Electrical Activity of the Heart

Question 1

Define excitation/contraction coupling.

Answer 1

The process in which muscle action potentials initiate calcium signals which in turn activate a contraction/relaxation cycle.

Question 2

What are T-tubules?

Answer 2

T-tubules are tube-shaped invaginations of the sarcolemma.

Question 3

Describe the contraction phase of EC coupling in cardiac muscle.

Answer 3

• an action potential originates spontaneously in the heart’s pacemaker cells and spreads into the contractile cells through gap-junctions
• the action potential then moves across the sarcolemma and into the t-tubules
• this opens voltage-gated L-type calcium ion channels in the cell membrane
• calcium ions enter the cell and open ryanodine receptor calcium ion release channels (RyR) in the sarcoplasmic reticulum
• stored calcium ions now flow out of the sarcoplasmic reticulum and into the cytosol
• this creates a calcium ion ‘spark’ that can be seen using special biochemical methods
• multiple sparks from different RyR channels summate to create a calcium ion signal
• calcium ions bind to troponin and initiate the cycle of cross-bride formation and movement

Question 4

Describe the relaxation phase of EC coupling in cardiac muscle.

Answer 4

• as cytoplasmic calcium ion concentrations decrease, calcium ions unbind from troponin
• myosin releases actin and the contracile filaments slide back to their relaxed position
• calcium ions are transported back into the sarcoplasmic reticulum with Ca2+-ATPase
• calcium ions are also exchanged for sodium ions via Na+-Ca2+exchanger (NCX)
• each calcium ion moves out of the cell against it’s electrochemical gradient in exchange for three sodium ions entering the cell in accordance to their electrochemical gradient
• the sodium that enters the cell during this transfer is removed by the sodium/potassium pump

Question 5

Give an alternative name for EC coupling in cardiac muscle.

Answer 5

• calcium ion induced calcium release (CICR)

Question 6

Give some features of myocardial muscles.

Answer 6

• myocardial muscle cells are branched
• have a single nucleus
• are attached to each other by specialised junctions, known as intercalated disks.

Question 7

Where does the right atrium receive blood from and send blood to?

Answer 7

• receives from the vena cava

- sends to the right ventricle

Question 8

Which chamber of the heart receives blood from the right atrium and where does it then send the blood to?

Answer 8

• Right ventricle

- sends blood to the lungs via the pulmonary trunk

Question 9

Which vessel returns blood from the lungs to the heart and to which chamber does it deliver blood to?

Answer 9

• pulmonary vein

- delivers blood to the left atrium

Question 10

Where does the left ventricle receive blood from and send blood to?

Answer 10

• receives from the left atrium

- sends blood to the body via the aorta

Question 11

Where does the vena cava receive blood from?

Answer 11

• systemic veins

Question 12

Where does the aorta send blood to?

Answer 12

• systemic arteries

Question 13

What surrounds the heart valves?

Answer 13

Fibrosis connective tissue rings.

Question 14

Give two functions of the fibrosis connective tissue rings.

Answer 14

• origin and insertion for cardiac muscle

- blocks electrical signal between atria and ventricles, acting as an electrical insulator.

Question 15

What kind of valve resides in the opening between each atrium and ventricle?

Answer 15

Atrioventricular valves

Question 16

Where would you find semilunar valves?

Answer 16

In the opening between each ventricle and artery.

Question 17

Describe the structure of atrioventricular valves.

Answer 17

• formed from thin flaps of tissues
• these tissues are joined at the base to the fibrous connective ring
• thin flaps are slightly thickened at the edge and connect on the ventricular side to collagenous tendons called the chordae tendineae.

Question 18

What do the chordae tendineae attach to?

Answer 18

• papillary muscles

- these are mound-like extensions of ventricular muscle

Question 19

What is the function of the papillary muscles?

Answer 19

• provide stability

- they cannot actively open/close the atrioventricular valves

Question 20

Name and describe the two atrioventricular valves.

Answer 20

• Tricuspid valve - has three valves and allows blood to flow from the right atrium to the right ventricle
• Mitral - has two flaps and allows blood to flow from the left atrium to the left ventricle

Question 21

Name and describe the structure of the semilunar valves.

Answer 21

• aortic valve
• pulmonary valve
• both have three cup-like leaves that snap shut when blood flows into the cups downwards from the veins

Question 22

Where does the signal for myocardial contraction come from?

Answer 22

• autorhythmic cells, also called pacemarker cells

Question 23

What two components form intercalated disks?

Answer 23

• desmosomes

- gap junctions

Question 24

What is the function of gap junction in intercalated disks?

Answer 24

They electrically connect cardiac cells to one another, allowing all heart muscles to contract almost simultaneously

Question 25

State the membrane potential of both contractile myocardium and autorhythmic myocardium.

Answer 25

Contractile myocardium - stable at -90mV

Autorhythmic myocardium - unstable pacemaker potential (usually starts at -60mV)

Question 26

Describe the refractory period of contractile myocardium.

Answer 26

• long

- due to resetting of Na+ channel gates delayed until the end of action potential