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HUBS191 > The Action Potential > Flashcards

The Action Potential Flashcards

1
Q

Chemical ion gating

A
  • chemical signaling (neurotransmitters) bind to receptive site on the channel
  • this causes a change in shape opening the channel
  • ions can cross membrane driven by their chemical gradient
  • signalling molecule unbinds
  • ion channel closes by changing shape again and membrane permeability is reduced
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2
Q

Voltage ion channel gating

A
  • common in areas of excitable membrane, crucial in generation and conduction of action potentials and synaptic transmission
  • channels are gated by voltage across them
  • may be opened by a change in voltage, then to pass an inactivated state so the can not open again until the membrane has been repolarized
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3
Q

Mechanical ion channel gating

A
  • channels are gated in response to physical / mechanical forces
  • critical in triggering signals in response to touch and pressure
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4
Q

Ion channels -neuronal distribution

A
  • dendridites exist to increase surface area of the cell - chemically gated ion channels
  • collection of axons brought together too form nerves
  • voltage gated Na+ channels and voltage-gated K+ channels all the way down the length of the axon
  • voltage gated Ca2+ Channels at axon (nerve) endings
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5
Q

Local potentials

A

(Aka graded potentials) defined as the opening of ion channels in response to chemical signalling from a synapse
- can be excitatory (depolarisation) or inhibitory (hyperpolarisation)
- not actively propergated - magnitude decreases at site away from initiation

  • interaction of a neurotransmitter with a receptor on the neuronal cell body typically results in a localised change in voltage across the membrane
  • the magnitude of the voltage change is related to the strength og the stimulus
  • these events are therefore regarded as both local and graded
  • the magnitude of a local potential decreases with distance away from the site of its initiation
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6
Q

Local potentials - summation

A
  • the effect of local potentials on cell membrane potential is summed over both time (temporal summation) and space (spatial summation)

(Re check slides for time stuff)

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7
Q

Initial segment

A
  • the point where the axon joins the neuron cell body
  • this region of membrane has a high density of voltage-gated Na channels
  • site where action potential is generated
  • for action potential to be generated, the net voltage at this point (summed local potentials) must exceed a minimal depolarisation (typically 10mV) the threshold
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8
Q

Action potential - threshold

A
  • depolarising local potentials may result in opening of voltage-gated Na channels
  • Na-channel opening drives further depolarisation
  • if sufficient Na channels open (at the initial segment) the depolarisation reaches a point at which large numbers of channels open resulting in a sudden large increase in Na influx - THRESHOLD
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9
Q

The acition potential

A
  • a nerve action potential is a reversal of trans-membrane voltage that is completed in 2-3ms
  • all life experiences are dependent on these 3ms changes in voltage across the nerve cell membranes
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10
Q

Action potential definition

A
  • a very brief increase in the permeability of the membrane to NA (inside of the cell becomes positive) followed by termination of the NA permeability and brief increase in K permeability ( restores inside negative)
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11
Q

Top off the curve?

A

voltage gated Na channels inactivated
Voltage gated K channels open - K exit, MP becomes more negative

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12
Q

Action potential diagrams and steps

A

Learn this

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13
Q

Action potentials - refractory periods

A
  • Absolute refractory periods: no matter how large the stimulus, another AP cannot be generated
  • Relative refractory periods: an AP can be generated, but only in response to a very large stimulus
  • caused by VG Na channel inactivation
  • prevents AP propagating ‘ backwards’
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14
Q

Action potentials - proporgation unmyelinated axons:

A

(Look at lecture slide diagrams)
1. As an action potential develops at the initial segment, the membrane potential at this site depolarised to +30mV

  1. As the sodium intros entering at spread away from the open voltage-gated channels, a graded depolarisation quickly brings the membrane in segment 2 to threshold
  2. An action potential develops in segment 2, the initial segment begins repolarisation (and is a now refractory)
  3. As the sodium ions entering at segment 2 spread laterllau, a graded depolarisation quickly brings the membrane in segment 3 to threshold.
    - the action potential can only move forward not backwards , because the membrane at the initial segment is in the absolute refractory period of replarisation
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15
Q

Action potentials - conduction velocity

A
  • APS are transmitted along unmyelinated axons relatively slowly - approx. 1-5m/s
  • Given our large bikies and need for line axons, slow AP conduction is not adequate for all out needs
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16
Q

Basic structure of myelin sheath

A

Myelin sheath:
- multiple Schwann cell is in line

Noted of ranvier:
- gaps between myelin
- increase conduction velocity

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17
Q

Action potentials - propagation myelinated axons

A
  1. An action potential develops at the initial segment
    - local current brings depolaration to following segment (meets threshold Na flux —-> AP propagation
    - prior segment then repolarises

-myelin act is as an insulator
- meaning gaps between myelin (nodes of ranvier) are excellent conductors ——>
- depolarisation essentially ‘jumps’ from node to node = ‘saltatory conduction’
- resulting in increased conduction velocity (20-100m/s)

18
Q

• Know what ion channels are and the various ways they can be gated

A

• Recall chemical, voltage, and mechanically gated ion channels, with chemical and voltage gating being
especially important in generation and propagation of action potentials

19
Q

• Know what local potentials are, and learn their basic features

A

• Small, variable, non-propagated changes in voltage, localized to site of generation. Can be excitatory or inhibitory, and can add if they arise close together in space and time.

