Cross bridge cycling; Excitation-Contraction Coupling

Question 1

What factor causes myosin to detach from actin?

Answer 1

ATP binding to myosin crossbridge is required for actin to detach from myosin head, and allowing myosin to assumes its original conformation.

Question 2

What causes the muscle cell to be excited?

Answer 2

Ca2+ ion binding to troponin

Question 3

What happens when Ca2+ binds to troponin?

What causes power stroke?

Answer 3

Ca2+ binds to troponin will physically pull troponin-trypomyosin from myosin binding site on actin.

Powerstroke is refer to the bending of myosin head, which pulls actin inwards. Phosphate is used up during power stroke and ADP released after power stroke.

Question 4

Ryanodine receptors

Answer 4

They are Ca2+ release channel on the lateral sac of sarcoplasmic reticulum

Question 5

Where is Dihydropyridine (DHP) receptors located in skeletal muscles vs DHP receptors in smooth or cardiac muscles

Answer 5

DHP receptors in skeletal muscles are voltage gated receptors on T tubules that physically connect with ryanodine receptors, located on the membrane of the sarcoplasmic reticulum. They’re physical coupling allows the opening of calcium channels.

In smooth muscles, DHP receptors are located on the plasma membrane.

Question 6

What event provides the energy for the power stroke of the cross bridge?

Answer 6

The splitting of ATP on the myosin head by myosin ATPase

Question 7

What are the stages in contractile activity is ATP required ?

Answer 7

1. ATP is required to bind to myosin to detach from actin.
2. ATP needs to split by myosin ATPase to provide potential energy for the next power stroke crossbridge cycle
3. ATP is required to transport Ca2_ back into the SR during relaxation, this depends on the energy derived from the breakdown of ATP

Question 8

Since there is no troponin in smooth muscle cells, how do cross-bridge cycle occur?

Answer 8

In smooth muscle cells, increased cytosolic Ca2+ in the sarcoplasm causes a chain of events that activates myosin kinase activity, which phosphorylates “light-chain” proteins on myosin head.

phosphorylated myosin allows myosin cross-bridge binding with actin

Question 9

What is the mechanism of Ca2+ action in smooth, skeletal and cardiac muscle cells?

Answer 9

Ca2+ induced a chemical change in myosin thick filament in smooth muscle cells. The chemical change brings about phosphorylation of myosin cross bridges so they can bind with actin

In skeletal and cardiac muscles, Ca2+ physically repositions troponin-tropomyosin complex to uncover actin-cross bridge binding sites.

Question 10

Unlike in skeletal muscles, in which excitation induced ca2+ released from SR, increase in cytosolic calcium in smooth muscles comes from two sources….

Answer 10

Extracellular fluid predominately and some from SR. This is due to no T-tubules, or myofibrils in smooth muscles.

Question 11

How is crossbridge activity different between smooth and skeletal muscle cells?

What is the function of dihydropyridine receptors in smooth muscles. How are they different from skeletal muscles?

Answer 11

Dihydropyridine receptors in skeletal muscles acts as receptors to open calcium channels. In smooth muscles, they open to allow the entry of ca2+ from outside of the cell. This entry of ca2+ triggers the release of ca2+ from the SR, thus influencing cross-bridge activity.

Question 12

How do smooth muscle becomes excited to contract? what opens the Ca2+ channels in the plasma membrane?

Answer 12

What opens ca2+ channels in plasma membranes of smooth muscle cells depends if it is a multi unit or single unit muscle cell.

Question 13

What does it mean when a cell (eg, smooth or heart muscle cells) are considered to be myogenic?

Answer 13

Myogenic means that the cells themselves are stimulated to contract by themselves rather than being stimulated by an outside event such as nerve stimulation.

Question 14

How does contraction occur in smooth muscle cell?

Answer 14

Smooth muscle contraction is caused by the sliding of myosin and actin filaments over each other. The energy for this to happen is provided by the hydrolysis of ATP. Unlike cardiac contractile and skeletal muscle, smooth muscle does not contain the calcium binding protein troponin.

Question 15

Which types of muscle cells are “self-excitable” ? (that it does not require nerves stimulation)

Answer 15

Single unit smooth muscle cells and auto rhythmic cells of the heart

Question 16

Where can you find single unit smooth muscle cells?

Answer 16

uterus, bladder and the gastro-intestinal tract, reproductive tract, and small blood vessels

Question 17

Where can you find multiunit smooth muscle cells?

