4- What is the role of the motor neurons ?

Question 1

How dose the physiology of contraction anatomically work ?

Answer 1

– 1st – synaptic transmission at the
neuromuscular junction
– 2nd – excitation-contraction coupling
– 3rd – contraction-relaxation cycle

Question 2

What are transverse (T) tubules ?

Answer 2

They are an extensive network of muscle cell membrane (sarcolemma) that invaginates deep into the muscle fiber.

Question 3

What are T tubules responsible for ?

Answer 3

They are responsible for carrying depolarization
from action potentials at the muscle cell surface to the interior of the fiber.

Question 4

What is the name of the voltage sensitive protein in T tubules ?

Answer 4

dihydropyridine receptor.

Question 5

How dose the excitation-contraction coupling work ?

Answer 5

1- Membrane depolarization opens the L-type Ca2+ channel (dihydropyridine receptor;DHP).
2- Mechanical coupling between DHP receptor and the Ca2+ release channel (ryanodine receptor;RyR) causes the RyR channel to open.
3- Ca2+ exits the SR via the RyR channel and activates troponin C, leading to muscle conctraction.

Question 6

What is a sarcomere ?

Answer 6

It is a structural unit of a myofibril in striated muscle, consisting of a dark band and the nearer half of each adjacent pale band.

Question 7

What dose sarcomeres contain ?

Answer 7

It contains contractile ( capable of contraction )units (myofilaments) such as thick and thin filaments.

Question 8

What is the thick filament composed of ?

Answer 8

primarily of a protein called myosin (+protein structures)

Question 9

What dose the thin filaments contain ?

Answer 9

contains Actin, Troponin, Tropomyosin (+protein
structures)

Question 10

What dose the SLIDING FILAMENT THEORY OF CONTRACTION contain ?

Answer 10

How is a muscle cell contacts

Question 11

What happens during the contraction of the muscle ?

Answer 11

The sarcomere shortens and the thin and thick filament overlap to a greater degree.

Question 12

Dose the length of the thick and thin change during contraction ?

Answer 12

No

Question 13

What is myodin ?

Answer 13

It is a protein molecule found in the thick filaments where in the muscle cells molecules are bundled together to form the thick filaments.

Question 14

What is the shape of myosin ?

Answer 14

It has a tail and two heads ( called cross bridges ).
- The head (cross bridge) has the ability to move back and forward.
- The flexing movement of the head provides the power stroke for muscle contraction.

Question 15

What is the purpose of the hinge in the tail of myosin ?

Answer 15

-allows vertical movement so that the cross bridge can bind to actin.

-The combination of two hinge points allows for necessary binding and power stroke of the cross bridges.

Question 16

How dose the myosin bind ATP and actin ?

Answer 16

wo important binding sites, one site specifically binds ATP the other one binds to actin (the thin filament).

Question 17

What happens when the binding of ATP into myosin molecule ?

Answer 17

molecule, transfers energy to myosin cross bridge. When ATP is hydrolyzed into ADP and phosphate, the energy is released and transferred to the myosin head.

Question 18

What are the three protein molecules of the thin filaments ?

Answer 18

1. Actin
2. Tropomyosin
3. Troponin complex

Question 19

What is the actin portion of the thin filament is composed ?

Answer 19

of actin subunits twisted into double helical chain, Each actin subunit has a specific binding site for myosin cross bridge (head).

Question 20

What is G-actin ?

Answer 20

Actin is a globular protein and, in this globular form it is called G-actin.

Question 21

How is F-actin is formed ?

Answer 21

In the thin filaments, G-actin is polymerized into two
strands that are twisted into an α-helical structure to form filamentous actin (F-actin).

Question 22

What happens when the muscle is at rest ?

Answer 22

the myosin-binding sites are covered by tropomyosin so that actin and myosin cannot interact.

Question 23

What is tropomyosin ?

