heart conduction system

Question 1

describe the conduction pathway and conduction speed

Answer 1

1. SAN originate (0.5m/s): spreads by 1. gap junctions 2. internodal bands
2. delay at AVN: fibrous tissue insulates 0.05m/s
3. Bundle of His in ventricular septum, splits to left and right branches (1ms)
   4 Purkinje fibre: fastest 5m/s
4. endocardia to epicardia - 0.5ms

Question 2

explain refractoriness by the fast Na channels

Answer 2

activated state: outside channel close (fast gate), inside channel open (slow gate)

depolarisation: fast gate open quick, slow gate close slow = split second open

inactivated state: slow gate shut completely = absolute refractory

recovering time: slow opening of slow gate: when impulse arrive, fast gate open, so Na conductance = relative refractory period

Question 3

why isnt there tetanus in heart muscle

Answer 3

heart muscle PEAK contraction DURING absolute refractory period. START RELAXING in that period

there is also 10ms latency between AP and actual contraction initiation

by the time AP can be generated in RELATIVE refractory period = most muscles are mostly relaxed = wont exist in a continual contracted state = no tetanus

ALSO there is latency

Question 4

describe the action potential phases in pacemaker tissues and the channels involved

Answer 4

TH is at -40mV
1. TH met: L-type Ca channel open = Ca influx = close to ECa, K conductance 0. Closure is spontaneous

2. Ca close = start repolarisation = slowly activated delayed rectifier, rapidly activated delayed rectifier and inward rectifier K channel (activated when polarised, inactivated when depolarised)
3. when Em approaches -40mV, funny Na channels open = funny current = cycle

Question 5

describe the AP phases in ventricular myocyte

Answer 5

1. at rest = -90 close to Ek = inward rectifier K channel
2. rapid depolarisation: Na fast channel open, depolarisation closes K channel = approaches ENa
3. Partial repolarisation: Na channel inactivate, transient outward K channel activate -
4. reaches -40MV: K channel shut, L-type Ca channel open = Em at 0mv maintained
5. repolarisation: spontaneous Ca closure, slowly and rapidly activating delayed rectifier K channel open, then inward rectifyer open