READ ME / 1: Origin and conduction of cardiac impulses Flashcards
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What is the function of the heart?
Muscle pump - contracts to push blood through the circulation
The pumping of the heart is controlled by electrical impulses.
Where in the body are they generated?
Within the heart
What is autorhythmicity?
The heart generates its own electrical impulses - doesn’t require anything else to beat
The heart can beat without any external stimuli - what is this called?
Autorhythmicity
Which chamber of the heart do electrical impulses start in?
What is the specific area called?
Which type of cell is found in this area?
Right atrium
Sino-atrial node (SA node)
Pacemaker cells
What do pacemaker cells initiate and where are they found?
Heartbeat
Found in SA node of right atrium
Where is the sino-atrial node (SA node) found?
Right atrium
Which great vessel of the heart is the SA node closely related to?
Superior vena cava (SVC)
The SA node normally determines the heart’s ___.
rate
A heart controlled by the SA node is said to be in which type of rhythm?
Sinus rhythm
What is the arrow pointing to?
Sino-atrial node (SA node)
What is the membrane potential of a cell?
Difference in ion concentrations inside and outside the cell, generating an electrical current
Pacemaker cells in the SA node (have / don’t have) a stable membrane potential.
don’t have a stable membrane potential
What are the two changes in membrane potential seen in SA node cells?
- Spontaneous pacemaker potential - slowly increases towards threshold
- Action potential - rapid change in potential which occurs when the threshold is reached
What happens after the threshold potential of SA node cells is reached?
Action potential - generation of a nerve impulse
Name A and B.
A - pacemaker potential
B - action potential
Is the action potential of an SA nodal cell a depolarisation, hyperpolarisation or repolarisation?
Depolarisation
(Hyperpolarisation - going from negative to even more negative
Repolarisation - going from positive back to negative
Depolarisation - going from negative to positive)
The pacemaker potential is caused by three changes in ion concentration - what are they?
1) Decrease in K+ efflux (less potassium OUT)
2) Na+<strong> </strong>and K+ influx (sodium and potassium IN)
3) Transient Ca2+ influx (calcium IN through T-type channels)
During spontaneous pacemaker potential, sodium and potassium ions enter the cell causing slow depolarisation - what is this called?
Funny current
During the spontaneous pacemaker potential, calcium ions enter the cell through which type of channel?
T-type Ca2+ channels
During the pacemaker action potential, what causes the rising phase (i.e the depolarisation) and which channel is responsible?
Ca2+ influx caused by the activation of L-type calcium channels
(Remember the T-type channels are involved in the spontaneous part only)
Two events cause the falling phase (i.e the repolarisation) of a pacemaker action potential - what are these and which channels are responsible?
1) Inactivation of L-type Ca++ channels - no more calcium IN
2) Activation of K+ channels - causes K+ efflux - more potassium OUT
There are two kinds of calcium channel - what are they called and which processes are they responsible for?
T-type - involved in spontaneous pacemaker potential
L-type - involved in pacemaker action potential
Where is the atrioventricular node (AV node) found?
Base of the right atrium
At the wall between the right and left atrium, also called the inter-atrial septum
How are action potentials carried from the SA node to the AV node?
Conduction between cells via Gap junctions
What structures allow action potentials to travel between cells?
Gap junctions
Which node is found at the base of the right atrium?
AV node
Which node is found in the upper right atrium?
SA node
The AV node is the only point of electrical contact between which two heart structures?
Atria
Ventricles
Does the AV node transmit electrical impulses immediately?
No - there’s a delay
Vital so that the heart contracts rhythmically
___ systole precedes ___ systole due to delayed conduction in the AV node.
(systole = contraction of muscle and ejection of blood)
Atrial systole precedes Ventricular systole
Through which structure do action potentials travel across the interventricular septum?
Bundle of His
Name the sequence of structures along which an action potential travels before reaching the terminal nerves of the heart (and include the name of them!)
SA node
AV node
Bundle of His
Right and left bundle branches
Terminal nerves = Purkinje fibres
The Bundle of His and Purkinje fibres allow ___ spread of action potential to the ventricles.
rapid
How do action potentials travel through the ventricular muscle of the heart?
Cell-to-cell conduction via gap junctions
Apart from pacemaker cells, a different type of cardiac cell generate action potentials - what are they called?
Cardiac myocytes
i.e the main bulk of cardiac muscle
The resting membrane potential of cardiac myocytes remains at __mV until the cell is excited.
