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Bio 222 Cell/Molec Neuroscience > Exam 1 > Flashcards

Exam 1 Flashcards

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1
Q

what are the two systems that are present in all cells, but are highly elaborated in neurons?

A
  1. Molecular machines for moving material around the neuron
  2. Ion channels/transporters and the machinery required to properly localize them and adjust their number
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2
Q

relationship b/w ions and water

A

ions in solution have layer of water around them, forming hydrated and dehydrated radius (both are important in determining whether ions can get in and out of cells through ion channels)

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3
Q

cations and anions important in neurophysiology

A

Cations: Na+, K+, Ca2+, Mg2+

Anions: Cl-

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4
Q

structure of phospholipids?

what can pass phospholipid bilayer?

A

polar head containing phosphate + nonpolar hydrocarbon tail

charged and polar molecules cannot pass through (need membrane protein)

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5
Q

electrically, phospholipids act as ____…..

A

as insulators, giving membranes the property of capacitance (can separate charges)

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6
Q

thickness of bilayer and membrane potentials

A

thickness is ~4nM, so a potential of 100mV has field strength of 25million V/m

voltages greater than ±200 mV cause bilayer to break down so all biological potnetials are less than this

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7
Q

amino acids: polar (hydrophilic) vs nonpolar (hydrophobic) mnemonics

A

nonpolar: Grandma Always Visits London In May For Winston’s Party

polar: Santa’s Team Crafts New Quilts Yearly

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8
Q

charged amino acids mneumonic

A

Dragons Eat Knights Riding Horses

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9
Q

which amino acid can form disulfide bonds

A

cysteine

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10
Q

which amino acids can be phosphorylated

A

Serine (Ser, S), Threonine (Thr, T), Tyrosine (Tyr, Y)

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11
Q

proper protein folding sometimes requires:

A

chaperone proteins or post-translational modification

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12
Q

post-translational modifications in neurons + enzymes

A

disulfide bonds link cysteines to constrain structure (reductase breaks bonds, oxidase forms bonds)

phosphorylation adds negative charge to temporarily alter structure/function (kinase adds PO4-, phosphatase removes)

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13
Q

mammals have hundreds of different neuronal types that can be characterized by what properties?

A
  • Pattern of connectivity to other neurons
  • Electrical properties
  • Biochemistry

–> New method is to create single cell gene expression profiles

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14
Q

Nobel prize in physiology/medicine in 1906: names

A

Golgi and Cajal

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15
Q

Camillo Golgi developed what?

A

golgi stain: first to reveal complexity of neuronal structure

fix/section tissue, soak in potassium dichromate, add silver nitrate → black participate stains few cells completely

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16
Q

what did golgi and cajal disagree on

A

disagreed on what images obtained from golgi method meant

Cajal: Neuron Doctrine (neurons unit of function of brain)

Golgi rejected neuron doctrine

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17
Q

what is special about golgi labeling/staining

A

staining is sparse, so axonal and dendritic geometry can be observed

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18
Q

dendrites: excitatory and inhibitory input

A

for pyramidal neurons:

most of the excitatory synaptic input is onto dendritic spines

most of inhibitory synaptic input is onto dendrite shaft or cell body

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19
Q

axons make their presynaptic contacts at:

A

enlargements referred to as boutons

boutons may be along the axon (en passant) or at the end of fine terminal branches of the axon

–> studied by Cajal

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20
Q

1899 Cajal: two major classes of cerebellar neurons

A

purkinje neurons: elaborate dendritic treets, planar

granule neurons: not planar

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21
Q

not all neurons have dendrites: two examples

A

spherical bushy cell of the cochlear nucleus

rat submandibular ganglion neurons

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22
Q

retrograde vs antergrade transport

A

retrograde: toward cell body
anterograde: toward nerve terminals

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23
Q

classic experiment to measure rate of axonal transport

A
  1. Inject radioactively labeled AAs into extracellular space of cell body
    1. Usually in spinal cord or DRG (sensory ganglion) to avoid spillover
  2. AAs are taken up into cell, used to synthesize proteins, some of which travel down axon
  3. Cut chunks of axon, isolate proteins using gel electrophoresis
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24
Q

