Heart physiology Flashcards
how is cardiac muscle arranged?
cardiac muscle cells of myocardium arranged in tightly bound layers encircling chambers
what does cardiac muscle do when the walls of the chamber contract?
exerts pressure on blood
contrast with skeletal muscle cells
skeletal muscle cells can be rested for prolonged periods and only a fraction are activated in a muscle per contraction
every heart cell contracts with every beat
1% of cardiac cells name and function
conducting system
electrical contact with cardiac muscle cells via gap junctions
initiates heartbeat and rapidly spreads action potentials
types of innervation received by heart
sympathetic and parasympathetic (in vagus nerves) nerve fibres
sympathetic innervation
sympathetic postganglionic fibres innervate entire heart adn release norepinephrine
parasympathetic innervation
fibres terminate ~ cells in atria
release acetylcholine
receptors for norepinephrine on cardiac muscle
beta-adrenergic
epinephrine - origin, effect
adrenal medulla
binds to same receptors as norepinephrine, exerts same actions
receptors for acetylcholine
muscarinic type
where does initial depolarisation arise?
sinoatrial node (SA node)
small group of conducting system cells
located in RA near entrance of SVC
sequence of excitation
SA node
internodal pathways/Bachmann’s bundle to RA/LA
AV node
(slight delay to prevent simultaneous contraction)
Bundle of His (down interventricular septum)
RBB & LBB
Purkinje fibres
purkinje fibres and their function
large diameter conductor cells
rapidly distribute impulse through most of ventricles
make contact with myocardial cells, which then pass on the impulse
what are the 2 main forces driving ions across cell membranes?
chemical potential and electrical potential
what is chemical potential?
ion moves down its concentration gradient
what is electrical potential?
ion moves away from ions/molecules of like charge
what is the transmembrane potential (TMP)?
electrical potential difference (voltage) between the inside and outside of cell
changes in TMP
net movement of +ve ions into cell -> TMP more +ve
TMP more -ve when movement of +ve ions out of cell
what do ion channels do?
maintain ionic concentration gradients and charge differentials between inside and outside
what are the properties of cardiac ion channels?
selectivity, voltage-sensitive gating, time-dependence
what is selectivity?
only permeable to single type of ion based on physical configuration
what is voltage-sensitive gating?
specific TMP range required for particular channel to be in open configuration
specific channels open and close as the TMP changes during cell depolarisation and repolarisation
what is time dependence?
some ion channels are configured to close a fraction of a second after opening - can’t open until TMP back to resting levels
what is an action potential?
electrical stimulation created by a sequence of ion fluxes through specialised channels in the membrane of cardiomyocytes. leads to contraction
what is the sarcolemma?
membrane of cardiomyocytes
what occurs in phase 4?
resting phase
resting potential is -90mV due to a constant outward leak of K+ through inward rectifier channels
Na+ and Ca2+ are closed at resting TMP
what occurs in phase 0?
depolarisation
action potential triggered in a neighbouring cardiomyocyte/pacemaker cell -> TMP above -90mV
fast Na+ channels open, Na+ leaks in, further raises TMP
reaches -70mV (threshold potential - self sustaining inward Na+ current)
large Na+ current depolarises TMP to 0mV - above for overshoot. fast Na+ channels close (time dependent)
L-type Ca2+ open when TMP is greater than -40mV - small but steady influx of Ca2+ down its concentration gradient
what happens in phase 1?
early repolarisation
TMP slightly positive
some K+ channels open briefly and an outward flow of K+ returns to TMP to ~ 0mV.
what happens in phase 2?
plateau phase
L type Ca2+ channel are still open and there is a small constant inward current of Ca2+.
K+ leaks out down its conc gradient through delayed rectifier K+ channels
2 countercurrents electrically balanced, TMP maintained at plateau below 0mV
what happens in phase 3?
repolarisation
Ca2+ channels gradually inactivated
persistent outflow of K+, now exceeding Ca2+ inflow, brings TMP towards -90mV.
