Chapter 12 Flashcards
Skeletal Muscle
-Skeletal muscles are connected to the bones they move by tough tendons, composed of closely-packed, parallel arrays of [mostly] collagen protein.
contraction is voluntary
As we will see later, contraction of skeletal muscle depends upon stimulation by motor neurons—this is very different from cardiac and smooth muscle, both of which are capable of spontaneous contraction, and are under autonomic control.
Myofiber
single cell, containing multiple bundles of contractile elements (“muscle fiber/cell”)
- run from one end of muscle to another
- multiple bound in a fascicles
- multinucleated (all fully functional), mitochondria, SR, T-tubule
Major elements of a myofiber:
1) Myofibrils (contractile filaments + regulatory prots)
2) Mitochondria
3) Sarcoplasmic reticulum- contains glycogen (storage form for glucose) granules in glycosomes
4) T-tubules- extension of cell membrane that run all the way through muscle cell
Muscle Cell Membrane
sarcolemma
muscles cytoplasm
sarcoplasm
-high concentrations of O2 binding protein = myoglobin
modified ER
sarcoplasmic reticulum
- plays a major role in excitation-contraction coupling –> the myofiber’s storage repository for calcium
- in response to a signal from a motor neuron, it releases calcium into the sarcoplasm for muscle contraction, then takes that calcium back up during relaxation.
what are the muscle contractile units?
myofibrils - bundles of contractile proteins within each muscle cell
Striations
These result from:
a) the regular arrangement of the contractile proteins in a given myofibril
b) the fact that parallel myofibrils are arranged in register with each other
c) the fact that all the myofibers are unbranched, i.e. they’re stacked parallel to each other
-muscle cells arranged in parallel, no branching, long, flattened nuclei = more noticeable striations
Z disc
center of each I band
H band
center of A band with no thin filament overlap
I band
only thin filaments, primarily ACTIN
A band
thick filament + thin filament overlap
thick filament= MYOSIN
Thick vs Thin filaments
Thick= aggregates of myosin—long fibrous “tail” connected to globular “head”
- hundreds of myosin molecules, joined end-to-end, and bundled together, with their “hinged” heads sticking out (form cross-bridge to link with acting to make contractions)
- remain stationary
thin= polymers of actin—made of 300-400 G-actin subunits in double row & twisted to form “double-helix”
-do sliding for contraction
sarcomere
the basic cellular unit of contraction
- A single sarcomere extends from one Z-line (disc) to the next.
- There are about 100,000 sarcomeres in your biceps muscle.
- In 3D, the sarcomere forms a hexagonal pattern
M lines
center of each A band; protein filaments that help hold down thick filaments
Titin
contributes to elastic recoil during relaxation (“spring”)
-runs from Z disc to M line
Contraction: Sliding Filament Mechanism
sarcomere’s get shorter
a) A bands do not shorten, but move closer together.
- Z bands get closer together
b) I bands shorten, but thin filaments do not.
c) Thin filaments slide toward H band
d) H band shortens or disappears
- -> muscle gets shorter
–>the thick filaments remain stationary, while the thin filaments slide inward (i.e. towards the M-line, lying at the center of each H-band)
Actin Monomer
G-actin (blue spheres) [G = “Globular”]
–> Each actin monomer in the filament has a myosin-recognition site to which an actin-binding site on the globular myosin head (see next slide) can bind. Under resting conditions, this site is obscured–covered up–by the tropomyosin, such that myosin cannot bind.
Actin Polymer
F-actin (joined spheres) [F = “Filamentous”]
Regulatory Proteins
Troponin & tropomyosin
-Running along the F-actin helices
There are three key features of the globular head of myosin which are critical for the mechanism of contraction:
- The tip of the head has the actin-binding site
- A second domain on the side of the head has an ATP-binding site, directly linked to a third domain, which has ATPase enzymatic activity
- The head is connected to the “tail” by a fourth domain which is a hinge, around which the entire head can pivot
- ->Pivoting of the globular head is triggered by ATP hydrolysis.
Myosin head: myosin ATPase
[ATPADP +Pi]
ATP ADP + Pi activates
globular myosin head, causing
it to pivot
Resting state….
…myosin binding sites on actin are BLOCKED by tropomyosin
–> this prevents crossbridge formation!
Calcium
Calcium binds to troponin –> tropomyosin moves away
from the myosin-binding site –> shifts 3D formation
-Ca2+ is critical to contraction—if it’s not present, myosin-actin interactions (crossbridges) cannot form, and no contraction is possible.
