Ch 1 - Skeletal Muscle Flashcards
(42 cards)
1
Q
Excitation-contraction coupling
A
AP -> binding of myosin and actin -> contraction
2
Q
Myosin
A
• Thick Filaments
• Large molecular weight
• Six polypeptides
- 2 heavy-chain myosin
• alpha-helical "tail"
- 2 pairs of light chains
• Globular "heads"
>> Actin-binding site
3
Q
Thin Filaments
A
• Anchored at the Z lines within the I bands
• Components
- Actin = has myosin-binding sites
- Tropomyosin = blocker
4
Q
Tropomyosin
A
- a filamentous protein
- runs along the groove of each twisted actin filament
- at rest, blocks the myosin-binding sites on actin, to prevent contraction
- must be removed for contraction to occur
5
Q
Troponin
A
- Complex of 3 globular proteins:
• Troponin T = attaches the troponin complex to tropomyosin
• Troponin I = along w/ tropomyosin, inhibits the interaction of actin and myosin by covering the myosin-binding site on actin
• Troponin C = Ca2+ binding site
6
Q
Troponin T
A
- T for tropomyosin
- Attaches the troponin complex to tropomyosin
7
Q
Troponin I
A
- I for inhibition
- along w/ tropomyosin, inhibits the interaction of actin and myosin by covering the myosin-binding site on actin
8
Q
Troponin C
A
- C for Ca2+
- Ca2+ binding site
9
Q
Sarcomere
A
- Basic contractile unit
- Delineated by Z disks (goes from one z disc to another)
- Full central A band with half of two I bands on either side
10
Q
A bands
A
- located in the center of the sarcomere
- contain the thick (myosin) filaments
- thick and thin filaments may overlap in the A band; potential sites of cross-bridge formation
11
Q
I bands
A
- located on either side of the A band
- contain the thin (actin) filaments, intermediate filamentous proteins, and Z disks
12
Q
Z disks
A
- Delineates the ends of each sarcomere
13
Q
Bare Zone
A
- NO POTENTIAL FOR CROSS-BRIDGING
- Center of each sarcomere w/ no overlapping filaments
14
Q
M line
A
- Bisects unit in 1/2; divides the sarcomere in half
15
Q
Cytoskeletal Proteins of Sarcomere
A
- Architectural proteins ensuring that the thick and thin filaments are aligned correctly and at proper distances w/ respect to each other
- “Scaffold”
16
Q
Titin
A
- longitudinal cytoskeletal protein
- associated w/ thick filaments
- maintains the integrity of the sarcomere during contraction
- center the thick filaments in the sarcomere
- biggest protein in the body
- extends from the M lines to the Z disks
17
Q
Nebulin
A
- longitudinal cytoskeletal protein
- associated w/ thin filaments
- serves as a “molecular ruler,” setting the length of thin filaments during their assembly
18
Q
alpha-Actinin
A
anchors the thin filaments to the Z disk
19
Q
Dystrophin
A
anchors the myofibrillar array to the cell membrane
20
Q
Duchenne Muscular Dystrophy
A
- due to a deletion of the DMD gene (Xp21); dystrophin
- symptoms usually appear in males before age 6
- progressive proximal muscle weakness of the legs and pelvis associated with a loss of muscle mass is observed first
- eventually weakness will spread to arms, neck, and other areas
- Gowers’ maneuver
- Pseudohypertrophy of calf = it increases in size and from the outside it looks like muscle growth but it is actually fat & connective tissue
- few survive beyond 20
21
Q
Becker’s muscular dystrophy
A
- milder form of DMD
- due to a mutation, not deletion
22
Q
Transverse Tubules
A
- conducts AP to interior of fiber allowing release of Ca2+ from Sarcoplasmic Reticulum
- communicates with the Sarcoplasmic Reticulum via the dihydropyridine receptor at the terminal cistern of the Sarcoplasmic Reticulum
23
Q
The transverse tubules communicate with the SR vi the
A
Dihydropyridine receptor at the terminal cistern of the SR
24
Q
Sarcoplasmic Reticulum
A
- Site of storage and release of Ca2+
- Ryanodine receptor = channel thru which Ca2+ is released
- Calsequestrin = a low-affinity, high-capacity Ca2+ binding protein within the SR
- Ca2+ ATPase (SERCA) = pumps Ca2+ into SR
25
Steps in Excitation-Contraction Coupling in Skeletal Muscle
AP in the muscle cell membrane initiate depolarization of the T Tubules -> causes a critical conformational change in its voltage-sensitive dihydropyridine receptor opening the Ca2+ release channels (ryanodine receptors) on the nearby SR into the intracellular fluid -> Intracellular [Ca2+] rises to above 10^-7 M -> Ca2+ binds to troponin C on the thin filaments leading to cross bridging events
26
Events in Cross Bridging
Ca2+ binds to Troponin C on the thin filaments -> Tropomyosin is moved out of the way -> actin & myosin bind, allowing thick and thin filaments to slide over each other, and ATP is hydrolyzed -> cross bridges break and a new molecule of ATP binds to the myosin head to begin a new cycle
27
What happens during muscle cell relaxation?
