Guyton chapter 5 - Membrane Potentials And Action Potentials

Question 1

What is the Nernst potential for an ion?

Answer 1

The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane.

Question 2

When a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors - which are they?

Answer 2

1. The polarity of the electrical charge of each ion
2. The permeability of the membrane to each ion
3. The concentrations of the respective ions on the inside and outside of the membrane

Question 3

Nerve action potential - what happens in the depolarisation stage?

Answer 3

At this time the membrane suddenly becomes permeable to sodium ions allowing tremendous numbers of positively charged sodium ions to diffuse to the interior of the axon. The normal polarised state of -90 milli volts is neutralised by the inflowing positively charged sodium ions with the potential rising rapidly in the positive direction. This is depolarisation.

Question 4

What happens in repolarisation stage in nerve action potential?

Answer 4

Within a second after the membrane becomes highly permeable to sodium ions, the sodium channels begin to close and the potassium channels open more than normal. Then rapid diffusion of potassium ions to the exterior re-establishes the normal negative resting membrane potential. This is called repolarisation.

Question 5

There is a plateau in some action potentials and this is caused by a combination of several factors - which are they?

Answer 5

In heart muscles, two types of channels enter into the depolarisation process:

1. The usual voltage activated sodium channels called fast channels
2. Voltage activated calcium-sodium channels which are slow to open and therefore called slow channels.

Opening of fast channels causes the spike portion of the action potential whereas the prolonged opening of the slow calcium-sodium channels mainly allows calcium ions to enter the fiber which is largely responsible for the plateau portion of the action potential as well.

Question 6

Describe the process that is necessary for spontaneous rhythmicity:

Answer 6

1. Some sodium and calcium ions flow inward
2. This increases the membrane voltage in the positive direction which further increases membrane permeability
3. Still more ions flow inwards and
4. The permeability increases more and so on until an action potential is generated.
   Then at the end of the action potential, the membrane repolarizes. After another delay of milliseconds or seconds, spontaneous excitability causes depolarisation again and a new action potential occurs spontaneously. This cycle continues over and over.