Exam I Review Flashcards
2 types of cells in the brain
Neurons - ~100b in the brain
Glia - 10-50x more glia
Neuron structure
- Polarized (has different functional regions)
- Divided into 4 anatomical/functional regions
- Dendrites
- Soma
- Axon
- Axon terminals
Functional neuron classification
- Sensory neurons (peripheral to CNS)
- Motor neurons
- Interneurons (largest #, most in brain, relay (projection)
Morphological neuron classification
- Unipolar (1 process)
- Bipolar (1 axon, 1 dendrite)
- Multipolar (1 axon, multiple dendrites)
DRG neuron
A special type of bipolar neuron (pseudo-unipolar neuron)
The stem axon separates into a peripheral and a central axonal branches
Neuronal Markers for parts of neuron
MAP2 - DENDRITES + SOMA
(NOT axons)
Tau - AXON
These proteins make excellent neural markers
==> can do double culture
Types of glial cells
- Microglia [clearing cells involved in disease, scavengers of the brain, pick up cell debris, activated after nerve injury]
- Macroglia
(1a) Schwann cells [form PNS myelin]
(2a) Oligodendrocytes [CNS myelin, single oligo can wrap around many cells]
(3a) Astrocytes [most abundant in CNS, various functions]
Schwann cell
Speed up AP velocity
Very little cytoplasm, basically just a lipid bilayer
Form many, many layers around
Functions of glial cells
[Most abundant in brain, =/ glue, located b/w neurons fill up much of brain]
- Structural support
- Form myelin sheath
- Microglia = scavenge/cell debris clean-up
- Help neuronal signaling
- - Don’t directly participate (no AP) but help maintain ionic conditions, buffer extracellular [K+], some astrocytes take up NTs - Guide neuron migration and axon outgrowth
- Form BBB
- Release GFs to nourish nerve cells
Glial stem cells
oligo and schwann cells have stem cells
can regenerate
some astro
Neural communication
electrical + chemical
Chemical signal converted back into electrical
AP –> SP –> AP [synaptic potential]
Nernst equation
Tells you equilibrium potential of an ion
E = RT ln (ion)o
/zF /(ion)i
Equilibrium potential
When chemical gradient = electrical gradient
The membrane potential where net flow = 0
Hypothetical cell
- Higher K inside
- Channels closed
- Then channels open, what happens?
- When closed - no membrane potential when equal #s of K+ and A- on each side
- Channels open
K+ ions flow out due to concentration gradient
Leaving behind negative charge
(outside becomes more pos, inside becomes more neg)
Then reaches equilibrium
GHK Equation
Calculates mem potential when multiple ions present
Resting membrane potential depends on GHK
Mem potential not governed by ion ion - it is established by the RELATIVE CONTRIBUTIONS of many
Equation
Na+
12 in, 145 out
More outside
K+
139 in, 4 out
More inside
Cl-
4 in, 116 out
More outside
Ca2+
0.1 micro in, 1.8 mm out
More out, but low overall (universal signaling molecule)
Mg2+
0.8 in, 1.5 out
More out
A-
138 in, 9 out
More in, very low out
A-: proteins, other organic molecules
RMP is most permeable to
K+
then Ca2+
Not Na+ though
What contributes to RMP?
- Leak K+ channels
- Nonselective cation channels
- Leak Na+ channels
What contributes to AP?
Voltage-dependent Na+ and K+ channels
Why do cells need ion potentials?
- Too much ENERGY needed to move a monovalent ion from water to lipid
- Ions are CHARGED so can’t go through hydrophobic lipid bilayer
- According to BOLTZMANN distribution, probability of it happening is basically none
Essential properties of ion channels
- Membrane proteins
- Selectivity (recognize/select particular ions)
- Conduction (pass ions passively and rapidly - no energy needed bc going down concentration gradient)
- Gating (open/close in controlled fashion, according to intra/extra -cellular cues)
Roderick MacKinnon
Nobel prize for chem in 2003
Found structure of ion channels
worked with Kcsa K+ channel
Cryo-electronmicroscopy (Cryo-EM)
Historically low resolution, improved to high res in 2013
Allowed for many channels to be discovered
What allowed for many ion channels to be discovered?
