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Flashcards in Muscle I Deck (18):

Skeletal Muscle cell description

Long cylindrical
Also called muscle fibers/myofibers (muscle cell).
Contain HUNDREDS of nuclei on PERIPHERY
Can be slow or fast twitch


Myofiber, Myofibril, and myofilament

Myofiber = muscle cell
Myofibril= collection of myofilaments
Myofilaments= actin and myosin


Cardiac muscle cell description

A single nucleus, much smaller in diameter and shorter than skeletal muscle.
Distinguishing feature: intercalated disc. Which physically ties together cells so they don't separate during contraction, and it has gap junction for electrical transmission from one cell to another (AP and heart beat)


Smooth muscle cell description

A single nucleus. But are thinner than cardiac cells.
Spindle-shaped, nucleus near center.
NOT striated. But use same proteins and sekeltal and smooth.



Each striated muscle cell has a series of repeating units called the sarcomere.
Basic unit of contraction.
From one Z line to the next.


How is force generated?

As thick and thin filaments slide past one another.


Myofibrils are covered by

Their own sarcoplasmic reticulum


2 main filaments

Thin: actin (f-actin)
Thick: mysoin


2 main regulatory proteins

They bind actin.


Thin filament

F-actin. Each thin filament is 1 um long.
Troponin and tropomyosin bind actin.
F-actin is like 2 strings of pearls twisted together (double stranded and helical).



Binds to tropomyosin at end.
To uncover the binding site..
Intracellular free Ca rises, troponin binds Ca and undergoes a conformational change.
Because troponin binds tropomyosin it can induce a conformational change that exposes the binding sites.



rod shaped, binds 6-7 actin mlcs of one strand.
Covers binding site for myosin on the actin.


Thick filament

Made of mysoin.
Myosin is made of 6 proteins that are really 3 pairs (1 pair of large heavy chains, 2 pairs small light chains).
Each heavy chain forms a long alpha-helical region with a globular head
These wrap around each other to form a long rod with globular heads near each other.
Short, light chains are associated with the globular heads (wrapped around like lever arm).
Myosin heads are staggered along the length of thick filament and around circumference.


Myosin head

is where you get interaction with actin.
Actin-myosin combine to contain ATPase activity.



This is myosin associating with actin.
In relaxed state binding is prevented because binding site on actin is covered by tropomyosin.
After conformational change of tropomyosin, myosin can bind actin.


Now let's talk about the contraction cycle

1.) Myosin binds actin
2.) Force is generated (NO ATP needed, spring is already compressed), energy released upon binding (myosin head rotates relative to neck region where the light chains bind. Pi leaves to generate movement.
3.) Get a shortening of sarcomere
ADP released
4.) Now myosin is stuck to actin until ATP binds. Binding of ATP allows myosin to dissociate from actin.
5.) ATP hydrolysis, not myosin is in a high-energy state (compressing the spring... head cocked in active state)


How do you get shortening greater than 8nm?

Get centimeters because
1.) Lots of sarcomeres in series summate linearly
2.) Many myosin-actin cycles occur during a single contraction


Rate of myosin turnover

Faster in fast twitch (20x/second) than slow twitch muscle (5x/second)