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Heart Muscle Physiology

• Contraction of Heart Muscle

  • Heart muscle is always in a state of contraction.

  • Conduction: the ability of heart muscle to pass electrical signals between cells.

  • Heart muscle responds to electrical stimuli; termed excitability or irritability.

  • Only some cells possess excitability: nerve cells and heart cells.

Autorethmicity

• Heart muscle demonstrates autorethmicity.

  • Meaning it can initiate contractions independently without external stimuli.

  • Though excitable, heart muscle cells can self-excite.

  • This contrasts with skeletal muscles which require nerve signals to contract.

Energy Requirements for Heart Function

• Heart relies on aerobic respiration for energy.

  • Requires glucose and oxygen.

  • Difficult to run out of glucose but easy to run out of oxygen.

  • Important metabolic processes: glycolysis, Krebs cycle, and electron transport chain (ETC).

  • Oxygen is crucial for ATP production; heart muscle cells store oxygen via myoglobin (distinct from hemoglobin in red blood cells).

  • Myoglobin is effective for muscle cells but does not share oxygen with other tissues.

The Heart's Conduction System

• Heart possesses a specialized conduction system made up of myocardial cells, not nerves.

  • Sinoatrial (SA) Node:

    • Known as the heart's pacemaker.

    • Sets heart rhythm (intrinsic firing rate of 60-100 beats per minute).

    • Can be influenced by external factors (extrinsic rates) like caffeine or stress hormones.

  • Atrioventricular (AV) Node:

    • Produces a delay in conduction to allow proper ventricular filling.

    • Acts like a traffic light to regulate electrical signals.

  • AV Bundle (Bundle of His):

    • Conduction continues from the AV node at a rate of 40-60 beats per minute in the absence of signals from the SA node.

  • Bundle Branches:

    • Carries impulses to right and left ventricles to facilitate synchronized contraction.

  • Purkinje Fibers:

    • Final specialized fibers that ensure spread of signals into the heart muscle (firing rate of 20-40 beats per minute if there’s no signal).

    • Ensures ventricles contract efficiently to pump blood.

Nervous Control of Heart Rate

• The cardiac rhythm is modulated by:

  • Sympathetic Nervous System:

    • Increases heart rate through stimulation of the SA node and AV bundle (e.g., during stress or exercise).

  • Parasympathetic Nervous System:

    • Slows heart rate by inhibition through acetylcholine release.

  • The heart operates in two phases:

    • Systole: when heart chambers contract.

    • Diastole: when heart chambers relax and fill with blood.

Structure and Function of Heart Chambers

• The heart consists of:

  • Two receiving chambers: atria (left and right).

  • Two pumping chambers: ventricles (left and right).

  • Atria and ventricles work in unison but are separate functionally to maintain efficient blood flow.