Roderick MacKinnon (first discovered ion channels) Cryo-EM
NDMA receptor
Important for learning and memory
Why are ion channel structures important?
- Gain better understanding of how channels work
- Can map disease-causing mutations onto channels
- Can study drug molecules to design better meds
Classification of ion channels
- Based on SELECTIVITY
2. Based on GATING MECHANISMS
voltage-gated
ligand-gated (NTs)
mechanically-gated
temperature-gated
gap-junctional channels (usually always open, but can be regulated; can allow ions and small molecules due to large pores)
Nonselective cation channels
Allow all POS ions to pass through
Typically K+ and Na+, sometimes Ca2+
Gating
Opening/closing of channel
Even LEAK CHANNELS can close
All ion channels go between these 2 states
4 ways to open/close a channel
- Voltage change (mostly opened by dep.)
- Ligand binding/unbinding
- Membrane stretch /mechanical force
- Temperature change
Inactivated state
Many channels have this stage
Allows AP to be unidirectional (refractory period)
I cannot be opened by stimulus
Tetrodotoxin
TTX
Puffer fish toxin
Na+ channel inhibitor
Saxitoxin
STX
Shellfish toxin
Na+ channel inhibitor
Tetraethylammonium
TEA
K+ channel inhibitor
Often used in research
Ba2+
K+ channel inhibitor
Dihydropyridines
Ca2+ channel inhibitor
Cd2+
Heavy metal
Ca2+ channel inhibitor
Many heavy metals can block channels
Curare
Blocks nicotinic ACh receptor on NMJ
β-bungarotoxin
Snake toxin
Blocks ACh receptor
Procaine
Local anesthetic
Na+ channel inhibitor
Blocks AP firing, (why you don’t feel pain)
Lidocaine
Local anesthetic
Na+ channel inhibitor
Blocks AP firing, (why you don’t feel pain)
3 Factors that determine size of single-channel currents
- Permeability
(determined by ion and channel - do they “like” each other?) - Ion concentration
- Membrane voltage
(higher v = more ions can pass through)
Single channel conductance equation
i = γ(Vm-Vrev)
γ = single-channel conductance Vm = membrane potential Vrev = reversal potential
Whole cell current determined by 4 factors
- Total # of channels on PM
- Single-channel conductance (γ)
- Single channel Po
- Electrochemical driving force
Clamp info
Add!
Reversal potential
Potential at which there is no net flow of currents
Open probability vs. open time
Open time: usually set threshold at 50% of amp; measure that as open time; numerical mean
Open prob: make open time histogram –> use distribution to find open time, do same thing for close time
Single-channel open probability
Sum(open times)
/total recording time
How is K+ channel so selective?
Puzzling sincle Na+ is smaller
Different water shells
Takes more energy to strip water away from Na+ than K+ (dehydration energy higher in Na ions)- this bc Na moles are smaller, ∴ water moles are able to exert stronger weight
When enter selectivity filter, surrounded by AAs that form surrogate hydration (no energy cost to ion due to physical-chemical arrangement of s.f.); matches arrangement
K+ entering process
When K+ ions approach, getting into pore, loses 4 H2O replaced by AA
Normal arrangement: 4 above, 4 below
Current-voltage relationship
Current grows bigger w/depolarization
Subunit movement
Formed by S6 segment + s.f.