slow vs fast axonal transport

A

slow: 0.2-8 mm/day

fast: 50-400 mm/day

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25
Q

can you use classic expermient to measure retrograde axonal transport

A

NO

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26
Q

Method that measures retrograde transport

A

Horseradish peroxidase (HRP) is an enzyme that is extremely stable and produces a colored product

can be endocytosed, transported anterogradely and retrogradely

injected into brain of living animal: cell bodies at distant locations (retrograde transport) are darkly stained

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27
Q

3 types of filamentous elements in all cells

A

microtubules (20nm) : highly expressed in axons/dendrites, transport

intermediate filaments (10nm): neurofilament prominent in axons/dendrites, structural rigidity

microfilaments (5nm): rare in axons/dendrites, except at their tips, transport

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28
Q

microtubules (macrofilaments) and microfilaments stucture

A

microfilaments: made up of actin filaments

microtubules: made up of tubulin filaments, which is made up of heterodimers (α and β subunit)

–> both have plus and minus end (polarized)

act as tracks for movement of molecules

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29
Q

molecular motors

A

kinesins and dyneins

move along micro/macrofilaments using ATP to translocate cargo to distant locations

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30
Q

microtubules length in axons

A

run entire length of axons

  • end at cell body

+ end at nerve terminals

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31
Q

kinesins

A

large family of motor proteins that run along tubulin tracks

use ATP to walk along microtubules using two “legs”

most kinesins (KIFs) are - to + motors (anterograde)

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32
Q

KIF1a

A

responsible for fast anterograde axonal transport of synaptic vesicles

w/o it, mice die

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33
Q

slow anterograde transport and kinesins

A

other KIFs (not KIF1) are slower

speed difference due to taking more stops

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34
Q

dynein

A

uses ATP to walk along microtubules using two “legs” but structure completely different from kinesins

+ to - motor (retrograde axonal transport only)

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35
Q

microfilaments in axons locations

A

mostly at axon terminals

move synaptic vesicles

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36
Q

movment in dendrites

A

microtubules w/ both polarities present, direction of transport is less clear

microfilaments are in dendritic spines

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37
Q

what are other functions of microtubules and microfilaments?

A

essential for axon growth during development

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38
Q

Nuclear pores

A

large pores allow transcription factors (protein) to enter and mRNA to exit

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39
Q

neurogenesis

A

neurons are post-mitotic

in mammals, most neurogenesis happens during prenatal period

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40
Q

promotors and enhancers

A

promotor: specific DNA sequences indicate the starting point for transcription into mRNA; on 5’ end

enhancer: can be anywhere in the vicinity, influence rate of transcription

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41
Q

transcription factors

A

proteins that bind to enhancer/promoter regions to make specific genes available for transcription

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42
Q

how is pre-mRNA processed

A

site specific RNA editing of some transcripts

splicing out introns

5’ capping

3’ poly A tail

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43
Q

ribosome vs rough ER translation

A

ribosome: protein remains in cell

rough ER: protein becomes membrane protein or exocytosed

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44
Q

in neurons, most translation is:

A

perinuclear (happens just around the nucleus) but also occurs in some dendrites

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45
Q

4 types of glia

A

oligodendrocytes (CNS)

Schwann cells (PNS)

Astrocytes

Microglia

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46
Q

difference b/w oligodendrocytes and schwann cells

A

both form myelin sheath, but oligodendrocytes can myelinate multiple axons but each schwann cell can only wrap one axon

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47
Q

function of myelination

A

support propagation along axon

different levels of myelination allow identification of different neurons in PNS

allow for rapid neuron communication (sensory neurons esp.)

