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Nervous System > Nerve conduction > Flashcards

Nerve conduction Flashcards

1
Q

extracellular concentrations of all the ions

A

sodium 140mM

potassium 4mM

Calcium 2.4 mM

Chloride 100mM

Bicarbonate 25mM

Hydrogen 40nM

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2
Q

Intracellular concentrations

A

sodium 5-25 mM

potassium 140mM

Calcium 0.1 μM

Chloride - less than extracellular

Bicarbonate 10-20mM

Hydrogen 50-100nM

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3
Q

Why does intracellular chloride not have a value?

A

Varies significantly between tissue

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4
Q

Membrane potential definition

A

An electrical potential difference between the inside of the cell and its surroundings

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5
Q

Explain the concept of gross electrochemical neutrality

A

The distribution of a single ion across the membrane is imbalanced, however the charge disparity is countered by an unequal distribution of an ion of an opposite charge

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6
Q

Explain the movement of potassium at resting potential

A
  1. 4mM outside the cell, 140 nM inside so there is a net diffusion of potassium ions through the potassium leak channels in the cell membrane
  2. However, the loss of the positive anions creates a charge separation, meaning that the intracellular matrix is more negative.
  3. This negativity pulls back potassium ions down their electrical gradient
  4. eventually reaches an electrochemical equilibrium, at -90mv
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7
Q

Electrical potential definition

A

The potential at which an equilibrium occurs where ion efflux is balanced by influx. E.g Ek = -90mV

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8
Q

What occurs to sodium ions at rest?

A

Sodium ion channels are shut, preventing the electrochemical diffusion into the cell

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9
Q

Nernst equation

A

RT/zF x Ln X+O/X+I

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10
Q

Explain what each symbol means

A 
R= Gas constant
T= temperature in kelvin
z= valency
F= Faraday's constant
X+O= ion concentration outside
X+I = ion concentration inside
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11
Q

What is valency? + an example

A

The charge so Ca2+ valency is 2+ whereas Cl- is -1

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12
Q

What temperature does it take place at?

A

37.5 degrees- body temp

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13
Q

What does the Nernst Equation calculate?

A

The membrane potential

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14
Q

How did Goldman vary Nernst’s equation?

A

took into account that the electric field would vary if there was a loss of ions across membranes

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15
Q

Requirements of the Donnan equation

A

Presence of an impermeant ion on one Side of the membrane.

osmotic balance

sodium is non diffusible

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16
Q

How was the impermeability of sodium disproved?

A

Radioactive iostope sodium-24 was injected, and thus could see travel of sodium in the frog’s sartorial muscle fibre. Shown that there was a steady gain of sodium

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17
Q

What counteracts this gain of sodium?

A

Sodium potassium ATPase pump

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18
Q

Explain the sodium potassium pump functions

A

pumps 3 sodium ions out and 2 potassium ions in, thus acts an electrogenic antiporter.

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19
Q

Apart from sodium and potassium ions, what else effects the resting potential?

A

Chloride ions

20
Q

How does the diffusion of chloride ions impact the resting potential?

A

Chemical gradient higher on the outside, so chloride wants to move in, making the membrane more positive. It’s electrical potential takes place at about -40mV, thus makes the resting potential less negative.

21
Q

What is the resting potential?

22
Q

How is an action potential generated?

A
  1. Stimulus occurs and exceeds a particular threshold. Potassium channels shut
  2. Voltage gated sodium channels open, allowing sodium channels to rapidly flood into the axon, depolarising it
  3. eventually becomes +42 mV, and sodium channels become inactivated.
  4. Delayed rectifier voltage gated potassium channels open, allowing potassium ions to flow out of the axon, repolarising it.
  5. Becomes too negative- hyper polarised.
23
Q

What is the refractory period?

A

A period of time in which an action potential cannot be generated?

24
Q

Two types of refractory periods + explained

A

Absolute refractory- all the voltage gated sodium channels are inactivated, so no action potential can form

relative refractory period- occurs during hyper polarisation. Action potential could form however would require a very strong stimulus to reach threshold.

25
Q

What form of feedback is the generation of an action potential?

A

Positive, more sodium ions, more voltage, more channels open.

26
Q

Threshold definition

A

The voltage at which an action potential is triggered. Occurs when the sodium influx exceeds background potassium efflux.

