Timing of heart

Question 1

Timing of the cardiac cycle + key requirements

Answer 1

• timing of the cardiac cycle is determined by the conducting system of the heart
• A single cardiac cycle is made up of a 2 sets of contractions & relaxations
  i. the atria contract (& then relax)
  ii. the ventricles contract (& then relax)
  Key requirements for timing:
  i. Very rapid propagation of cardiac muscle cell action potentials so that all atrial muscle contracts together & all ventricular muscle contracts together.
  ii. a delay between activation of atria and activation of ventricles.
  This timing is controlled and coordinated by the conducting system of the heart.

Question 2

The conducting system of the heart = what’s included

Answer 2

• The conducting system of the heart consists of pacemaker cells and conducting cells
• pacemaker cells
  - auto-active
• conducting cells
  - have specialised conducting properties
• Although the AV node is the secondary pacemaker, under normal circumstances, it functions as the conduit of electrical activity from the atrium to the ventricles
• AV = key role in timing of heart
• The cells that make up the conducting system have far fewer contractile proteins in those cells than normal contractile cells = main role not to contract but to auto-activate/ conduct action potentials

Question 3

Rapid propagation : cardiac muscle cells form a functional syncytium

Answer 3

• Cardiac muscle cells are “joined together” – i.e. they are physically connected to their adjacent “neighbours” via intercalated discs.
• When one cell is activated, the action potential is propagated very rapidly to its adjacent neighbours by diffusion of ions through the intercalated discs, depolarising the neighbouring cells to threshold.
• This produces a cascade of activation, so that after the first action potential fires off in the SA node, the entire myocardium (heart muscle) is activated within ¼ second.
• This is known as a functional syncytium

Question 4

Intercalated discs

Answer 4

• The intercalated discs allow rapid propagation of action potentials b/w adjacent cardiac muscle cells
• The intercalated disc (ICD) joins adjacent cardiac muscle cells and helps to strongly bind the muscle cells together.
  - The ICD contain desmosomes; networks of adhesion proteins that form very tight junctions b/w adjacent cells.
  - Important in preventing heart muscle cells from tearing apart during contractions.
• ICD also contain gap junctions; specialised membrane proteins (connexins) that form a connection b/w the cytoplasm of the two cells.
  - similar to channels = more bigger + rapid diffusion
• Gap junctions allow the rapid passage of ions (such as Na+& Ca2+) b/w adjacent cells
  - via diffusion