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48
Q

example of demyleinating disease

A

multiple sclerosis

immune system attacks myelin and oligodendrocytes

impairs conduction of APs along axons

can affect muscle movement, coordination, speech, pain, fatigue

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49
Q

grey vs white matter

A

white: myelinated axons and oligodendrocytes

grey: neuronal cell bodies, unmyelinated axons, astrocytes

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50
Q

astrocytes

A

play role in development and regulation of neuronal communication

help guide axons to destination during development

form connections b/w neurons and blood vessels

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51
Q

microglia

A

immune cells of nervious system (macrophages of CNS)

phagocytose apoptotic neurons, debris

first line of defence if infectious agents cross the BBB

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52
Q

what are molecular markers for glial cells

A

tag different glia by inserting fluorescent tag to marker in gene sequence

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53
Q

name the markers for each type of glial cell

A

oligodendrocyte & schwann: MBP (myelin basic protein)

astrocyte: GFAP (glial fibrillar acidic protein)

microglia: IBA1 (ioninzed calcium binding adaptor molecule 1)

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54
Q

consanguinous families

A

mating b/w family members

have higher probability of recessive mutations showing up

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55
Q

neural circuit for pain perception

example: what happens when you step on a tack

A
  1. Tissue damage causes foot sensory neuron endings to initiate generator potential
  2. Causes action potential in foot sensory neuron axons
  3. Synaptic potential in spinal cord neuron dendrites
  4. Action potential in motor neuron axons (muscles move)
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56
Q

what gives membranes the property of conductance

A

open channels, that are a pathway for ionic current flow

symbolized by “g”, units Seimens (S)

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57
Q

channels open in an _____ manner

A

all or none manner

always open to same amplitude signal (measured using patch clamp), are either open or closed for long time or short time

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58
Q

channels in response to a stimulus: amplitude

A

channels increase probability of opening in response to a stimulus

under patch clamp conditions, signal channel currents sum linearly, so we see amplitude vary b/w (for example) -16 pA, -32, -48, etc.

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59
Q

types of stimuli that can change the probability of a channel opening from low to high

A

membrane potential change (usually depolarization)

binding of a small molecule

physical activation

covalent modfication

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60
Q

a few types of channels have a high probability of being open in the absence of ____

A

a stimulus

most of these types of channels can be shut by a stimulus

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61
Q

how are transporters triggered

A

binding of a molecule triggers a conformational change that allows the molecule to unbind and be released on the other side

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62
Q

max flux through transporter vs ion channel

A

max flow through electrogenic transporter is about 0.1% of a typical ion channel

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63
Q

some transporters are electro___ and others are electro____

A

electroneutral: transport neutral substances
electrogenic: transport charges

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64
Q

what does movement against electrochemical gradient require for transporters?

A

conformational change of some transporters is coupled to an energy source

primary active transporters use ATP

secondary active transporters use gradient established by primary

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65
Q

18th century discoveries underlying bioelectricity

A

Franklin, Galvani, Volta’s discoveries provide theoretical underpinning for how ions flow through channels

franklin: defined + vs - charges

galvani: animals use electricity in nerves and muscles to signal

volta: invented battery, compared to biology

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66
Q

19th century discoveries underlying bioelectricity

A

Kirchhoff and Kelvin’s discoveries provide theoretical underpinning for how electrical signals spread in axons and dendrites

kirchhoff: two laws explain how current flows in branching circuits

kelvin: theory for current flow (transatlantic telegraph cable)

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67
Q

voltage definition

A

when you separate charges, you create electrical potential energy (voltage)

device that does this is a battery

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68
Q

keeping charges separated requires:

A

an insulator (non conducting material) b/w the charges

the ability to separate charges is capacitance (C), units farads (F)

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69
Q

conductance: ionic solutions and cell membranes

A

degree to which substance allows flow of charges (g)

ionic solutions have high conductance

cell membranes have low conductance (unless ion channel proteins open)

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70
Q

resistance

A

ability to prevent flow (R), units Ohms (Ω)

reciprocal of conductance (R = 1/g)

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71
Q

current definition + determined by what

A

movement of particles per unit time (Q = # of charges in coulumbs)

determined by two factors: potential energy in the system (ΔE) and number/size of conductive pathways (g)