27
Q

Which ions can generate an action potential + differences?

A

Sodium- AP lasts around 1 millisecond

Chloride- AP lasts around 100 milliseconds

28
Q

How is an action potential propagated across an unmyelinated axon?

A
  1. Sodium influx in an active patch of membrane generates strong local depolarisation
  2. If the depolarisation is great enough to exceed threshold, all the voltage gated sodium channels open.
  3. adjacent regions become depolarised, allowing a current to flow, any ions can carry the charge within the axon, therefore the electrodiffusion can be mediated by any ion
  4. Behind the regions of depolarisation, sodium channels become inactivated and delayed rectifier potassium channels open, repolarising the membrane
29
Q

Difference between leak of charge and direction of action potential

A

Charge leaks in both directions. Both forward- orthodromic and reverse antidromic directions

Due to inactivated sodium channels, AP can only be propagated in the orthodromic direction

30
Q

structure of voltage gated sodium channels

A
  • transmembrane spanning protein
  • pore through centre enables ions to travel
  • narrow region acts as a selectivity filter
  • charged mobile voltage sensing region, formed of positive amino acids
  • activation gate coupled to voltage sensors controlling the opening and closing of the channel
  • inactivation particle serves to plug open channels
31
Q

cycle of voltage gated sodium channels

A
  1. closed
  2. voltage sensing region detects a change in PD, opening the activation gate -OPEN
  3. Change in PD detected, inactivation particle blocks pore.- INACTIVATED
  4. channel then must become closed again ti enable to be reactivated again
32
Q

All or nothing principle definition

A

Size of action potential is independent of the strength of the stimulus, as long as it is above threshold. If not above threshold no action potential is generated.

33
Q

conduction velocity definiton

A

The speed at which an electrochemical impulse propagates down a neural pathway

34
Q

Factors governing conduction velocity

A

fibre diameter, myelination and temperature

35
Q

Explain each of the factors

A

Fibre diameter- Internal resistance is inversely proportional to the axon cross sectional area, therefore the greater the diameter, the greater the velocity as less leak of ions.

Temperature- ions have more kinetic energy so move faster, membranes more fluid

Myelination- enable saltatory conduction

36
Q

Explain each of the factors

A

Fibre diameter- Internal resistance is inversely proportional to the axon cross sectional area, therefore the greater the diameter, the greater the velocity as less leak of ions.

Temperature- ions have more kinetic energy so move faster, membranes more fluid

Myelination- enable saltatory conduction and decreases membrane capacitance

37
Q

Explain saltatory conduction

A

The generation of an action potential is restricted to the nodes of Ranvier, Therefore the current flows from node to node, which activates the voltage gated channels only there. Amplifies the signal at each node

38
Q

Explain structure of Nodes of Ranvier

A

Clusters of potassium and sodium voltage gated channels so depolarisation can only take place there

39
Q

Myelinated vs unmyelinated nerve fibre sizes and conduction velocities- sensory neurones

A

myelinated- above 0.5μm- up to 120m/s

unmyelinated- below 0.5μm- 0.5-2m/s

40
Q

Explain compound action potentials in peripheral nerves

A

Occurs in efferent neurons.

Idealises the summation of a group of almost simultaneous action potentials from several muscle fibres in the same area.

41
Q

Two electrical constants

A

Time constant and length constant

42
Q

Explain time constant

A

T= RmCm, where Rm is the membrane resistance and Cm is the membrane capacitance

Defines how fast local current flow can depolarise the membrane.

Faster time constant means membrane ahead of impulse reaches threshold quicker, therefore faster conduction velocity

43
Q

Explain length constant

A

√ Rm/Ri. where rm is resistance of membrane and ri is resistance of inside

Longer length constant means that more distant areas of membrane ahead of the impulse can be depolarised to threshold, thus increases the conduction velocity

44
Q

Two ion channel blockers and their effects

A

Tetrodotoxin TTX- sodium channel blocker, inhibits action potentials forming and thus muscles contracting

Tetraethyammonium ions TEA- blocks autonomic ganglia, blocks voltage gated potassium channels

45
Q

How does TTX work?

A

Binds to site 1 of the voltage gated sodium channel, which is present in the extracellular pore opening.

46
Q

how does TEA work?

A

TEA binds externally, causing inactivation by a foot-in-the-door mechanism

Nervous System (9 decks)

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