L5.3 Neural control of the heart

Question 1

Electrical conduction pathway

Answer 1

• SA → AV → Bundle of His → Purkinje fibres
  
  • AP spreads passively through neighbouring gap junctions

Question 2

Cardio ventricular AP

Answer 2

• (1 AP = 1 contraction)
• Fast Na channel opens
• L type channels close
• L type channels open
• K channels open
• Stable RMP

Question 3

Intrinsic pacemaker AP

Answer 3

• Threshold = -40
• No fast Na current
• T-type Ca channel
• I-funny (I-f) channels open Na entry
  
  • T-type & I-f channels propagates AP to threshold → I-f closes & L-type Ca opens → Ca influx (depol) → K channel opens (repol)

Question 4

Which is the dominant pacemaker?

Answer 4

• Most cardiac cells capable of becoming pacemaker
• Normally cells in SA that is fastest takes lead (well connected together)
• Depol & injured cells risk becoming ectopic pacemaker → causes arrthymia
  
  • E.g. Fom ischemia

Question 5

Autonomic innervation dominance

Answer 5

• Blocks NS to the heart → Nerves might regrow but not necessarily → usually leads to increase SNS or PNS depending on dominance
  
  • I.e. if all N are cut in situ and an increase in HR is exhibited → Means PNS was dominant before (ACh > NA)
  • Dominance of PNS increases with training

Question 6

Sympathetic regulation

Answer 6

• INcrease Ca depol current
• NA activates G-protein signaling → change in conformation
• Mech:
  
  • GTP binds to a-subunit → dissociates from b & gamma subunit → activates adenylyl cyclase → generates cAMP → Phosphorylate and stimualtes I-Ca (T&L channels) & I-f → Net Na flux

Question 7

PNS regulation

Answer 7

• Increase hyperpol K current → slower depol
• GTP binds to a-subunit (Gai) → dissociates from b & gamma subunit → b & gamma binds K-channels → directly increase conductance (NO SECONDARY MESSENGERS)

Question 8

Refractory period

Answer 8

• Unresponsive to stimuli
• Skeletal muscles refractory period ~10ms
  
  • Period not as important →force generated through tetanus
• Cardiac muscle refractory period ~250ms
  
  • AP is longer from incoming Ca before K repol
• Refractory period due to:
  
  • Delay inactivation of L-type Ca channel
  • Protects against early re-excitation
  • Allows 1 AP = 1 contraction

Question 9

Baroreceptor reflex

Answer 9

• High pressure R at aortic arch & carotid sinus → controls BP
• Stretched → Decrease BP (Vice versa)

Question 10

Bainbridge reflex

Answer 10

• Low range pressure R, controls BV
• Atria, pulmoanry A, RV
• Increases VR = increased stretch → renal vasodilation → increase urine → decrease BV → decrease stretch of R