biol155 lecture 27

Structural similarities of skeletal and cardiac myofibers

Both are striated (contain sarcomeres) and contain myosin and actin filaments.

Structural differences between skeletal and cardiac myofibers

Cardiac myocytes have a branched structure and one nucleus per cell, while skeletal myocytes span the whole muscle and have multiple nuclei.

Physiological similarities of skeletal and cardiac myofibers

Both are excited through action potentials and require depolarization, calcium release, and repolarization to contract.

Physiological differences between skeletal and cardiac myofibers

Cardiac myocytes cannot produce tetanus due to a longer action potential.

Phases of a cardiac action potential

0: depolarization (sodium channels open); 1: height of depolarization (sodium channels close, potassium channels open); 2: repolarization starts (calcium channels open); 3: more repolarization (calcium channels close); 4: resting potential.

Coupling in skeletal vs cardiac muscle

Skeletal muscle uses mechanical coupling while cardiac muscle uses biochemical coupling.

Reason cardiac muscles cannot produce tetanus

The prolonged action potential length leads to a longer absolute refractory period.

Conduction pathways of the heart

Made of highly modified myocytes.

Direction of action potentials in the heart

Sinoatrial node > internodal pathways > atrioventricular node > AV bundle/bundle branches > Purkinje fibers.

SA node

A 'pacemaker cell' generating an intrinsic rhythm of depolarization and repolarization.

Membrane potential changes in SA node

Pacemaker potential driven by funny channels and action potential triggered by T-type voltage gated calcium channels.

AV node

Triggers ventricular contraction and has pacemaker properties with a slower rhythm.

Speed of depolarization in AV node

Slower due to fewer gap junctions.

ANS impact on SA node

Parasympathetic activation slows heart rate; sympathetic activation increases heart rate.

SA and AV node impact on heart contractions

SA node excites atrial contraction; AV node delays and forwards signals to ventricles.

ECG

An electrocardiogram measuring electrical impulses of the heart.

ECG points and contraction cycle

P wave: atrial depolarization; Q wave: beginning of ventricular depolarization; QRS complex: completion of ventricular depolarization; T wave: ventricular repolarization.

Arrhythmia

Unusual patterns of cardiac electrical activity.

Tachycardia

Heart rate above 100 bpm.

Bradycardia

Heart rate below 60 bpm.