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Biosci 107 > Excitable tissue: neurons > Flashcards

Excitable tissue: neurons Flashcards

1
Q

How is a message transmitted from one excitable cell to another?

A

a) through chemical synapses (most often)

b) through electrical synapses (e.g. the retina)

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2
Q

Synaptic transmission occurs between neurons at the?

A

Axo-dendritic synapse

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3
Q

What are the main stages of synaptic transmission?

A
  1. Pre-synaptic terminal invaded by action potential
  2. Increased permeability of Ca2+ (voltage gated ca channels open) Ca2+ influx occurs into synaptic knob.
  3. This activates molecular events that results in movement of vesicles to synaptic membrane and synaptic cleft and release via exocytosis
  4. Reaction of transmitter with post synaptic receptors
  5. activation of synaptic channels
  6. Post synaptic action potential
  7. Post synaptic action potential (or end plate potential) due to increased pemeability of postsynaptic membrane to both Na+ and K+, results in AP along muscle fibre
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4
Q

What is a non selective cathion channel?

A

A channel that facilitates movement of any cation.

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5
Q

What are the two main types of chemical synapses in the CNS?

A

a) Excitatory synapses

b) Inhibitory synapses

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6
Q

Main neurotransmittor in excitatory synapses?

A

Glutamic acid or ACh

Transient opening of channels selective for Na+, K+, and sometimes Ca2+.

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7
Q

Main neurotransmittor in inhibitory synapses?

A

GABA (gamma-aminobutyric acid) or gylcine

You get opening of either K+ or Cl- channels (chemically gated)

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8
Q

Which ion has the same EP as resting membrane potential?

A

Chloride

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9
Q

2 classifications of neurotransmitters

A

Small molecule NT (classical NT)

Neuropeptides (neuromodulators)

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10
Q

What are the factors determining type of synaptic action?

A

a) Type of neurotransmitter

b) Type of neurotransmitter receptor expressed in the post synaptic membrane

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11
Q

NMDA receptor also permeable to which ion?

A

Ca2+

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12
Q

Consequence of too much glutamate release?

A

Excessive depolarization and over activation of neurons. Long term opening of NMDA receptors can result in neurotoxicity.

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13
Q

How does the neurotransmitter become inactivated?

A

a) diffusion away from the synapse
b) Enzymatic degredation

c) Re-uptake (for most of aa and amines) and re-cycling!
This involves glia cells which act as neurotransmitter transporters in the pre-synaptic membrane against conc. gradient. eg glutamate transporter.

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14
Q

Integration of synaptic inputs by neurons

A

Neurons receive up to 100000 synapses!
Each individual synapse, when activated, produces only very small (=0.1 mV or less) postsynaptic potentials at axon initial segment.
In order to depolarise the initial segment the EPSPS need to be enhanced through temporal and spatial summation.

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15
Q

Why do neurons have dendrites

A

To create greater surface area for synaptic contacts

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16
Q

Where is the AP generated?

A

Mostly AP generated in axon hillock

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17
Q

Where is the information in nerve impulses coded?

A

Information is coded in frequency of AP; temporal and spatial summation of post synaptic potentials at axon initial segment

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18
Q

How many neurons in the human brain?

A

~100 billion

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19
Q

Most prolific motor neuron (classification)

A

Multipolar

20
Q

Most prolific sensory neuron

A

Unipolar

21
Q

Write Nernst equation

A

Eion= 61.5mv x log (ion)outside/(ion)inside

22
Q

RMP rule

A

The higher the membrane permeability to a particular ion the closer the membrane potential will be to that ions equilibrium potential.

23
Q

Write goldman equation

A

Vm = 61.5mv x log Pk(K+)outside + PNa (Na+)outside/ Pk(K+)inside + PNa (Na+)inside

24
Q

What is the voltage of RMP?

A

~-65mv

25
Q

What voltage is threshold?

