Electrical Properties of the Heart Flashcards
What do gap junctions do
Permit cell-cell conduction of excitation hence myocardial cells form a functional syncytium
=> ALL OR NONE CONTRACTION
What is the RMP due to
Increased concentration of K+ in IC fluid
Increased permeability of cell membrane to K+ as compared to other ions
What is the RMP of a cardiac cell
-90 mV which is close to the equilibrium potential for K+ (-94mV) but there is an inward background current of Na+
What is happening during plateau phase
Cell is ELECTRICALLY INEXCITABLE (absolute refractory period) for 200-400 ms - active contraction only lasts 200-250 ms
(fused series of contraction in cardiac muscle would be fatal)
What does the plateau phase do
Initiates contraction
AND influences strength of contraction
Ca2+ entry => stronger force of contraction
What is the relative refractory period
Period of gradual return -> full excitability (50ms)
m vs h gate of Na+ channel
m - activation
h - inactivation
Stage 4 of pacemaker AP = PRE-POTENTIAL/PACEMAKER POTENTIAL
Initial potential of SA node = -60/-70mV
Spontaneously and slowly becomes more +ve
Name of stage 0
Upstroke
Name of stage 3
Repolarisation
What is the decay of pacemaker potential over time caused by
- Na+ current (if) progressively depolarising cell
- Gradually falling membrane permeability to K+
=> ik decreases progressively allowing if to dominate
How is the AP triggered
threshold of -50 to -40mv is reached iCaT contributes to final 1/3 of pacemaker potential (channels are voltage gated)
Why is upstroke (0) slow in rising and small in amp
Cells of SA and AV nodes lack functionally fast Na+ channels and also slow ICa
Progressive depolarisation is opposed by iK
What is the late repolarisation (3) due to
iK
How are changes in rates of SA node discharge achieved
Primarily by autonomic nervous activity
What is a change in HR controlled by
Frequency of SA pacemaker discharge
Parasympathetic innervation of SA node
what does it do
Vagus nerve - decreases HR
decreases slope of pacemaker potential and hyperpolarises SA node => threshold reached later => HR slows
Sympathetic innervation of SA node
Cardiac nerves - increase HR
What happens to pacemaker potential following sympathetic stimulation
The slope of the pacemaker potential increases
Threshold is reached faster
Spontaneous rate of SA node firing increases HR
Noradrenaline is released from sympathetic nerve endings and binds to beta 1 adrenergic receptor
What conductances increase
GNa and GCa
therefore iCa and if increase
IC signalling of pacemaker potential
- NA binds to beta 1 adrenergic receptor
- causes an increase in conc of cAMP
- cAMP increases if, which is the inward Na+ current
- cAMP activates PKA which phosphorylates the Ca2+ channel and thereby increases iCa
IC signalling of hyperpolarisation
- ACh binds to its receptor (Muscarinic M2)
- activation of ACh-sensitive K+ channel
- Gk increases
- decreased IC conc of cAMP
- decreased effect of sympathetic stimulation
- decreased if, iCa => decreased slope of pacemaker potential
