cardiovascular system - electrical activity of heart

Question 1

role of ion channels

Answer 1

-bring in various ions to change the electrical activity of the heart
-control level of Ca2+ and other ions

Question 2

excitable cells (able to generate their own electrical activity)

Answer 2

-myocytes
-skeletal muscle
-nerves

Question 3

diads

Answer 3

-where T-tubule is connected/coupled to the sarcoplasmic reticulum

Question 4

L-type Ca2+ channel

Answer 4

-sits on T-tubule
-in close proximity to ryanodine receptor

Question 5

ryanodine receptors (RyR)

Answer 5

-part of sarcoplasmic reticulum that stores Ca2+

Question 6

calcium-induced calcium release
(C-I-C-R)

Answer 6

-ions come in from outside through the L-type Ca2+ channel
-RyR senses Ca2+ influx causing Ca2+ release from SR
-Ca2+ required to come in from outside which induces a much larger release from the internal store (SR)

Question 7

percentage of Ca2+ required for contraction

Answer 7

-approx. 75% of Ca2+ from internal structures (sarcoplasmic reticulum)
-approx. 25% comes from extracellular space (through L-type Ca2+ channels)

Question 8

Ca2+ conc. in cell

Answer 8

-when completely relaxed: ~0.1µM
-when contracting: ~10µM

Question 9

excitation-contraction coupling (brief overview)

Answer 9

-Ca2+ enters cell through L-type Ca2+ channel
-CICR occurs
-Ca2+ interacts with troponin-C, causing troponin-I to uncover myosin binding site on actin
-intracellular Ca2+ reabsorbed or removed by respective mechanisms
-as intracellular Ca2+ decreases, Ca2+ dissociates from troponin-C
-myosin binding site on actin is inhibited
-ATP required to remove myosin from actin and reset sarcomere to normal length

Question 10

Ca2+ removed from cell

Answer 10

-ATP-dependent Ca2+ pump
-Na+/Ca2+ exchanger (and Na+/K+ pump to prevent Na+ overload)

Question 11

Ca2+ reabsorbed into sarcoplasmic reticulum (SR)

Answer 11

-sarco-endoplasmic reticulum Ca2+ ATPase (SERCA) pump (also ATP dependent)

Question 12

calcium and force production

Answer 12

-Ca2+ binds to troponin-C (TN-C) on actin thin filament
-troponin-I (TN-I) exposes site on actin so it is available for myosin head to bind
-ATP hydrolysis supplies energy for actin-myosin conformational change
-‘ratcheting’ of actin-myosin and shortening of sarcomere occurs
-Ca2+ dissociates from TN-C and myosin unbinds from actin with energy from ATP

Question 13

phospholamban and SR calcium uptake

Answer 13

-SR reabsorbs Ca2+ via SERCA
-disinhibition of phospholamban increases rate of Ca2+ uptake
-cytosolic Ca2+ decreases, Ca2+ removed from TN-C
-excess Ca2+ removed from cell by other processes

Question 14

phospholamban

Answer 14

-a protein that inhibits the activity of SERCA
-allows rate of relaxation to be regulated
-can be phosphorylated to dissociate the interaction between itself and SERCA

Question 15

inhibiting/slowing down relaxation

Answer 15

-allow phospholamban and SERCA to interact
-slows down activity of SERCA as the rate at which Ca2+ is removed is slowed

Question 16

electrogenic pumps

Answer 16

-when the molecules that they move have different charges
-(Na+/Ca2+ exchanger (3:1), removes 3 +charges in exchange for 2 +charges)
-impacts electrochemical gradient but also affects electrical activity of the heart

Question 17

sinoatrial node (pacemaker)

Answer 17

where electrical activity of the heart originates

Question 18

what is a feature of all the (contractile) cells within the heart

Answer 18

-generate electrical activity
-generate an action potential

Question 19

electrical conductivity of the heart

Answer 19

-starts at right atrium
-spreads across rest of the atria
-allowing atria to contract first
-reaches atrioventricular node to allow electrical activity to pass from atria to ventricles
-insulating tissue separating atria from ventricles so elec.act. cannot pass any other way, other than through the atrioventricular node
-elec.act. passes down the interventricular septum in the bundle of His
-elec.act. reaches purkinje fibres, this is where the contractile activity of the heart is generated

Question 20

what is conductance ?

Answer 20

the permeability of the cell membrane to a given ion

Question 21

what is the nernst potential ?

Answer 21

the voltage required to stop flow of ions due to conc. gradient

Question 22

nernst equation

Answer 22

E = (RxT/zF) x log([c⌄i]/[c⌄o])

R: gas constant (8.31 JK-1mol-1)
T: temperature (310k)
z: charge/ion
F: Faraday constant (9.6x10^4 Cmol-1)
c⌄o: extracellular conc.
c⌄i: intracellular conc.

Question 23

factors that determine membrane potential

Answer 23

-conc. gradients of K+, Na+ and Ca2+
-relative permeability (elec. conductance) of membrane to their respective ions
-electrogenic ion pumps

Question 24

ion pumps in plasma membrane that maintain gradients

Answer 24

-Na+/K+ ATPase pump
-Na+/Ca2+ exchanger
-Ca2+ ATPase pump

Question 25

Na+/K+ ATPase pump

Answer 25

• 3Na+ out, 2K+ in
  -net negative charge inside cell

Question 26

Na+/Ca2+ exchanger

Answer 26

• 1Ca2+ out, 3Na+ in
• net positive charge inside cell

Question 27

Ca2+ ATPase pump

Answer 27

• 1Ca2+ out
• net negative charge inside cell