A

~-55mv

26
Q

Three key stages to an AP

A

1) Fast depolarisation
2) Repolarisation
3) After hyper polarisation

27
Q

Which kind of neuron has faster transmission of the AP, myelinenated or non myelinated.

A

Myelinated

28
Q

What is the speed of an AP in a non-myelinated axon

A

~1 m per second

29
Q

Why is action potential slow in non-myelinated axon?

A

Because the AP has to be generated at every point in the axon.

30
Q

How fast is AP transmitted in myelinated axon?

A

20-100 m/sec

31
Q

How does myelination of axons increase speed of AP transmission?

A

Myelination increase the speed of passive spread of current flow by providing insulation of current in-between nodes of ranvier

32
Q

Is saltatory conduction in myelinated or non-myelinated axons?

A

Myelinated

33
Q

Current flows passively in both directions, why under physiological conditions does the AP then only travel in one direction?

A

Because of the absolute refractory period; passive flow of ions does occur backwards but they cannot open the voltage gated Na+ channels as they are in the ARP

34
Q

Which direction os orthodromic

A

From cell body to axon terminal (physiological direction)

35
Q

Which direction is antidromic

A

Towards cell body (electrode stimulated)

36
Q

In sensory ions what are the channels in the sensory neuron terminals?

A

Mechanically gated.

37
Q

Steps of AP generation in sensory neurons

A

1) Mechanically gated channels are stimulated creating local depolarisation that is graded dependant on the strength of the stimulation; this is known as the receptor potential
2) The receptor potential travels passively to distally located trigger zone where AP is generated if the receptor potential is great enough

38
Q

Steps of neuron AP transmission

A

1) Presynaptic AP
2) AP induces voltage gated Ca2+ channels in synaptic knob to open
3) Influx of Ca2+ causes vesicles containing the neurotransmitter to fuse with membrane and neurotransmitter is released via excytosis.
4) Chemically gated non-selective ion channels are opened and there is an influx of Na+ and an outflow of K+ this creates an end plate potential
5) End plate potential is enough to trigger voltage gated Na+ channels to open and a consequent AP

39
Q

Excitatory postsynaptic potentials:

Name neurotransmitters, ionic mechanisms, and effect

A

Glutamate, ACh
Open Na+, K+ and sometimes Ca2+ channels
Depolarises membrane towards threshold

40
Q

Inhibitory postsynaptic potentials:

Name neurotransmitters, ionic mechanisms, and effect

A 
Gaba or glycine
Open Cl- and K+ channels
Hyperpolarises membrane (K+) or decrease cell membrane resistance (Cl-) reducing effectiveness of current induced by EPSP
41
Q

Describes the two types of mechanisms of gating using neurotransmitters

A

Direct- where the neurortransmitter binds to the receptor/channel and opens it, fast short action.
Indirect- where the neurotransmitter binds to receptors such as G protein coupled or metabolic receptors activating second messengers such as cAMP which activates protein kinases which phosphorylate ion channels causing them to open or close. Slow and prolonged effect.

42
Q

Describe temporal summation

A

When more than one EPSPS occurs within quick succession the membrane potential in the initial segment of the axon reaches threshold and an action potential can occur.

43
Q

Describe spatial summation

A

When more than one EPSP occurs simultaneously in the axon the membrane in the initial segment reaches threshold and an AP can occur.

44
Q

Describe small molecule neurotransmitters

A

Usually fast action (within milliseconds) and direct on post synaptic receptors eg: Amino acids: glutamate, GABA, glycine
ACh
Amines; norepinephrine, dopamine, serotinin
Others; ATP, nitric oxide

45
Q

Describe neuropeptides

A

Large molecule chemical messengers that have an indirect (metabotropic) or modulatory action on other neurotransmittters. Slow (seconds to minutes) and usually more diffuse actions (volume transmission)
Several dozens of neuropeptides have been identified which maybe involved in communication between neurons. Eg neuropeptide Y, Substance P, endorphnis.

Biosci 107 (8 decks)

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