Exam (elaborations)
Electrical events in the cardiac cycle.
- Course
- Institution
Electrical events in the cardiac cycle.
[Show more]
Seller
Follow
modockochieng06
Content preview
Electrical events in the cardiac cycleDescribe resting membrane potential, depolarisation & action potentials - ANS-The
membrane force at which the chemical & electrical forces are balanced & there is no net
movement of ions = equilibrium potential -->
K=-90mV
Na=70mV
Ca=100mV
Resting membrane potential = tends towards -90mV as most cells are far more
permeable to K than Na or Ca + pumps actively remove Na & Ca outside the cell
For depolarisation to occur the membrane potential becomes less negative so that a
threshold is reached & voltage-sensitive, ion-selective channels open = triggering an AP
--> membrane permeability to Na & Ca is rapidly increased, permeability to K is
decreased
At rest = K channels open, Na & Ca channels closed = membrane potential -90mV -->
when a stimulus enters & depolarises the membrane potential (AP from neighbouring
cell, chemical transmitter) = voltage threshold reached = K channels closed, Na
channels open first, then Ca channels
Depolarisation occurs = constant influx of positive ions increases membrane voltage to
+30mV
Describe the 3 intervals on ECG & implications if they are altered
1. PR interval
2. ST segment
3. QT interval - ANS-1. Atrial depolarisation to ventricular depolarisation = start of
P-wave to start of QRS complex (as Q often not visible) --> time for conduction of AP
from SA node to ventricular muscles
- normally 0.12-0.2 seconds (0.16 at normal HR)
- prolonged PR interval (0.2-0.35) = first degree heart block = electrical signal travels
through AV node slower than normal = impaired conduction at AV node
2. no change in electrical activity (phase 2 plateau) = flat segment between s-wave &
t-wave
- period when ventricles are fully depolarised = isoelectric = no current (plateau)
, - position relative to rest of ECG can indicate ischaemia (depressed) or infarction
(elevated)
3. ventricular systole = QRS complex to end of t-wave --> total time for which ventricles
depolarise to repolarise - varies with HR, age, gender
- should be 0.4sec at 70bpm
- if >50% = prolonged Q-T syndrome = predispose to ventricular arrhythmia, fibrillation
Describe the 5 phases of an AP in a ventricular myocyte - ANS-Phase 4 = resting phase
= myocyte at rest - membrane voltage -90mV --> K channels open (steady K efflux),
other channels closed
Phase 0 = depolarisation --> stimulus raises the membrane potential to -75mV (flow of
ions from gap junctions) = triggering fast voltage-gated Na channels to open & K
channels co close = rapid Na influx, K efflux ceases = rapid depolarisation to peak of
+20/30mV (all or nothing - if threshold isn't reached AP doesn't occur)
Phase 1 = initial repolarisation = fast voltage-gated Na channels close = depolarisation
finishes --> then voltage-fated K channels open = small K efflux (transient increase in
permeability to K) = small drop in membrane potential
Phase 2 = plateau phase (0.2sec) = slow voltage-gated Ca channels open = Ca influx
(counter-balances positive K efflux) = prolonged depolarisation (unique to cardiac
ventricular muscle) --> Ca influx initiates ventricular contraction
Phase 3 = repolarisation = slow voltage-gated Ca channels close (& any remaining
open Na channels close), K channels remain open = K efflux = net outward positive
current = decreasing membrane potential back to resting (-90mV)
some K channels close & the resting phase begins again
Describe the absolute & relative refractory period - ANS-Absolute refractory period
(200-250ms) = voltage-gated Na channels are inactivated & unavailable for another
depolarisation --> contraction cannot be stimulated regardless of stimulus strength
Relative refractory period (50ms) = some voltage-gated fast Na channels are activated,
some remain inactivated (proportion of ready channels increases overtime) -->
contraction can be stimulated by a higher-than-usual strength stimulus = as not the full
population of Na channels are available so a strong stimulus is required for
depolarisation
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller modockochieng06. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $7.99. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
75759 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy study notes for 14 years now