Biology of the Brain
Voltage (V)
-Difference in electrical potential
List the three criteria that define a neurotransmitter.
1. substance must be synthesized and present within synaptic terminal with amounts sufficient enough to produce a response. 2. Substance must be released in response to synaptic depolarization (Ca dependent) 3. Specific receptors for substance need to be present on postsynaptic cell
Capacitance (C)
A measure of the ability to separate (and store) electrical charge
What are the similarities and differences in the gating properties, kinetics and calcium permeability of AMPARs and NMDARs?
AMPARs mostly only permeable to Na and K, but some are also permeable to calcium, basis for many forms of synaptic plasticity, whereas NMDA is permeable to Na, K, and Ca and requires glycine
What is the 'function' of an action potential?
Electrical signal, Rapid, non-linear depolarization in membrane potential followed by rapid repolarization
Nernst F
Faraday's constant (96,500 coulombs/mole)
What are typical values of equilibrium potentials for Na ions in neurons?
In: 5-15 Out: 145
Dendrites
Input region of the cell, receive signals.
What are the functional differences between neurons and glial cells?
Neurons are excitable (membranes) whereas glial are not. Neurons can generate action potentials, but glials cannot. Neurons are for long distance signaling. Glial can still carry out complex signaling. Both have cell-type diversity, but neurons have exceptional diversity.
Explain the reversal potential of the NMJ EPC.
Straight diagonal line, linear
Nernst T
Temperature (K)
How are tetrodotoxin and tetraethylammonium used to separate ionic currents?
Tetrodotoxin blocks Na+ currents, TEA blocks K+ currents
Explain how to determine the role of specific ions using the voltage clamp?
This separates the currents, you need to eliminate other ions to see the specific currents via ion substitution
What techniques allow us to record ionic currents?
Voltage-clamp & patch clamp
What is a reversal potential?
the membrane potential at which there is no net (overall) flow of that particular ion from one side of the membrane to the other.
neural coding
the way in which information is represented in the brain
What are the limitations of the Nernst equation for predicting the resting membrane potential?
you need very large differences in ionic concentrations between the inside and outside of the cell in order to shift Vm
List in order, the steps that comprise the synaptic vesicle cycle.
(1) Neurotransmitters are actively transported into synaptic vesicles (2) Synaptic vesicles cluster and dock at the active zone (3) In the active zone, vesicles are primed for Ca2+ -dependent fusion (4) After fusion, synaptic vesicles undergo endocytosis via clathrin-coated pits and recycled (5) Recycled vesicles are re-acidified and re-filled with NT either directly or after passing through an early endosome
Describe the movement of ions at electrochemical equilibrium.
-58mV for K+ ions
Explain the relationship between reversal potential for a NT receptor-mediated synaptic current and AP threshold in predicting whether an NT is excitatory or inhibitory
-A NT is excitatory if Erev for the receptor is more depolarized than threshold -A NT is inhibitory if Erev for the receptor is more hyperpolarized than threshold
What are the factors that influence the integration of synaptic inputs onto dendrites?
-location of inputs (spatial summation) -timing of inputs (temporal summation) -neurochemistry of inputs (excitatory or inhibitory) -passive cable properties -active conductances
In what way is the NMJ EPC Ohmic? What accounts for gEPC?
-the relationship between current and voltage (the I-V plot) is linear and therefore is described as "Ohmic" (follows Ohm's law)
List the steps leading from an action potential in a presynaptic cell to the binding of neurotransmitter on the postsynaptic cell.
1. NT is synthesized and stored in synaptic vesicles 2. AP invades presynaptic terminal 3. Depolarization of presyn terminal, opens VGCCs 4. Influx of calcium through VGCCs 5. Calcium causes vesibles to fuse with presyn membrane 6. NT is released into synaptic cleft via exocytosis 7. NT binds to NT receptors in postsyn membrane 8. Current flows through NT receptors 9 Postsynaptic depolarization 10. retrieval of vesicle from plasma via endocytosis
Describe the lifecycle of ACh from synthesis to degradation.
