Electrical properties of the heart pt.1 and pt.2. Flashcards
Differences between cardiac muscle and skeletal muscle?
true syncytium formed (IN SKELETAL) - where multiple muscle cells fuse together to form a larger muscle fibre (which will then have lots of nuclei) .
Then each of those skeletal muscle fibres is independent from the other one.
functional syncytium- don’t have fusion of cardiac cells as they’re all independent from each other. But connected electrically (gap junctions) and physically (desmosomes)
difference in action potential- in skeletal muscle v short and cardiac is longer
Function of gap junctions?
narrow channel running through the centre- which connects the cytoplasm of one cell to the cytoplasm of another.
Some small signalling molecules can pass through this channel.
But also so when one cell depolarises that depolarisation will then spread to its neighbour and it will depolarise as well.
Function of desmosomes?
they stitch all of the cardiac muscles together so that they contract as one big muscle
Describe what makes an intercalated disc?
gap junction then desmosome and repeat
Where is the actin anchored together?f
at the z lines
What is the membrane of the muscle cell called?
the sarcolemma
What are the invaginations in the muscle membrane called?
T tubules
What is the job of the sarcoplasmic reticulum?
job to act as a store for intracellular calcium
What is the reason for cardiac muscle action potential being longer than skeletal muscle?
as well as being mediated by voltage gated sodium channels, also have voltage gated calcium channels- open when the cell depolarises and we get calcium flooding into muscle cell
What two things is the calcium flooding into the muscle cell going to do?
depolarise the cell
bind to binding sites on the troponin and increase the no. cross bridges formed between the actin and myosin.
** this doesn’t happen in skeletal muscle as the release of calcium from the stores in the sarcoplasmic reticulum is enough to fully saturate the binding sites in troponin and you always get a full twitch contraction
Describe refractory period of cardiac and skeletal muscle?
skeletal muscle action potential short and refractory period short as well. Don’t have to wait long until muscle can be stimulated again.
What happens after latency period in skeletal muscle vs cardiac?
TETANIC CONTRACTION
full twitch muscle contraction and this lasts substantially longer than the length of the action potential.
Because of consecutive twitch contractions, they then can summate. - allows muscle to stay contracted
In cardiac-long refractory period and therefore wouldn’t be able to stimulate cardiac muscle again and get it to fire another action potential. NOT CAPABLE OF TETANIC CONTRACTION
Can cardiac muscle muscle modulate how much calcium is coming into the cell from outside?
yes. and will affect the no. crossbridges you get between actin and myosin
What is resting membrane potential of skeletal muscle cells?
-90mV
Non pacemaker vs pacemaker action potentials?
NON PACEMAKER - Stable resting membrane potential (-90mV)
- don’t depolarise until told to by neighbours.
When they do there is a then a very, very, rapid depolarisation phase and long plateau phase and they will eventually repolarise.
PACEMAKER-
don’t have a stable resting membrane potential.
Instead just spontaneously depolarise to their threshold of -40mV.
They’ll fire an action potential and once they’ve repolarise they can then spontaneously depolarise and then fire another action potential.
Describe the non pacemaker action potential?
at resting membrane potential of -90mV
(leaky k channels opened)
cell told to depolarise by neighbours and we get really rapid depolarisation phase
increase in permeability to sodium ions and voltage gated sodium channels are triggered to open (and calcium but these take longer to open)
depolarises cells to 20mv
sodium channels close and there is decrease in membrane potential
plateau phase: decrease in permeability of the cell to potassium and this is because some of leaky potassium channels close and increase in permeability to calcium through calcium channel (L-TYPE)
Calcium channels close and cell rapidly begins to repolarize (potassium ions flowing once again out of cell) to resting membrane potential
What does L-type mean?
large channels , long
Describe pacemaker action potentials?
pre potential part (pacemaker part)
leaky potassium channels close and cell depolarises and voltage gated sodium channels open (If)
late increase in permeability of cell to calcium ions by voltage gated calcium channels (T-type)
What does T type mean?
don’t open for very long
-T-tiny
tiny depolarisation
What triggers If voltage gated sodium channels to open?
triggered to open by repolarisation phase of previous action potential
What explains autorhythmicity?
pacemaker cells
What does sinoatrial node contain?
the fastest of pacemakers- pacemaker of heart
What after pacemaker cells in sinoatrial node depolarise to threshold first?
that depolarisation will spread cell to cell via electrical connections (gap junctions)
Wave of depolarisation spreading through atria and atria will contract
What is annulus fibrosis?
ring of non conducting tissues
prevent depolarisation immediately spreading from atria to the ventricles
What is atrioventricular node?
acts as a delay box
allows time for depolarisation of atria and to squeeze blood into ventricles before ventricles then contract
What is the right and left bundle of His and Purkinje fibres?
rapid conduction system
ensures coordination of contraction of heart
Describe the special conduction system’s stages?
pacemaker cells in the sinoatrial node spontaneously depolarising to threshold and then gradually throughout the atria
atria contract-pushing blood into ventricles
Then depolarisation spreads to the atrioventricular node and it allows atria plenty time to depolarise and contract before ventricles contract
After the AV node, the depolarisation spreads very rapidly through bundle of His and Purkinje fibres and you get ventricular contraction
then ventricles relax
How is electrocardiogram produced?
An action potential in a single myocyte evokes a very small extracellular electrical potential
However, lots of small extracellular electrical potentials evoked by many cells depolarising and repolarising at the same time can summate to create large extracellular electrical waves
These can be recorded at the periphery as the electrocardiogram
Describe ECG?
p wave (atrial depolarisation ) then QRS complex (ventricular depolarisation) and T wave (ventricular repolarisation)
What is normal ECG known as ?
sinus rhythm
RR interval of three squares, what is their heart rate?
large squares x 0.2 (their time)= seconds of beat
60/ seconds of beats =bpm
What is 1st degree heart block?
the PR interval has increased
(distance from p wave to r wave)
still getting conduction from atria to ventricles but is just taking longer
should be 0.2 s normally
What is second degree heart block?
PR interval gradually increasing and increasing until that conduction fails and we get a qrs complex being dropped
Occasionally that depolarisation isn’t spreading from the atria to AV node (Mobitz type 1)
What is third degree heart block?
no AV conduction
normal p waves
Why are there still QRS complexes on the trace in third degree heart block?
pacemakers in ventricle take over and cause it to depolarise and contract
How can complete heart block be treated?
fitted pacemaker in heart
What is atrial flutter? (disorders of rhythm)
atria depolarise and contract a lot faster than they should
many p waves before QRS complex
What is atrial fibrillation?
failure of pacemaker to spread wave of depolarisation through atria
uncoordinated contraction
no clear p waves and QRS complexes happening at random intervals
What is ventricular fibrillation?
uncoordinated contraction but this time in the ventricles
What does a defibrillator do?
depolarise all cells at same time
all enter refractory periods at same time#
gives sinoatrial node an opportunity to restart sinus rhythm