Ineuron = gion channels * ΔElectrical potential (V)

72
Q

definition of a negative current through channels

A

cell becomes more positive: positive charges entering or negative charges leaving

also called inward current

negative/inward current causes depolarization

73
Q

apparatus for recording electrical signals consists of:

A
  • Display device that shows signal as function of time (oscilloscope or computer screen)
  • Electrode that detects small voltage/current fluctuations
  • Amplifier to match sensitivity of the electrode to the displace device
  • Interface to communicate w/ computer
74
Q

electrode that’s used to record electrical signals: two types of recordings

A

must have small tip (0.001 mm) and held stably in place near the cell by a micromanipulator

extracellular recording: can be metal wire or glass tube filled w/ conductive soln; signal is a small fraction of actual membrane potential

intracellular recording: always glass tube filled w/ conductive soln (usually KCl); signal is actual membrane potential or current

75
Q

resting potential can vary from __ to __ but generally ___

A

varies from -10mV to -90mV

usually around -70mV

76
Q

in non-excitable cells, there is a ___

A

steady resting potential

(also in skeeltal muscle fibers and some neurons)

77
Q

many neurons are ____ active

A

spontaneously:

action potentials

synaptic potentials (too small to detect w/ extracellular recording) due to input from nearby neurons

78
Q

what is the cause of membrane conductance vs capacitance

A

conductance: ion channels
capacitance: lipid bilayer (insulator)

79
Q

a cell is behaving passively when:

A

conductance does not change in response to a stimulus (doesn’t cause more channels to open)

80
Q

properties of two basic electrical circuits in neurons

A
  1. a cell body w/ no axon/dendrites acts as a conductor and capacitor in parallel (g-C or R-C circuit)
  2. an axon disconnected from cell body acts like complex network of resistors and capacitors
81
Q

what is the hyrdraulic equivalent of a capacitor

A

tube w/ an elastic membrane

applying hydraulic pressure causes membrane to bulge, there is movement (current flow) until the force is insufficient to stretch the membrane any more

like capacitor (stores charge up until a certain amount)

82
Q

Galvani’s work in late 1700s

A

from time of galvani, it was clear that nervous system can propgate electrical signals

mechanistic explanation of bioelectricity lagged b/c measuring tools not available (metal probes killed living cells, didn’t have glass probes)

83
Q

Young in 1930ss

A

zoologist discovered squid’s giant axon, large enough for scientists to thread metal wires down it instead of penetrating it

84
Q

Hodgkin and Hukley’s work in 1930s-1950s

A

Huxley student of Hodgkin

HH used squid giant axons

two wires threaded down length of axon w/o killing it

electrode 1 passed current, electrode 2 recorded voltage

results: squid giant axons have stable negative resting potential that needs stimulus to reach threshold to fire

85
Q

Nernst 1920

A

discovered nernst equation

86
Q

key facts of logarithms

A

log(1) or ln(1) = 0

ln(x) = 2.303*log(x)

87
Q

diffusion across permeable barrier

A

concentrations of ions become equal inside/outside

88
Q

diffusion across semipermeable membrane

A

can cause separation of charge

diffusional force and electrical force eventually stabilize

89
Q

when is equilibrium achieved

A

diffusional force pushing in = electrical force pushing out

90
Q

active transports establish ____ but do not ____

A

establish ion gradients, but don’t directly cause membrane potnetial

91
Q

concentrations of ions outside vs inside in mammalian neuron

A

Na, Cl, Ca more concentrated outside

K more concentrated inside

Mg equal inside and outside

92
Q

How do we test if a single class of ion channels is responsible for the resting potential?

