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Y Physiology I > Week 04 Lect. 3 - Origin/Spread of Cardiac Excitation > Flashcards

Week 04 Lect. 3 - Origin/Spread of Cardiac Excitation Flashcards

1
Q

What is the volume of cardiac output per minute?

A

5-6 L/min

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2
Q

What is the mean arterial pressure under resting conditions?

A

90-95 mmHg

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3
Q

What is the route of cardiac excitation starting from its pacemaker onward?

A

SA node > AV node > bundle of His > bundle branches (Tawara) > Purkinje fibers + ventricular myocytes

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4
Q

What are the two kinds of APs generated in cardiac tissues and which tissues are they created in?

How do these action potential graphs look?

A
  1. Slow Response AP - nodal (SA/AV) cells
  2. Fast Response AP - atrial/ventricular myocardia
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5
Q

How do the intrinsic pacemaking frequencies of different heart tissues compare?

SA node…

Atrial/Ventricular myocytes…

AV node/bundle of His…

Bundle branches/Purkinje fibers…

A

SA node: 70-80 bpm

Atrial/Ventricular myocytes: none

AV node/bundle of His: 40-60

Bundle branches/Purkinje fibers: 20-40

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6
Q

What is the current responsible for the “pacemaker potentials” of the heart?

Via what channel + how is it activated?

What ions/which direction?

A

If (funny) current

  • HCN4 channel - activated via hyperpolarization (< -50 mV) and increased cAMP
  • Na+ ions diffuse into nodal cells
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7
Q

What is the current responsible for the initial depolarization of the SA node?

Via what channels + how?

What ions + in which direction?

A

ICa,T or Transient Current (b/c it inactivates)

  • T-type VDCC - -55 mV threshold
  • Na+ and Ca++ ions diffuse into cell
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8
Q

What is the current responsible for the “depolarization” phase of SA node potentials?

Via what channels + how?

What ions + in which direction?

A

ICa,L (long-lasting) current

  • L-type VDCC - -25 mV threshold
  • Ca++ ions diffuse into cell
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9
Q

What is the current responsible for repolarizaton of SA node APs?

Via what channels + how?

What ions + what direction?

A

Ik current

  • Multiple types of K+ channels (Ito, Ikis, Ikr, Ikur)
  • K+ ions diffuse out of the cell
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10
Q

What is the current responsible for hyperpolarization of the SA node?

Via what kind of stimulation?

What channels?

What ion + which direction?

A

IK,ACh Current

  • via ACh signals from the vagus nerve
  • GIRK1/4 channels
  • K+ ions diffuse out of cell
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11
Q

What common drug is used to slow heart rate via the SA node and how?

A

Ivabradine

  • specific inhibition of the “funny” channel slows the prepotential created by If
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12
Q

What common cardiac arrhythmia drug effects the SA node and how?

A

Verapamil

  • blocks the T-type calcium channel affecting the It current of early SA node depolarization

(although Wikipedia says it’s an L-type blocker… lecture slides say T-type)

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13
Q

What is the ion channel associated with funny current (acronym and full name)?

What two ways can it be activated?

A

HCN - Hyperpolarization-activated cyclic nucleotide-gated ion channel

  1. hyperpolarization below -50 mV (voltage sensor, subunit 4)
  2. cAMP binding
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14
Q

How long do the two channels responsible for the majority of depolarization of the SA node remain open?

A

T-type - 25 ms

L-type - 100 ms

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15
Q

What are the two different ways that parasympathetic innervation can effect heart contraction via two different sites of innervation?

Via what nerve?

A

Right Vagus > SA node = neg. chronotropic effect

Left Vagus > AV node = neg. dromotropic effect (slows speed of conduction)

  • note that vagus does not innervate ventricles
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16
Q

What two aspects of pacemaker potentials are affected by autonomic innervation?

A
  1. Minimum Diastolic Potential - lowest Em value during the AP
  2. Slope of Prepotential - rate of diastolic repolarization via funny current
17
Q

Describe the mechanism of sympathetic regulation of heart rate…

What is the neurotransmitter, receptor, G protein and effects?

A
  • Norepinephrine
  • β1 receptor
  • Gs activation
    • ​increased cAMP
    • increased If
    • increased ICa,L permeability
18
Q

Describe the mechanism of parasympathetic regulation of heart rate.

Give the neurotransmitter, receptor, G protein and effects.

A
  • Acetylcholine
  • M2 muscarinic receptor
  • Gi (α subunit)
    • decreased cAMP
    • decreased funny current
    • decreased ICa,L current
  • Gi (βγ subunits)
    • ​IACh,K current via GIRK channels causes hyperpolarization
19
Q

What did the lecture’s example of “pharmacological denervation” of the heart show regarding autonomic control of heart rate?

How?

A

Parasympathetic innervation has a greater effect on heart rate than sympathetic.

  • Using the muscarinic (parasympathetic) blocking agent Atropineraises heart rate much more than using the adrenergic (sympathetic) blocking agentPropranolol lowers it
20
Q

What happens if SA node pacemaking fails?

A

Other HCN Channel-containing cells take over

  • the cells with the highest endogenous frequency take over

In order of frequency:

  1. AV node
  2. Bundle of His
  3. Tawara Branches
  4. Purkinje fibers
21
Q

How does cardiac excitation spread between cells of heart tissues?

A

via gap junctions

22
Q

What determines the rate of propagation of APs in the heart?

A

Strength of the depolarizing current

  • higher/faster depolarization = faster conduction
  • therefore “slow response” tissues such as the nodes have slower conduction
23
Q

What causes the fast depolarization seen in “fast response” cardiac tissues?

A

voltage-gated Na+ channels

24
Q

In what heart tissues is conduction fastest?

Why?

A

Bundle of His + Purkinje Fibers

  • these cells have a large diameter and thus conduct faster

(they are also “fast response” tissues displaying quicker depolarization phases)

25
Q

What delays contraction of the atria in comparison to the ventricles?

How?

A

The AV node is the site of delay

  • the connective tissue “fibrous skeleton” of the heart between the atria and ventricles insulates the electrical activity initiated by the SA node
  • the AV bundle passing through this insulation has only a few, thin “penetrating fibers” which conduct slowly due to their small diameter
26
Q

How is the speed of conduction regulated by the two branches of the autonomic nervous system?

What is this effect called?

A

Dromotropic Effect

Sympathetic: Positive Dromotopic effect

Parasympathetic: Neg. Dromo. effect (Left vagus > AV node)

Y Physiology I (17 decks)

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