Unit 4.2: Initiation of Cardiac Contraction Flashcards
True or False. Like other cells, the heart requires input from the nervous system for contraction.
False. The heart DOES NOT require input from the nervous system for contraction.
Why doesn’t the heart require input from the nervous system for contraction?
It contains specialized cells called autorhythmic cells (pacemaker cells)
Where are pacemaker cells located?
In sinoatrial node (SA node), in the right atrium, near superior vena cava
True or False. Pacemaker cells have a membrane potential of -60 mV when they are at rest.
False. Membrane potential of pacemaker cells are never at rest.
What ion channels to pacemaker cells contain that others do not?
If (funny channels)
They are permeable to Na and K
What is the flow of ion when the membrane potential of a pacemaker cell is negative?
Na influx > K efflux -> net influx of + charge -> slow depolarization of membrane
What is the flow of ion when the membrane potential of a pacemaker cell goes towards positive?
I(f) channels close; Ca2+ channels open -> continued depolarization -> threshold reached -> many ca2+ channels open and rapid influx of ca2+ -> steep depolariation phase of action potential
What does the flow of ions look like at the end of depolarization?
Ca2+ channels close and K+ channels open slowly; efflux of K+ causes repolarization
What is a major difference between action potentials and the pacemaker potentials in the pacemaker cells?
PP - Na+ and Ca2+ influx
AP - only Ca2+ influx
What modulates heart rate?
The autonomic division modulates RATE of pacemaker potentials
How does the autonomic division INCREASE heart rate?
- Norepinephrine released from sympathetic neurons and epinephrine released from adrenal medulla
- Bind to B1 adrenergic receptors on pacemaker cells (coupled to G proteins)
- release of cAMP through signalling pathways which binds to open I(f) channels -> channels stay open longer -> increased permeability to Na+ and Ca2+
- Increase depolarization rate increases rate of action potentials which increases heart rate
How does the autonomic division DECREASE heart rate?
- Acetylcholine released from parasympathetic neurons beind to muscarinic receptors
- Increase K+ permeability which hyperpolarizes the cell
- Pacemaker potential starts at more negative value there it takes longer to reach threshold potential
- Heart rate decreases
How does depolarization spread to nearby cardiac cells?
Via gap junctions in the intercalated discs
What are the events of conduction in the heart?
- Action potential fired in the SA node
- Rapid spread through cells of internodal pathway
- Signal passed through AV node ONLY at AV junction
- Signal slightly delayed by AV node to ensure that atria have fisnihed contracting
- Signal is carried to bottom of heart through bundle of His
- Purkinje fibres transmit signals VERY fast to ensure that all contractile cells at apex contract together
Why is it necessary to conduct signals ONLY through the AV node and bundle of His?
- So that the signal for contraction initiates at the bottom of the heart
- This ensures that the contraction drives the blood up since it leaves the heart at the top
When can arrhythmia (abnormal heart beat) develop?
- when another part of the heart takes over as pacemaker
- when the SA node develops an abnormal rate/rhythm
- when the normal conduction pathway is interrupted
What are the 4 types of arrhythmia studied?
- Bradycardia
- Tachycardia or tachyarrhythmia
- Ventricular fibrillation
- Atrial fibrillation
What is bradycardia?
slow heartbeat, less than 60 BPM
What are the symptoms of bradycardia?
Fatigue, dizziness, light-headedness, fainting
What is tachycardia or tachyarrhythmia?
Rapid heart beat, more than 100 BPM
What are the symptoms of tachycardia or tachyarrhythmia?
Palpitations, dizziness, light-headedness, fainting
What is ventricular fibrillation?
Disorganized contraction
Ventricle quivers, heart can’t pump blood
Collapse and sudden death follows unless medical help is provided immediately
How to we treat ventricular fibrillation?
Can be converted into normal rhythm with electrical shock (defibrillator)
What is atrial fibrillation?
Atria quiver instead of beating effectively
Blood is not pumped out completely when heart beats
Allows blood to pool and clot