A

test with ion substitution experiments for a single ion (Na, K, Ca, Mg, or Cl), then compare this to nernst equation prediction

we can usually only manipulate extracellular ions

93
Q

ion substitution experiments

A

change extracellular concentration of the ion

predict what Ex will be for various outside concentrations using nernst, then compare to experimental data

94
Q

10 fold change in outside concentration leads to how much change in Ex

A

60 mV change (for monovalent)

outside conc smaller: becomes more positive

inside conc smaller: becomes more negative

95
Q

conclusions of ion substitution experiments if predicted potentials do not match actual data

A

ion does not contribute to resting potential (at resting potential, all ion channels are closed)

96
Q

when can goldman hodkin katz equation be used and what are the assumptions

A

only when monovalent cations/anions are under consideraton

assumptions (not true): treats membrane as homogenous - assumes one pathway/channel w/ variable permeabilities to each ion

97
Q

an alternative approach (from goldman-hodgkin-katz equation) to predict resting potential is to:

A

solve an equivalent circuit for multiple conductances in parallel

98
Q

at any stable potential, we can ignore the:

A

capacitor

only sum of resistors matter

99
Q

driving force

A

the electrical force acting on an ion in a channel

driving force = ΔV = Vm - Ex

cation: negative driving force pushes ion in
anion: negative driving force pushes ion out

100
Q

at resting potential, there is no net ___

A

current (Itotal = 0)

example: if only Na and K channels open, then Itotal = INa + IK = 0

101
Q

at resting potential of a certain cell gna/gk = 0.14

what does this mean

A

a few sodium channels are open (gna is low)

many potassium channels are open (gk is high)

currents are equal and opposite because the sodium driving force is large (-130 mV) and the potassium driving force is small (+18 mV)

102
Q

resting potential is a _____ not an ____

A

steady state, not an equilibrium

if ATP is removed, it is not stable

103
Q

effect of addding a Na/K pump inhibitor

A

oubain:

no change in resting potential for a while nor AP amplitude, but eventually the gradients dissipate and cell beings to lose its resting potential and action potential amplitude decreases

104
Q

significance of what would happen if sodium conductance dominated as much as potassium conductance usually does

A

a very rapid increase in gNa while gK remains the same is the basis of the rising phase of an action potential

105
Q

it is impossible to go ____ to Ena and _____ to Ek unless:

A

positive, negative

unless current is applied from an external source (any potential b/w ENa and Ek can be achieved by cells)

106
Q

at resting potential gNa ____ gK

A

gNa << gk thus Vm is near Ek

at the peak of AP, this flips so Vm is near ENa

107
Q

threshold definiton

A

at sufficient level of stimulus, AP generated in most neurons

defined as the stimulus level where one gets an action potential on some, but not all trials

108
Q

shape of passive responses

A

stimulus is a square of current pulse, of varialbe amplitude

response voltage traces a curve that eventually stabilizes

exact shape of curved response depends on cell shape (isolated cell body is exponential - parallel R-C circuit)

109
Q

shape of passive responses: final response amplitude is:

A

linearly proportional to stimulus amplitude

for a passive response ΔV/ΔI is defined as the input resistance

110
Q

the speed of the passive response (response to current from stimulating electrode) indicates the speed at which:

A

a neuron will be able to integrate synaptic inputs

111
Q

details about the shape of the passive response (to current from stimulating electrode) can indicate:

A

where active synapses are located on the dendritic tree

what part of the neuron the rcording was made from

112
Q

the amplitude of the passive response (to current from stimulating electrode) can indicate

A

how big the neuron is

how much synaptic input is required to drive the neuron past threshold

113
Q

passive behavior of an isolated cell body

A

parallel g-C circuit

if you close switch at t=0 after it has been open for a long time: V(t) = IR*(1-e-t/τ​) (don’t memorize)

charge accumulates on capacitor, current flowing through capacitor decreases

τ = Rcell*Ccell (don’t memorize)

114
Q

time constant (τ) of an exponential function determines:

A

the rate of change

115
Q

greater time constant (τ) means:

A

it takes longer to reach its “max” value

reaches “max” after 5 time constants

116
Q

how to find time constant (τ) from graph

A

at t=τ (at one time constant) a rising exponential function has reached 63% of the final amplitude

at t=τ, the amplitude of the falling function is 37% of the final amplitude

117
Q

once threshold is exceeded, all action potentials have:

