Exam 1 Flashcards
(175 cards)
1
Q
what are the two systems that are present in all cells, but are highly elaborated in neurons?
A
- Molecular machines for moving material around the neuron
- Ion channels/transporters and the machinery required to properly localize them and adjust their number
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)
3
Q
cations and anions important in neurophysiology
A
Cations: Na+, K+, Ca2+, Mg2+
Anions: Cl-
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)
5
Q
electrically, phospholipids act as ____…..
A
as insulators, giving membranes the property of capacitance (can separate charges)
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
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
8
Q
charged amino acids mneumonic
A
Dragons Eat Knights Riding Horses
9
Q
which amino acid can form disulfide bonds
A
cysteine
10
Q
which amino acids can be phosphorylated
A
Serine (Ser, S), Threonine (Thr, T), Tyrosine (Tyr, Y)
11
Q
proper protein folding sometimes requires:
A
chaperone proteins or post-translational modification
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)
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
14
Q
Nobel prize in physiology/medicine in 1906: names
A
Golgi and Cajal
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
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
17
Q
what is special about golgi labeling/staining
A
staining is sparse, so axonal and dendritic geometry can be observed
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
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
20
Q
1899 Cajal: two major classes of cerebellar neurons
A
purkinje neurons: elaborate dendritic treets, planar
granule neurons: not planar
21
Q
not all neurons have dendrites: two examples
A
spherical bushy cell of the cochlear nucleus
rat submandibular ganglion neurons
22
Q
retrograde vs antergrade transport
A
retrograde: toward cell body
anterograde: toward nerve terminals
23
Q
classic experiment to measure rate of axonal transport
A
- Inject radioactively labeled AAs into extracellular space of cell body
- Usually in spinal cord or DRG (sensory ganglion) to avoid spillover
- AAs are taken up into cell, used to synthesize proteins, some of which travel down axon
- Cut chunks of axon, isolate proteins using gel electrophoresis
24
Q
slow vs fast axonal transport
A
slow: 0.2-8 mm/day
fast: 50-400 mm/day
25
can you use classic expermient to measure retrograde axonal transport
NO
26
Method that measures retrograde transport
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
27
3 types of filamentous elements in all cells
**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
28
microtubules (macrofilaments) and microfilaments stucture
**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
29
molecular motors
kinesins and dyneins
move along micro/macrofilaments using ATP to translocate cargo to distant locations
30
microtubules length in axons
run entire length of axons
- end at cell body
+ end at nerve terminals
31
kinesins
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)
32
KIF1a
responsible for fast anterograde axonal transport of synaptic vesicles
w/o it, mice die
33
slow anterograde transport and kinesins
other KIFs (not KIF1) are slower
speed difference due to taking more stops
34
dynein
uses ATP to walk along microtubules using two "legs" but structure completely different from kinesins
+ to - motor (retrograde axonal transport only)
35
microfilaments in axons locations
mostly at axon terminals
move synaptic vesicles
36
movment in dendrites
microtubules w/ both polarities present, direction of transport is less clear
microfilaments are in dendritic spines
37
what are other functions of microtubules and microfilaments?
essential for axon growth during development
38
Nuclear pores
large pores allow transcription factors (protein) to enter and mRNA to exit
39
neurogenesis
neurons are post-mitotic
in mammals, most neurogenesis happens during prenatal period
40
promotors and enhancers
**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
41
transcription factors
proteins that bind to enhancer/promoter regions to make specific genes available for transcription
42
how is pre-mRNA processed
site specific RNA editing of some transcripts
splicing out introns
5' capping
3' poly A tail
43
ribosome vs rough ER translation
ribosome: protein remains in cell
rough ER: protein becomes membrane protein or exocytosed
44
in neurons, most translation is:
perinuclear (happens just around the nucleus) but also occurs in some dendrites
45
4 types of glia
oligodendrocytes (CNS)
Schwann cells (PNS)
Astrocytes
Microglia
46
difference b/w oligodendrocytes and schwann cells
both form myelin sheath, but oligodendrocytes can myelinate multiple axons but each schwann cell can only wrap one axon
47
function of myelination
support propagation along axon
different levels of myelination allow identification of different neurons in PNS
allow for rapid neuron communication (sensory neurons esp.)
