Origin of the heartbeat Flashcards
The heart muscle is composed of two major cell types - what are these?
- contractile cells (majority of heart cells) = straited
- non-contractile cells with unstable membrane potentials (autorhythmic cells) = aid in electrical conduction
Membranes of non contractile cells depolarises slowly until it reaches a threshold value to generate what?
- generates an action potential
Once an action potential has been generated (after slow depolarisation of non-contractile cell membranes) what happens to the action potential?
- Action potential spreads through cellular gap junctions causing contraction of contractile cells
Where is the primary pacemaker region?
- the sinoatrial node (SA node)
The SA node has the fastest rate of depolarisation why?
- sets pace of heart
Pacemaker activity can be regulated by what?
- can be regulated by autonomic NS
What does the heart not need to contract?
- doesn’t need nerves to contract as it has an intrinsic contraction
What is the cardiac skeleton?
- fibrous connective tissue NOT bone
What does the cardiac skeleton provide?
- provides rigidity and site of attachment for musculature and values
The cardiac skeleton electrically insulates each chamber meaning it needs what for AP propagation?
- wiring for AP propagation
What is the wiring for the AP propagation?
- the conduction system
The SA node spontaneously depolarises - resting heart rate does what between species?
- varies
Autorhythmic cells form a specialised conduction system what does this system allow for?
- faster conduction than possible through gap junctions
- introduce an important delay from atria to ventricles
List the order of conduction through the heart?
- SA node
- Atrial muscle
- AV node
- Bundle of HIS
- Purkinje fibres
- Ventricular muscle
The membrane of the SA node is what?
- unstable
How does the SA function as a pacemaker - describe what happens within the node?
- gradual and slow depolarisation of the membrane potential
- reaches threshold potential triggers rapid depolarisation (action potential)
- action potential in SA node terminated
- gradual depolarisation and drift back towards threshold potential
- these repeated slow depolarisations are called pacemaker potentials
Describe the pattern of a membrane potential (this would be shown on a graph):
- Gradual depolarisation (becomes more positive)
- peak of depolarisation and termination
- repolarisation (becomes more negative)
- starts again
Cell membranes keep ions at different concentrations internally vs externally.
A difference in charge across the membrane does what to it?
- polarizes it
Most animal cells have a stable … what resting membrane potential?
- a stable negative resting membrane
= inside is negative compared to the outside
If ions were allowed they would diffuse .. their concentration gradient
- if ions were allowed they would diffuse down a concentration gradient
if more ions were diffusing in then out of a cell what would happen?
- depolarisation
If more ions were diffusing out a cell than in what would happen?
- repolarisation
What would sodium moving into the membrane do?
- depolarise
Ca moving in would do what to the membrane?
- depolarise
Potassium moving out would do what to the membrane?
- repolarise
- What do leaky f channels allow sodium to do?
- allow sodium to diffuse down its concentration gradient = slow depolarisation
= if or funny current
Are cells in the pacemaker impermeable?
- not impermeable
- As the cell depolarises, f channels close but what happens to continue the depolarisation?
- ca channels open continuing depolarisation
- When the membrane reaches threshold, another type of ca channel opens allowing for what?
- rapid influx - steep depolarisation (the AP)
- At the peak of the depolarisation, ca channels close and what happens next?
- slow potassium channels open = slow repolarisation
- When repolarized, the potassium channels shut and what happens next?
- f channels re-open, starting the next pacemaker potential
Spontaneous depolarisation isn’t unique to SA node - what has the second highest rate of depolarisation?
- The AV node
These are other autonomic foci with their own intrinsic rate - what are these?
- atrial foci (60-80 bpm)
- junctional foci ( 40-60 bpm)
- ventricular foci (20-40 bpm)
The SA node has the fastest intrinsic rate - so what does it do to others and what are the other important in?
- over rides others to set pace
- important in cases of arrhythmia
What does cardiac muscle look like?
- straited muscle that is involuntary
- sarcomeres with actin and myosin
- one central nucleus
- many mitochondria
- good blood supply
Cardiac muscle is a functional syncytium - how does its structure aid this function?
- branching cells connected by intercalated discs
- cells (myocytes) are coupled via intercalated discs
= electrically by gap junctions
= mechanically by desmosomes
Why does the myocardium need a continuous supply of ATP?
- to support contraction
What does branching of fibres in the heart help with?
- helps AP spread
Cardiac myocytes are driven - what does this mean?
- no slow depolarisation
- they wait for an action potential to reach them via gap junction
- the endpoint is contraction
What are the characteristics or stages of a membrane potential curve?
- stable resting potential (more -ve)
- rapid depolarisation - fast Na channels open
- notch - fast Na channels close
- plateau phase - ca2 enters (voltage- sensitive calcium channels) potassium permeability is low
- repolarisation - potassium leaves (K channels opens calcium channels close)
What is the process required for systole?
- action potential arrives
- plateau phase and ca2 entry
= directly raises cytosol ca2
= also mediates ca2 release from SR - binding to troponin, change tropomyosin and reveal myosin binging sites on actin
- contraction
What is the process required for diastole?
-Ca2 - ATPase (enzyme that breaks down ATP) pumps ca2 outside of cell and recycles to SR
- facilitated transport ca2/Na antiporter (exchanger) 3:1
Excitable cells have a refractory period - what happens here?
- they are unable to respond to new stimulus
In skeletal muscles what is the refractory period like?
- quite short
Why is it useful to have short refractory periods in skeletal muscle?
- enables new contraction to be initiated before the force of the old one has subsided = tetnay
- useful for posture, locomotion
In cardiac muscle the long plateau phase and timing of tension development is important.
Why do we need a relatively long refractory period in cardiac muscle
- cannot respond to further stimulation
- allows atria/ventricles sufficient time to empty and refill before next contraction
When the heart in systolic what happens within the chambers?
- they are emptying
When the heart is diastolic what is happening in the chambers?
- they are filling
What is an electrocardiogram (ECG)?
- measurement of electrical activity of the heart (difference of electricity)
Describe the contraction and filling of the heart:
- atrial contraction
- isovolumetric contraction
- rapid ejection
- reduced ejection
- isovolumetric relaxation
- rapid filling
- reduced filling
What are heart sounds associated with?
- associated with valves closing
What are S1 and S2 sounds?
- S1 - (lub) = closure of AV valves at (ventricular) systole
- S2 - (dub) = closure of semilunar valves at (ventricular) diastole
What are the other sounds of the heart that can also be heard?
- S3
- S4
Heart sounds depend on what?
- species
- age
What heart sounds do horses have and dogs and cats?
- horses have all 4 sounds
- cats and dogs usually just S1 and S2
Abnormal sounds (murmurs) can be due to what?
- leaky valves
What are the main features of an ECG?
- p-wave = atrial depolarisation
- QRS complex = ventricular depolarisation
- T-wave - ventricular repolarisation
On an ECG what does a PQ interval equal?
- AV conduction time
What cells are not reflected on an ECG?
- pacemaker cells
What can an ECG give info on?
- heart rate, rhythm, origin of excitation
- spread and decay of excitation
- anatomical orientation of heart
- relative chamber sizes
What can an ECG not give you info on?
- cannot give you info on contractile properties or pumping activity of the heart
