Lecture 7: Cardiovascular 2: Cardiac fxn and control

Question 1

What 2 types of muscle cells compose the myocardium?

Answer 1

1.Contractile cells:NEED STIMULATION
-99% of cells, AP required for contraction to occur

2. Auto rhythmic cells: SELF POLARIZE
   -1% of cells
   -Modified non-contractile cells
   -Concentrated in specific regions of heart
   -Spontaneously generate AP
   -Audtorhythmicity =pace maker

Question 2

what are gap junctions?

Answer 2

In heart, adjacent cells are connected by water-filled pores forming open connections = GAP JUNCTION
-This allows ions to move freely from 1 cell to other = electrical activity can pass from cell to cell VERY FAST

Question 3

What are the major players in the electrical conduction system in the heart? What are the different nodes?

Answer 3

-Generated by the pacemaker cells = auto rhythmic
-Sino-Atrial (SA) node: Command centre determines heart contraction, rhythmical self excitation
-Atrio-Ventricular (AV) node: have auto rhythmic ability BUT slower pace so UNDER SA CONTROL. Gateway for electrical conduction b/w atria and ventricles
-Bundle of HIS and Purkinjie fibres: Help to quickly propagate electrical activity from the AV node to the rest of the ventricles

Question 4

What is the role of the electrical system and the clinical relevance?

Answer 4

-Maintain appropriate heart rate
-Coordinate contraction of atria and ventricles
-Coordinate contraction of each chamber

Clinical reference:
-Use electrocardiogram (ECG) to determine heart rhythm
-Problems with conduction-abnormal rhythm (arrhythmia)

Question 5

What is the sequence of excitation?

Answer 5

1. SA node self excitation (generation of APs)
2. AP’s propagate through atria = atrial contraction
3. AV node activated by AP wave, transmit electrical activity to the bundle of His and Purkinje fibres with a little delay (allows packing of blood in ventricles and closure of the AV valves)
4. Electrical activity propagate through ventricles = ventricular contraction

Question 6

What happens to generate an AP in the SA node under 1. normal condition and 2. by the ANS?

Answer 6

1. Cells from SA node gradually depolarize. This drift in potential is caused by the leakage of Na+ inside the cell and reduced diffusion of K+ outside the cell
   -When threshold is met an AP is generated
   -cycle is repeated = Peacemaker potential
2. Sympathetic fibres: reduce the time required to reach threshold = faster pace
   Parasympathetic fibres: prolong the time required to reach the threshold = slower pace

Question 7

How does AP of contractile cell differ from AP of auto rhythmic cells or muscle cells?

Answer 7

Action potential of contractile cell, has a stable resting membrane potential (no drift)
-Differs from skeletal muscle AP in duration skeletal muscle is very short 1 millisecond and cardiac muscle is long 100’s of millisecond
Length of refractory period; skeletal muscle is shorter than cardiac muscle

So the muscle doesn’t cramp: have to make sure after contraction its relaxed so we can have another contraction

Question 8

What are the 5 steps of an AP of contractile cells?

Answer 8

Phase 1: Resting membrane potential
Phase 2: Rapid depolarization, mainly due to influx of Na+ into cell
Phase 3:Short phase of repolarization; loss of K+ from cell
Phase 4: Plateau phase-influx of Ca2+ into cell
Phase 5: Repolarization phase-outward movement of K+ from cell

Question 9

what are the steps of calcium-stimulated-calcium-release?

Answer 9

1. Depolarization
2. Voltage-gated Ca++ channels open
   3.Ca++ entry induces an avalanche of Ca++ release from sarcoplasmic reticulum (10fold increase in Ca++)
3. Large increase in intercellular Ca++ triggers contraction
4. Ca++ pumped back in its pre-stimulation compartment

Question 10

What is the effect of the sympathetic nervous system on the control of the heart contraction?

Answer 10

-Stimulates the heart rate; stimulates the firing of the SA node which stimulates the velocity of the AV node conduction (shorter delay between them)
-Increases the contraction force; Increases the release of Ca2+ from sarcoplasmic store
-Reduces the contraction time; increases speed of Ca2+ transport (reduces the plateau length) ie movement of Ca+ is increased to be ready for increase heart rate

Question 11

What is the effect of the parasympathetic nervous system on the control of the heart contraction?

Answer 11

-Decreases the heart rate; reduces the firing of the SA node, Decreases the velocity of the AV node contraction (longer delay)
-Actions mediated by binding of acetylcholine on muscarinic receptors in auto rhythmic cells
-this increase K+ permeability, hyper polarization increase time required to reach the AP threshold

Question 12

What is an electrocardiogram (ECG)?

Answer 12

-In heart lots of cells are firing an AP simultaneously
-Generate a strong current, and current conducted through body fluid to the skin
-ECG= measure heart membrane potential throughout the cardiac cycle
-Electrode placed at different location on the skin will read the progression of the electric current wave

Question 13

How many waves does the standard ECG have?

Answer 13

3 waves

P wave= Depolarization of the atria
QRS wave= depolarization of the ventricles
T wave= repolarization of the ventricles