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A Intro to Neuro > Intro to neurons/Resting membrane potential > Flashcards

Intro to neurons/Resting membrane potential Flashcards

1
Q

What are the two broad categories of neurons

A
  • Neurons
  • Glia
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2
Q

How many neurons in the human brain?

A

Approx. 86 billion

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3
Q

Neuron structure and function of each part

A
  • Dendrites are inputs. Often there are many. They taper in diameter.
  • Soma (cell body) sums all the inputs from the dendrites.
  • The axon is the output (only one but it can branch.) Axons have constant diameter.
  • Some neurons don’t have an axon or dendrites (but still function as neurons).
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4
Q

Structure of an idealized neuron

A
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5
Q

Communication within and between neurons

A
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6
Q

Interactions at Synapses

A

A neuron isn’t purely pre- or post-synaptic. It can be pre-synaptic to many others and post- synaptic to many others (even itself)

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7
Q

Dendrites

A
  • “trees”
  • unique to neurons
    Dendrites
  • most synapses end on dendrites (i.e. dendrites are neuron inputs)
  • neurotransmitters bind to special receptor proteins in the cell membrane of dendrites
  • ion channels in membrane allow electrical signals to reach soma
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8
Q

Dendritic Spines

A
  • some dendrites are spiny, others are not
  • synapses often end on spines
  • shape and density of spines determine synapse strength, timing properties, etc
  • spines change in development and with learning
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9
Q

Rats: dendritic spine changes associated with environmental enrichment

A
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10
Q

Humans: dendritic spine abnormalities associated with intellectual impairment

A

Example 1: general intellectual impairment - long and abnormally spindly spines

Example 2: Fragile X syndrome (most common genetic cause of autism)
- long and abnormally dense spines

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11
Q

Neuron Cytoskeleton

A

Scaffold proteins affect neuron shape and function (common to eukaryotic cells):
* microfilaments – 5nm
* neurofilaments – 10nm
* microtubules – 20nm (transport
molecules up and down axon)

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12
Q

Microtubule Associated Proteins (MAPs)

A

Regulate assembly and function of microtubules
e.g. the MAP known as tau protein links microtubules

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13
Q

Alzheimer’s Disease macroscopic pathology

A
  • Cell death, gyri shrink, sulci & ventricles expand
  • Cognitive deficits – memory loss, confusion, difficulty with speech and navigation
  • Innumerable factors are reported to influence occurrence, but cause is unknown
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14
Q

Alzheimer’s Disease microscopic pathology

A

Two key proteins are abnormally clumped

  • Tau protein forms neurofibrillary tangles inside neurons
    (number of tangles correlates with cognitive decline)
  • Plaques made of β-amyloid protein form outside neurons
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15
Q

Alzheimer’s Disease Progression (β amyloid hypothesis)

A
  1. abnormal secretionof βA by neurons; βA clumps into plaques
  2. βA triggers tangle formation: - shape of tau changes
    (excess phosphate molecules attach to tau) - microtubules fall apart
  3. neuron dies
  4. tau accumulates (tangles)
  5. distorted tau and βA spread to nearby neurons
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16
Q

Axons

A
  • unique to neurons
  • axons have few or no ribosomes (little or no protein synthesis). Needed proteins must come from the soma
  • ion channels in the axon membrane are critical for electrical conduction of action potentials
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17
Q

Axoplasmic Transport

A

Transport of organelles and molecules between soma and synapses along axons
* Anterograde – toward axon terminal
* Retrograde – away from axon terminal

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18
Q

Axoplasmic transport- microtubules

A
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19
Q

Herpes

A
  • Enters nerve terminal through broken skin → retrograde transport to soma
  • Viral replication occurs in soma
  • Anterograde transport of HSV causes cold sores recurrence on lips
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20
Q

Rabies

A
  • Enters nerve terminal through bite → retrograde transport to soma
  • Viral replication occurs in soma
  • Cell death (no anterograde transport)
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21
Q

Glia

A

“glue”
* Brain is ~50% glia
* Generally small soma, 5 - 20 μm (neuron somas are 5-100μm)
* Roles: electrically insulate neurons, protect neurons from infection, nourish neurons

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22
Q

Astrocytes

A
  • one of the most common glia
  • fill spaces between neurons
  • regulate ion concentrations around
    neurons
  • guide neurons in development
  • protect neurons by taking up toxins
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23
Q

Myelinating Glia

A
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24
Q

Myelination: Oligodendroglia & Schwann Cells

A
  • Schwann cells are in the PNS (peripheral nervous system) – each insulates about a 100 micron length of a single axon
  • Oligos are in the CNS (central nervous system) – each insulates many neurons (up to about 50).
  • Why the two types?
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25
Q

Are Schwann cells in the PNS or CNS?

A

PNS

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26
Q

Are Oligos in the PNS or CNS?

A

CNS

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27
Q

CNS Tumors

A
  • CNS tumors are generally glia cancers. Most common are astrocytomas (most serious is glioblastoma)
  • Why don’t we see neuron cancers?
    Rare because neurons don’t generally divide in the adult brain
  • Symptoms of a glial tumor include nausea, headache, vomiting, and functional deficits associated with the affected part of the brain
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28
Q

Brain electrical activity

A
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29
Q

Electrical activity of individual neurons

A
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30
Q

The neuronal membrane at rest

A
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31
Q

Potential difference in electrical circuits

A
32
Q

Potential difference in neurons

A
33
Q

Ion concentrations in axoplasm and extracellular fluid mM)

A
34
Q

How are the ion concentration gradients established and maintained?