A

the same peak amplitude and time course

118
Q

intensity of a stimulus is encoded by:

A

the frequency and pattern of action potentials

frequency is limited by the refractory period

119
Q

the maximum firing frequency is:

A

the reciprocal of the absolute refractory period

example: if absolute refractory period is 0.004 s, max firing rate is 1/0.004 = 250 Hz

120
Q

relationship between sodium and potassium equilibrium potentials compared to resting potential

A
121
Q

Hodkin and Katz:

A

measured peak of the action potential at various extracellular sodium concentrations using squid axons

122
Q

Hodkin cycle:

A

depolarization of the axon opens sodium channels, sodium influx, which further depolarizes the cell, opening more channels, etc.

123
Q

what causes the initial depolarization that triggers an action potential?

A

the anatomical location of excitation determines the site of initiation (of depolarization) and the direction of spike propagation

124
Q

orthodromic vs antidromic propagation

A

orthodromic: the direction the AP propagates normally
antidromic: opposite direction (normally, does not happen b/c of refractory period)

if you stimulate axon part way b/w cell body and terminals, spikes travel in both direction

125
Q

hodgkin and huxley perfected the ____ that allowed them to break postive feedback loop of sodium

A

voltage clamp: breaks positive feedback cycle by preventing channel opening (due to changing membrane potential)

126
Q

Hodgkin Huxley analysis of squid AP mechanisms

A

papers explained why they needed to use voltage clamp, showed how to separate currents from voltage clamp recordings

127
Q

how does voltage clamp work

A

experimenter sets VCMD (command)

device measures Vm

feedback amplifier rapidly generates whatever current is necessary to make Vm = VCMD

current generated by voltage clamp equal/opposite to whatever currents opening/closing of channels is generating

128
Q

voltage clamps: voltage steps

A

family of currents: measure different test potential

w/in 200 µs, feedback amplifier charges membrane capacitance and reestablishes Vm = VCMD

129
Q

voltage recording vs voltage clamp recording

A

voltage recording = current clamp

130
Q

key qualitative results of voltage clamp analysis of the squid axon (Hodgkin and Huxley)

A

channels that cause resting potential are voltage indepedent K and Na channels (leak channels); distinct from each and from channels that cause APs

channels that cause APs are voltage dependent Na and K channels

131
Q

what determines the waveform of voltage clamp currents

under voltage clamp: what is constant

A

Vm, ENa, and Ek are all constant

all the change in wave is due to changes in gNa, gk, or both

separating currents allows us to see what currents are flowing (done by lowering extracellular sodium to different extents)

132
Q

separation of currents by changing outside sodium concentration

A

green: normal solution
blue: low sodium solution (current only due to potassium channels, can find potassium conductance from this)

sodium current can be found by subtracting green-blue

133
Q

TTX

A

tetrodotoxin: made by microorganisms that live symbiotically w/ aquatic animals (japanese pufferfish and frogs/salamanders of Americas)

blocks some voltage gated sodium channels (but not in heart, zombie)

134
Q

STX

A

saxitoxin, very similar to TTX, blocks voltage gated sodium channels

135
Q

potassium currents under voltage clamp when TTX is present

A

using toxin allows you to see K currents w/o needing to do many solution changes

driving force is postive for all depolarizations

sigmoid activation, gets faster w/ depolarization

potassium currents maintained as long as depolarization is

136
Q

how do you find conductance from voltage clamp recordings using toxins

A

voltage clamp measures family of currents

plug that into ohms law to find peak potassium conductance

can form graph of conductance as a function of voltage (you can see the probability of being open at different potentials)

137
Q

K: activation and inactivation gates

A
138
Q

TEA

A

blocks K currents, allowing one to observe Na currents directly

139
Q

sodium currents under voltage clamp

A

using TEA

ENa = 60 mV, so driving force starts very negative then gets smaller and eventually reverses w/ depolarization

fast sigmoidal activation

eventually channels show complete inactivation

140
Q

Na: activation and inactivation gates

A
141
Q

gating changes during an action potential

A
142
Q

Seymour Benzer (1960, 2017)

A

study effects of genes on behavior by mutating fruit flies

discovered shaker and ether a go go mutants: two different K channel genes (seizure prone)

143
Q

how do molecular neuroscientists identify residues involved in gating and seletivity?