48
example of demyleinating disease
multiple sclerosis
immune system attacks myelin and oligodendrocytes
impairs conduction of APs along axons
can affect muscle movement, coordination, speech, pain, fatigue
49
grey vs white matter
**white:** myelinated axons and oligodendrocytes
**grey:** neuronal cell bodies, unmyelinated axons, astrocytes
50
astrocytes
play role in development and regulation of neuronal communication
help guide axons to destination during development
form connections b/w neurons and blood vessels
51
microglia
immune cells of nervious system (macrophages of CNS)
phagocytose apoptotic neurons, debris
first line of defence if infectious agents cross the BBB
52
what are molecular markers for glial cells
tag different glia by inserting fluorescent tag to marker in gene sequence
53
name the markers for each type of glial cell
**oligodendrocyte & schwann:** MBP (myelin basic protein)
**astrocyte:** GFAP (glial fibrillar acidic protein)
**microglia**: IBA1 (ioninzed calcium binding adaptor molecule 1)
54
consanguinous families
mating b/w family members
have higher probability of recessive mutations showing up
55
neural circuit for pain perception
example: what happens when you step on a tack
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)
56
what gives membranes the property of conductance
open channels, that are a pathway for ionic current flow
symbolized by "g", units Seimens (S)
57
channels open in an _____ manner
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
58
channels in response to a stimulus: amplitude
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.
59
types of stimuli that can change the probability of a channel opening from low to high
membrane potential change (usually depolarization)
binding of a small molecule
physical activation
covalent modfication
60
a few types of channels have a high probability of being open in the absence of \_\_\_\_
a stimulus
most of these types of channels can be shut by a stimulus
61
how are transporters triggered
binding of a molecule triggers a conformational change that allows the molecule to unbind and be released on the other side
62
max flux through transporter vs ion channel
max flow through electrogenic transporter is about 0.1% of a typical ion channel
63
some transporters are electro\_\_\_ and others are electro\_\_\_\_
electroneutral: transport neutral substances
electrogenic: transport charges
64
what does movement against electrochemical gradient require for transporters?
conformational change of some transporters is coupled to an energy source
primary active transporters use ATP
secondary active transporters use gradient established by primary
65
18th century discoveries underlying bioelectricity
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
66
19th century discoveries underlying bioelectricity
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)
67
voltage definition
when you separate charges, you create electrical potential energy (voltage)
device that does this is a battery
68
keeping charges separated requires:
an insulator (non conducting material) b/w the charges
the ability to separate charges is capacitance (C), units farads (F)
69
conductance: ionic solutions and cell membranes
degree to which substance allows flow of charges (g)
ionic solutions have high conductance
cell membranes have low conductance (unless ion channel proteins open)
70
resistance
ability to prevent flow (R), units Ohms (Ω)
reciprocal of conductance (R = 1/g)
71
current definition + determined by what
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
definition of a negative current through channels
cell becomes more positive: positive charges entering or negative charges leaving
also called **inward current**
negative/inward current causes depolarization
73
apparatus for recording electrical signals consists of:
* 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
electrode that's used to record electrical signals: two types of recordings
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
resting potential can vary from __ to __ but generally \_\_\_
varies from -10mV to -90mV
usually around -70mV
76
in non-excitable cells, there is a \_\_\_
steady resting potential
(also in skeeltal muscle fibers and some neurons)
77
many neurons are ____ active
spontaneously:
action potentials
synaptic potentials (too small to detect w/ extracellular recording) due to input from nearby neurons
78
what is the cause of membrane conductance vs capacitance
conductance: ion channels
capacitance: lipid bilayer (insulator)
79
a cell is behaving **passively** when:
conductance does not change in response to a stimulus (doesn't cause more channels to open)
80
properties of two basic electrical circuits in neurons
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
what is the hyrdraulic equivalent of a capacitor
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
Galvani's work in late 1700s
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
Young in 1930ss
