Electrical activity and the heart Flashcards
Define membrane potential
difference in electrical potential (voltage) between the exterior and interior of a cell across its membrane
difference due to unequal distribution of ions between the inside and outside of the cell creating a difference in potential on either side of the membrane
Describe resting membrane potential
inside of the cell is always negative relative to the outside in an unexcited cell, this potential = resting membrane potential
What is the resting membrane potential in most cells, cardiomyocytes and cardiac pacemaker cells?
most cells = -30 - -100 mV
cardiomyocytes = -80 - -90 mV
cardiac pacemaker cells = -50 - -70 mV
How is a resting membrane potential generated?
semi-permeable nature of cell membrane:
- under resting conditions the membrane is more permeable to potassium ions than any other ions
- some leak potassium ion channels that are open during the resting conditions
presence of ion channels and pumps (active transport)
both factors lead to unequal distribution of ions between the inside and outside of the cell generating a negative charge inside of the membrane compared with outside
What 3 ion channels do cardiomyocytes mainly express?
K+
Na+
Ca2+
Which ion is the most important in controlling resting membrane potential at rest?
K+
K+ channels are open and this allows ions out of the cell down the concentration gradient to generate the negative potential
What is an action potential?
in muscle cells/tissue, action potentials are when the cell/tissue depolarise from the resting membrane potential and this causes contraction
What does auto-rhythmicity mean?
cardiac tissue does not require any neural or hormonal input in order to beat - it generates its own action potential from cardiac pacemaker cells
Name 2 types of cardiac muscle cells in the heart
cardiac myocytes
pacemaker cells
Describe cardiac myocytes
not auto-rhythmic and do not normally create action potentials on their own
do conduct action potentials to adjacent cardiac myocytes via gap junctions
Describe pacemaker cells
auto-rhythmic - initiate and conduct action potentials for the adjacent cardiac myocytes cells
the rate of producing action potentials is modulated by autonomic inputs
predominantly located in the SA node and some in the AV node
Where are pacemaker cells located?
predominantly in the SA node and some in the AV node
Describe the propagation of depolarisation in the heart
pacemaker cells in SA node initiate an action potential (depolarisation) and under normal circumstances, heart rate is governed by the rate of this discharge
this is conducted across the atria to the AV node where this conduction is delayed about 100 msec (AV nodal delay)
the depolarisation then spreads down the bundle of His and Purkinje fibres to the ventricles
What does the delay at the AV node allow?
allows time for complete ventricular filling after atrial contraction before ventricular contraction
What mechanisms allow all heart tissue to depolarise together and cause a uniform wave of tissue contraction?
gap junctions, T-tubules and intercalated disks
connections between cardiac myocytes = intercalated disk contain gap junctions that directly connect myocytes to allow direct ion flow between cells
T tubules penetrate deep in cells and allow for quick conduction of action potentials between and into cells