- Ca2+ is reaccumulated in the SR by the Ca2+ ATPase of the SR membrane (SERCA)
- Ca2+ concentration decreases to less than 10^-7 M -> insufficient Ca2+ for binding to troponin
- Ca2+ is released from troponin C
- Tropomyosin returns to its resting position preventing myosin interaction w/ actin
28
Length-Tension Relationship
Refers to the effect of muscle fiber length on the amount of tension the fiber can develop
29
Isometric contraction
A contraction in which a muscle exerts force (tension) but does not change in length
The muscle is allowed to develop tension at a preset length (called preload) but is not allowed to shorten
Yoga & stretching
30
Isotonic contraction
Shortening of muscle at a constant afterload
| Actually doing reps with the barbell
31
Passive Tension
Tension developed by simply stretching a muscle to different lengths
Generate greatest tension at its resting length; tension of an unstimulated muscle
32
Active Tension
Represents the active force developed during cross-bridge cycling
Active tension = total tension - passive tension
Proportional to the number of cross-bridges that cycle
33
Force-Velocity Relationship
The velocity of shortening reflects the speed of cross-bridge cycling
As the afterload on the muscle increases, the velocity will be decreased b/c cross-bridges can cycle less rapidly against the higher resistance
As the afterload increases to even higher levels, the velocity of shortening is reduced to zero
34
Smooth Muscle
- unstriated; thick & thin filaments, while present, are not organized in sarcomeres
- found in GI Tract, bladder, uterus (strongest muscle in body), bronchioles, etc.
35
Types of Smooth Muscle
Unitary Smooth Muscle: located in GI tract, bladder, uterus; contain gap junctions which allows for the coordinated fashion of organ contraction, permits electrical coupling between cells. Slow waves which sets a characteristic pattern of APs within an organ, which then determines the frequency of contractions
Multiunit Smooth Muscle: located in iris, ciliary muscles of the lens, and in the vas deferens. Each muscle fiber behaves as a separate motor unit (similar to skeletal muscle), and there is little or no coupling between cells. Densely innervated by postganglionic fibers of the para & sympathetic nervous systems
36
What is the regulating factor of the myosin-actin bridges in smooth muscle?
Calmodulin rather than troponin is the regulating factor of the myosin-actin bridges
- Ca2+ binds to calmodulin instead of troponin C
- In turn, Ca2+ - calmodulin regulates myosin-light-chain kinase, which regulates cross-bridge cycling
37
Steps in Excitation-Contraction Coupling in Smooth Muscle
1. AP occur in the smooth muscle cell membrane
2. Opening of voltage-gated Ca2+
3. Ca2+ flows down its gradient into the cell (intracellular Ca2+ concentration increases)
4. Ca2+ binds to calmodulin
5. The Ca2+ - calmodulin complex binds to & activates myosin-light-chain kinase
6. Myosin becomes phosphorylated
7. Cross-bridge formation
38
Relaxation of smooth muscles occurs when
The SR re-accumulates Ca2+, via the Ca2+ ATPase, and lowers the ICF [Ca2+] below the level necessary to form Ca2+ - calmodulin complexes
39
Mechanisms that Increase Intracellular Ca2+ Concentration in Smooth Muscle
1. Voltage-gated Ca2+ channels are sarcolemmal Ca2+ channels that open when the cell membrane potential depolarizes
2. Ligand-gated Ca2+ channels also are present in the sarcolemmal membrane
3. IP3-gated sarcoplasmic reticulum Ca2+ channels also are opened by hormones & neurotransmitters
40
- associated w/ thick filaments
- maintains the integrity of the sarcomere during contraction
- center the thick filaments in the sarcomere
Titin
41
- associated w/ thin filaments
| - serves as a "molecular ruler," setting the length of thin filaments during their assembly
Nebulin
42
Calsequestrin
A low affinity, high capacity Ca2+ binding protein within the SR
At rest, by binding to Ca2+, intracellular Ca2+/free Ca2+ is maintained low
