HUBS192 - Lecture 9 - The excitable heart Flashcards
what are the 2 cells of the heart?
1) contractile cells
2) electrical cells
how many percent of cells of in the heart are contractile cells?
99%
what are 2 features of contractile cells?
- striated appearance
- high actin and myosin (for cross bridges)
how many percent of the cells in the heart are electrical cells?
1%
what are 2 features of electrical cells?
- pale striated appearance (due to low actin and myosin)
- low number of actin and myosin
what is the function of electrical cells?
move an electrical signal from cell to cell to cell as quickly as possible throughout the heart
what is the relationship between actin and myosin and the speed of an electrical impulse in contractile cells?
actin and myosin in the contractile cells slow down the electrical impulse
what does SAN stand for?
sinoatrial node
what is the function of the SAN?
the origin of the electrical signal in the heart
what starts at the SAN in terms of conduction?
depolarisation starts at the senatorial node (SAN)
what is the process of action potential propagation along the surface membrane of electrical and contractile cells?
1) depolarisation starts at the SAN as the electrical signal originates here
2) the electrical signal passes into the electrical cells initiating a heart beat
3) the signal is sent from cell to cell to cell throughout the heart as quickly as possible
4) sends the signal to the neighbouring contractile cells (in contact with the electrical cells) rapidly to initiate contraction
what is present in a contractile cell?
1) increased cytosolic Ca2+ level
2) cross bridge attachment
3) contraction
how is the signal from the electrical cells passed to neighbouring contractile cells?
through intercalated disks (ICD’s) that contain gap junctions that are between the electrical and contractile cells
what are the 2 features of intercalated disks (ICD) that allow the electrical signal to pass through?
1) through pores with low resistance to ionic current
2) that allow current flow between adjacent cells
can you pass an electrical signal through a contractile cell to another contractile cell?
yes, through gap junctions between contractile cells however this is less rapid
what is a functional syncytium?
millions of cardiac cells that behave as one because of the increased speed of impulse throughout the heart
what is the location of the SA node (sinoatrial node)?
bundle of cells on top of the right atrium
what is the SA node known as?
the pacemaker of the heart because the cells are highly specialised to let out a continuous and spontaneous release of electrical energy
what is the only thing the heart requires to perform a heartbeat without being connected to anything else in the body and how does this work?
the heart can function independently to being connected to anything as it only requires blood to provide the ions required and energy. This allows the SA node to be able to trigger the electrical events to make a heart beat therefore the SA node is the only thing required to perform a heartbeat without connection
what is the first step of the conduction pathway of the heart?
-begins at the SA node
the electrical impulse is sent in 3 different directions
1) first some of the electrical impulse is sent directly into the right atrium
2) secondly, sends some of the electrical impulse across the interatrial bundle and over to the left atrium
3) thirdly, through the internodal bundles that lead down to the AV node (atrioventricular node)
where is the interatrial bundle located?
above the 2 ventricles but in between the arteries up in the aorta area
what is the result for the first 2 locations where the electrical impulse is sent from the SA node?
causes the left and right atrium to contract first before the ventricles
what is the location of the AV node (atrioventricular node)
between the atria and the ventricles
what is the second step of the conduction pathway?
when the electrical impulse is sent to the AV node it collects the signal and holds it, briefly pausing the signal before sending it down the AV bundle (atrioventricular). This allows the the atrium to finish contracting simultaneously and then relax, allowing the ventricles to start contracting through the AV bundle
what is the importance of the brief pause of the electrical impulse?
without the brief pause the atrium and the ventricles would contract simultaneously at the same time and the blood flow would not be unidirectional
what is the AV bundle?
one stretch of cells and spilts into 2 halves (left and right), 1 for each ventricle
what is the third step of the conduction pathway?
the electrical impulse travels down through the septal wall and down to the bottom of the heart
what is the septal wall of the heart?
thick wall that separates the 2 ventricles
what is the fourth step of the conduction pathway?
where the electrical impulse is brought up to walls of the heart through the purkinje fibres from the bottom of the septal wall, triggering a simultaneous ventricular contraction
what is the function of purkinje fibres in the fourth step of the conduction pathway?
the purkinje fibres send the electrical impulse deep into the ventricular walls ultimately triggering a simultaneous ventricular contraction
what is the reason that the conduction pathway causes the ventricle walls to contract from bottom to top?
if the electrical impulse is bought down to the bottom and then contracts from the bottom and up to the top then the blood is moved most efficiently in the direction of the aorta to be ejected from the heart
what colour represents depolarisation?
yellow
what is depolarisation?
electrical signal arriving at some part of the heart triggering the heart to contract
what colour represents repolarisation?
red
what is repolarisation?
electrical signal that leaves an area of the heart initiating relaxation
what does the colour grey represent?
no electrical activity is occurring
what is the 1st step of the excitation and the conduction pathway?
the electrical impulse originated from the SA node arrives and spreads directly into the right atrium and over the interatrial bundle to the left atrium causing depolarisation
what is the excitation and the conduction pathway?
the combination of the how the electrical events cause the mechanical actions of the heart
what is the 2nd step of the excitation and the conduction pathway?
the 2 atria are now fully depolarised triggering simultaneous atrial contraction while the AV node briefly pauses the signal
what is the 3rd step of the excitation and the conduction pathway?
the atria repolarise as the electrical impulse leaves causing relaxation, while the AV node sends the electrical impulse down the AV bundles, down to the septal wall of the heart starting to go to the purkinje fibres
what is the 4th step of the excitation and the conduction pathway?
-atria are now fully repolarised
the ventricles are fully depolarised and ventricular contraction occurs where blood ejection occurs (isovolumetric ventricular contraction phase)
what is the 5th step of the excitation and the conduction pathway?
once around 60% of the blood has been ejected the ventricles begin to repolarise causing relaxation
what is the 6th step of the excitation and the conduction pathway?
the ventricles have fully repolarised and relaxed therefore the heart is ready to re-enter another cycle of the conduction pathway again
what is the function of an ECG?
the ECG detects changes in the electrical state of a chamber in the heart
what are the 3 components of an ECG reading?
1) P wave
2) QRS complex
3) T wave
what causes the P wave?
atrial depolarisation, initiated by the SA node
what causes the QRS complex?
ventricular depolarisation begins at apex combined with atrial repolarisation
what causes the T wave?
ventricular repolarisation begins at the apex
what does the purple line represent?
atrial pressure
what does the blue line represent?
ventricular pressure
what does the green line represent?
aortic pressure
what are the 2 sounds of the heart?
1) ‘lubb’
2) ‘dupp’
what causes the ‘lubb’ sound of the heart?
the sound of the AV (atrioventricular) valves shutting to prevent blood back flow into the atria
what causes the ‘dupp’ sound of the heart?
the sound of the SV (semilunar) valves shutting
