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Neuroscience (own deck) > Excitability of cells > Flashcards

Excitability of cells Flashcards

1
Q

what are the 3 general functions of neurons?

A
  • collect signals
  • integrate them
  • transmit/output them to produce a response
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2
Q

what does the nervous system do?

A
  • system of communication that allows can organism to react rapidly and modifiably to changes in the environment
  • electrical signals provide rapid, reliable and flexible means for neurons to receive, integrate and transmit signals
  • chemical messengers and receptors between and within cells provide more flexibility via inhibition
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3
Q

what are the 2 electrical properties of neurons?

A

Action potentials

  • all or nothing (fixed size)
  • travel in one direction
  • can’t summate
  • long distances
  • coded by frequency

Graded potentials

  • variable size
  • can travel in both directions
  • can summate
  • local signals
  • coded by amplitude/size
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4
Q

when is a neuron said to be at rest?

A
  • when it is not generating either APs or GPs

- when the inside of the neuron is more negative than the outside

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5
Q

what is the voltage of the membrane at resting potential?

A

-65mV

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6
Q

how is potential difference measured?

A
  • connect neuron to voltmeter
  • microelectrode filled with KCl is inserted into the neuron
  • another electrode made of silver chloride is inserted into fluid surrounding the neuron
  • there is no potential difference in the extracellular solution (0mV)
  • as soon as the microelectrode enters the resting cell, the voltage changes to between -65mV and -90mV
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7
Q

why do neurons have a resting potential?

A
  • selectively permeable membrane
  • unequal distribution of charged ions
  • physical forces
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8
Q

how is the membrane selective and how does it maintain unequal charge?

A
  • channels confer selectivity

- pumps assist unequal charge distribution e.g. sodium-potassium pump (3Na+ out, 2K+ in)

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9
Q

how is the membrane selective and how does it maintain unequal charge?

A
  • channels confer selectivity

- pumps assist unequal charge distribution e.g. sodium-potassium pump (3Na+ out, 2K+ in)

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10
Q

which 2 forces control flow of ions across the neuronal membrane?

A
  1. Diffusion:

2. electrical fields

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11
Q

how does diffusion effect ion flow?

A

Diffusion:

  • net movement from high conc to low conc
  • lipid bilayer provides barrier to diffusion to form concentration gradient
  • open ion channels provide route for ions to flow down conc gradient by passive diffusion
  • diffusion continues until equilibrium is reached
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12
Q

how do electrical fields effect ion flow?

A
  • opposite charges attract and like charges repel
  • because ions are charged, they form an electric current (measured in Amperes)
  • current flow depends on:
    • electrical potential (voltage)
    • electrical conductance (measured
      in siemens)
    • electrical resistance (ohms)

if no channels are open, conductance is 0

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13
Q

what are the key ion pumps?

A

K+/Na+ ATPase:

  • exchanges internal sodium for extracellular potassium against their conc gradients
  • requires ATP

Ca2+ pumps:

  • transports Ca2+ out of neurons to maintain low intracellular calcium
  • important as calcium is a signalling ion
  • changes in calcium conc is detected by proteins and used to control cell functions
  • high intracellular calcium is toxic so would kill neurons
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14
Q

why are ion pumps essential?

A
  • they set up ionic gradients across neurons

- they enable the existence of resting potential

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15
Q

what is the equilibrium potential (Eion)?

A
  • the membrane potential that would be achieved if the membrane were selectively permeable to that ion
  • established when the electrostatic force counteracts the diffusional force
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16
Q

what is the Nernst equation used to calculate?

A
  • the equilibrium potential (Eion) of an ion
  • RT = temperature
  • z = charge of ion
  • F = Faraday constant
  • log(ion conc outside/ion conc inside)
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17
Q

how is the Nernst equation limited?

A
  • it does not take into account the permeability of other ions
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18
Q

What is the equilibrium potential of a membrane at rest?

