Lecture 9: Skeletal Muscle Function Flashcards
What type of synapse is the NMJ considered?
Cholinergenic Synapse as it releases the neurotransmitter Acetyl Choline.
What does Acetyl Choline do?
It interacts with receptors on the muscle fiber sending an action potential across the surface of the Transver tubular network.
Explain the events occurring at the cholinergic synapse.
Action potential arrises at the voltage gatted calcium channels. Calcium then diffuses into the axon terminal causing the release of Acetyl Choline via regulated exocytosis. ACh diffuses across the synaptic cleft, space between postsynaptic membrane and axon terminal, onto an ACh-gated sodium ion channel producing a graded depolarisation. Depolarisation ends as ACh is broken down into constituent parts.
What happens to the action potential created on the muscle fiber?
The AP runs along the sarcolemma and down the tubular membranes. Once the AP reaches the end of the tubular membrane it reaches a point called the triad. The triad is a structure of tubular membrane and two terminal cisternae on either side. Terminal cisternae are basically bulges of the sarcoplasmic reticulum which is a structure that holds calcium ions. The reversal of membrane potential at the triad causes Ca channels to open in the sarcolemma membrane.
Where do the calcium ions move from the sarcoplasmic reticulum.
Down there conc. gradient to an area of low concentration within the sarcoplasm.
Explain the interaction between troponin and tropomyosin when calcium is introduced.
Troponin binds to tropomyosin which binds to actin. When calcium is introduced it binds to troponin-C causing it to change shape. When troponin changes shape it results in tropomyosin rolling off actin exposing myosin binding sights.
Explain the state myosin is in at the start of the cross bridge cycle.
Initially the myosin is in a cocked position. ADP and Pi remain bound to the myosin globular head.
Explain the cross bridge cycle.
Once the actin myosin binding site is exposed myosin binds to actin creating a cross-bridge. Once formed myosin the DP and Pi causing it to change shape making it flex while bound to actin. This is called the power stroke and results in slight movement. ATP binding site is exposed on myosin head causing ATP to bind to it. This results in the cross bridge breaking. ATP is then converted into ADP + Pi on the myosin head causing it to return to the position it was in in the start of the cross bridge cycle.
Myosin and Actin overlap
Amount of force which can be produced by a sarcomere is when the overlap between myosin and actin is optimal. As the sarcomere lengthens, overlap between actin and myosin is reduced, so number of cross bridges is reduced, and force falls almost to zero when there is no actin-myosin overlap. Force also declines as myofilament overlap increases because the thin actin filaments overlap in the centre of the sarcomere and interfere with optimal cross bridge formation. This means that each muscle has an optimal length where it will be strongest, and when either longer or shorter than that length, it will be weaker