S4 moves in response to depolarization
S4 movement pulls on S4/S5 linker
S4/S5 linker causes change in S5/S6, that will open/close pore
Capsaicin
chili pepper, spicy molecule
TRPV1
Cation channel, R for capsaicin
Important for temp
Can have lower gate and s.f. - the 2 can open/close independently
Channels can have…
1 or 2 gates
At different locations
And s.f. can act as gate
Ligand-gate relationship
Ligand binding site is far away from gate
Causes wave of conformational change
Functions of ion transporters
- Create/maintain ion gradient
2. Uptake NTs
Ion channels vs. ion transporters
- Transporters need ATP, channels don’t
- Transporters are slower
- Transporters move at least ion ion Against conc.gradient, channels only conduct down
Ion channel vs. transporter relationship
Channels are essential for setting up RMP
If open…dissipation of ion [ ] gradient
–> ∴ need TRANSPORTERS to restore gradient
2 types of ion transporters
- Ion pumps (ATPases)
Na/K pump, Ca pump, H pump - Ion exchangers
Antiporters, cotransporters
Ion pumps mechanism
Use ATP hydrolysis energy to transport ions
Na/K: High Na out, high K in
Ion exchangers mechanism
Indirectly use ATP, use gradients to drive ions in/out
(1) Antiporters - ions go against conc.gradient in diff directions (2) Cotransporters - ions go in same direction
Ion exchangers mechanism
Indirectly use ATP, use gradients to drive ions in/out
(1) Antiporters - ions go in different directions (2) Cotransporters - ions go in same direction
Na/Ca transporter
[Antiporter]
Na transported in…
Exchanger uses this energy…
….to take Ca out
Na/H transporter
Only occurs when inside of cell becomes acidified (bc usually pH similar on both sides)
Na in…
Exchanger uses this energy to take
…H out
Ion transporter properties
- (Some) ions transported against conc.gradient
- Requires ATP, either directly or indirectly
- Some ion transporters are ELECTROGENIC (mem pot. can be affected by electrical activity)
Jens Sken
Discovered Na/K pump
Na/K pump - discovery and importance
Originally discovered by Jens Sken
Without Na/K pump - no RMP, to AP, no LIFE
Na/K pump is electrogenic
Meaning…
Oubain
Poison made in adrenal glands, also found in plants
Drug used to treat certain types of heart failure (↑↑ pumping)
Experimental effects…
Palytoxin
Can bind where oubain binds and makes pump permanently active
Which ions?
Physiological role of Ca pump
Keep intracellular [Ca2+] low
or restore concentration (working overtime)
How do ions eventually cause voltage/signaling?
Ion movement produces current
Current changes voltage
Membrane voltage used for signalling
3 types of transient voltage changes
- Receptor potential (produced by external sensory stimuli, mainly in specialized sensory organs)
- Synaptic potential (produced at synapses)
- AP
RP and SP are usually local voltage changes, usually passive propagation/summation
Where are signals produced in neuron
Local signals produced in dendrites/soma
Axon hillock = trigger zone
Traveling from soma to hillock = passive propogation
Transient electrical signals - 3 important characteristics
- Magnitude
- Time course (kinetics)
- Propagation (depends on distance and speed)
Passive propagation is determined by…
Membrane properties of neuron
Current equation
I = V/R
Current = voltage/resistance
Current-voltage relationship
Linear
What factors determine magnitude of voltage?
ΔVss = I * Rin
I we control, Rin = input resistance of cell
- Conductance
- Size (bigger cell = bigger membrane = more channels = higher conductance = lower resistance)
- Higher open prob = lower resistance
- Channel density
- Single-channel conductance
- Size of cell
- Open probability
Conductance formula
g = 1/R
Conductance is the inverse of resistance
Specific membrane resistance (Rm)
Size-independent
Resistance of a given area of membrane
Factors:
- Po
- # of channels
- PM conductance/resistance
Capacitance take-home
Cell mem is a capacitor
2 conducting plates w/insulator in between
The larger the capacitance, the more charges you can store
If PM was just a conductor, membrane-voltage-Δ would be instantaneous; instead it is delayed
Dialectic constant
Water has high DC (80)
Oil has low (2)
How does size affect capacitance?
Magnitude and time course greatly affect AP firing
- larger capacitance = slower the response = timing/frequency would decrease
- smaller capacitance = can be charged more quickly = faster response = timing/frequency would increase
What determines passive conduction?
