Medical Physiology - Guyton & Hall (ed. 13)
Summary of Chapters 5, 6 and 7. On action potentials and skeletal muscle conctraction.
Summary is in English. Used during year 2 theme 1 of Bachelor of Medicine at the RUG (in 2017).
Membrane potentials and action potentials
Medical Physiology: Chapter 5 (p. 61-73)
Calculating potentials
The Nernst potential for an ion is the diffusion potential of a membrane that is exactly as big as the
net diffusion of a particular ion through the membrane. The greater the ratio of the concentrations
inside and outside the cell, the greater the tendency to diffusion and therefor the greater the Nernst
potential.
The Nernst Potential (in mV) is calculated with the following formula at 37 °C:
Z is the electrical charge of the ion. The Nernst potential is the potential for inside the membrane. So
if the potential is positive then the positive ion diffuses from outside to the inside of the membrane.
The Nernst potential is the potential for one ion. If you want to measure the diffusion potential for
the whole membrane (and its different ions) then you have to use the Goldman equation.
C is the concentration. P is the permeability of the membrane for an ion.
A positive concentration gradient from inside the membrane to outside causes electronegativity
inside the membrane. That’s because the positive ions leave the inside of the membrane and leave
the negative ions inside. The negative ions can’t diffuse to the outside and make the inside of the
membrane negative.
Action potential
The propagation of an electrical signal happens in both directions of the stimulation.
● Resting stage:
The membrane is polarized: it has a -90 mV membrane potential.
There is more sodium (Na+) outside than inside the nerve. There is more potassium
(K+) inside than outside the nerve. They leak to the opposite side but are pumped
back through a Na-K-pump that needs ATP to function.
● Depolarization stage:
The inside of the membrane becomes less negative. If it reaches a certain threshold (-
65 mV) it will cause the voltage-gated sodium channels to open. This causes the
inside of the membrane to become even less negative and this opens even more
sodium channels until they are all open (positive feedback channel). Due to the
inflow of sodium the membrane potential rises in the positive direction:
depolarization. There is an overshoot that causes the membrane potential to become
positive (+35 mV).
● Repolarization stage:
The sodium channels start closing at the same time as they open, the closing just
takes a bit longer than the closing. The potassium channels start opening at the same
time as the sodium channels but they are slightly delayed. This causes the opening of
the potassium channels to happen at the same time as the closing of the sodium
channels. The potassium ions diffuse to the outside while the sodium ions stop
, diffusing to the inside. This results in a normal resting membrane potential. This is
called repolarization.
Afterwards, the sodium and potassium need to return to their own spots. This
happens with the help of the Na-K-pump.
Absolute refractory period: period during which the nerve cannot be stimulated
again because it hasn’t repolarized enough.
Relative refractory period: the stimulus that is needed for the action potential is
higher than normal. It’s hard but it’s possible.
Plateau in some action potentials
Sometimes the membrane doesn’t repolarize immediately after depolarization. It stays at a plateau
just below its max depolarization. This prolongs the period of depolarization and is seen in heart
muscle fibers.
The plateau is caused by the slow-opening sodium channels (there are also fast-opening sodium
channels) that stay open longer than normally. Besides that, the potassium channels are also slower
to open than normal.
Rhythmicity
Rhythmical discharges are found in heart muscles, the peristalsis of the intestines and in breathing.
For spontaneous rhythmicity to happen, the membrane must be permeable enough to sodium ions.
This causes resting membrane potential to be -60 mV, which is not enough to keep the sodium and
calcium channels completely closed. Therefore, some sodium and calcium ions flow inwards. This
causes the membrane potential to rise in positive direction and more ions flow inwards. This
continues to happen until an action potential is generated.
Towards the end of the action potential, the membrane becomes permeable for potassium. This
leaves more negativity inside than it normally would and causes hyperpolarization. During
hyperpolarization, self-re-excitation won’t happen. But the increased permeability for potassium
disappears and the membrane potential increases again up to the threshold.
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