Answer 17

1. in the walls of large blood vessels
2. small airways to the lungs
3. in the muscle of the eye that adjusts the lens for near or far vision
4. in the iris of the eye, which alters the pupil size to adjust the amount of light entering the eye;
5. At the base of hair follicles, contraction of which causes “goose bumps”

Question 18

What is the difference between multiunit and single unit smooth muscle cells?

Answer 18

Like skeletal muscles, Multiunit smooth muscle cells are neurogenic, that is, the contraction in these muscles is initiated by the nerve responsible for stimulating theseits corresponding muscles.

Single Unit smooth muscles are made up of fibers that contract as a unit when stimulated. They are electrically linked by gap junctions.

Question 19

Resting membrane potentials

Answer 19

the potential or ion differences between the inside and outside of the cell. and that’s what creates the RMP. In skeletal muscles, neural stimulation of the muscle cell alters the ion permeability on the membrane of the cell, which changes the charge of the membrane, leading to contraction of the muscle cell.

Question 20

What is slow wave potentials? and where is this found in the body?

Answer 20

It is an alternating polarizing and depolarizing swings in potential

caused by automatic cyclic changes in the rate at which sodium ions are actively transported across the membrane. If threshold is reached, results in a burst of AP. But slow wave potentials can oscillate without generating action potential until triggered by either local or neural factors.

It occurs in the gastrointestinal tract

Question 21

What is slow wave potentials? and where is this found in the body?

Answer 21

It is an alternating polarizing and depolarizing swings in potential

caused by automatic cyclic changes in the rate at which sodium ions are actively transported across the membrane. If threshold is reached, results in a burst of AP. But slow wave potentials can oscillate without generating action potential until triggered by either local or neural factors.

Slow wave potentials smooth muscles found in the gastrointestinal tract

Question 22

What causes pacemaker potentials?

Answer 22

The pacemaker potentials is found in self-excitable cell membrane, like a heart cell, and once this threshold is reached, it leads to an action potential. For depolarization to occur, a very small net inward currents of calcium ions crosses into the cell membrane which give rise to the action potential.

Question 23

Why is there no tetanus in cardiac muscle?

Answer 23

Cardiac muscle cell contractions last roughly 10 times longer than those of skeletal muscle fibers…as a result cardiac muscle tissue cannot undergo tetanus(sustained contraction). This properties important because a heart in tetany could not pump blood.

Question 24

What is the function of papillary muscles?

Answer 24

Helps to prevent leakage through the the AV valves during cardiac systole

Question 25

What causes the AV valves to close shut?

Answer 25

The rising ventricular pressure greater than atrial pressure causes the AV valves to close.

Question 26

What cause the Aortic and pulmonary semilunar valves to close ?

Answer 26

Ventricular relaxation creates a backward pressure gradient and closes the valve

Question 27

What are the 3 layers of the heart walls

Answer 27

endothelium
myocardium
epicardium

Question 28

The electrical activity in cardiac contractile cells undergoing an action potential is described in 5 phases.

Describe the ion movement that occurs in the

1) Rising phase = rapid depolarization
2) Dropping phase = 1st repolarization phase
3) Plateau phase
4) Steep decline phase = Repolarization phase
5) Resting phase

Answer 28

Once the myocardial cell is excited to threshold,

1. Voltage gated Na+ channels opens. It opens so quickly that Na+ rushes out causing a rapid depolarization.
2. Na+ gates inactivates. 1) And voltage gated K+ channels open and K+ moves out quickly – No more Na+ ions moving out and K+ moving in contributes to making the cell more positive.
   3) Plateau phase: is caused by Slow L-type Ca2+ channels opening and Ca2+ slowly move into the cell. L-type channels are slow…
   4) This re-polarization continues more steeply once the ca2+ channels close and the voltage gated k+ channels open
   5) Once the cell membrane potential reaches about -90 mV, voltage gated K+ closes and the cell is “resting” This occurs in between action potential.

Question 29

The electrical activity of cardiac pacemaker cells undergoing an action potential is described in 3 phases.
Describe the ion movement that occurs int th

1. Slow depolarization/pacemaker potential
2. Depolarization
3. Repolarization

Answer 29

Pacemaker cells have unsteady membrane potentials. Since they are self-excitable cells, they can spontaneously depolarize.

1) slow depolarization - Na+ and K+ move into the cell, and the cell membrane potential moves toward threshold (-50) Then K+ channels closes and T-type Ca2+ channels open, which moves the cell to threshold
2) rapid depolarization occurs when L-type Ca2+ channels open and an action potential is generated.
3) Repolarization occurs when Ca2+ closes and and voltage gated K+ channels open and K+ exits the cell, bringing the cell back down to negative.