Cardiac muscle

Question 1

what is cardiac muscle composed of

Answer 1

cardiac myocytes (myocardial muscle cells) which are shorter, branched cells and usually contain a single nucleus.

Question 2

what connects cardiac myocytes?

Answer 2

intercalated disks– composed of desmosomes and gap junctions

Question 3

how do the t tubules in cardiac muscle compared to sk muscle?

Answer 3

fewer T tubs, but larger and wider t tubs

Question 4

Smaller amounts of sarcoplasmic reCculum -cardiac muscle requires the entry of extracellular Ca2+
An abundance of mitochondria (~1/3 of cell volume) -oxidaCve metabolism for ATP producCon

Answer 4

ya

Question 5

how is contraction initiated in myocytes?

Answer 5

not by neurons; generated by autorhythmic cells; depol starts at SA node; APs generated spontaneously

Question 6

describe the AP pattern in autoryth cells

Answer 6

funny current channels open at -60 mV, net influx of Na; around -45 mV, some t-type Ca channels open and If channels close; at -40 mV, L-type Ca channels open; Ca induced Ca released– lots of L type channels open allowing for a sharp depol to +15 mV; Ca channels close, K opens–> efflux of K to -60 mV; then If channels open again, etc

Question 7

why is the SA node the dominant pacemaker?

Answer 7

has lots of HCN channels so reaches threshold faster that the rest of network—>region with most HCN channels set HR

Question 8

describe an AP in a myocardial cell

Answer 8

Ions that entered from an AP are transferred to adjacent cells through gap juncCons leading to depolarizaCon of the cell.

• Once the adjacent cell reaches threshold voltage gated Na+ open causing the Na+ spike of acCon potenCal (green). (This AP also spreads to myocardial cells next to it through gap juncCons).
• The posiCve voltage change of the AP slowly opens L-type Ca2+ channels. -There is a brief repolarizaCon from fast K+ channels (blue) before the Ca2+ influx through the L-type Ca2+ channels cause the K+ channels to close and lead to a sustained depolarizaCon (the plateau) (yellow)
• once the Ca2+ channels close, slow K+ channels then open and hyperpolarize the cell (pink).

Question 9

Excitation-contraction coupling in cardiac muscles

Answer 9

L-type Ca channels (DHP receptors) are not mechanically linked to RyR; depol flows through the membrane and causes Ca to enter through the L-type channels and bind to RyR on the SR to release more Ca–>sparks sum together–> this allows Ca to bind to sarcomeres; Ca-induced Ca released

Question 10

relaxation in cardiac muscle

Answer 10

Ca unbinds from troponin; some Ca is pumped out into the ECF (moreso than in sk muscle) MOST of the Ca is taken back up into the SR

Question 11

what regulates the SERCA pump?

Answer 11

PLN–PLN is active and activates the pump when phos;

Question 12

how quicker relaxation(=quicker subsequent contraction) achieved?

Answer 12

reuptaking Ca faster

Question 13

how is contractile force enhanced in cardiac muscle?

Answer 13

NOT through summation; in increase in ICF Ca in myocytes enhances contractile force;increased Ca2; length tension relationship

Question 14

cardiac muscle is capable of graded single twitch contractions.

Answer 14

ya

Question 15

how is contractile force enhanced in cardiac muscle through increased ICF Ca?

Answer 15

additional troponin complexes activated and

increased cross bridge formation leading to increased force of contraction

Question 16

how is contractile force enhanced in cardiac muscle through length tension rlnshp?

Answer 16

cardiac muscle generates a higher force when slightly stretched

Question 17

what does the sympathetic NS affect in the heart?

Answer 17

increases HR/conduction velocity

affects autorhyth. cells and contractile cells

Question 18

how does the parasymp NS affect the heart?

Answer 18

decrease HR/conduction velocity– affects on autorhyth cells

Question 19

how does the symp NS modulate contraction?

Answer 19

1. Phosphoryla5on of Ca2+ channels causes the calcium channels to remain open longer during ac5on poten5als.
2. Phosphoryla5on of ryanodine receptors enhances sensi5vity to Ca2+, increasing release of Ca2+ from the sarcoplasmic re5culum.
3. Phosphoryla5on of SERCA (PLN) increases the speed of Ca2+ re-uptake which increases Ca2+ storage

Question 20

explain the length tension rlnshp

Answer 20

1. A slightly stretched sarcomere increases the Ca2+ sensi5vity of the
   myofilaments.
   -a stretched sarcomere has a decreased diameter which may reduce the distance that Ca2+ needs to diffuse increasing probability of cross-bridge cycling.
2. A slightly stretched sarcomere puts addi5onal tension on stress- ac5vated Ca2+ channels, increasing Ca2+ entry from extracellular space and increasing Ca2+ induced Ca2+ release.
   when stretched, has a reduced diameter—>smaller distance of diffusion for Ca; more troponin complexes activated etc

Question 21

Parasympathe5c neurons containing ACh mainly innervate the SA and AV node influencing autorhythmic myocardial cells, decreasing the frequency of ac5on poten5als (decreasing heart rate).

Answer 21

ACh acts on muscarinic cholinergic receptors opening K+ channels and closing T-type Ca2+ channels and HCN channels.
-since the pacemaker ac5vity does not usually reside within the AV node, these mechanisms act at the AV node to decrease conduc5on velocity.

Question 22

Sympathe5c modula5on of pacemaker ac5vity

Answer 22

Beta1 adrenergic receptors can be ac5vated by NE released from sympathe5c neurons or epinephrine from the adrenal medulla

• increase Na+ conductance through HCN channels: cells reach threshold more rapidly
• increase Ca2+ conductance through Ca2+ channels:
• cells reach threshold more rapidly, shorter dura5on AP

Question 23

what type of control is HR under?

Answer 23

tonic control

Question 24

what is tonic control?

Answer 24

never really rests? stays at RMP for long?