Flashcards in Resting + Action Potentials and Receptors (Chapter 15) Deck (45)
What do neurones transmit?
Electrical impulses, which travel very rapidly along the CSM from one end of the cell to the other
What are electrical impulses (signals)?
Brief changes in the distribution of electrical charge across the CSM called action potentials, caused by the very rapid movement of Na+ and K+ into and out of the axon
Describe the resting potential
- In a resting axon, the inside of the axon always has a slightly negative electrical potential compared with the outside
- ∴ the resting potential is the potential difference (the difference between the potential of the outside and inside)
What is normally the value of the resting potential?
-70 to -60 mV - ∴ the electrical potential of the inside of the axon is between 60 and 70 mV lower than the outside
What is the resting potential produced and maintained by and how do these work?
Sodium-potassium pumps (membrane proteins) in the CSM which constantly move Na+ out of the axon and K+ into the axon against their conc gradients, using energy form the hydrolysis of ATP
How do the sodium-potassium pumps work?
1) 3 Na+ are removed from the axon for every 2 K+ brought back in
2) the membrane has protein channels for K+ and Na+ which are open all the time - there are far more of these for K+ than for Na+
3) ∴ some K+ diffuses back out again much faster than Na+ diffuses back in
4) however, there are many large, negatively charged molecules inside the cell that attract K+, reducing the chance that they will diffuse out
5) the result of these effects is an overall excess of negative ions inside the membrane compared with the outside
Why does Na+ move into the axon during an action potential, despite the fact that the membrane is relatively impermeable to Na+?
The electrochemical gradient - a double gradient consisting of 1) a steep concentration gradient and 2) the inside of the membrane being negatively charged, which attracts positively charged ions
What happens when the axon is stimulated? (summary of action potential)
1) the p.d. across the CSM of the axon suddenly switches from -70mV to +30mV
2) it then swiftly returns to normal after a brief 'overshoot'
What is an action potential and how long does it take?
- A rapid, fleeting change in p.d. across the membrane
- The whole process takes roughly 3s
What is an action potential caused by?
Changes in the permeability of the CSM to Na+ and K+
What is a voltage-gated channel?
- Channels that allow Na+ and K+ to pass through
- They open and close depending on the electrical potential (voltage) across the membrane
- When the membrane is at its resting potential, these channels are closed
How does depolarisation occur in an action potential?
1) a stimulus causes the opening of a few voltage-gated channels in the CSM which allow Na+ to pass through
2) bc there is a much greater [Na+] outside the axon than inside, Na+ begins to enter through the open channels
3) this changes the p.d. across the membrane, which becomes less negative on the inside
4) this depolarisation triggers more channels to open so that more Na+ enter ∴ there is more depolarisation
5) if the p.d. reaches between -60mV and -50mV (threshold potential), many more Na+ channels open and the inside reaches a potential of +30mV
How does repolarisation occur in an action potential?
1) After roughly 1ms, all the Na+ voltage-gated channels close (∴ Na+ stop diffusing into the axon) and K+ channels open
2) K+ diffuse out of the axon, down their conc gradient
3) the outward movement of K+ removes positive charge from inside the axon to the outside ∴ making the p.d. briefly more negative than normal (hyperpolarisation) and then returning the p.d. to normal (-70mV) - this is repolarisation
4) the K+ channels then close and the Na+ channels become responsive to depolarisation again
Why does the Na-K pump continuously pump Na+ out and K+ in when the axon is at rest?
To help maintain the distribution of Na+ and K+ across the membranes so that many more action potentials occur
How do action potentials help to transmit information along a neurone when a stimulus is applied somewhere along an axon experimentally?
1) an action potential at any point in an axon's CSM triggers the production of an action potential in the membrane on either side of it
2) the temporary depolarisation of the membranes at the site of the action potential causes a local circuit to be set up between the depolarised region and the resting regions on either side of it
3) these local circuits depolarise the adjoining regions ∴ generating action potentials in them too
Why, in reality, do action potentials begin at one end and 'new' action potentials are generated ahead, not behind?
Because the region behind it will still be recovering from the action potential it has just had and the Na+ voltage-gated channels are hut tight and cannot be stimulated to open, however great the stimulus
What is the period of recovery where the axon is unresponsive called?
The refractory period
What does the existence of refractory periods mean?
1) action potentials are discrete events - they do not merge into one another
2) there is a minimum time between action potentials occurring at any one place on a nerve
3) the length of the refractory period determines the maximum frequency at which impulses are transmitted along neurones
What can you say about the size of the action potential?
1) action potentials do not change in size as they travel or according to the intensity of the stimulus
2) ∴ the action potential will continue to reach a peak value of +30mV inside all the way along
What can you say about the speed at which action potentials travel?
It does not vary according to the size of the stimulus
What is the effect of strong/weak stimuli on action potentials?
Action potentials have different frequencies resulting from strong and weak stimuli
What does a weak stimulus result in?
1) fewer action potentials per second
2) action potentials passing along just one or two neurones
What is the effect of a strong stimulus?
1) produces a rapid succession of action potentials, each one following along the axon just behind its predecessor
2) likely to stimulate more neurones than a weak stimulus
3) can produce action potentials in many neurones (more than one or two)
What can the brain interpret about action potentials and stimuli?
1) the frequency of action potentials arriving along the axon of a sensory neurone
2) the number of neurones carrying action potentials, to get information about the strength of the stimulus being detected
3) the nature of the stimulus is deduced from the position of the sensory neurone bringing the information
How much slower is the speed of conduction in unmyelinated neurones?
0.5 m/s vs 100 m/s
How does myelin speed up the rate at which action potentials travel?
By insulating the axon membrane
Explain how myelin speeds up the rate at which action potentials travel
1) Na+ and K+ cannot flow through the myelin sheath ∴ it is not possible for depolarisation or action potentials to occur in parts of the axon which are surrounded by the myelin sheath
2) ∴ action potentials can only occur at the nodes of Ranvier, where all the channel and pump proteins are concentrated
3) so the local circuits exist from one node to the next ∴ action potentials jump from one node to next (saltatory conduction)
4) this can increase the speed of transmission in a myelinated axon by up to 50 times that in an unmyelinated axon
What is saltatory conduction?
When action potentials jump from one node to the next
What affects the speed of transmission?
Diameter - thick axons transmit impulses faster than thin ones bc their resistance is much less