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Heart Muscle Physiology

Contractility and Conduction

• All muscles are contracted; heart muscle is defined by its contraction.

• Heart muscle cells possess conduction ability, allowing electrical signals to pass from one cell to another.

• Excitability/Irritability: Heart muscle can respond to electrical stimuli, a property shared with nerve cells.

Autorythmicity

• Heart muscle is autorhythmic, meaning it can initiate its own contractions without an external stimulus.

• Despite its autonomy, it remains influenced by the central and peripheral nervous systems.

Comparison with Skeletal Muscle

• Skeletal muscle requires an external signal (nerve stimulus) to contract.

• Severing the nerve supply stops skeletal muscle contraction, while heart muscle continues to beat autonomously.

Energy Requirements for Heart Function

• Heart muscle relies on aerobic respiration to function effectively, necessitating:

  • Glucose: Initiates energy production.

  • Oxygen: Essential for ATP production.

• The process of aerobic respiration includes:

  • Glycolysis

  • Krebs Cycle

  • Electron Transport Chain (ETC)

• Oxygen is critical; muscle cells store it in a protein called myoglobin, which is specific to muscle function.

• Myoglobin vs Hemoglobin:

  • Hemoglobin carries oxygen in blood; myoglobin stores oxygen specifically for muscles.

Conduction System of the Heart

Overview of Conduction Cells

• Heart conduction system comprises specialized myocardial cells (not nerves) that transmit electrical signals.

• These cells operate similarly to nerves but are distinctly different in structure and function.

Sinoatrial Node (SA Node)

• Known as the heart's pacemaker; sets the rhythm and intrinsic firing rate of 60-100 beats per minute.

• The SA node depolarizes and repolarizes; it dictates heart rate but can be influenced by external factors (extrinsic control).

Atrioventricular Node (AV Node)

• Serves as a traffic light, regulating electrical signal flow and causing a brief delay to allow the heart chambers to fill with blood before contracting.

Bundle of His and Bundle Branches

• After the AV node, the signal travels to the Bundle of His, which splits into right and left bundle branches.

• The intrinsic firing rate for the Bundle of His is approximately 40-60 beats per minute; it takes over in cases where signals from the SA node fail to reach the ventricles.

Purkinje Fibers

• Found at the end of the bundle branches, these fibers penetrate deep into heart muscle to ensure effective contraction across the entire myocardium.

• Purkinje fibers can fire independently at a rate of 20-40 beats per minute, only if there is no signal from the bundle branches.

The Role of the Nervous System in Heart Regulation

• The sympathetic nervous system can increase heart rate by stimulating the SA node, while the parasympathetic nervous system decreases heart rate by inhibiting it.

• Both systems work together to ensure appropriate heart functioning and response to physiological needs such as stress or rest.

Heart Phases

• The heart operates in two main phases:

  • Systole: When the heart chambers contract and pump blood.

  • Diastole: When the heart chambers relax and fill with blood.

• Atria and ventricles work cooperatively to maintain efficient heart function and blood circulation.