Action Potentials: Generation and Transmission

Question 1

What is an action potential caused by?

Answer 1

A fluctuation in membrane potential caused by a transient opening of voltage-gated ion channels (mainly Na+ and K+) which spreads, like a wave, along parts of the neuron.

Question 2

When do action potentials occur

Answer 2

After the membrane potential reaches a certain voltage along the threshold.

Question 3

What is the significance of action potentials

Answer 3

1. The frequency of action potentials encodes information (a language by which neurons communicate)
2. Are a key element of signal transmission along (often very long) axons.

Question 4

What are the three stages of action potentials

Answer 4

1. fast depolarisation
2. repolarisation
3. after-hyperpolarisation

Question 5

Label all four

Answer 5

1. stimulus
2. fast depolarisation
3. repolarisation
4. AHP

Question 6

What happens when the voltage threshold is reached

Answer 6

Sodium channels open and Na+ ions move into the cell along both the concentration and electrical gradient.

Question 7

Influx of Na+ slows down and stops in AP when

Answer 7

1. The inside potential becomes positive (moves towards Ena) and thus attracts Na+ less
2. Na+ channels inactive

Question 8

What happens at stage 1: fast depolarisation

Answer 8

Voltage-gated ion Na+ channels open very fast. PNa»»Pk

Question 9

What happens at stage 2: repolarisation

Answer 9

Na+ channels inactivate and voltage-gated K+ channels open. Pk»>PNa.

Question 10

What happens at stage 3: after-hyperpolarisation

Answer 10

Voltage gated K+ channels remain open for a while and then close. Pk»>PNa, then Pk»PNa.

Question 11

How do voltage gated Na+ channles activate and inactivate

Answer 11

1. RMP
2. depolarisation to threshold
3. after a fraction of millisecond, channel inactivation
4. back to resting state when membrane repolarises.

Question 12

Describe what it means when each action potential is an all or none event

Answer 12

The amplitude of action potenials is usually constant (~100 mV) and does not depend on the ‘stimulus’ as long as the stimulus is ‘suprathreshold’.

Question 13

How can APs be evoked?

Answer 13

Positive ions accumulate under the cathode. If depolarisation is high enough, Na+ ions can go in which inititates the AP. This allows Na+ in and back to resting.
If Na ions cause sufficient depolarisation to the adjacent part of the membrane. Voltage gated ion channels open, which generate another AP (threshold reached -> another AP).

Question 14

What are the two main paths of current

Answer 14

1. outside the axon from + to -
2. across the membrane and inside axon (this path only changes RMP)

Question 15

What happens when a current is generated as a circuit by an outside source and flows through the cell membrane from outside to inside.

Answer 15

Local hyperpolarisation at anode (MP becomes more negative)

Question 16

What happens when current flows from inside to outside

Answer 16

Local depolarisation (MP becomes less negative)

Question 17

How are APs generated ‘physiologically’ in CNS neurons

Answer 17

APs are first generated in the axon initial segment (axon hillock) which has the lowest threshold, and thus serves as the ‘trigger zone’ for APs.

Question 18

How is depolarisation to threshold evoked

Answer 18

By excitatory postsynaptic potentials (EPSPs) which spready mainly passively from dendrites.

Question 19

What happens once APs are generated

Answer 19

APs are transmitted actively along the axon, away from the cell body.

Question 20

What is ‘passive’ spread of current

Answer 20

1. (subthreshold) depolarisation at one region of the membrane
2. passive current flow (inside and outside the axon)
3. depolarisation of adjacent parts of the membrane

Question 21

Why does current only spread passively over a short distance?

Answer 21

Local (subthreshod) depolarisation induced by current injected into an axon by a glass microelectrode. Current quickly dissipates as it flows along the axon.

Question 22

Why is the conduction velocity slow in unmyelianted axons

Answer 22

AP must be regenerated at every point on the membrane. This takes tim and therefore conduction velocity is slow

Question 23

Why does AP propagate much faster in myelinated axons than unmyelinated>

Answer 23

Due to the insulating properties of myelin, there is less current dissipation as it flows along the axon.

Question 24

Structure of neurons with myelinated axons

Answer 24

Myelin sheath formed by oligodendrocytes in the CNS and Schwann cells in the PNS.
Myelination is discontinuous, interrupted at nodes of Ranvier.

Question 25

Why does myelination increase action potential conduction velocity

Answer 25

Increases speed of AP conduction by increasing the efficiency of passive spread, and the gact that APs do not need to be regenerated at every part of the cell membrane.

APs are generated only at nodes of Ranvier (current flows passively between nodes). Process called saltatory conduction.

Question 26

Why does AP conduct in only one direction under physiological conditions

Question 27

What does the PNS contain

Answer 27

• axons and cell bodies of sensory neurons
• axons of motoneurones
• neurons forming the ‘autonamic nervous system

Question 28

How are APs generated in sensory neurons

Answer 28

When stimulus acts of receptors in sensory neurons, it does not immeidately evoke APs
* first evokes a graded depolarisation known as the ‘receptor potential’
* the receptor potential spreads passively to more distally located ‘trigger zone’ where APs are generated.
* APs then spread along the axon towards the CNS

Question 29

What is the trigger zone

Answer 29

Where there is a high density of voltage gated ion channels

Question 30

Where is information about the strength of a stimulus coded in

Answer 30

In the amplitude of the receptor potential and the frequency of the APs.