zoologist discovered squid's giant axon, large enough for scientists to thread metal wires down it instead of penetrating it
84
Hodgkin and Hukley's work in 1930s-1950s
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
Nernst 1920
discovered nernst equation
86
key facts of logarithms
log(1) or ln(1) = 0
ln(x) = 2.303\*log(x)
87
diffusion across permeable barrier
concentrations of ions become equal inside/outside
88
diffusion across semipermeable membrane
can cause separation of charge
diffusional force and electrical force eventually stabilize
89
when is equilibrium achieved
diffusional force pushing in = electrical force pushing out
90
active transports establish ____ but do not \_\_\_\_
establish ion gradients, but don't directly cause membrane potnetial
91
concentrations of ions outside vs inside in mammalian neuron
Na, Cl, Ca more concentrated outside
K more concentrated inside
Mg equal inside and outside
92
How do we test if a single class of ion channels is responsible for the resting potential?
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
ion substitution experiments
change extracellular concentration of the ion
predict what Ex will be for various outside concentrations using nernst, then compare to experimental data
94
10 fold change in outside concentration leads to how much change in Ex
60 mV change (for monovalent)
outside conc smaller: becomes more positive
inside conc smaller: becomes more negative
95
conclusions of ion substitution experiments if predicted potentials do not match actual data
ion does not contribute to resting potential (at resting potential, all ion channels are closed)
96
when can goldman hodkin katz equation be used and what are the assumptions
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
an alternative approach (from goldman-hodgkin-katz equation) to predict resting potential is to:
solve an equivalent circuit for multiple conductances in parallel
98
at any stable potential, we can ignore the:
capacitor
only sum of resistors matter
99
driving force
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
at resting potential, there is no net \_\_\_
current (Itotal = 0)
example: if only Na and K channels open, then Itotal = INa + IK = 0
101
at resting potential of a certain cell gna/gk = 0.14
what does this mean
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
resting potential is a _____ not an \_\_\_\_
steady state, not an equilibrium
if ATP is removed, it is not stable
103
effect of addding a Na/K pump inhibitor
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
significance of what would happen if sodium conductance dominated as much as potassium conductance usually does
a very rapid increase in gNa while gK remains the same is the basis of the rising phase of an action potential
105
it is impossible to go ____ to Ena and _____ to Ek unless:
positive, negative
unless current is applied from an external source (any potential b/w ENa and Ek can be achieved by cells)
106
at resting potential gNa ____ gK
gNa \<\< gk thus Vm is near Ek
at the peak of AP, this flips so Vm is near ENa
107
threshold definiton
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
shape of passive responses
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
shape of passive responses: final response amplitude is:
linearly proportional to stimulus amplitude
for a passive response ΔV/ΔI is defined as the input resistance
110
the speed of the passive response (response to current from stimulating electrode) indicates the speed at which:
a neuron will be able to integrate synaptic inputs
111
details about the shape of the passive response (to current from stimulating electrode) can indicate:
where active synapses are located on the dendritic tree
what part of the neuron the rcording was made from
112
the amplitude of the passive response (to current from stimulating electrode) can indicate
how big the neuron is
how much synaptic input is required to drive the neuron past threshold
113
passive behavior of an isolated cell body
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
time constant (τ) of an exponential function determines:
the rate of change
115
greater time constant (τ) means:
it takes longer to reach its "max" value
reaches "max" after 5 time constants
116
how to find time constant (τ) from graph
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
once threshold is exceeded, all action potentials have:
the same peak amplitude and time course
118
intensity of a stimulus is encoded by:
the frequency and pattern of action potentials
frequency is limited by the refractory period
119
the maximum firing frequency is:
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
relationship between sodium and potassium equilibrium potentials compared to resting potential
121
Hodkin and Katz:
measured peak of the action potential at various extracellular sodium concentrations using squid axons
122
Hodkin cycle:
depolarization of the axon opens sodium channels, sodium influx, which further depolarizes the cell, opening more channels, etc.