- # of leak channels on membrane(if current very leaky most of the current you inject will leak out)
- Diameter
(how much current can go through; charges bumping each other quickly) - Resistance of cytoplasm
(higher resistance=more likely current is to x)
Bigger SA, more channels, more leaking
- Resistance/conductance of PM
Conduction of sea animal vs. mammalian axon
Squid giant axon
Sea squid cyto should be MORE conductive
- Sea animal cyto has very high salt; high salt = more conductive
- Larger diam - so rho (resistance) for squid axon is very low
SEE NOTES
Rho/ϱ
Resistance of 1cm^3 of cyto
Constant
Passive membrane propogation
Propagates passively without activating voltage channels, ∴ subthreshold
Factors:
- Membrane input resistance
- Capacitance (slows down mem voltage)
- Time course
Distance depends on
- Mem properties
- Size of processes (dendrites, axons)
- Intrinsic properties
First ones to record APs
Alan Hodgkin
Andrew Huxley
Used electrodes to record APs on squid axon
Able to completely describe biological phenomenon through math
What can we learn from single-channel recordings?
We can see single-channel conductance
- Plot amplitude of currents against voltage to get i-V relationship
4 Patch Clamp Configurations
- Cell-attached
- Whole-cell
- Inside-out
- Outside-out
Cell-attached patch clamp
1.
Whole-cell patch clamp
2.
Inside-out patch clamp
3.
Outside-in patch clamp
4.
What is reversal potential?
Potential at which there is no net flow of ions across membrane
Refers to the fact that a change of membrane potential on either side of the equilibrium potential reverses the overall direction of ion flux
Electrogenic
Producing a change in the electrical potential of a cell
Electrogenic
Producing a change in the electrical potential of a cell
How is Na/K+ activity affected by extracellular [K+]?
-
Train of axons
-
How is ouabain used as a heart medication?
Increases heart pumping
Inhibits Na/K+ pump, [Na] inside cardiac cell ↑slightly
The ↑[Na] will inhibit the Na/Ca exchanger (antiporter), making it go slower; ∴ less Ca pumping out
↑ in Ca inside makes muscle contractions stronger
Na/K+ pump
3 Na out, 2K in
Ouabain effect on AP
Blocks Na/K+ pump
If pump is blocked with ouabain, AP will burst sooner and faster
Input resistance
Rin= Vss / i
Reflects the extent to which membrane channels are open.
A low resistance (high conductance) implies many open channels, while high resistance implies many closed channels
Tau
Time constant
A quantitative measure of how fast the membrane responds to current flow
Make sure you know how to do this
τ = Rin * C
Change in voltage over time
ΔV(t) = ΔVss (1-e^(-t/τ)
Factors for AP conduction velocity
Conduction velocity is directly proportional to nerve fiber diameter and degree of myelination
(The larger the diameter of the nerve the lower the resistance there is to the flow of current along its length)
The larger the fiber type diameter and the more abundant the myelin, the faster the nerve conduction velocity.
λ
Lambda/length constant
Mathematical constant used to quantify the distance that a graded electric potential will travel along a neuron via PASSIVE electrical conduction
The greater the value of the length constant, the farther the potential will travel
Po response to voltage
Po increases with depolarization –> reaches max level (~80%)
Overall: voltage-dependent parabola
Conductance response to voltage
Conductance increases further and further w/depolarization because more and more channels opening faster and faster
Feedback Cycle
Diagram in notes
AP trajectory and diagram
In notes
K+ equilibrium potential
-80 mV
Na+ equilibrium potential
+60 mV
Driving force
When an ion is not at its electrochemical equilibrium, an electrochemical driving force (VDF) acts on the ion, causing the net movement of the ion across the membrane down its own electrochemical gradient
Vdf = Vm − Veq
Push to reach equilibrium
What does RMP reflect in regard to ion equilibrium potentials?
In general, what determines where it settles?
RMP is -60, which is closer to Ek than Ena - this is because there are many K+ leak channels on PM and few Na
Thus the membrane is many more times permeable to K+
In general, depends on:
- # of channels
- open probabilities
Refractory periods
x
3 factors that affect AP conduction speed
- Myelination
- Axon diameter (more diam = lower resis)
- Extracellular K+
Myelin made by…
Macroglia
Schwann cells in PNS
Oligos in CNS
(single oligo can wrap around many cells)
