Force Generation of the Heart

Question 1

Cardiac muscle is ______.

Answer 1

Striated

Question 2

There are no neuromuscular junctions in cardiac muscle so how does charge move between cells?

Answer 2

cardiac myocytes are electrically coupled by GAP JUNCTIONS

Question 3

What are gap junctions and what purpose do they serve?

Answer 3

Protein channels which form low resistance electrical communication pathways between neighbouring myocytes They ensure that each electrical excitation reaches all the cardiac myocytes (All-or-none Law of the heart)

Question 4

What are desmosomes function?

Answer 4

The desmosomes within the intercalated discs provide mechanical adhesion between adjacent cardiac cells (gives strength to tissues)

Question 5

Each muscle fibre (cell) contains many MYOFIBRILS. These are the contractile units of muscle, made of what?

Answer 5

Actin and Myosin Within each myofibril: actin and myosin are arranged into SARCOMERES

Question 6

Describe actin filaments

Answer 6

(thin filaments) causes the lighter appearance in myofibrils and fibres

Question 7

Describe myosin filaments

Answer 7

(thick filaments) causes the darker appearance in myofibrils and fibres

Question 8

How is muscle tension produced?

Answer 8

Sliding of actin filaments on myosin filaments

Question 9

Force generation in the heart depends on what?

Answer 9

ATP-dependent interaction between thick (myosin) and thin (actin) filaments

Question 10

What is needed for contraction of the heart muscle?

Answer 10

ATP and Ca++

Question 11

___ is required to switch on cross bride formation in-between actin and myosin

Answer 11

Ca++

Question 12

Why is Ca++ needed to switch on cross bride formation?

Answer 12

Myosin binding site on actin are covered by regulatory proteins troponin/tropomyosin Calcium binds to troponin  conformational change  reveals binding site

Question 13

Where is calcium mainly stored?

Answer 13

Ca++ mainly stored in lateral sacs Ca++ is released from the sarcoplasmic reticulum (SR)

Question 14

What is the release of calcium from the SR dependant on?

Answer 14

the presence of extracellular Ca++

Question 15

what dos the binding of actin and myosin cross bridge trigger?

Answer 15

a power stroke that pulls thin filament inward during contraction

Question 16

when Ca++ binds with troponin what happens?

Answer 16

pulls troponin-tropomyosin complex aside to expose cross-bridge binding site; cross-bridge binding occurs

Question 17

When the muscle fibre is relaxed what happens?

Answer 17

no cross-bridge binding because the cross-bridge binding site on actin is physically covered by the troponin- tropomyosin complex

Question 18

What is the refractory period?

Answer 18

the period following an action potential in which it is not possible to produce another action potential

Question 19

During the plateau phase of ventricular action potential what happens to the Na+ channels?

Answer 19

the Na+ channels are in the depolarised closed state i.e. they are not available for opening

Question 20

During the descending phase of action potential the __+ channels are open and the membrane can not be depolarised

Answer 20

K

Question 21

Why does the heart need a long refractory period?

Answer 21

is protective for the heart preventing generation of tetanic contractions in the cardiac muscle

Question 22

Contraction of ventricular muscle ejects the ________

Answer 22

Stroke Volume (SV)

Question 23

Define the stroke volume

Answer 23

the volume of blood ejected by each ventricle per heart beat