1. Precursors aceytl CoA and choline 2. Synthesized by choline acetyltransferase 3. packaged in vesicles by VAChT
Acetylcholine
ACh in synaptic cleft is degraded by this in synaptic cleft, choline is transported back into terminal
What are the major forms of active transport? How do they each contribute to electrochemical gradients?
ATPase pumps: maintains transmembrane concentration gradients (like high K and low Na in the cell) Ion exchangers: energy is obtained from electrochemical (concentration) gradient in different directions Co-transporters: provides the energy to transport other ions in same direction
Conductance (g)
Ability of charge to flow
How do you measure the reversal potential using a voltage-clamp? What is the significance of the reversal potential measured in this way?
Ability to measure currents, flow in and out of cell
What is the difference between absolute and relative refractory period?
Absolute: Is the period of time during which a second action potential ABSOLUTELY cannot be initiated, no matter how large the applied stimulus is. Relative: Is the interval immediately following the Absolute Refractory Period during which initiation of a second action potential is INHIBITED, but not impossible.
In which part of the neuron are action potentials initiated?
Axon, the "output"
Why did investigators study EPPs under conditions of low [Ca2+]?
By lowering calcium, release probability is lowered
In a voltage clamp, What happens when you depolarize Vm from -65 mV to 0 mv?
Captive current spikes, then there is a transient inward current dipping below 0, with a delayed outward current going up to +1
How does the IV relationship for cations differ from that of anions?
Cations go with the flux, but anions go opposite of the ion flux (movement of positive charges). For example, Cl- moving one way would cause the current to go the other way.
Relate the molecular structure of a channel to its function.
Channel structure defines what ion will be brought across the membrane
In a voltage-clamp experiment, what happens to the current when you hyperpolarize Vm from -65 mV to -130 mV?
Charge is redistributed, and a captive current is formed as a sudden dip but then returns to 0
How is a closed state different from an inactivated state?
Closed is not allowing any ions into the cell whereas inactivating is the process of closing the channel.
Describe the components of the SNARE complex and their role in vesicle fusion.
Docking (vesicular SNARE synaptobrevin comes into contact with plasma membrane SNARES), Priming (they wind around each other into a SNARE complex), SNARE complex (vesicle is brought in close prozimity to the plasma membrane and configured for fusion), Calcium entry(VGCCs open), Fusion (SNARE complex drives membrane fusion to release vesicle contents into synaptic cleft)
Define driving force.
Driving Forcex = (Vm-Ex)
Are dendritic spines associated with excitatory or inhibitory synapses?
EXCITATORY!
The Nernst Equation
Ex=RT/zF*ln([x]out/[x]in)
Name two network motifs involved in the myotatic reflex microcircuit?
Feedforward excitation, Feedback/recurrent inhibition
Current (I)
Flow of electrical charge
Metabotropic receptors
G-protein coupled receptors, activation initiates a second messenger signaling cascade, may couple downstream ion channels, slower timescale
Describe the structure and function of the gap junction. What electrophysiological feature(s) can help you identify a gap junction (electrical synapse)?
Gap junctions are electrical synapses, they allow ionic current flow passively from one neuron to another. It's bi-directional and very rapid. They're formed by connexon channels
Nernst R
Gas constant (8.314 joules/°mole)
Ohm's Law
I = V/R, V= IR
How would you rearrange Ohm's Law to include conductance?
I=g/V
Describe how hydrated K+ ions pass through the selectivity filter of KcsA.
In the "selectivity filter" of most K+ channels, ions lose their waters of hydration and co-ordinated by backbone carbonyl groups
What is the molecular structure of a 'typical' ion channel?
Integral membrane proteins, multiple areas for intra- and extracellulatory areas, can span membranes, contain repeated subunits, a pore domain (ion permeation pathway), often have auxiliary subunits
Explain the reversal potential for GABAARs in terms of ion flux and current.
Ionic current has a negative relationship with ionic flux; current goes linear positive, flux is linear negative. Forms an X
Explain why the Na/K ATPase is electogenic?