-90mV
Remember membrane potential of potassium is -90, membrane potential of sodium is +60
Cell is always trying to get rid of sodium via sodium-potassium pump, so resting membrane potential is usually CLOSER to potassium’s (i.e really negative)
What causes the rising phase of action potential (i.e depolarisation) in cardiac myocytes?
Fast Na+ influx
After depolarisation, the membrane potential of cardiac myocytes has changed from -__mV to +__mV.
- 90mV to +20mV
i. e potassium’s potential to sodium’s potential (because a ton of sodium has just entered the cell)
What happens in Phase 0 of the action potential in cardiac myocytes?
Fast Na+ influx causing depolarisation
This graph shows the membrane potential of ventricular muscle during an action potential - describe what happens to cause the numbered phases shown on the graph.
Phase 0: fast influx of Na+
Phase 1: closure of Na+ channels and transient efflux of K+
Phase 2: mainly Ca++ influx through L-type channels
Phase 3: closure of Ca++ channels and K+ efflux
Phase 4: resting membrane potential
In cardiac myocytes, the membrane potential is maintained near the peak of action potential for a few hundred milliseconds - what is this phase called?
Plateau phase
The plateau phase is a unique characteristic of which cell’s action potential?
Cardiac myocytes
What causes the plateau phase of action potentials in cardiac myocytes?
Which Phase of the action potential does it line up with?
Influx of Ca2+ through L-type channels
Phase 2
What causes the falling phase of action potential (i.e the repolarisation) in cardiac myocytes?
(Closure of L-type Ca2+ channels)
K+ efflux
Heart rate is mainly influenced by the _____ nervous system.
autonomic
(Autonomic nervous system supplies the MOTOR aspect of the internal organs and is split into the sympathetic and parasympathetic arms, lots more of this throughout the year so dw)
Which kind of autonomic nervous stimulation increases heart rate?
Sympathetic stimulation
(think “fight or flight”)
Sympathetic stimulation (increases / decreases) heart rate.
increases
Which kind of autonomic nervous stimulation decreases heart rate?
Parasympathetic
Parasympathetic stimulation (increases / decreases) heart rate.
decreases
Which cranial nerve gives the parasympathetic supply to the heart?
CN X
(Vagus nerve)
The vagus nerve (CN X) exerts a continuous influence on which node of the heart?
SA node
The vagal tone slows the heart rate from around 100bpm to its normal resting heart rate of __bpm.
70 bpm
What is the normal range for resting heart rate?
60 - 100 bpm
When a patient’s heart rate is found to be < 60 bpm they are said to be…
bradycardic
When a patient’s heart rate is found to be > 100 bpm they are said to be…
tachycardic
What are two effects of vagal stimulation on heart excitation?
1) Slows heart rate
2) Increases AV nodal delay
What is the post-synaptic parasympathetic neurotransmitter for the heart?
Which receptor does it act on?
Acetylcholine
M2 muscarinic receptor
Which drug is used to treat patients with extreme bradycardia and why is it effective?
Atropine
Competitive inhibitor of acetylcholine, reducing vagal bradycardia
Vagal stimulation decreases the frequency of action potentials - why?
It causes the pacemaker cell to hyperpolarise, so it takes longer to reach the same threshold potential to trigger an action potential
Vagal stimulation reduces the heart rate, so it is said to have a ___ chronotropic effect on the heart.
negative
Which parts of the heart are supplied by sympathetic nerves?
AV node
SA node
(to increase heart rate)
Myocardium
(to increase force of contraction)
What are the three effects of sympathetic stimulation on the heart?
1) Increases heart rate
2) Decreases AV nodal delay
(so the opposite of parasympathetic stimulation)
3) Increases force of contraction
(note that parasympathetics have NO EFFECT on contractility)
What is the post-synaptic sympathetic neurotransmitter for the heart?
Which receptor does it act on?
Noradrenaline
ß1 adrenoceptors
Sympathetic stimulation increases the frequency of action potentials - why?
Pacemaker cell depolarises, so it reaches threshold potential quicker, so more action potentials occur and heart rate increases
What is an electrocardiogram (ECG)?
“A record of depolarisation and repolarisation cycles of cardiac muscle obtained from the skin surface”
Graph of heart’s electrical activity which can be used to diagnose disease
___ tone dominates under resting conditions.
Vagal
to keep the heart rate between 60 - 100 bpm
As it increases heart rate, sympathetic stimulation has a ___ chronotropic effect on the heart.
positive
