INTS 8: Membrane Potentials Flashcards

1
Q

Why does the membrane potential exist?

Give an example of some common membrane potentials and their concentration inside and outside cells

A
  • the membrane potential exists due to the differences in concentration of ions on either side of the membrane
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2
Q

Define diffusion

A
  • the movement of molecules (or ions) down their concentration gradient until equilibrium is achieved
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3
Q

Define the electrochemical gradient

A
  • an electrochemical gradient shows an electrochemical potential difference
  • allowing an ion to move across a membrane
  • comprises of two parts:
  • the chemical gradient: difference in concentration across the membrane
  • the electrical gradient: difference in charge across a membrane
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4
Q

How do neuronal membranes allow ions to diffuse across the membrane?

A
  • neuronal membranes contain channels
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5
Q

Describe the properties of membrane channels

Describe the different types of membrane channels

Look at images

A
  • channels are proteins embedded in the neuronal membranes that are selectively permeable to one ion
  • they allow passive diffusion of ions down their electrochemical gradient
  • no energy is required
  • two types:
    1. voltage-gated: open in response to changes in the membrane potential
    2. ligand-gated: open in response to binding of a chemical messenger or ligand
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6
Q

What is the action potential and when and where can they be produced?

A
  • the action potential is a specialised potential
  • it can be produced in the neurons when the membrane potential reaches a certain level
  • which is called the threshold potential
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7
Q

What is the equilibrium potential?

A
  • when the diffusion potential (or driving force) balances or opposes the tendency of ions to move down their concentration gradients
  • the movement of ions across the membrane creates a diffusion potential
  • each potential is unique to each ion and is determined by a set of ion concentrations on either side of the membrane
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8
Q

Describe the general principle of the equilibrium potential for potassium (K+)

A
  • there are two compartments, inside and outside, (check image) with sodium and potassium ions and a concentration gradient
  • if a potassium channel is added, the potassium ions will be diffuse down the concentration gradient and move outside the cell
  • a net negative charge develops inside the cell
  • initially, the net force is diffusion down the concentration gradient, but as the inside fluid becomes more negative, the positive potassium ions are pulled back in by electrical force
  • eventually the electrical force counterblances the diffusion force
  • this is the equilibrium potential
  • electrical and diffusion forces are equal and opposite
  • no net movement of potassium ions into the channel
  • the charge difference between the two sides is the equilibrium potential and for potassium, it is approx -80mV
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9
Q

What is the resting potential?

A

-70mV (negative)

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10
Q

List the five phases of the action potential

  • how long does this take?
A
  1. Rising phase
  2. Peak phase
  3. Falling phase
  4. Hyperpolarisation
  5. Refractory period
    - this whole process is rapid and takes around 4ms
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11
Q

Describe how an action potential is generated, using Na+ as an example

A
  • when the permeability of the membrane to ions, such as sodium ions, changes, the membrane potential is altered
  • if the potential reaches the threshold, an action potential can be generated
  • this threshold is around -55mV, less negative than resting potential
  • this depolarisation occurs when sodium ion channels open and sodium ions move across the membrane into the cell
  • this depolarisation is rapid and the membrane potential eventually reverses to about +20mV
  • the inactivation of sodium channels prevents further influx of sodium ions
  • the opening of potassium ion channels allow efflux of potasssium ions out of the cell, repolarising the membrane potential
  • there is an overshoot when the membrane potential becomes more polarised than the resting potential and the potential eventually becomes resting state
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12
Q

Why does the refractory period occur?

Explain absolute refractory period and relative refractory period

Use sodium and potassium ions as an example

A
  • the refractory period is due to the inactivation of sodium ion channels and the speed with which potassium channels close
  • while the sodium channels are inactivated, depolarisation will not open them
  • when most of the channels are in this state, this is the absolute refractory period
  • following this period, enough Na+ channels are in the active state and they are capable of being activated by depolarisation
  • however, because the speed with which voltage-gated potassium channels close is slower, a large stimulus is required to induce a further action potential
  • this is relative refractory period
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13
Q

What are the distinct states of the voltage-gated Na+ channel?

When does each occur?

A
  • closed: when the membrane is at the resting membrane potential
  • open: during the depolarisation phase
  • inactivated: after initial opening, the channels rapidly adopt a special inactive conformation, in which they cannot open even though the membrane is still depolarised
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14
Q

Why does the pore of a channel (e.g. sodium ion channel) have an inactivated state when depolarised?

A
  • once the membrane is depolarised, the channel opens, but the inactivated conformation is more stable
  • so, after a brief period spent in the open conformation, the channel becomes temporarily inactivated and cannot open until the membrane is re-polarised again
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15
Q

Once an action potential has started to progress, can it travel backwards?

A
  • it has to continue in the same direction, travelling only forawrd from the site of polarisation
  • the channel inactivation prevents the advancing front of depolarisation from spreading backwards
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16
Q

What can increase the speed with which action potentials are transmitted along the axon?

A
  • the presence of myelin
  • it is wrapped around the axon
17
Q

Where is myelin produced?

A
  • glial cells
  • the Schwann cell in the peripheral nervous system
  • one cell can myelinate only a single axon
  • oligodendrocyte in the central nervous system
  • can myelinate many axons
18
Q

What are the unmyelinated portions of myelinated axons called?

A
  • node of Ranvier
19
Q

What diseases are loss of myelin a hallmark of?

A
  • some neurodegenerative diseases
  • e.g. multiple sclerosis
20
Q

Why do action potentials travel at great speed along myelinated axons?

What speed?

A
  • due to saltatory conduction
  • myelinated axons conduct speeds up to 120m/s
  • unmyelinated axons conduct speeds of up to 5m/s
21
Q

Study these images on myelinated and unmyelinated axons

A
22
Q

Examine this image of the classification of nerve fibres by their size and conduction velocity

A
23
Q

What determines whether the post-synaptic potential is excitatory or inhibitory?

A
  • the type of ion channel which opens on the post-synaptic membrane
24
Q

Describe excitatory post-synaptic potentials (EPSP)

A
  • these post-synaptic potentials bring the membrane potential closer to the firing threshold by making it less polarised
  • i.e. they increase the likelihood of an action potential being generated
  • these are mediated by the influx of Na+ ions
25
Q

Describe inhibitory post-synaptic potentials (IPSP)

A
  • these post-synaptic potentials take the membrane potential further away from the firing threshold by making it more polaised
  • i.e. they decrease the likelihood of an action potential being generated
  • these are mediated by the influx of Cl- ions
26
Q

What is temporospatial summation?

A
  • a single post-synaptic potential will not bring the membrane potential to the firing threshold
  • but the cumulative effect of many potentials may generate an action potential
  • this is temporospatial summation
  • it relies on receiving synapses being close together and also receiving input within a short time frame
27
Q

Give examples of excitatory neurotransmitters

A
28
Q

Give examples of inhibitory neurotransmitters

A