Resting potentials and action potentials

Question 1

How do cells communicate?

Answer 1

• juxtacrine signalling
• paracrine signalling
• autocrine signalling
• neural signalling
• endocrine signalling

Question 2

What is juxtacrine signalling?

Answer 2

• A signalling molecule (ligand) attached to the surface of one cell interacting with a receptor molecule on the immediate neighbour cell.

Question 3

What is paracrine signalling?

Answer 3

• A signal molecule leaving one cell and interacting with a receptor molecule on the immediate neighbour cell

Question 4

What carries out paracrine signalling?

Answer 4

• local hormones such as prostaglandins and histamine

Question 5

What is autocrine signalling?

Answer 5

• the signal travels out of the cell and binds to the outer membrane

Question 6

Give an example of autocrine signalling

Answer 6

T cells produce IL-2, which bind to its own receptors, activating the cell

Question 7

What is neural signalling

Answer 7

• signalling between neurons

Question 8

What are some examples of neurotransmitters?

Answer 8

acetylcholine, noradrenaline

Question 9

What is endocrine signalling?

Answer 9

• a signal molecule leaving one cell and interacting with a receptor molecule on the immediate neighbour cell

Question 10

List the chemical communications from fastest to slowest

Answer 10

• juxtacrine = fastest
• neural
• autocrine
• paracrine
• endocrine = slowest

Question 11

Why is juxtacrine the fastest?

Answer 11

• constantly occurring when 2 cells are in contact

Question 12

Why is neural signalling faster than paracrine?

Answer 12

the synaptic cleft is very small

Question 13

What are ions?

Answer 13

Ions are charged particles, in this case, the single atoms (of salts) that have either kept or lost an electron when separated from their partners. Important ions in maintaining cell membrane potentials are sodium ions (Na+), potassium ions (K+), and chloride ions (Cl-).

Question 14

Why is the inside of the cell negative relative to the outside?

Answer 14

The inside of the cell is negative relative to the outside, even though the outside has more chloride. This is due largely to a high concentration of amino acids (A) inside the cell that are negatively charged.

Question 15

How does the sodium-potassium channel work?

Answer 15

The balance is constantly being reset by the sodium-potassium pumps, which expend ATP to actively reset the balance, pushing 3 sodium ions out for every 2 potassium ions in (both types of ions are being pushed against their chemical gradients).

Question 16

How do chloride channels work?

Answer 16

Chloride channels are more complicated, as there are many different types and they are not generally being used to maintain resting potential of a cell. We tend to learn about them when we discuss certain specific cell functions. One example is in the action of GABA (an important neurotransmitter). GABA opens ligand-gated chloride channels, changing post-synaptic membrane potential.

Question 17

What is electrical potential?

Answer 17

“Potential” is simply the potential to do work. When we talk about electrical potentials, we measure this in voltage. The potential to do work comes from the separation of charged particles. If we remove the barrier that separates them, they will flow. This flow can be used to do work. This is the basis for electrical appliances. It’s also the basis for your cell functions. You eat food to get the energy that you need to separate charges across cell membranes. Once these are set, the barriers can be removed to allow the electrons to flow and for work to happen.
If I stick two electrodes in a solution of ions, I will measure zero volts because the ions (saline bath) are not separated – their concentration is equal at each electrode.

Question 18

What is membrane potential?

Answer 18

If I stick one electrode in side the cell and the other outside, I will see a difference in their electrical charge and this is a potential (measured in millivolts). Because a cell is ‘normally’ in this state, we call it the “resting” potential. Different cell types have different resting potentials, but a typical value is around -70 mV. (NB: membrane potentials are measured from the inside out, by convention, so the outside is given the reference value of zero, while the inside is measured relative to this.

Question 19

How is the resting potential maintained?

Answer 19

It is maintained through ion gradients, particularly by the Na+ and K+ pump

Question 20

What are the different channels that respond to different stimuli?

Answer 20

• normal channel
• voltage-gated channel
• ligand-gated channel
• signal-gated channel

Question 21

What is a normal channel?

Answer 21

• always open

Question 22

What is a voltage-gated channel?

Answer 22

• opens in response to change in the membrane potential

Question 23

What is a ligand-gated channel?

Answer 23

• opens and closes in response to a specific extracellular neurotransmitter

Question 24

What is a signal-gated channel?

Answer 24

• opens and closes in response to a specific intracellular molecule

Question 25

How can voltage open a channel?

Answer 25

• Channels are made of proteins and proteins have charged regions. These regions may change shape if the voltage across them changes. In the case of a sodium ion channel, there are two factors that can stop ion flow: 1) is the dark blue region (in this diagram) that pinches off the channel when it is “closed”, 2) is a “ball and chain” region that can cover the channel when it is “inactivated”.

Question 26

What is an action potential?

Answer 26

An action potential can be broken down into several stages. When this is done in introductory-level biology books the activation of sodium and potassium channels are often shown operating at different times (as we see here). While these channels are, indeed, open at different times, it does not explain how different events can be occurring at the same voltage at different times. For example, at 0 mV on the left side of the peak, the sodium channels are open but, at 0 mV on the right side, they are closed. How would the membrane ‘know’ the difference if they are both set to occur at 0 mV?

Question 27

How would the membrane ‘know’ the difference if they are both set to occur at 0 mV?

Answer 27

The reason that this really works is that two types of voltage gated ion channels allow their ions through at different rates.
In each, the ions will flow down their particular electro-chemical gradient.

The action potential works because the sodium and potassium channels are initially affected at the same time, but the channels that allow Na+ through do so very efficiently. Therefore, the permeability (P) increases sharply – notice that, in this graph, there are two y-axes. The orange and green traces represent relative permeability while the red trace shows membrane potential.

The membrane at 55degrees is more permeable to sodium ions than to potassium ions

Question 28

How does an action potential take place?

Answer 28

Action potentials are initiated according to an all-or-none principle
•Either depolarization does not reach threshold = therefore no action potential
•or it reaches threshold and elicits an action potential that is always the same
•Threshold for an action potential is reached when the opening of voltage-gated sodium channels stimulates other channels to open in a positive feedback loop

Question 29

What is the absolute refractory period?

Answer 29

Absolute refractory period is when new stimulus can have no greater effect.

Absolute refractory period.
• Cannot open regardless of size of the applied stimulus
• lasts for 1-2 ms
• Determines maximum action potential frequency

Question 30

What is relative refractory period?

Answer 30

Relative refractory period is when the sodium channels can be reopened, but the PNa+ needs to exceed the PK+ to get another AP, so it is harder to do.

Relative refractory period.
• Before closure of K+ channels; PK+ is large
• Therefore, larger stimulus needed to make PNa+ > PK+
• Therefore, increased threshold
• This period lasts for 3-15 ms