Class: Kin 305
KIN 305 TUTORIAL #1 Questions
1) Why does a membrane potential exists in the cardiac myocytes.
Action potentials require the membrane potential in order to exist. The
membrane potential allows sodium ions to flow down the electrochemical
gradient, commencing the AP. The membrane potential also exists due to
different membrane permeability for potassium, sodium, and calcium ions.
The high permeability for potassium ions allows more potassium efflux
down a concentration gradient. This creates a membrane potential that is
closest to the potassium ion equilibrium potential (-90mV).
2) How much pericardial fluid do we have and why?
We have 50 mL of pericardial fluid. It is there to reduce friction between the
pericardium and the Epicardium of the heart
3) How thick are the ventricles and why?
The ventricle walls are 8-9 mm thick for the left ventricle; 3-4 mm for the
right ventricle. The left ventricle needs to be thicker because it needs to
pump blood out to various parts of the entire body, whereas the right
ventricle is only pumping blood to the lungs. Since the left ventricle must
pump blood to far greater distances, it endures more pressure and must be
4) What are the relative permeabilities of Ca, K, and Na and why?
K>>Na>Ca. The permeability for K is highest because we need a very
negative resting membrane potential. This is so that a lot of sodium ions can
flow into the cell, once voltage gated sodium ion channels are open, very
quickly to create an action potential.
5) What does the Nernst equation do and what are the major variables?
The Nernst equation calculates the equilibrium potential for given amounts
of intracellular, and extracellular ion concentrations for one particular type
of ion. The equation assumes that P(permeability) = 1, meaning that the
membrane is fully permeable to that ion. It also assumes that that ion is the
only present ion in the system. The major variables are:
V = valency of the ion type
T = temperature in kelvins
[X]o = extracellular ion concentration
[X]i = intracellular ion concentration 6) What is rectification as discussed in class?
Rectification is the correction of imbalances in a cell that were caused by an
action potential. Rectification involves mainly the efflux of potassium ions to
return the membrane potential back to resting levels. It also involves the
sodium potassium ATP pump that corrects sodium and potassium ion
concentration levels both inside and outside the cell.
7) What limits the overshoot in phase 0-1?
Decreased conductance of sodium ions due to closing h-gates. Ito (Transient
outward potassium ion current) allows K to flow outwards, also limiting the
overshoot. Relatively high potassium ion membrane permeability also limits
the overshoot. The IK1 channel allows potassium efflux as well, but it has
very minimal effect in the latter stages of phase 0.
8) What does TTX (tetradoxin) do and how does it change the shape of
the slow AP?
It doesn’t do anything! It only blocks fast Na channels in fast myocardial
9) What are the directions of the chemica