ANP 1105 Lecture Notes - Lecture 5: Myosin Head, Endoplasmic Reticulum, Threshold Voltage
2 May 2021
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LAB 1. SKELETAL MUSCLE
At Subthreshold Stimuli There Is no Muscle Response Despite the Application of Stimuli. Explain
Why This Occurs at a Cellular Level
!When stimulus is below threshold, Na+ channels do not open and an action potential is not generated. As a
result, calcium channels will not open, thus no calcium will be available to activate the cross bridge cycle. "
When Threshold Is Met, Summarize the Sequence of Events That Occur at a Cellular Physiology
Level That Lead to Muscle Shortening
•Voltage gated Na channel opens which generates action potential that spreads along sarcolemma and then down T
tubules, This changes the shape of voltage-sensitive proteins on T tubules"
•This stimulates sarcoplasmic reticulum to release Calcium ions"
•Calcium binds to troponin and move the troponin-tropomyosin binding complex away from the myosin binding site to
form a cross bridge. "
• $The shift will expose the active sites on the actin (myosin binding sites). Myosin head is bound to an ADP and
phosphate. When the myosin dephosphorylates, it will bind to the active sites on actin forming a cross bridge"
•Myosin will release the ADP and the myofilaments glide past each other propelled by the head called a “power stroke” "
•ATP will stop the gliding when bound to myosin. Hence, separating the myosin head from the actin active site to be
bound again"
•This cross bridge cycle causes muscle contraction = shortening"
!
How Similar Are Each Recorded Value When Comparing Individual Twitches at a Constant
Stimulus?
!Factors that affect the strength of a contractions are changes in frequency, number of recruited muscle fibers,
and muscle fiber size. The strength of the applied stimulus remained constant for each individual twitch. This means that
the number of recruited muscle fibers and frequency of each twitch are the same. Since the stimulus was applied to the
same area of the muscle each time, the muscle fiber size did not vary. The factors that would affect the value of each
twitch remained constant, which leads to the expectation that the values for latency period, amplitude, contraction time
and relaxation time to be similar for each twitch. "
Should the Recorded Times Change With an Increase in Applied Stimuli?
Latency: No change observed; Time between stimulus and starting cross bridge cycling is always the same"
Contraction and relaxation times: No change observed; Amount of Ca released is the same each time."
