Heart And Cell Activation Flashcards
What are cardiac myocytes
They are specialised cardiac cells
What are cardiac myocytes composed of
They are composed of bundles of myofibrils
How big is one cardiac myocyte
25u in diameter and 100u in length
What are the myofibrils composed of in cardiac cells
Myofillaments
What are myofibrils in cardiac myocytes
They are distinct repeating contractile units called sacromeres
What are sacromeres composed of
They are composed of thick and thin filaments
What are the thick and thin filaments composed of
Myosin and actin
What do the interaction between myosin and actin provide
Provide some evidence to support the sliding filament theory
What is myosin
Myosin is a protein which makes up the thickl filaments in sacromeres they have 2 heads which are able to interact with the binding sites on actin
When myocytes die what do they release
Troponin
What is the clinical relevance of cardiac myocytes dying
When they die they release troponin, high levels of troponin in the blood can aids the diagnosis of a Myocardial infarction
What do they chemical and physical interactions cause
They cause the length of the sacromeres to shorten
What are the thin filaments of actin composed of
They are composed of tropomyosin and troponin which is a regulatory protein complex
What is actin
Actin is a globular protein made of a chain of repeating units which makes up 2 strands which become an alpha helix
What is found between the 2 strands of alpha helix
In between is a rod shaped protein- tropomyosin
How many subunits of troponin are there
3
What are the 3 subunits of troponin
Troponin-T
Troponin-C
Troponin-I
What does troponin-T do
This attaches to tropomyosin
What does troponin-I do
This inhibits myosin binding
What does troponin-C do
This is the bonding site of Ca2+
When calcium binds to the troponin what happens
This binding causes a conformational change to the troponin complex where troponin-I moves away from the myosin binding site allowing myosin to bind
When calcium is removed from the troponin complex what is unable to bind
Myosin
When the myocyte is stretched what does this cause
This causes an increase of ventricular preload which increases the force of the contraction
What is the membrane of the heart permeable to
K+ Ions
What is the membrane of the hear muscle cell determined by
It is determined only by ions that can cross the membrane
Is the membrane of the heart muscle cell positive or negative
Negative
Why is the heart muscle cell membrane negative
Because k+ ions diffuse onwards from a high concentration to a low concentration so anions are unable to follow which causes a build up inside the cell and generates a negative potential within the cell
What is the Nernst equation
E= E0-RT/ZEinQ
E= reduction potential
E0=standard potential
R=universal gas constant
T=temperature in kelvin
Z= ion charge
F= faraday constant
Q=reaction quotient
What goes in and out in the myocyte membrane pump
In= K+ ions
Out= Na+ and Ca2+
What type of transport does the myocyte membrane pump use
Active transport with the use of ATP
What does phase 0 show on the action potential graph
Rapid depolarisation due to Na+ ion inflow
What does phase 1 show on the action potential graph
Partial repolarisation which is caused by K+ ions out and Na+ ion inflow stops
What does phase 2 on the action potential graph show
A plateau, slow inflow of Ca2+ ions
What does phase 3 show on the action potential graph
Repolarisation
K+ ions flow out of the cell
Ca2+ ions flow into the cell
What does phase 4 show on the action potential graph
Pacemaker potential
Na+ ions in flow
Slow K+ ion outflow
What type of drugs are quinidine and procainamide
They are class Ia and they are Na+ ion channel blockers which when taken cause a prolonged conduction repolarisation
What type of drugs are lignocaine and what do they cause
They are class Ib drugs which are Na+ channel blockers which cause no effect to the conduction or repolarisation
What type of drugs are flecainide
This is a class Ic drug which is am Na+ channel blocker which causes a prolonged conduction when taken
What is a type of beta blocker
Sotalol
What does the drug amiodarone causes when taken
This causes prolonged repolarisation due to it blocking the K+ channels
How is action potential propagated
The action potential spreads over the cell membrane due to the positive charge from the Na+ ions affecting the adjacent cells which causes depolarisation via gap junctions
Where is the conduction of electrical activity fastest
Purkinje fibres
How fast is the electrical current in the purkinje fibres
4m/s
What normally determines the rate of the heart beat
Sinoatrial node
What is the resting membrane potential at the sinoatrial node
Resting membrane potential is around -55 to -60mV
How does the resting membrane potential increase towards the threshold
Na+ channels close changing the potential of the membrane
What does the atrioventricular node do
It transmits the cardiac impulse between the atria and ventricles, which causes a delay in impulse to allow for the atria to empty all the blood into the ventricles
What are characteristics of the AV node
There are fewer gap junctions and have smaller fibres then in the atria
What does rapid conduction of the signal cause in the HIS-purkinje system
Coordinated contraction of the ventricles
What are characteristics of the HIS-purkinje system
They have large fibres and have high permeability at gap junctions
What is a comparison of cardiac muscle and skeletal muscle
Cardiac muscle contraction lasts up to 15 times longer than that of a skeletal muscle due to the slow calcium channels which decrease permeability of membrane to the potassium after the action potential
Why does the heart muscle have a refractory period
To further the stimulation of the action potential due to the fast Na+ +/- slow Ca2+ channels close which inactivates the gates
What is the average refractory period for ventricles and what does it prevent
0.25s but is shorter for atria which prevents excessively frequent contractions
What does a refractory period allow
This allows for adequate time for the heart to fill
What is the relative refractory period
This happens after the absolute refractory period due to some of the Na+ channels are still closed and K+ channels still open so only strong stimuli can cause action potentials
What does sympathetic control cause in relation to the heart
Increased heart rate which is positively chronotropic
Increased force of contraction is positively inotropic
Increased cardiac output
When the sympathetic control increases the cardiac output how much is it increased by
Up to 200%
What dose parasympathetic control cause in relation to the heart
Decrease in heart rate which is negatively chronotropic
Decrease in force of contraction negatively inotropic
Increase in cardiac output
How much is the cardiac output decreased with parasympathetic stimulation
Up to 50%
What is sympathetic stimulation controlled by
Adrenaline
noradrenaline
Type 1 beta adrenoreceptors
Increased amount of adenyl cyclase in sympathetic stimulation causes
An increase of cAMP
Decrease in sympathetic stimulation causes
Decrease in heart rate and force of contraction which causes a decrease in cardiac function by 30%
What is parasympathetic stimulation controlled by
Acetylcholine and M2 receptors
What does M2 receptors cause
An inhibits adenyl cyclase reducing cAMP
Negative resting membrane potential is determined by
By Na+/K+ pump pumping out the K+ ions
What is depolarisation caused by
Rapid influx of Na+ ions and prolonged influx of Ca2+ ions slowly
What causes repolarisation
K+ ions exiting cells
What causes propagation of action potential
Positive Na+ ions depolarising adjacent cells via the gap junctions
What is excitation-contraction coupling
The contraction of the heart muscle requires the delivery of Ca2+ ions into the cytoplasm through surface ion channels which is then amplifies with NaCa
