Electrical and Chemical Signalling: Propagation of the AP and the synaptic transmission Flashcards

1
Q

how does resting state -> propogation of an action potention occur of an unmyelinated axon occur?

where is the trigger zone?

A

continous propogation:

trigger zone: axon hillock / aka axon initial segment

resting state: high conc of Na+ outside cell, extracellular env: +ve. -70mV inside cell

initiation: voltage gated Na+ channel opens: Na+ go to site A. creates a local zone in both extraceullar and intracellular fluid with a sudden change in charge.THEN go to Site B (which is an area of high conc negative charge) and sodium Na+ ions open here

propogation: Na+ at Site B attracted to -ve chrge –> go to site C. Also attracted to site A, but cant go back bc thats an absolute refractory period: only goes in one direction

propogation continues: Na+ go along site D etc. Site A is now region of repolarisation (new AP can occur)

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2
Q

what does myelin do?

what are gaps of myelin called?

what is concentrated at ^?

A

myelin: provides high resistance to ion flow across the membrane

gaps of myelin: Node of Ranvier: concentration of voltage gate sodium and potassium channels

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3
Q

how can we think of AP occuring as in myelinated axons? name for this?

(what is size of internode? node?)

A

AP jumps from one to node to the next: aka saltatory conduction

internode: 1.5mm
node: 1 um

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4
Q

why / how do action potentials jump down axon (myelinated?)

A

schwann cell insulates axon: no ions leak out of axon

at node of Ranvier: high conc of voltage gated sodium and k channels

when depolarisation occurs: Na+ in at Node of Ran. charge cell moves along unimpeded to next unmyelinated area: Node of Ranvier. electrical signals jump down much faster

repolarisation and hyperpol: open at delayed time at previous node of Ran

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5
Q

would you be able to have one long myelin sheath?

A

no - would get some charge dissapation. need nodes of Ranvier to top up the depolarisation effect

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6
Q
A
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7
Q

why do action potentials travel in one direction only?

what is the absolute refractory period of the AP?

A
  1. the membrane immediately behind (in direction of cell soma) is in the absolute refractory period
  2. this is the time in which a 2nd AP cannot be generated
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8
Q

what is relationship between current, voltage and resistance?

how does this relate to axons?

(dont fret about knowing too much i think)

A

V = IR

  • current flowing along axon can leak across the cell membrane or travel through the cytoplams depening on path of least resisitance
  • resistance decreases with increased diameter

- larger axons conduct faster

- Membrane resistance is increased with insulation (myelinated fibres have greater membrane resistance due to insulation). for the same fibre diameter as unmyelinated, myelinated have greater conduction velocity.

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9
Q

what myelinates axons in a) PNS b) CNS?

A

a) Schwann cell: PNS
b) Oligodendrocytes: CNS

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10
Q

why do we have different conduction velocities and how do they achieve this?

A

fastest conduction velocities have widest axon diameter and myelination:

(size goes down in order - see photo)

Aα fibres are the fastest: motor neurons, muscle spindles, golgi tendon organs

Aβ fibres: touch / mechanical pressure:

Aδ fibres: pain (fast), temp

C fibres: pain (slow), temp and itch. unmyelinated

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11
Q

how does schwann cell form myelin sheath around axon ?

for awareness probs

A

wraps around around around

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12
Q

what are the two different types of synapse?

A
  1. electrical
  2. chemical: (could be thought of as electrochemical) converts electric signal into chemical singal.
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13
Q

how does chemical synapse work?

A
  • AP passes down axon: depolarisation of presynaptic terminal.
  • Ca2+ voltage gated channels open
  • increase in cellular Ca2+ is the trigger to release synaptic vesicles
  • Synaptic vesicle bind to membrane and release neurotransmitter across synaptic cleft
  • NT binds to ionotropic or metabotropic receptors (change from closed to open channels)
  • change in membrane potental (hyper or dep) of post synatpic terminal

ONLY ONE DIRECTION

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14
Q

how does electrical synpase work?

A
  • AP in presynaptic terminal goes through gap junction channel
  • coupling potential

BOTH DIRECTIONS

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15
Q

what else is present in presynaptic cleft of chemical synapses?

A

mitochondria ! energy required

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16
Q

what makes it easier for Ca2+ to diffuse across into presynaptic cleft?

A

calcium concentration outside of cell is 10 000x time higher than inside - large inward concentration and electrical gradient allows free calcium to pass into cell

17
Q

what is the process called when NT bind to presynaptic cleft?

what is ^ mediated by?

A
  1. = docking
  2. docking is mediated by SNARE complex - ensures membrane fusion of vesicle
18
Q

what is the process of that NT move across synaptic cleft?

A

endocytosis

19
Q

what is the change in signal like at presynaptic cleft?

what does this mean for what happens on post synaptic cleft?

A

- at presynaptic terminal: AP is converted into an analogue signal (chemical NT)

  • at post synaptic cleft: different NT cause excitation or inhibition (depending on the receptor on post synaptic neuron)
20
Q

how do different excitatory or inhibitory NT work together / agaisnt each other on post-synaptic cleft.

what is the name for this?

A
  • analogue (aka graded) potentials can arrive at trigger zone together and their sum create a suprathreshold signal to cause an AP

OR

  • could get one inhibitory and two excitatory -> summed potentials are below threshold so no AP !

addition of excitatory and inhibtory signals = summation

21
Q

name 4 important NT that are collectively called monoamines

where each expressed?

A

adrenaline: fight or flight. PNS

noradrenaline: concentration and arousal. PNS. undersupply can depress mood

dopamine: pleasure, learning attention and momevent. CNS. XS: schizophrenia. depleted: Parkinsons

serotonin: mood, hunger and arousal. CNS and gut. depression

= collectively known as monoamines

22
Q

what is acetylcholine involved with? with NS?

A

CNS: memory and learning

PNS: neuromuscular junctions (muscle action)

involved in the preganglionic

deteriote in AD

23
Q

name three amino acid NTs?

name a peptide NT

A

amino acids:

gaba: calming. major inhib NT. CNS. undersupply: seizures, tremors

glutamate: memory. major excitatory NT. XS: oversupply: overstimlate brain - migraines

glycine: major inhibitory NT in spinal cord

peptides:

endorphins: euphoria

24
Q

which of gaba and glutamate are excitatory / inhibitory?

A

gaba: inhibtory
gluatamate: excitatory

25
Q

Excitatory post synapatic potential (EPSP) and Inhibitory PSP (IPSP).

A

EPSP:

  • depolarisation at post synaptic neuron: increases liklihood of AP
  • (e.g. glutamate channels cause an net influx of Na+ & depol)

IPSP:

  • hyperpolarisation decreases ability of the membrane to reach threshold: decreases liklihood of AP
  • (e.g. gaba and glycine cause net influx of Cl-)

summation of IPSP and EPSP gives probabilty of firing an AP

26
Q

what is temporal summation?

what is spatial summation?

A

temporal summation

  • post synaptic potentials at same syanpse (A&A) occur in rapid succession
  • first potential doesnt have time to dissipate: next potentials add to previous once

spatial summation

  • multiple postsynaptic potentials from different synapses (A+B) occur same time and add
  • alone, EPSP not strong enough to cause AP. reinforce each other = AP.
27
Q

which type of summation is this?>

A

spatial summation: different synapses occur at same time (A+B)

28
Q

what type of summation is this?

A

temporal summation: same synapse!!

29
Q

what does summation allow?

A

summation allows neurones to integrate large amounts of information

30
Q

what can excess firing of AP lead to?

A

seizures

31
Q

which NT is most common in brain?

A

glutamate !!