Cardiac Muscle

Contractile myocardial cells give the heart muscle its striated appearance as they contain lots of thick and thin filaments.  They make up the walls of the ventricles and atria.

Pacemaker cells are small, pale and with few organelles and thick and thin filaments.  They drive the electrical activity of the heart (where the heartbeat originates).  They are found at the sinoatrial node and atrioventicular node.

Striated

Involuntary

Conductors are short, broad, oriented end to end with few lateral connections.  They are insulated on the outside and have gap junctions which allows electrical signals to pass from cell to cell.  They are found at the Bundle of His and Purkinje Fibres.

• An action potential starts at the sinoatrial node.  The wave of excitation spreads over the atria (the electrical signal passes through gap junctions between cells) and causes depolarisation of the atrioventicular node.  The action potential set up at the AN node passes to the Bundle of His down to the apex of the heart, along the Purkinje fibres and to the cardiac cells that make up the ventricles.  
• The Bundle of His delays the contraction of the ventricles till after the contraction of the atria.  This is essential to have an effective pump system.

Autonomic nervous system

Pacemaker cells and contractile cells

• Pacemaker cell
• 1.  RMP sits at about -50mV (compared to -70mV in skeletal muscle) so they have a relatively high RMP.  There is a gradual depolarisation as there is a net influx of positive charge (Na⁺) - this is referred to as the pacemaker potential.  During this time fast gated Na⁺ channels are being opened so by the time the cell gets close to threshold almost none of these channels are open as they are also fast to close.  There the AP is not brought about by opening Na⁺ channels.
• 2.  AP is brought about by opening voltage gated Ca²⁺ channels and an inflow of Ca²⁺ ions.  Ca²⁺ channels are not fast channels so the AP lasts around 100ms.
• 3. Repolarisation occurs by closing Ca²⁺ channels and opening K⁺ channels.

• Contractile cell
• 1.  There is a stable RMP so the majority of cells will remain inactive unless driven by the sinoatrial node.  When they are stimulated there is an explosive depolarisation (similar to what we see in nerves) as fast voltage gated Na⁺ channels are opened and Na⁺ moves into the cell.
• 2.  Initial repolarisation occurs because the fast voltage gated Na⁺ channels close.
• 3.  Mainted depolarisation (Plato phase) occurs in cardiac muscle because voltage gated Ca²⁺ channels are opened, which are slow to close and so will maintain depolarisation.  The Plato phase lasts around 100ms.
• 4.  Repolarisation occurs via closing Ca²⁺ channels and opening K⁺ channels to bring the membrane potential back down to the stable RMP.

• The refractory period
• During this time an AP cannot be generated.  This is important as it gives time for the heart to fill with blood again before the next contraction (you do not want to be able to summate the mechanical activity of the heart to maintain a continued contraction).

• Get calcium from the extracellular fluid
• AP opening calcium voltage gated channels
• Opening of calcium gated calcium channels in the sarcoplasmic reticulum

• Slope of pacemaker cells
• Change the threshold of the cell
• Change the minimum membrane potential

Increased stretch of the ventricular muscle due to increased diastolic filling leads to an increase in the force of contraction

The resting range in cardiac muscle sits at a sub-optimum level (unlike skeletal muscle).  The heart cannot operate with its resting range at its optimum range.  If it did when you exercised and increased your blood pressure more blood would flow into the heart but it wouldn't all be expelled because as the muscle stretches past its optimum you get a weaker force of contraction, so the heart stops acting as a pump.

• a) increase in heart rate (and force of contraction)
• b) decrease in heart rate