Sequence of events at a single crossbridge (assume that we start at the end of the previous contraction):
- The myosin head is bound to the binding site on actin. (by hydrolysis of bound ATP- allows binding of head to actin and “energizes” the head –> cocking head like a trigger)
- ATP binds to the myosin head—this triggers release from the actin (i.e. the crossbridge is dissolved)
- ATP is hydrolyzed to ADP—this energizes the myosin head, and it engages with the binding site on actin
- Release of inorganic phosphate (Pi) causes the myosin head to move, which drags the bound actin along with it.
- A new ATP molecule binds to the myosin head and the cycle starts again.
- -> impossible w/o Ca, because tropomyosin MUST be moved away
*After power stroke, ADP is released and a new ATP binds. This makes myosin release actin—and the cycle begins again, continuing until the sarcomere has shortened.
Where does Ca come from?
ALL Ca comes from the sarcoplasmic reticulum in skeletal muscle
—SR is major reservoir for Ca2+ storage & release
Ca and SR
- SR is modified ER that stores Ca2+ when muscle is at rest
- Upon stimulation, Ca2+ diffuses out of calcium release channels (ryanodine receptors)
- At end of contraction, Ca2+ actively pumped back into the SR.
- ->SR is extensive: ensures that all the bundles of myofibrils can be potentially be exposed to a flood of calcium
T-tubules
extensions of cell membrane (sarcolemma)that associate with ends (terminal cisternae) of sarcoplasmic reticulum.
–> brings action potentials into interior of muscle fiber
= important for excitation-contraction coupling
-proximity of SR to T-tubules: this ensures that an action potential traveling down the tubule is quickly transduced into a signal for calcium release.
How nerves signal a muscle:
- Neurotransmitter released: acetylcholine depolarizes
end-plate region of the muscle fiber
-When it binds to its receptor, it triggers opening of a Na+ channel (ligand-gated sodium channel). - Na moves in! Depolarization initiates A.P. in the muscle fibercontraction
–> that local change in voltage next triggers opening of so-called voltage-gated Na+ channels, they in turn let more Na+ in and that changing voltage is propagated along the membrane (sarcolemma) and passes down into the T-tubules.
–> each skeletal muscle fiber is innervated my a different nerve fiber, but be separately stimulated by a single motor neuron (no electrical connection like cardiac muscle)
Excitation-contraction coupling in skeletal muscle
conversion of a nerve signal (“excitation”) into a muscle contraction. The molecule that “couples” these two processes is CALCIUM
A.P.’s conducted along T-tubules –> voltage-gated Ca2+ channels –> open Ca2+ release channels in s.r.–> Ca2+ stimulates contraction
*volatage-gated Ca channel = dihydropyridine (DHP) receptors
For this class I will refer to them as DHP receptors, rather than voltage-gated calcium channels, because they really don’t let calcium in. Instead, they are physically connected to so-called ryanodine receptors that are located in the membrane of the s.r. The ryanodine recptors are calcium release channels. When the DHP receptors are activated by an action potential, they change their conformation and that in turn, open the ryanodine receptors, allowing calcium to flow out of the s.r. into the sarcoplasm, and flood the contractile apparatus with calcium.
NOTE: In skeletal muscle, ALL the calcium for contraction comes from the s.r.—none comes from the extracellular space. This is a difference between skeletal and cardiac and smooth muscles—in the latter two muscle types some calcium DOES come from outside, via “L-type calcium channels.”
Muscle Relaxation
-Action potentials stop; no further calcium release from s.r.
-Active pumping of Ca2+ back into s.r. via SERCA pump
(Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase; requires ATP)
ATP is required for both initiation & termination of contraction:
- Unbinding of myosin head from actin at the end of a contraction cycle
- Energization (activation) of myosin head
- Pumping of Ca2+ out of sarcoplasm back into s.r.
Muscle Contraction Steps
- Acetylcholine from motor neuron
- End plate potentials
- Action potentials (propagated down T-tubules)
- T-tubule V-gated calcium channels (DHPR) activated
- Opening of calcium release channels (ryanodine receptor) in s.r.
- Calcium released, binds to troponin.
- Contraction
Muscle Relaxation Steps
- Action potentials cease.
- Calcium release channels close
- Ca2+-ATPase pumps (SERCA) move Ca2+ back into s.r.
- No more Ca2+ bound to troponin C
- Tropomyosin blocks myosin binding sites on actin