A
  • Hypothesis generation based on general chemical principles (hydrophobicity, charge in transmembrane domains)
  • Hypothesis generation based on sequence comparisons (residues that are highly conserved are likely functionally importnant)
  • Direct observation of structure by X-ray crystallography or cryo-electron microscopy
144
Q

how can you test hypotheses about what amino acid residues are involved in gating/selectivity?

A

make mutant proteins, then perform electrophysiological analysis:

site directed mutagenesis to change the cDNA that encodes individual AAs, or remove whole domain of a protein

express mutant proteins in non-native cell (that doesn’t express channels at all); transfect cell w/ plasmid that has promotor to drive expression

145
Q

Shaker potassium channel protein

A

gene cloned by sequence walking first

protein of about 50 kD

found to have 6 α-helical TM domains

S4 less hydrophobic bc has positive charges

146
Q

subunit organization in shaker

A

observed using EM, biochemical experiments: all potassium channels are tetramers

some are homotetramers (all 4 subunits the same, each 50 kD)

some are heterotetramers (two types of similar subunits)

147
Q

shaker voltage sensor

A

S4 has 7 positive charges

Lysine or Arginine spaced at every 3rd position

if any one S4 is mutated, channel is altered in voltage sensitivity, if all removed, voltage sensing lost

148
Q

how was sodium channel gene identified

A

made STX radioactive w/ Iodine, binds to sodium channels

grind up source of tissue (electric eel have lots of sodium channels)

extracted proteins and sequenced N terminal AAs, use this to find clone that encodes protein

149
Q

voltage gated sodium channel structure

A

Na channels are pseudotetramers, not exactly the same, but similar

pore loop controls selectivity (activation/inactivation gates)

S4 determines voltage sensitivity

150
Q

properties of APs based on Hodgkin Huxley Analysis

A
  • All or none
  • Threshold more positive than resting potential
  • Transient reversal of potential at peak
  • Afterhyperpolarization (more negative than resting potential)
  • Maximum frequency set by absolute refractory period
    • Relative refractory period: time until last K channels return to baseline
151
Q

Action potential propagation experiment + results

A

stimulate at position 1, record at V1, V2, and V3

substantial depolarization required to elicit AP

time until AP occurs at V1 depends on stimulus intensity

once AP, spreads at constant velocity

152
Q

Lord Kelvin and Hodgkin

A

solved electrical circuit for an insulated coppor wire sitting in the sea to hel pdesign the Transatlantic Telegraph

Hodgkin applied to electrical circuit of unmyelinated axon

153
Q

passive behavior of an isolated axon

A

the more distant the recording site from the site of stimulation, the slower the rate of change in Vm

154
Q

space constant (λ)

A

indicates how far passive responses will spread and how fast action potentials will propagate

155
Q

gi vs gm

A

gm = membrane conductance = how many channels are open

gi = internal conductance = how many ions are present

156
Q

how does gi and gm scale w/ big vs small axons

FOR UNMYELINATED AXONS

A

gi is proportional to axon cross sectional area (scales with diameter2)

gm is proportional to axon circumference (scales linearly with diameter)

therefore, λ scales with sqrt(diameter), and since conduction velocity of AP depends on λ, it also scales with sqrt (diameter)

157
Q

how fast action potential propagates depends on

A

space constant, λ

158
Q

Local circuit mechanism of action potential propagation: hodgkin

A

during depolarized phase of an AP, most of current flows in/out locally, but some current flows down inside of axon to depolarize regions ahead and behind

AP only goes in one direction bc other direction is still in absolute refractory period

159
Q

all ____ make myelin

A

vertebrates (invertebrates do not make myelin)

