Lecture 2 - EC coupling

Question 1

function of intercalated discs

Answer 1

allow impulses to travel rapidly between adjacent cells so they function as one rather than individual cells

Question 2

what is E-C coupling

Answer 2

converting an electrical stimulus into a mechanical response

action potential –> contraction

Question 3

Define ‘ action potential’

Answer 3

a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls following a consistent trajectory

Question 4

list some features of action potentials

Answer 4

• resting and threshold potential
• occurs in excitable cells ( neurons / muscle cells )
• generated by special voltage-gated ion channels
• depolarisation / hyperpolarization

Question 5

AP stage 1

Answer 5

resting potential -90mV transient potentials move the depolarisation threshold (-67mv)

Question 6

AP stage 2`

Answer 6

• 67mV threshold reached and action potential initiates

- rapid voltage change due to influx of Na+ ions

Question 7

AP stage 3

Answer 7

+50mV efflux of K+ ions

Question 8

AP stage 4

Answer 8

hyperpolarization due to excess K+channels remaining open

depolarisation is not possible during this stage

Question 9

Sodium concentrations

Answer 9

Extracellular 135-145mmol/L

Intracellular 10

Question 10

Potassium concentrations

Answer 10

EC 3.5-5.0

IC 155

Question 11

Chloride

Answer 11

EC 95-110

IC 10-20

Question 12

Absolute refractory period

Answer 12

impossible to evoke another action potential

Question 13

Relative refractory period

Answer 13

a stronger than usual stimulus required

Question 14

Ventricular AP phase 0

Answer 14

resting potential -85-95

depolarizing impulse activates fast Na+ channels and inactivates K+ channels

Question 15

V AP Phase 1

Answer 15

early repolarization phase

transient opening of K+ and closing of Na+

Question 16

V AP phase 2

plateau phase

Answer 16

Ca2+ channels open

key difference between nerve AP

Question 17

V AP Phase 3

Repolarization phase

Answer 17

Ca2+ channels inactivate

K+ channels open

Question 18

V AP refractory period

Answer 18

Na+ channels are inactive until membrane is repolarised

Question 19

SA node pacemaker potential phase 0

Answer 19

Rapid depolarisation

due to L-type Ca2+ channels opening

Question 20

SA node pacemaker potential phase 1-3

Answer 20

rapid hyperpolarisation due to K+ channels

Question 21

SA node pacemaker potential phase 4

Answer 21

unstable resting potential
due to funny/fast Na+ channels
MP-reaches threshold potential

Question 22

define automaticity

Answer 22

a pacemaker cell’s ability to spontaneously depolarise reach threshold and propagate an AP

Question 23

How are action potentials conducted to myocytes

Answer 23

via gap junctions

Question 24

role of calcium in EC coupling

Answer 24

depolarization of muscle cell surface to release of Ca2+ in SR
controls Ca2+ within muscle and Ca2+ controls force of contraction

Question 25

Role of calcium in muscle contraction

Answer 25

1. in the plateau phase Ca2+ enters–> there is small increase in intracellular Ca2+
2. Ryanodine receptors ( SR surface )detect this positive feedback induced releasing more Ca2+ –> produces calcium sparks
3. spatial + temporal summation of 30,000 sparks leads to a cell-wide increase in cytoplasmic Ca2+ concentration

Question 26

Whats in muscle contraction?

Answer 26

1. Calcium binds to troponin C, moving the tropomysin complex off the actin binding site allowing the myosin head to bind to the actin filament
2. The myosin head uses ATP hydrolysis to pull the actin filament towards the center of the sarcomere
3. Intracellular Ca2+is taken up by SR ATPase pump back
4. intracellular calcium concentration drops and troponin complex returns over the active site of the actin filament which ends contraction

Question 27

Length of refractory period in cardiac muscle

Answer 27

Long, therefore the cardiac muscle cannot sum action potentials or undergo tetanus

Question 28

How long does the effective refractory period last for?

Answer 28

duration of the AP

prevents tetanic contraction so the muscle relax allowing venous blood to refill the heart

Question 29

Purpose of a shorter EPI AP?

Answer 29

ensures ENDO is still refractory during EPI repolarisation and cannot be activated by EPI

Question 30

Features of ENDO refractory period

Answer 30

longer than time taken for the AP to propagate from ENDO to EPI plus the EPI AP duration

Question 31

What happens if EPI repolarization occurs outside the ENDO refractory period

Answer 31

depolarised EPI tissue may reactivate the ENDO causing a re-entry cyclic arrhythmia to be set up