Chapter 6: Contraction of Skeletal Muscle Flashcards
Percent composition of the body that is skeletal muscle
40%
Percent composition of the body that is smooth and cardiac muscle
10%
The diameter of skeletal muscle fibers
10-80 micrometers
Except for about 2% of the muscle fibers, each fiber is usually innervated by only one nerve ending, located near the _________ of the fiber.
A. Middle
B. Tendinous End
C. Muscle Spindle
A. Middle
The thin membrane enclosing a Skeletal Muscle Fiber
Sarcolemma
What makes up the outer coat of the sarcolemma?
Thin layer of Polysaccharide material that contains numerous thin collagen fibrils
At each end of the muscle fiber, the polysaccharide layer of the sarcolemma fuses with a _________ fiber.
tendon fiber
The tendon fibers, in turn, collect into bundles to form the muscle tendons that then connect the muscles to the bones.
Each myofibril is composed of about how many adjacent myosin and actin filaments?
1500 myosin (thick filaments) 3000 actin (thin filaments)
What causes the alternating light and dark bands of the myofibrils?
interdigitation of the myosin and actin filaments
The light bands contain only actin filaments and are called:
I bands because they are isotropic to polarized light
The dark bands contain myosin filaments, as well as the ends of the actin filaments, where they overlap the myosin, and are called:
A bands because they are anisotropic to polarized light.
These are the projections from the sides of the myosin filaments:
cross-bridges
Which parts of the myofibrils interact to cause contraction
It is the interaction between these cross-bridges
and the actin filaments that causes contraction
The ends of the actin filaments are attached to a ______ from which these filaments extend in both directions to interdigitate with the myosin filaments
Z disk
At which length of the sarcomere is the muscle capable of generating its greatest force of contraction?
2 micrometers
The length of the sarcomere is about 2 micrometers. At this length, the actin filaments completely overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another. As discussed later, at this length, the muscle is capable of generating its greatest force of contraction.
This part of the myofibril is composed of filamentous proteins different from the actin and myosin filaments, passes crosswise across the myofibril and also crosswise from myofibril to myofibril, attaching the myofibrils to one another all the way across the muscle fiber.
Z disk
The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z disks is called a:
sarcomere
What is the length of the sarcomere when the muscle is contracted
2 micrometers
Which filamentous molecules keep the myofilaments in place?
Titin Filamentous Molecules Keep the Myosin and Actin Filaments in Place
This protein forms the filamentous molecules that maintain the side-by-side relationship between myosin and actin filaments
Titin
Each titin molecule has a molecular weight of about ________, which makes it one of the largest protein molecules in the body.
3 million
These springy molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomeres will work
Titin
To which portion of the sarcomere does the elastic end of titin attach to?
Z disk
To which portions of the sarcomere do the ends of the titin molecule attach to?
Myosin thick filament Z disk (elastic end)
This molecule may also act as a template for the initial formation of portions of the contractile filaments of the sarcomere, especially the myosin filaments
Titin
What is the intracellular fluid between myofibrils?
Sarcoplasm
The intracellular fluid between myofibrils contains large quantities of which ions?
Potassium, Magnesium and Phosphate
plus multiple protein enzymes
Which organelles lie parallel to the myofibrils providing large amounts of energy during contraction?
Mitochondria
Also present in the sarcoplasm are tremendous numbers of mitochondria that lie parallel to the myofibrils. These mitochondria supply the contracting myofibrils with large amounts of energy in the form of adenosine triphosphate (ATP) formed by the mitochondria.
This specialized organelle surrounding myofibrils of each muscle fiber is extremely important in regulating calcium storage, release, and reuptake; hence, these are especially extensive in rapidly contracting types of muscle fibers
Sarcoplasmic Reticulum
In the general mechanism of muscle contraction, which channels are opened as Acetylcholine acts on a local area of the muscle fiber membrane?
acetylcholine-gated CATION channels
Upon opening of acetylcholine-gated channels during muscle contraction, what ions diffuse to the interior of the muscle fiber membrane causing local depolarization prior to action potential?