160
Q

conduction velocity depends on what factors in myelinated axons

A

spacing b/w nodes

thickness of myelin (number of wrappings) - each layer decreases Cm and increases Rm (time constant unchanged but space constant gets longer)

161
Q

vertebrate axons that are less than ___ in diameter are unmyelinated

A

1 μm

162
Q

voltage sensing K channels: how are they activated

A

in K chanels, all four of the S4 domains must be in “up” position for the channel to open (in steady state, prob. of any one being up at particular voltage is n, prob of all four being up is n4 → sigmoid curve)

163
Q

some K channels can inactivate

A

example: first potassium channel to have its cDNA cloned and sequenced was Shaker

Shaker inactivate/activate at a speed thats intermediate b/w Na and normal K channels

Shaker slows down rate at which APs can fire

164
Q

Mechanism of inactivation in shaker

A

Ball is set of residues close to N terminal that has many positive charges (globular structure)

Chain has many polar residues, not charged

when channel has fully opened (S4 all up), negatively charged residues are exposed and ball binds (activation kinetics are also n4)

165
Q

tests of inactivation model for shaker: ball deleted and chain length variation

A

if ball deleted, inactivation eliminated

if chain is shorter, faster activation

if chain is longer, slower activation

166
Q

Sodium channel inactivation

A

ball and chain mechanism, but different location of ball from shaker (have four balls that can block current flow)

sodium channels have only one ball per sodium channel, chain is short so rate of inactivation is fast

167
Q

S4 channels in sodium channels

A

only three of four S4 voltage sensors need to be up position for channel to open, but all four need to be up to expose the ball binding site

m3 kinetics, exponentional NOT sigmoidal

168
Q

sodium channel diversity and naming

A

pore forming subunits called α subunits, genes that encode them are named SCN#A

protein family called Nav

SCN1A, SCN2A…SCN5A correspond to Nav 1.1, 1.2…Nav 1.5

BUT SCN6A/SCN7A encodes a sodium selective channel (not voltage gated), so not a Nav

SCN9A codes for Nav 1.7

169
Q

Pakistani people without pain

A

mutations are in SCN9A gene, which encodes Nav 1.7 protein

three independent mutations (in the three families they studied), which all make truncated nonfunctional proteins

170
Q

potential mechanisms for action when Nav 1.7 is absent

A
  1. mutation causes developmental problem
    1. not supported, adult knock out mice also fail to sense pain
  2. Nav 1.7 protein essential in axon and for propagation to the spinal cord
    1. not supported, spikes propagate fine once initiated
  3. Nav 1.7 is essential in peripheral terminals of pain sensing neurons (no AP in senses) (true)
  4. Nav 1.7 is essential in central terminals of pain sensing neurons (true)
171
Q

what did recent studies of Nav null mutant mice and humans find

A

Nav 1.7 inhibitors were predicted to prevent pain

  • inhibitors given to normal people still experience pain
  • when naloxone (opioid receptor blocker) can make people who don’t normally feel pain feel some
172
Q

current concept of people with insensitivity to pain

A

long term adaptations of sensory neurons to absence of Nav 1.7 signaling that produces insensitivity to pain

  • It’s more difficult to initiate spikes in response to painful stimuli in people w/o Nav 1.7
  • In spinal cord, opiate receptors in spinal cord are overloaded with opiates, suppressing transmission of pain signals
173
Q

what would happen if Nav was overactive instead of inactive

A

chronic pain

174
Q

primary erythermalgia

A

childhood onset of burning pain in feet/lower legs

variety of SCN9A mutations are known to cause this disorder, all are dominant

S214T mutation is one (serine replaced with threonine), located near one of the voltage sensors

175
Q

S241T mutation in primary erythermalgia results in:

A

small negative shift in voltage at which Nav 1.7 activates

-ramp stimulation given to mutant and WT, found that this change makes neurons more spontaneously active, causing pain

Bio 222 Cell/Molec Neuroscience (7 decks)

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