Answer 23

It is a regulatory protein it a part of the thin filament. Tropomyosin twists around the actin. In the unstimulated muscle, the position of the tropomyosin, covers the binding site of the actin subunits and prevents myosin cross bridge binding.

Question 24

What happens in the troponin complex ?

Answer 24

expose the binding sites open for myosin binding the tropomyosin molecule must be moved aside. This is faciliated by the presence of a 3th molecule, called troponin (troponin complex) Troponin is attached and spaced periodically along the tropomyosin strand.

Question 25

What dose Troponin is a heterotrimer consisting of ?

Answer 25

1- Troponin T : binds to a single molecule of Tropomyosin. 2- Troponin I : binds to Actin and inhibits contraction Facilitates the inhibition of myosin binding to actin by tropomyosin. 3- Troponin C : binds Ca2+ promotes the movement of tropomyosin.

Question 26

What are the SIX STEPS OF SINGLE CROSS BRIDGE CYCLING ?

Answer 26

Step 1: Exposure of binding sites on actin Step 2: Binding of myosin to actin Step 3: Power stroke of the cross bridge Step 4: Disconnecting the cross bridge Step 5 : Re-energizing and re-positioning of the cross bridge Step 6: Removal of calcium ions

Question 27

What happens in the exposure of binding sites on actin ?

Answer 27

Presence of an action potential in the muscle cell membrane. ↓ Release of calcium ions from the terminal cisternae. ↓ Calcium ions rush into the cytosol and bind to the troponin. ↓ Conformational change in the troponin-tropomyosin complex. ↓ Tropomyosin moves away from the myosin binding sites on actin.

Question 28

What happens in the binding of myosin to actin ?

Answer 28

When a binding site of actin exposed, energized cross bridge can bind to actin.

Question 29

What happens in the power stroke of the cross bridge ?

Answer 29

The ADP and Pi are released from the myosin. ↓ The myosin head (cross bridge) tilts backward. ↓ The power stroke occurs as the thin filament is pulled toward the center of the sarcomere.

Question 30

What happens in the disconnecting the cross bridge ?

Answer 30

In order to disconnect the cross bridge from actin, an ATP molecule must bind to site on the myosin cross bridge.

Question 31

What happens when re-energizing and re-positioning of the cross bridge ?

Answer 31

The release of myosin cross bridge triggers the hydrolysis of ATP into ADP and Pi. Energy is transferred from ATP to myosin cross brigde which returns its high energy state.

Question 32

What happens in the removal of calcium ions ?

Answer 32

Calcium ions fall off the troponin. ↓ Calcium is activelly transported from cytosol into the sarcoplasmic reticulum by ion pumps. ↓ Tropomyosin again covers the binding sites on actin.

Question 33

What is rigor mortis ?

Answer 33

Rigor Mortis is the stiffening of the body after death because of a loss of Adenosine Triphosphate (ATP) from the body's muscles. Rigor Mortis begins throughout the body at the same time but the body's smaller muscles - such as those in the face, neck, arms and shoulders - are affected first and then the subsequent muscles throughout the rest of the body; those which are larger in size, are affected later.

Question 34

When dose relaxation occur ?

Answer 34

when Ca2+ is reaccumulated in the sarcoplasmic reticulum by the Ca2+ATPase of the sarcoplasmic reticulum membrane (SERCA).

Question 35

What happens when there is insufficient CA2+ ?

Answer 35

binding to troponin C, tropomyosin returns to its resting position, where it blocks the myosin-binding site on actin.

Question 36

What happen when the relaxed muscle cell ?

Answer 36

the concentration of calcium ions are about 10,000 lower in the cytosol than in the SR.

Question 37

How dose ATP plays a key role in the contraction of muscle ?

Answer 37

1. Energizing the powerstroke of the myosin cross bridge, 2. Disconnecting the myosin cross bridge from the binding site on actin at the conclusion of a power stroke, 3. Actively transporting calcium ions into the sarcoplasmic reticulum.