4.2 - Initiation and Coordination of Contraction Flashcards
The heart contains specialized cells known as ____
autorhythmic cells (pacemaker cells)
Where are the autorhythmic cells found in the heart?
- sinoatrial node (SA node)
- Right atrium, near superior vena cava
What is the function of pacemaker cells?
generate APs without input from the nervous system
What is the pacemaker potential?
- starting point of -60mV
- unstable membrane potential slowly drifts upwards from pacemaker potential until reaches threshold and initiates AP
Why do they have unstable membrane potentials?
Because they have different membrane channels than other excitable cells
What are funny channels? What are they permeable to?
- specialized If channels (I=current, f=funny channels)
- permeable to K and Na
When membrane potential is -ve ______
Na+ influx > K+ efflux = net influx of +ve charge = slow depolarization of membrane
What happens when funny channels close?
Ca2+ channels open = continued depol. = threshold reached = many more Ca2+ channels open, rapid influx of Ca2+ = steep depol. phase of AP
What happens at the end of depol. ?
Ca2+ channels close, K+ channels open slowly, efflux of K+ causes repol.
What is the major difference between AP and PP in pacemaker cells?
Influx:
A.P = only Ca2+
P.P = both Na+ and Ca2+
What modulates the rate of pacemaker potentials?
The autonomic division
- sympathetic: stressful situations
- parasympathethic: rest and digest
Explain how the release of norepinephrine and epinephrine leads to heart rate increasing
- Norepinephrine (sympathetic neurons) and epinephrine (adrenal medulla) bind to b1 adrenergic receptors
- release of cAMP through signalling pathway binds to open funny channels
- channels stay open longer, increasing permeability to Na+ and Ca2+
- increased depol. rate increases rate of action potentials, thus increasing heart rate
Explain how the release of acetylcholine leads to heart rate decreasing
- ACh released from parasympathetic neurons, binds to muscarinic receptors
- increases K+ permeability, hyperpolarizing the cell
- pacemaker potential starts at more negative value, taking longer to reach threshold potential, decreases heart rate
How does electrical communication take place in the heart
- autorhythmic/pacemaker cells initiate the electrical excitation of the heart
- depol. spreads to neighboring cardiac cells via gap junctions in intercalated discs
Explain how the events of conduction take place
- action potentials are fired at the SA node, spreads to adjacent cells
- rapid spread through cells of internodal pathway, slow spread through contractile cells of atrium due to cytoplasmic resistance
- signal passed through AV node ONLY at AV junction: fibrous connective tissue acts as insulator preventing electrical signals from atrium to the ventricle
- AV node ONLY pathway for action potential
- signal delay at AV node
- signal carried to bottom of heart through bundle of His
- Bundle of His divides into left and right branches: purkinje fibres transmit signals rapidly to ensure all contractile cells at apex contract together