123
what causes the initial depolarization that triggers an action potential?
the anatomical location of excitation determines the site of initiation (of depolarization) and the direction of spike propagation
124
orthodromic vs antidromic propagation
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
hodgkin and huxley perfected the ____ that allowed them to break postive feedback loop of sodium
voltage clamp: breaks positive feedback cycle by preventing channel opening (due to changing membrane potential)
126
Hodgkin Huxley analysis of squid AP mechanisms
papers explained why they needed to use voltage clamp, showed how to separate currents from voltage clamp recordings
127
how does voltage clamp work
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
voltage clamps: voltage steps
family of currents: measure different test potential
w/in 200 µs, feedback amplifier charges membrane capacitance and reestablishes Vm = VCMD
129
voltage recording vs voltage clamp recording
voltage recording = current clamp
130
key qualitative results of voltage clamp analysis of the squid axon (Hodgkin and Huxley)
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
what determines the waveform of voltage clamp currents
under voltage clamp: what is constant
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
separation of currents by changing outside sodium concentration
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
TTX
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
STX
saxitoxin, very similar to TTX, blocks voltage gated sodium channels
135
potassium currents under voltage clamp when TTX is present
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
how do you find conductance from voltage clamp recordings using toxins
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
K: activation and inactivation gates
138
TEA
blocks K currents, allowing one to observe Na currents directly
139
sodium currents under voltage clamp
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
Na: activation and inactivation gates
141
gating changes during an action potential
142
Seymour Benzer (1960, 2017)
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
how do molecular neuroscientists identify residues involved in gating and seletivity?
* 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
how can you test hypotheses about what amino acid residues are involved in gating/selectivity?
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
Shaker potassium channel protein
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
subunit organization in shaker
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
shaker voltage sensor
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
how was sodium channel gene identified
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
voltage gated sodium channel structure
Na channels are pseudotetramers, not exactly the same, but similar
pore loop controls selectivity (activation/inactivation gates)
S4 determines voltage sensitivity
150
properties of APs based on Hodgkin Huxley Analysis
* 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
Action potential propagation experiment + results
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
Lord Kelvin and Hodgkin
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
passive behavior of an isolated axon
the more distant the recording site from the site of stimulation, the slower the rate of change in Vm
154
space constant (λ)
indicates how far passive responses will spread and how fast action potentials will propagate
155
gi vs gm
gm = membrane conductance = how many channels are open
gi = internal conductance = how many ions are present
156
how does gi and gm scale w/ big vs small axons
FOR UNMYELINATED AXONS
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
how fast action potential propagates depends on
space constant, λ
158
Local circuit mechanism of action potential propagation: hodgkin
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
all ____ make myelin
vertebrates (invertebrates do not make myelin)
160
conduction velocity depends on what factors in myelinated axons
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
vertebrate axons that are less than ___ in diameter are unmyelinated
1 μm
162
voltage sensing K channels: how are they activated
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
some K channels can inactivate
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
Mechanism of inactivation in shaker
**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
tests of inactivation model for shaker: ball deleted and chain length variation
if ball _deleted_, inactivation eliminated
if chain is _shorter_, faster activation
if chain is _longer_, slower activation
166
Sodium channel inactivation
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
S4 channels in sodium channels
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
sodium channel diversity and naming
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
Pakistani people without pain
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
potential mechanisms for action when Nav 1.7 is absent
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)
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what did recent studies of Nav null mutant mice and humans find
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
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current concept of people with insensitivity to pain
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
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what would happen if Nav was overactive instead of inactive
chronic pain
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primary erythermalgia
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
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S241T mutation in primary erythermalgia results in:
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