Sodium ions
The opening of acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This action causes a local depolarization that in turn leads to the opening of voltage-gated sodium channels, which initiates an action potential at the membrane.
Which channels of the sarcolemma open and generate action potential during muscle contraction?
Voltage-gated sodium channels
What molecular mechanism initiated by calcium ions is theoretically known as the process of muscle contraction?
Sliding Filament Mechanism
What forces cause the actin filaments to slide inward among the myosin filaments?
This action is caused by forces generated by interaction of the cross-bridges from the myosin filaments with the actin filaments.
Where does the energy needed for the process of muscle contraction come from?
High-energy bonds in the ATP
Molecular weight of each myosin molecule
480,000
2 heavy chains (200,000 each) = 400,000
+
4 light chains (20,000 each) = 80,000
How many polypeptide chains are there in a myosin molecule?
6 polypeptide chains
How many heavy and light chains does a myosin molecule have?
2 heavy chains and 4 light chains
What is the molecular weight of 1 myosin heavy chain?
200,000
What is the molecular weight of 1 myosin light chain?
20,000
Which chains form the tail of a myosin molecule?
2 heavy chains wrapping spirally around each other to form a double helix
One end of each of the heavy chains is folded bilaterally into a globular polypeptide structure called a:
myosin head
How many free heads are there at one end of the double helix myosin molecule?
2 heads
How many light chains are there in one head of a myosin heavy chain?
2 light chains to each head
At least how many myosin molecule are there in a myosin filament
at least 200
The tails of the myosin molecules form which part of the thick filament?
Body
The central portion of one of these filaments is shown in Figure 6-6B, displaying the tails of the myosin molecules bundled together to form the body of the filament, while many heads of the molecules hang outward to the sides of the body.
The part of the myosin molecule that hangs to the side along with the head is known as the:
arm
Which parts of the myosin molecule, together, form the cross-bridges?
arm and head
These are the 2 points of flexibility in each cross-bridge
hinges:
one where the arm leaves the body of the myosin filament and the other where the head attaches to the arm
Hinged portion of the myosin participate in the contraction process?
Hinged head
**The hinged arms allow the heads either to be extended far outward from the body of the myosin filament or brought close to the body. The hinged heads, in turn, participate in the contraction process, as discussed in the following sections.
The total length of each myosin filament is uniform, almost exactly _____ micrometers
1.6 micrometers
Because the hinged arms extend away from the center, there are no cross-bridge heads in the center of the myosin filament for a distance of about _____ micrometer
0.2 micrometer
To ensure that the cross-bridges extend in all directions around the filament, the myosin filament is twisted. By how much degree is each successive pair of cross-bridge displaced from the previous pair?
120 degrees
Which part of the myosin functions as an enzyme?
Myosin head:
Another feature of the myosin head that is essential for muscle contraction is that it functions as an adenosine triphosphatase (ATPase) enzyme
Which protein molecule forms the backbone of the actin filament?
F-actin protein molecule
**The backbone of the actin filament is a double-stranded F-actin protein molecule. The two strands are wound in a helix in the same manner as the myosin molecule.
Which protein molecule polymerizes to form a strand of F-actin?
G-actin molecule
The molecular weight of a G-actin molecule
42,000
These molecules are believed to be the active sites on the actin filaments with which the cross-bridges of the myosin filaments interact to cause muscle contraction
ADP molecules
**Attached to each one of the G-actin molecules is one molecule of ADP.
How many active sites are there on the overall actin filament about every 2.7 nanometers?
1
The active sites on the two F-actin strands of the double helix are staggered, giving one active site on the overall actin filament about every 2.7 nanometers.
The approximate length of an actin filament:
1 micrometer
**Myosin filament is 1.6 micrometers
The molecular weight of a tropomyosin
70,000
The length of a tropomyosin
40 nanometers
These molecules wrap around the sides of the F-actin helix lying on top of the active sites of the actin strands
Tropomyosin
These protein molecules are complexes of three loosely bound protein subunits, attached intermittently along the sides of the tropomyosin molecules
Troponin
The subunits of the troponin molecules have strong affinity with which specific structures?
Troponin I = Actin
Troponin T = Tropomyosin
Troponin C = Calcium