Skeletal muscle organization and mechanics Flashcards Preview

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Flashcards in Skeletal muscle organization and mechanics Deck (60)
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The alpha motorneuron acts as the final common pathway for motor unit activation because?

It integrates excitatory and inhibitory inputs


Where are cell bodies for motor neurons?

ventral horn


What is included in a "motor unit"?

motor neuron plus all the muscle fibers it innervates


What is the difference between slow twitch motor units and fast twitch motor units?

slow twitch: small motor neuron bodies with innervate relatively few muscle fibers which have smaller cross sectional area.
Fast twitch: large motor neuron body that innervates a large number of fibers with larger cross sectional area


What causes release of Ach from presynaptic nerve?

Action potential reaches the end of the nerve fiber, opens voltage-gated calcium channels, which leads to influx of calcium into neuron and fusion of vesicles with the presynaptic membrane --> release of ACh


What kind of receptors are on muscle fibers? autonomic ganglion?

Nm; Nn


Describe nicotinic Ach receptors

ligand gates Na channels that bind 2 Ach


Binding of 2 Ach molecules to nicotinic Ach receptors cause _____.

influx of Na into the muscle fiber and a graded depolarization


Describe how action potential is generated in the muscle fiber after Ach has bound receptors and opened Na channels?

1. A sufficient number of receptors must be activated in order to trigger an action potential in the muscle fiber (individual graded depolarizations from NM channels must be summed and reach threshold)
2. The action potential propagates along the muscle fiber membrane like it would in an unmyelinated nerve axon
3. Because the muscle fiber is really long and really thin, the action potential moves almost completely parallel to the long axis of the muscle fiber


How is Ach degraded? What happens to the products?

1. Acetylcholinesterase in the synaptic cleft degrades acetylcholine into acetate and choline
2. Choline is the transported back into the nerve terminal and combined with acetyl-CoA to generate new acetylcholine


What are T-tubules

Invaginations of the muscle fiber membrane which pass through the cell and bring the membrane close to the SR.


What is the sarcoplasmic reticulum?

Specialized endoplasmic reticulum that stores a butt load of calcium which would be toxic to the cell if allowed to remain in cytoplasm for very long


What steps couple excitation to contraction?

Depolarization of T-tubule--> conformation changes in voltage sensitive dihydropyridine receptor--> activates ryanodine receptor--> allows efflux of Ca


What happens to Ca and associated receptors after action potential?

The dihydropyridine receptor resumes its role of inhibiting the ryanodine receptor. Calcium is pumped back into SR into active transport.


Define sarcomere.

subunit of muscle fiber contractile machinery spanning from one Z line to the next Z line


What is the Z line?

In middle of I band, where thin filaments are anchored


What does cross section through I band show?

ordered hexagonal array of thin filaments anchored to Z line


What does cross section through A band show?

ordered hexagonal arrangement of thick filaments


What does cross section through section where A and I band overlap show?

Interdigitation of thick and thin filaments


T or F. The muscle fiber can expand/contract withouth the A band changing length at all.



Components of thick filament

1.Myosin heavy chain: large, with filamentous tail, globular head. a) tend to form dimers b) Dimers polymerize to form thick filaments
2. Myosin light chains: 2 per head a) Role appears to be permissive and modulatory for ATPase activity of heavy chain globular head


What are the 3 parts of thin filament?

Actin, tropomyosin, troponin


_____ are fibrous proteins that extend along length of thin filaments, covering up the mysoin binding sites on actin moleucles.



What are the components of troponin ternary complex?

Troponin C (Ca binding), T (tropomyosin binding), I (keeps troponin complex in position covering the myosin binding sites when calcium is not present in high concentration.)


When does myosin bind actin?

When myosin has ADP bound to its head, it changes conformation into a high-affinity binding state for binding actin


Power stroke occurs when there is a conformational change of what part of myosin?

hinge region


What happens to Z lines in power stroke?

Z lines are pulled closer to each other


what causes release of myosin from actin?

Once power stroke has occurred (bending of myosin at hinge region), myosin has a higher affinity for binding ATP than ADP--> ATP displaces ADP--> Causes myosin to bind with lower affinity to actin --> release


Active force is maximum at a specfic _______.

optimal muscle fiber length


What happens if muscle fiber is too short?

there is too much baseline overlap between thick and thin filaments: only a small amount of force can be generated from that position (steric hindrance of too many proteins in a tight space)