electrical activity of the heart Flashcards
what is the membrane round the heart muscle fibres called?
sorcolemma
what is the function of the of the sarcoplasmic reticulum?
it is a calcium ion store, this calcium is released during an action potential to cause contraction by binding to troponin
how would you describe the cardiac muscle in terms synchronisation?
it is a functional syncytium
in what way the the heart a functional syncytium?
the cells are connected electrically and physically so contract in syncrony
what feature of cardiac cells allows them to be connected electrically?
gap junctions between cells
what feature of cardiac cells allows them to be connected physically?
desmosomes
describe the make-up of the intercalated discs in cardiac muscle?
pattern of desmosome the gap junction the desmosome etc.
what is the function of t-tubules?
allows depolarisation of all muscle cell
is the action potential longer or shorter in cardiac muscle than skeletal muscle?
longer
why is the AP longer in cardiac muscle than in skeletal muscle?
because in cardiac muscle Ca+ ions must also move in through form outside the cell
how is the strength of contraction of the heart controlled?
the amount of calcium that enters from outside the cell
which cardiac cells have unstable resting membrane potentials?
pacemaker cells
describe the graph of membrane potential in a non-pacemaker cardiac muscle cell
when the action potential is fired the membrane depolarises rapidly from -90mV to above 0, then plateaus, repolarising very slowly. after this it repolarises rapidly to -90mV
describe the graph of membrane potential in a pacemaker cardiac muscle cell
the cell depolarises slowly until it reaches a threshold membrane potential ~-40mV at this point it depolarised rapidly to above 0mV. after this the cell repolarises to around -60mV before slowly starting to depolarise for the process to begin again.
what is the action porential length of skeletal muscle?
~2 msec
what is the action potential of cardiac musle?
~250 msec
what cause resting membrane potential in non-pacemaker cells?
high permeability to K+
what causes the initial depolarisation in non-pacemaker cells?
increase in permeability to Na+, caused by synchronized depolarisation of other cardiac cells and
what causes the plateau in the membrane potential of non-pacemaker cells?
caused by the increased permeabilty of the membrane to Ca2+ caused by the opening of voltage gated L-type Ca2+ channels
also caused by a decrease in potassium permeability by closing of voltage gated potassium channels
what causes the repolarisation in the membrane of non-pacemaker cells?
a decrease in the permeability of the membrane to calcium ions by the closing of the voltage gated type-L Ca2+ channels
also an increase in K+ with the opening of the
voltage gated potassium channels
what causes the pacemaker potential (pre-potential) in pacemaker cells
- a gradual decrease in the permeability to K+, this is caused by the closing of “leaky” K+ channels
- an early increase in Na+ permeability caused by the opening of PF Na+ channels which open in response to the previous action potential
- a late increase in Ca+ permeability by the opening of T-type Ca2+ channels
what allows autorhythmicity?
pacemaker cells
which pacemaker cells control the heart rate?
the ones with the fastest rate as they depolarise the rest of the cadiac cells before the others can
what factors affect electrical activity in the heart?
-sympathetic and parasympathetic systems
- drugs
temp.
-plasma K+ conc.
-plasma Ca2+ conc.
what do Ca2+ channel blockers do to heart rate?
decrease the force of contraction
what do cardic glycocides do to heart rate?
increase it, at a rate of about 10 beats per minute per degrees C
how does hyperkalemia affect heart rate?
causes fibrillation and heart block as cells depolarise in an abnormal pattern
how does hypokalemia affect heart rate?
causes fibrillation and heart block (not as expected, would expect repolarisation but actually causes depolarisation)
what is heart block?
AP can’t get from atrium through to ventricle
how does hypercalcemia affect the heart?
increased HR and force of contraction
how does hypocalcemia affect the heart?
decreased HR and force of contraction
where is the sinoatrial node located?
in the right atrium
what is the function of the sinoatrial node?
it is a pacemaker and contains the fastest depolarising pacemaker cells in the heart
what is the rate of depolarisation of the sinoatrial node?
0.5m/sec
what is the annulus fibrosis and is it conducting?
the layer between the atria and the ventricles, it is non-conductiong
what is the atrioventricular node?
it is the “delay box” that slows the movement of action potential through the heart as it is slow at conduciong
what is the conducting speed of the atrioventricular node?
0.05m/sec
which structures cause contraction of the myocardium of the ventricles?
the bundles of His and purkinje fibres
are the purkinje and bundles of His fast or slow conducting? and what is the conducting speed?
fast, 5m/sec
describe the wave of depolarisation in the heart during a beat
- pacemaker cells in sinoatrial node depolarise
- this causes depolarisation of the atria
- the atrioventricular node is depolarised
- the bundles of His and purkinje fibres depolarise
- this causes the depolarisation of the ventricular walls
can cardiac cells exhibit tetanus?
no
what does the P wave of an ECG correspond to?
atrial depolaristion
what does the QRS complex correspond to?
ventricular depolarisation
what does the T w ave correspond to?
repolarisation of ventricular walls
what does first degree block look like on an ECG?
time between P and Q is too long as there is impaired conduction between atria and ventricles
what does second degree block look like on an ECG?
time between P and Q increases until the ventricles just don’t contract and the cycle repeats
what does third degree block look like on an ECG?
there is no conduction between the atria and ventricles, there is no apparent association between P waves and QRS complex
what does atrial flutter look like on an ECG?
P wave is superimposed on T wave, HR >150bpm
what does ventricular fibrillation look like on an ECG?
P wave not discernable