A
  • neuronal membrane is permeable to K+ at rest

- therefore the membrane potential is close to Ek

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19
Q

what are the properties of K+ channels and how do they set up ionic gradients?

A
  • have 4 subunits, each with 6 transmembrane domains
  • because membrane is highly permeable to K+ at rest, changes in K+ conc can have massive effects
  • increasing extracellular potassium causes a shift in Ek
  • this causes Ek to become more positive, causing depolarisation
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20
Q

what is the Goldman equation used for?

A
  • for calculating the real membrane potential by taking into account the permeability of other ions
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21
Q

how is information encoded in the nervous system?

A
  • action potentials

- graded potentials

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22
Q

what are the characteristic features of an action potential?

A
  1. rising phase: stimulus causes rapid depolarisation of the membrane
  2. overshoot: membrane potential is above 0mV (around +40mV)
  3. falling phase: rapid repolarisation of membrane
  4. hyperpolarisation: membrane potential becomes too negative
  5. undershoot: sodium-potassium pump returns membrane potential to resting
23
Q

how fast is AP generation and what is its max frequency?

A
  • 2 milliseconds to be generated

- max frequency = 500Hz

24
Q

how are APs generated?

A
  • a stimulus (e.g. activation of stretch receptor, or an excitatory neurotransmitter) cause sodium voltage-gated ion channels to open
  • influx of Na+ causes depolarisation of the neuron
  • if sufficient depolarisartion allows membrane potential to reach threshold, an AP is trigered
  • if the depolarising stimulus is prolonged, a train of APs will be generated
25
Q

what are the properties of APs?

A
  • transient, rapid and reversible change in membrane potential from - to +
  • different types of excitable cell may have different types of AP
  • all APs are the same size
  • they travel in one direction
  • they do not decrease in size as they travel down the axon
26
Q

how does change in membrane permeability cause an AP?

A
  • a neuron is depolarised when the permeability to Na+ is inccreased (Ena) - +62mV
  • if a cell at rest is Ek (-80mV), the net driving force on Na+ will be -80 - 62 = -142mV
  • large potential difference forms a large conc gradient which allows Na+ infux
  • causes rapid depolarisation
  • after 1ms, Na+ channels shut ad K+ current dominates, resulting in outward potassium
27
Q

what is the structure of voltage-gated Na+ channels?

A
  • 6 transmembrane domains
  • 4 other domains
  • one transmembrane domain has many positive charges due to it containing positive amino acids
  • when membrane is depolarised, amino acids undergo conformational change and move to the outside
  • this causes the opening of the pore so Na+ can move in
28
Q

How do Na+ channels open in response to depolarisation?

A
  • conc of charge near plasma membrane affects voltage sensors
  • if the charge reaches threshold due to depolarisation, conformational change is caused to open the channel
29
Q

How do Na+ channels become inactivated?

A
  • occurs in 1ms - why APs are so brief
  • membrane potential must repolarise so channels can be activated again
  • absolute refractory period
30
Q

what are delayed rectifying channels?

A
  • these are potassium channels which open slowly with a delay of 1ms
  • this is to rectify/restore the membrane potential
  • relative refractory period occurs while voltage-gated channels are open
31
Q

give examples of useful poisons involved in APs

A
  • Tetraethylammonium, TEA. K+ channels
  • Lidocaine Na+ channels
  • Saxitoxins, STX. Dinoflagellates (sp.) Na+
  • Tetrodotoxin, TTX. Puffer fish (sp.) (fugu). Na+ channels
32
Q

how are APs conducted along an axon?

A
  • spread of Na+ causes local depolarisation of neighbouring parts of the axon
  • although Na+ spreads in both directions, the Na+ channels behind the conduction are inactivated - refractory period
  • the channels ahead of the conduction are open, so AP travels down axon in one direction to the terminal
33
Q

in what manner are APs propagated?

