltage-
gated calcium channels open ACh released to synaptic cleft. 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
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.
o Why is this? Tropomyosin blocks active sites and myosin heads of thick filaments cannot
attach.
• When sarcoplasmic calcium increases from its release from the SR (sarcoplasmic reticulum),
what specifically does it eventually bind to in the sarcomere? Troponin-C
o What change in the thin filament occurs? Troponin changes shape and moves
tropomyosin away from myosin-binding sites.
o What can the thick filament now do? Myosin heads bind to actin and undergo power
stroke causing sarcomere shortening and contraction.
• What happens when nervous system (motor neuron) stimulation of the skeletal muscle cell
ceases? The muscle relaxes.
o What produces the decline in sarcoplasmic calcium concentration?
▪ SR pump name? SERCA pumps.
▪ Sarcoplemmal? Sarcolemnal calcium pumps.
• How is the position of the troponin-tropomyosin complex affected by:
o High sarcoplasmic calcium? Tropomyosin blocks active sites on actin of thin filaments.
o Low sarcoplasmic calcium? Troponin changes shape and moves tropomyosin away from
myosin-binding site..
• Define cross-bridge formation. High-energy myosin attaches to thin filaments.
o Briefly describe the process of cross-bridge cycling and the change in location of the z-
discs relative to the m-line. Energized myosin head attaches to an actin myofilament,
forming a cross bridge. Power stoke is the bending and twisting of the myosin head
which pulls the thin actin filament toward the M-line of the sarcomere. Pi is released
from the myosin head initiating power stroke. ADP is released toward the middle/end.
After ATP attaches to myosin, the link between myosin and actin weakens, and the
myosin head detaches. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to
its prestroke high-energy, or “cocked,” position.
o What zones/bands are affected by the shortening of the sarcomeres?
o What zones/bands are Not affected by shortening of the sarcomeres?
• What sarcoplasmic ion concentration allows for cross-bridge cycling to continue in the
contracting muscle cell? The cycle continues as long as ATP is available and Calcium is bound to
troponin.