20
Q

• Learn what action potentials are, and how they are generated and propagated

A

• 2-3msec reversal in membrane polarity, typically arising by crossing threshold (point at which VG Na channels
open). Depolarisation due to Na entry, repolarization due to K exit. Conducted along axon by spread of local
current inside the axon resulting in opening of V-gated channels ahead of the wave of depolarization, causing
regeneration of the event. Unlike local potentials, APs do not vary in size and are often referred to as “all-or-
none”

21
Q

• Know and account for the two types of refractory period of the axon

A

• Absolute RP (no new AP can be generated) due to VG Na channel inactivation, relative RP when a very large excitatory stimulus at the initial segment may generate a second AP before the membrane is fully repolarised. RPs prevent the AP propagating “backwards”.

22
Q

Explain how the speed of conduction of the action potential can be enhanced by myelin

A

• Myelin prevents short range local currents across the membrane, so current spreads further along the inside of
the axon to depolarize membrane to threshold at next Node of Ranvier. Increase CV up to 20-100m/sec.

23
Q

Na and K inside / outside the cell

A

Na is high in the efc
K is low in the efc

24
Q

Local potentials

A

Interactions of neurotransmitters with a receptor on the neuronal cell body typically results in a localised change in voltage across membrane
- the magnitude of the voltage is related to the strength of the stimulus
- these events are therefore local and graded

25
Q

Dedridites exist to

A

Increase surface area of cell
- contain chemically gated ion channels

26
Q

The end of the axons contain:

A

Voltage gated calcium channels

27
Q

The local potentials are ______ A____ P______

A

Local potentials are not actively propergated
- the magnitude of local potential decreases with distance away from its sit of initiation

28
Q

Summination

A
  • the effect of local potentials on cell membrane is summed over both time (temporal summination) and space (spatial summination)
  • adds together if two active at same time- if lots are active at the same time
  • if lots are active at the same time, big voltage is produced and maybe bring to threshold.
29
Q

The initial segment

A
  • the point where the axon joins the neuron cell body
  • region has a high density of voltage gated Na channels
  • AP is generated here
  • for an AP to be generated, the net voltage at this point (summed local potentials) must exceed a minimal depolarisation
  • 10mV the THRESHOLD
30
Q

How threshold is reached

A
  • depolarising local potentials may result in opening of voltage- gated Na channels
  • Na channel opening drives further depolarisation
  • if sufficient Na channels opens (at initial segment) the depolarisation reaches a point at which large numbers of channels open resulting in a sudden larger increase in Na influx
  • threshold
31
Q

A nerve cell action potential is:

A

A reversal of trans-memebreane voltage that is completed in 2ms -1

32
Q

Action potentials

A
  • A very brief increase in the permeability of the membrane to Na (inside of the cell becomes positive) followed by termination of Na permeability and brief increase in K permeability (restores inside negative)
33
Q

Local potential

A

A graded depolarisation brings an area of exciteable memebreane to threshold

34
Q

At the peak (+30mV) of the refractory period

A
  • sodium channels close, voltage gated K channels open,
  • K ions move out of cell - replorisation begins
35
Q

Relative refractory period

A
  • voltage gated potassium channels begin closing
  • near threshold the voltage gated sodium channels beigong reactivating and the memebreane soon returns to its normal resting state
36
Q

Absolute refractory period

A
  • no matter how large the stimulus, another AP cannot be generated
37
Q

Relative refractory period

A
  • an AP can be generated, but only in response to a very large stimulus
  • caused by V.G Na channel inactivation
  • prevents A.P from proper sting backwards
38
Q

Propergation of action potentials

A
  • an action potential develops at the initial segment, the membrane potential at this site depolarises to +30mV
  • as the sodium ions entering at the initial segment spread away from the open voltage gated channels - a graded depolarisation quickly brings the membrane in segment two to threshold
  • an action potential develops in segment two (because the local potential bought it to threshold) - the initial segment begins to repolarise (and is now in refactory - to prevent backwards proporgation)
  • as the sodium ions entering at segment two spread laterally, a graded depolarisation quickly brings the membrane in segment 3 to threshold
  • the action potential can only move forward, not backwards, because the membrane in the initial segment is in the absolute refractory period of repolarisation
39
Q

Two types of summation

A

Temporal: arrival of stimulus immediately after each other resulting in addition of them together
- where SPS’s that occur in quick succession ADD up

Spatial summation: addition of multiple stimulus from multiple neurons to a single postsynaptic neuron
- where PSP’s from different ares of the input zone ADD UP

40
Q

Action potential propergation across myelinated axons

A

Action potential propergation across myelinated axon
- a local current produces a graded depolarisation that brings the axolemma at node 1 to threshold
- action potential develops at node one
- the initial segment begins repolarsation (refactory)
- local current produces a graded depolarisation that brings axolemma at node 2 to threshold

41
Q

What an action potential is, how they are generated and propergated

A

2-3ms reversal in membrane polarity, typically arising by crossing threshold (point at which VG Na channeks ion)
- depolarisation due to NA influx
- repolarisation due to K exit
Conducted along axon by spread of local current inside axon resulting in opening of V-gated channels ahead of the wave of depolarisation, causing regeneration of the event.
DONT VERY IS SIZE - ALL OF NOTHING

HUBS191 (36 decks)

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