Cardiac Cycle- Electrical function of the heart Flashcards
Describe how an action potential is generated and propagated in cardiac muscle.
- Generation: (same as skeletal) pacemaker cells which possess automaticity = spontaneously depolarise and generate Ads
- Propagated (differently) from cell to cell via:
a) branching of the cells
b) cell:cell communication through intercalated disks which contain “gap junctions” which allow positively charged ions (cations) to move between cells to depolarise the next cell
What does the system of intercalated disks mean with regards to APs?
AP is transmitted from cell to cell v v v fast, enabling neighbouring cardiac muscle cells to contract as one unit –> functional SYNCYTIUM (acts as one cell)
Why is the SAN the pacemaker and where do they lie?
because these cells depolarise most quickly. Lie in walls of right atrium
What is arrhythmias?
a clinically serious abnormal rhythm: where other cells in the myocardium develop automaticity after injury = ectopic pacemakers.
What are 2 differences in action potentials in the cardiac muscle and action potentials in the skeletal muscle
- skeletal: AP initiated by somatic motor neurones and because skeletal muscle cells are insulated from each other, is limited to the cell stimulated. So when multiple cells contract, each must have received neurological stimulation to do so
- Cardiac muscle AP generated in absence of innervation
How is the heart innovated
By autonomic nervous system which affects frequency and speed of APs
- Sympathetic speeds up heart and inc force of contraction
- parasympathetic slows down and dec force of contraction
Describe the pathway electrical excitation follows in the normal mammalian heart
- AP generated in SAN
- propagated across atria (cell to cell 1m/s) causing near simultaneous atrial contraction
- Annulus fibrosus electrically insulates V from A
- Impulse reaches AVN where conduction is slowed down (0.1m/s)
- Gap between atrial and V contraction, enabling A systole to optimise V filling
- Impulse from AVN to bundle of His
- BoH impulse takes 2 paths through inter ventricular septum = left bundle branch and right bundle branch
- small offshoot of the right bundle branch crosses the lumen of the right V via the septomarginal band to the right V free wall.
- LBB subdivides into anterior and posterior fascicles to supply the thick wall of the left V
- L and R BBranches divide further into the branching PURKINJE fibres = v fast conduction to ensure V contract as a unit, starting at apex, finishing at base OF V
WHOLE PROCESS = 0.2 seconds
What is the name of the fibrous skeleton that insulates the A from V
Annulus fibrosus
What is the nae of the fibrous skeleton that supports the valves
Annulus fibrosus
Why is there a pause of 0.1-0.15 m/sec when impulse reaches AVN
Fibres in AVN are narrower
Where is the SAN located
In the Free wall of the right atrium
Explain how the pacemaker potential is generated.
by slow inward current of sodium through “funny” Na channels.
K+ kept inside cell
Where does the bundle of his travel
down and into proximal inter ventricular septum
Are purkinje fibres nerves
NO = modified cardiomyocytes
What does eptopic mean
In an abnormal location
What does eptopic pacemaker mean
pacemaker In an abnormal location
What ahs the potential to be more damaging and why? an atrial ectopic pacemaker or ventricular
Atrial = AP still follows same pathway through ventricles so still good V contraction Ventricular = more damaging as AP can go in wrong way = affect CO
What does ECG stand for and what does it record. what does it plot
Electrocardiogram.
Records electrical activity of the heart by comparing voltage at a positive electrode with voltage at negative electrode.
Electrodes placed on skin surface (fore and hind limbs)
Plots a graph of voltage (Y axis) against time (X axis): how much pot difference (amplitude) was there and how long did that last for
What are the ECG waveforms?
P = atrial depolarisation (atrial repolarisation = not seen = lost in other waves.
QRS complex = ventricular depolarisation
T = ventricular repolarisation
What does the height and width of deflection indicate in ECG
H = The size of the electrical potential difference W = time taken for membrane potential difference to return to 0
