The Action Potential (Prof. Pedarzani)

Question 1

How is information encoded in the Nervous System ?

Answer 1

As AP frequency (often codes for simulus intensity).

Question 2

What are the 4 phases of the AP ?

Answer 2

The rising phase = raid depolarization
Overshoot = part of the AP where the inside is positive w/ respect to the outside
Falling phase = rapid depolarization of the membrane until is is actually
Undershoot = hyperpolarisation of the membrane

Question 3

What is the consequence of Na+ influx (e.g. if stretch-activated Na+ permeable channels open) ?

Answer 3

The membrane is depolarized, which results in a generator potential (or graded potential).
Depolarization alone NEVER generated APs.
Applying increasing depolarization to a neuron only has a graded, local effect until it crosses the threshold.

Question 4

How do APs work ?

Answer 4

They are “all or none” : if the generator potential achieves a critical level, the threshold, the membrane will generate an action potential.

Question 5

What does the firing frequency of APs reflect ?

Answer 5

The magnitude of depolarization, and thus often the intensity of the stimuli.

Question 6

What will a neuron do if it is continuously stimulated beyond threshold ?

Answer 6

It will fire many APs in succession.

Question 7

What is the limit of AP fequency and why ?

Answer 7

About 1000Hz.
After each AP there is a refractory period during which the threshold is higher than normal. It is thus harder to generate another AP during this phase.

Question 8

What are the 2 phases of the refractory period ?

Answer 8

Absolute RP = aprox 1ms, threshold infinite

Relative RP = several ms, AP can be generated if stimulus is sufficiently strong

Question 9

How do the conductances for the different ions vary during the AP ?

Answer 9

During the rising phase, gNa+»gK+
During the falling phase, gK+»kNa+
During the undershoot gK+»Na+ but a little more than usual to restore the negative interior of the call. The increase in K+ conductance falls more slowly than the increase in Na+ conductance.

Question 10

How long do Na+ channels stay open before they inactivate ?

Answer 10

1 ms

Question 11

What is the accepted molecular model for the inactivation for the inactivation of the Na+ channel ?

Answer 11

The hinge and lid model (NOT the ball and chain model).
Basically, the “lid” is made of Ile, Phe and Met on one of the intracellular loops that swing on the movable “hinge”, the Gly, Gly pair.

Question 12

Why does the AP occur so quickly ?

Answer 12

Because of the rapid opening of Na+ channels in response to depolarization and their rapid inactivation.

Question 13

What is TTX (Tetrodotoxin)?

Why is it useful ?

Answer 13

TTX is a toxin that was isolated from the ovaries of the puffer fish that selectively block Na+ channels by clogging the pore upon tight binding to a specific site outside of the channel.
Ingested TTX = fatal
It is however useful to understand the function of specific Na+ ions channels, deduce their structure and discover different sites.

Question 14

Why does the AP not turn back on itself ?

Answer 14

Because of the refractory period (rapid inactivation of Na+ channels).

Question 15

What factors influence conduction velocity ?

Answer 15

Axon width and membrane resistance (if it has lots of leak channels or not).

Question 16

Why do humans not have giant axons ?

Answer 16

Because they would be too NGRetically costly. Instead, Schwann cells in the PNS and oligodendrocytes in the CNS myelinate the axons.

Question 17

Where are the ions situated on myelinated axons ?

Answer 17

At breaks in the myelin sheet called the nodes of Ranvier. The inter-nodal distance is usually 0.2-2mm. This allows saltatory conduction.

Question 18

Where does the AP initiated in a neuron ?

Does this place vary according to the type of neuron ?

Answer 18

At the Spike-Initiation Zone (SIZ), which os often located at the axon hillock (part of the neuron where an axon originates for the soma) e.g. pyramidal neurons
However, most sensory neurons have their SIZ located at the nerve ending

Question 19

How do we call propagation going rom SIZ to presynaptic terminal ? And vice versa ?

Answer 19

Orthodromic conduction

Antidromic conduction

Question 20

If AP always have the same amplitude, how is the graded nature of the input signals translated at the SIZ ?

Answer 20

Into a frequency code of APs.

Question 21

What does the duration of the stimulus determine ?

Answer 21

The number of APs.

Question 22

How can we know how much neurotransmitter will be released into the cleft ?

Answer 22

The total number of APs in a given period of time determines exactly how much neurotransmitter will be released by the cell.

Question 23

How can each neuron or type of neuron have a sepcific signature AP ?

Answer 23

The firing pattern of each neuron is determined by the prop, nb and subcellular distribution of many different ion channels.
Each neuron may express well over a dozen ions channels, and their complex interactions create the eclectic electric signature of each class of neurons.