Electrical Activity of the Heart Flashcards
What is the thick muscle filament?
Myosin
What is the thin muscle filament?
Actin
How does muscle contract?
By actin and myosin filaments moving over each other
What surrounds muscle cells and what is its structure?
Sacrolemma It has deep invaginations (T tubules)
What is the big calcium store in the muscle cell called?
Sacroplasmic reticulum
Describe briefly what happens at the NMJ
AP reaches NMJ and causes an influx of Ca via Ca voltage gated channels This causes release of Ach into the extracellular space This causes an end plate action potential which travels along the T tubules and triggers the release of Ca from the sarcoplasmic reticulum
What is the action of Ca in the contraction of muscle?
It binds to troponin which pulls tropomyosin
When muscle relaxed myosin binding sites blocked by tropomyosin
When the Ca concentration reaches a certain concentration troponin pulls tropomyosin away to expose the myosin binding sites
What are the individual components of muscle called?
Myofibrils
They are composed of thin and thick filaments
Cardiac muscle acts as a functional synsitum - what does this mean?
Acts as one big cell
How are the cardiac muscle cells joined together?
Gap junctions
These are links in the cytoplasm that allow the spread of current (small signalling molecules travel through)- if one cell depolarises the other also depolarises
What feature allows the cardiac muscle to be a syncytium?
Intercalated discs
What are the intercalated discs composed of?
Gap junctions and desmosomes
(1 desmosome, 1 gap fjunction, 1 desmosome…)
What are desmosomes?
Strong phyiscal connections
How long is the contraction time of the heart?
250msec (long compared to 2msec in skeletal muscle)
Can cardiac muscle regulate the strength of contraction?
Yes (unlike skeletal muscle it can) - Ca released produces submaximal contraction so you can regulae the strength of contraction by controlling the concentration of Ca that enters the cell
Ca released does not saturate troponin so regulation of Ca release can be used to vary strength of contraction
Define twitch (of muscle)
Contraction caused by firing of one AP
Some of the cardiac muscle cells have a property which allows them to act as pacemaker cells - what is this?
Unstable resting membrane potential
What are the differences in how pacemaker and non-pacemaker cells depolarise?
Non-pacemaker cells are depolarised due to neighbouring pacemaker cells (but sit with steady RMP until that point)
Pacemaker cells spontaneously rise to threshold
NB may be a case that there is a spectrum of these cells as opposed to two distinct groups
What changes in the cardiac muscle cells allow depolarisation and repolarisation to occur?
Changes in their ion channels
What is the RMP of non-pacemaker cells like?
High resting permeability to potassium ions
The resting membrane potential is due to leaky K channels
What occurs in the initial depolarisation of non-pacemaker cells?
There is an increase in permeability to sodium (from neighbouring cells as voltage gated sodium channels open)
Then there is a plateau - where there is an increase in permeability to Ca (L-type channels (large/long type) get a lot of Ca in and decrease permeability to K)
What occurs during repolarisation of non-pacemaker cells?
Decrease in permeability to Ca and increase in permeability to K
What causes the pacemaker action potential?
An increase in permeability to Ca (slower depolarisation caused by L type channels)
First there is a pacemaker potential (pre-potential) - due to gradual increase in permeability to K, early increase in permeability to Na and late increase in permeability to Ca (T-type)
What is the function of the T-type Ca channels?
= tiny type
Only let a little bit of Ca in to help push over threshold
What are the modulators of electrical activity in the heart?
Sympathetic & parasympathetic
Drugs
Temperature
High or low K/Ca
What drugs impact electrical activity in the heart?
Ca channel blockers - decrease the force of contraction by targetting L type Ca channels
Cardiac glycocides - increase force of contraction (inc. amount of Ca released from internal stores)
How does temperature affect electrical activity in the heart?
Increases HR by around 10bmp/degree Celcius
How does hyperkalaemia affect electrical activity in the heart?
Causes fibrillation and heart block (reduced conc gradient for K so cell starts to depolarise –> fibrillation)
How does hypokalaemia affect electrical activity in the heart?
Fibrillation and heart block (anomalous)
How does hypercalcaemia affect electrical activity in the heart?
Increases HR and strength of contraction (when you open the Ca channels you will get more Ca flowing in and faster)
How does hypocalaemia affect electrical activity in the heart?
Decreases HR and force of contraction
Where are the fastest pacemakers?
SA node
The heart will always beat at…
The rate of the fastest depolarising cells
How fast do the cells depolarise in the SA node?
0.5m/s
What is the pacemaker of the heart?
SA node
What is the annulus fibrosis?
Non-conducting insulator between the atria and ventricles
What is the function of the AV node?
Delay box - slows down AP (to around 0.05m/s
Once APs are evoked, there is a wave of depolarisation that travels rather quickly/slowly
Slowly
What is the function of the bundle of his?
Electrical conductor
Conducts APs from the AV node to the purkinje fibres
What is the function of purkinje fibres?
Rapid contraction system (5m/s) provides conduction to ventricles at set pace
How is this activity picked up on an ECG?
An AP in a single myocyte evokes a v small extracellular (transmembrane) electrical potential - however, lots of small extracellular electrical potentials evoked by many cells depolarising & repolarising at the same time can summate to large extracellular electrical waves that can be picked up on the ECG
What does the P wave correspond to?
Atrial depolarisation
What does the QRS complex correspond to?
Ventricular depolarisation
What does the T wave correspond to?
Ventricular repolarisation
Draw the electrical conduction system of the heart