A
  • The sodium-potassium pump! (and other ion pumps)
  • It moves 3 Na+ out of the cell and brings 2 K+ into the cell
  • The pump runs constantly to establish and maintain Na+ and K+ concentration gradients
  • Dysfunction causes various motor and seizure disorders. Complete failure of the pump would be fatal.
35
Q

The sodium-potassium pump

A
  • The sodium-potassium pump moves ions
    against their concentration gradients
  • This process takes a lot of energy. The brain uses 20% of the bodies ATP (but is only 2% of mass).
  • The pump uses about 70% of the brain’s ATP!
36
Q

Ion channels allow ions to diffuse across membrane

A
  • The cell membrane blocks the diffusion of ions across the membrane
  • Ions can cross the membrane through ion channels (and ion pumps)
  • Ion channels are composed of proteins that span the membrane (transmembrane subunits)
  • Ion channels have a pore that passes the ion (if open)
37
Q

Selective ion channels

A
  • Some ion channels are “selective”; i.e. they only pass one type of ion
  • Selectivity is based on the size of the pore and the subunits the channel is made from
  • Most ion channels have two states, open and closed
38
Q

Ion movement through ion channels is governed by two forces: Diffusion and Electrical

A
  1. Diffusion force: ions move down concentration gradients (away from high concentration toward low concentration)
  2. Electrical force: ions with like charge repel and ions with opposite signs attract
39
Q

Ions diffuse down concentration gradients through open channels

A
  • Add table salt to water on one side of membrane
  • Both Na+ and Cl- channels are open
  • Both ions diffuse across membrane down concentration gradients until equal concentrations on both sides of membrane
  • When both positive and negative ions diffuse, there are equal numbers of the two on each side of the membrane
40
Q

When is equilibrium reached?

A

When diffusion force equals electrical force

41
Q

Equilibrium

A
  • Na+ diffuses to the right down concentration gradient
  • Positive charge builds up on right side and negative charge on left side (bc Cl- can’t cross)
  • As Na + diffuses, Vm becomes more negative on the left
  • Na+ on right starts repelling the flow of more Na+
  • Eventually, electrical repulsion stops the diffusion. At this point, the membrane is in equilibrium.
42
Q

In reality, the small difference in + and – charges appears right at the cell membrane

A
  • The asymmetry in the number of ions on the two sides of the membrane involves a small percentage of all the ions
  • Solutions away from the membrane are effectively neutral
43
Q

Equilibrium Potential & Ionic Driving Force

A
44
Q

What is the driving force of an ion?

A

The difference between the current membrane potential and the ion’s equilibrium potential

45
Q

The higher the driving force, the ___ ions will move across the membrane

A

Faster

46
Q

When the membrane potential is at the equilibrium potential for an ion, what is the driving force?

A

0

47
Q

Equilibrium Potentials of Important Ions

A
48
Q

The greater the concentration gradient, the ___ Eion is away from zero (for ions with same valence).

A

Greater

49
Q

Driving force at resting membrane potential

A
50
Q

Driving force at different membrane potentials (table)

A
51
Q

At a low negative membrane potential (typical resting potential), the driving force is much higher on ___ than on ___

A

Much higher on Na+ than on K+

52
Q

At positive membrane potentials the driving force becomes higher on ___ than on ___

A

Higher on K+ than on Na+

53
Q

For a positively charged ion, a positive driving force means ___ and a negative force means ___ (easy to remember with salty banana)

A

Outwards, inwards

54
Q

How is equilibrium the point Eion determined?

A

If we know Eion, we can predict the forces on ions at any membrane potential (very useful!)
* Walther Nernst (1864 – 1941) figured out how to calculate Eion

55
Q

What does the Nernst Equation calculate?

A

Equilibrium potential for a particular ion

56
Q

Nernst equation

A
57
Q

Example of Nernst equation

A
58
Q

Valence of different ions

A
59
Q

Fun with the Nernst Equation!

A
60
Q

What is the ionic driving force?

A

Energy pushing ions in or out of cell

Vm – Eion

61
Q

What is ionic conductance?

A

Capability of an ion to cross the membrane

gion

62
Q

Conductance vs. permeability

A
  • Permeability is a relative measure comparing different ions
  • High conductance or permeability means ions could cross the membrane
63
Q

What is ionic current?

A

Conductance x driving force

gion(Vm – Eion)

Ion movement across membrane

64
Q

Permeability, equilibrium, and Vm (Sodium Sammy and Potassium Penny)

A
65
Q

If the cell membrane is permeable to only Na+, what will happen to Vm?

A

Vm will increase until it reaches ENa

66
Q

If the cell membrane is permeable to only K+, what will happen to Vm?

A

Vm will decrease until it reaches Ek

67
Q

Permeability to ions at rest

A

At rest, neurons are typically about 40x more permeable to K+ than Na+, so Vm at rest is much closer to Ek

68
Q

Effects of aberrant resting potential (Weaver mice)

A
  • Weaver mice” have a K+ channel mutation in the cerebellum that allows the channel to pass both K+ and Na+
  • Because of increased Na+ permeability, resting potential is more positive than normal and neuron function is compromised
  • Called “Weaver” because the mutation causes abnormal posture and movement (“weaving”). They also die prematurely (after death of motor neurons).
69
Q

Axoplasmic transport moves various kinds of material along ___

A

Microtubules

70
Q

The output elements of neurons are generally ___

A

Neurons

71
Q

The formulation of neurofibrillary tangles in Alzheimer’s disease is triggered BY what factor?

A

Accumulation of beta-amyloid

72
Q

Which ion is most concentrated inside the neuron compared to outside?

A

K+

73
Q

The resting membrane potential of neurons is closest to the Eq for which ion?

A

K+

74
Q

Describe the insulation of oligodendroglial cells

A

An oligodendroglial cell insulates portions of multiple axons

75
Q
A

A Intro to Neuro (17 decks)

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