Cardiac Muscle Physiology

Published: 20/09/2020

This is my summary of the key information from the ClinicalPhysio webinar on Cardiac Muscle Physiology by Dr Jack Hurley. This is to be used as a revision summary only. 

Please go to ClinicalPhysio website or check out their instagram @ClinicalPhysio

Summary of topic areas

• Macroscopic organisation

• Microscopic organisation

• Muscle contraction cycle

Macroscopic organisation

Heart structure

The heart is surrounded by a fibrous sack called the pericardium. This pericardium works as an anchor to adjacent structure to prevent the heart from moving inside your thorax. 

Diagram from Socratic.org

Functions of specific structures and further notes

• Pericardial cavity (or fluid) - ~ 50ml of fluid which acts as a lubricants whilst the inner layers of the heart contract and relax
• Serous pericardium (visceral layer), visceral meaning an organ

Heart valves

• Atrioventricular values (between the atria and ventricles) - Right valve = Tricuspid, Left valve = Mitrial or Bicuspid
• Semilunar valves (in the blood vessels taking blood away from the heart)

Heart sounds

Lub - Dub

• Lub = AV valves closing
• Dub = Semilunar valves closing

Nerve structure and supply

Sinoatrial node is the hearts contraction initiation centre. It beats without external influence and would fire 100 times a minute if it wasn't for parasympathetic nervous system intervention. An adults normal resting heart rate is 60 to 80 bpm.

The Sinoatrial (SA) node's firing rate is influenced by regulatory centres in the brain (medulla) and spinal cord. These centres either slow down or speed up the bpm.

Conduction pathway:

1. Sino-atrial node
2. Atrio-ventricular node
3. Bundle of His
4. Right and left Bundle branches
5. Purkinje fibres

Diagram from: radiopedia.org

The parasympathetic nervous system (medulla) suppress the SA node and is active during rest. The sympathetic nervous system (T1 to 4) increases the SA node firing rate to increase the heart rate due to exercise or external factors (e.g. stress)

Parasympathetic pathway

1. Medulla
2. Parasympathetic nervous system
3. Vagus Nerve (CN X)
4. Acetylcholine
5. Decreased heart rate and contractility

Sympathetic pathway

1. T1 to 4
2. Sympathetic nervous system
3. Cardiopulmonary splanchnic nerves
4. Noradrenaline
5. Increased heart rate and contractility

Diagram from: AHA Journals

Microscopic organisation

The cardiac muscles look red (after the addition of microscopic dye) as these fibres predominantly rely upon aerobic respiration to achieve their energy requirements.

The branch like structure (demonstrate by the 3D view) aid in the heart contraction to a small volume (rather than just length shortening).

Diagram from: Tes Teach

Function of specific structures in the cardiac muscle fibres

• Mitrochondria - site of ATP synthesis
• Nucleus - Brain on the muscle fibre
• Intercalated disc - contains Adherent type junctions, desmosomes and gap junctions
• Desmosomes - mechanically links the structures (muscle fibres etc) together
• Gap junctions - electrical synapses which allow the passage of electrical signals
• Adherent type junctions - anchor muscle fibres together

Muscle contraction cycle

Contraction of muscle

Diagram from: en.wikipedia.org

Clinical note: Troponin (T) is measured at specific time intervals after a cardiac event to indicate the likelihood of an heart attack. (Measured as a % rise in levels)

Contraction process

1. Calcium binds to troponin
2. Creates a cross-bridge between the Thin and thick filament
3. Power stroke --> myosing head bends to 45 degrees
4. ATP binding
5. ATP hydrolysis

Requires a high concentration of ATP and calcium to contract. Removal or reduction in one of these, particularly clacium, impacts on the muscle fibres ability to contract.

Diagram from: ClinicalPhysio

General muscle fibre contraction process
Step 1. An action potential travels across the T-tubules in the muscle fibres
Step 2. This causes the calcium protein channels to open to allow the transport of calcium to the muscle fibres
Step 3. After the action potential has finished the calcium return to its original position ready for the next contraction

Cardiac muscle fibre contraction process
Works by calcium induced calcium release
In the cardiac muscle fibres the T-tubules contain a level of calcium ions. These calcium ions bind to the calcium receptors to induce calcium release. 

Step 1. calcium ions binds to calcium receptors to enable calcium ion movement to the muscle fibres
Step 2. Receptors release calcium ions to the muscle fibres to enable contraction to occur
Step 3. Calcium ions move to original position, ready for next muscle contraction

Final words

Please go to the ClinicalPhysio website and look at their hanbooks (available for purchase) and webinars. These provide great revision and consolidation on a variety of topics with only the information you need to know for clinical practice.