A
  • in a non-decremental manner

- meaning axons can generate APs along their entire length

34
Q

how fast is conduction velocity?

A

10m/s

35
Q

what factors influence conduction velocity?

A
  • diameter
  • myelination
  • permeability of membrane
36
Q

how does diameter affect Cv?

A
  • resistance to flow of current is inversely proportional to the cross-sectional area of the axon
  • the larger the diameter, the faster the conduction
  • axons involved in life-threatening information tend to be large in diameter
37
Q

How does myelination affect Cv?

A
  • myelin is made by wrapping glial cells around axon membrane
  • prevent current loss along axon by increasing membrane resistance and space constant = increase Cv
  • forms nodes of ranvier = saltatory conduction
38
Q

how does permeability of membrane affect Cv?

A
  • conduction is slower if the membrane is discontinuous (holes in it)
  • it tkaes longer for sufficient Na+ to accumulate down the axon to reach threshold
  • leaky
  • amplitude of AP decreases as distance increases if the membrane is leaky
39
Q

examples of different conduction velocites:

A
  • smallest unmyelinated axons = 0.2-1.5um = 0.5-2m/s
  • most myelinated axons = 1-20um = 5-120m/s
  • squid giant axon (unmyelinated) = 100-um = 25m/s
40
Q

which is easier: myelination or increasing diameter?

A

myelination

41
Q

what is saltatory conduction?

A
  • breaks in myelination form nodes of ranvier

- nodes of ranvier have high conc of Na+ channels there, allowing APs to jump from node to node

42
Q

differences between axons and dendrites in conduction:

A
  • dendrites have voltage-sensitive channels but don’t usually produce APs
  • axons generate APs at axon hillock near the cell body
  • sensory neurons lack as many dendrites, and their axon hillocks are located at sensory nerve endings
43
Q

how are APs coded?

A
  • frequency of APs is dependent on size of the stimulus
  • the stronger the stimulus, the higher the frequency of APs
  • enables encoding of stimulation intensity
44
Q

what is the absolute refractory period?

A
  • there is a limitation to the frequency of APs that can be generated
  • no matter the strength of the stimulus, the neuron is incapable of generating another AP after one has just been produced
45
Q

what is the absolute refractory period?

A
  • there is a limitation to the frequency of APs that can be generated
  • no matter the strength of the stimulus, the neuron is incapable of generating another AP after one has just been produced
46
Q

what is the relative refractory period?

A
  • when another AP can be fired, but requires a stronger stimulus due to the threshold being raised
47
Q

what are graded potentials?

A
  • not all-or-nothing
  • can be excitatory or inhibitory
  • caused by opening of neurotransmitter-gated ion channels
  • opening/closing of K+ channels
  • can summate
48
Q

how can GPs summate?

A
  • they integrate info from multiple inputs
  • they add EPSPs and if it reaches threshold, causes AP
  • neurons can have 200000 synapses = lots of integration
49
Q

what is spatial summation of GPs?

A
  • when multiple EPSPs arrive at several separated neurons, the AP produced overall is stronger
50
Q

what is temporal summation of GPs?

A
  • relies on high frequency of APs in a short time, close to each other on one axon, to cause large depolarisation
51
Q

how can EPSPs be shunted?

A
  • if the membrane is leaky, EPSPs become dissipated
  • the shunting effect is caused by opening of selective cation channels which allow cations to leave the axon and decrease membrane potential
52
Q

what are electrical synapses?

A
  • pore between 2 neurons
  • rapid - immediate depolarisation of adjacent axon
  • can travel both ways
  • examples: retinal neurons, glial junctions, cardiac muscle, smooth muscle
53
Q

what are electrical synapses?

A
  • pore between 2 neurons
  • rapid - immediate depolarisation of adjacent axon
  • can travel both ways
  • examples: retinal neurons, glial junctions, cardiac muscle, smooth muscle

Neuroscience (own deck) (14 decks)

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