It generates an electrical current due to the ion flow
Compare and contrast key features of gK and gNa.
K+ activation is delayed, Na+ is rapid and decreases rapidly following activation
Vm is negative to Ek in Nernst equation?
K+ flows into cell
Vm is positive to Ek in Nernst equation?
K+ flows out of cell
What are the key differences between the early, inward current and the late, outward current observed by Hodgkin and Huxley in the squid giant axon?
Late currents rise very quickly with no dips, early current has a dip in membrane potential
What is the difference between a microcircuit and a macrocircuit?
Macrocurcuits perform complex neural functions by a massive ensemble of neurons, while microcircuits are operations performed by a small number of neurons.
How are macroscopic currents and microscopic currents related?
Macroscopic currents are the sum of microscopic currents (currents through a single ion channel or small 'patch' of membrane)
Describe the role of Ca2+ in transmitter release. How can you confirm the importance of Ca++ in transmitter release experimentally?
Measuring membrane potential, calcium causes current to both drop and raise sharply
What is an MEPP? What information do MEPPs provide about transmitter release?
Miniature end-plate potentials, they're non-stimulated voltage deflections observed in muscle and appear spontaneously with a small amplitude, might reflect release of neurotransmitter in discrete packets (quanta)
Explain how small-molecule neurotransmitters and neuropeptides can be released differentially by a single cell
Multiple transmitters release depending on following: activity, calcium concentration, calcium 'nanodomains', specific release machinery
Describe Mg2+ block of the NMDAR pore.
NMDAR pore binds extracellular to Mg, at hyperpolarized Mg blocks pore
In a voltage step from -65 mV to +44 mV, we fail to see any inward current despite a large gNa. Why?
Na+ could be blocked
What is the purpose of the blood-brain barrier?
Necessary for separating blood from extracellular fluid of brain, protects brain from changing blood composition, the physical barrier results from tight junctions between endothelial cells
When do you use the Nernst Equation vs. the Goldman Equation?
Nernst is used for when there's a single membrane potential for an ion to be calculated, whereas Goldman is multiple permanent ions to membrane potential.
Compare and contrast the passive 'cable' properties of an axon with an axon that is generating action potentials.
Passive generation will cause Vm to decrease exponentially as you get away from the site of the stimulus. The stimulus will not be detectable several lengths away. However, with an axon that is generating action potentials, there is no decreasing of amplitude, and the transmission can occur faithfully.
Describe the structure and function of a chemical synapse.
Rapid release and diffusion from a substance from one cell to another, which binds to a receptor, resulting in a change in the receiving cell
Resistance (R)
Resists flow of charge
What is the relationship between the Goldman Equation and reversal potential?
Reverse potential (equilibrium) can be sound for one ion but not the others, which would throw off equilibrium if looking at multiple ions.
Explain how the patch-clamp method works
Same circuitry of voltage-clamp, but using an electrode (glass micro-pipette with a very fine tip) to tightly seal onto small area of a cell membrane known as a "patch"
Describe the similarities and differences between neurotransmitters ACh, glutamate and GABA.
Similarities: ion channel pore part of their structure, domains for ligand binding, different subunit comps can confer unique properties Differences: structure
How can you differentiate between small-molecule neurotransmitters and neuropeptides?
Small: synthetic enzymes transported to terminals, NTs synthesized in terminals, then packaged into vesicles Neuropeptides: synthesized in cell body, packaged into large dense-core vesicles, vesicles transported to terminals
Describe the lifecycle of glutamate.
Synthesis: Major precursor is glutamine, released by glial cells, taken up by neurons
Use the Nernst Equation to test hypotheses concerning the role of specific ions in generating the resting membrane potential
The Nernst equation allows you to calculate the electrochemical equilibrium potential for X (a single ionic species), or Ex
In what ways do GABAARs resemble nAChRs?
They're pentameric receptors with varying subunit composition
What factors affect action potential conduction velocity?
Time & Distance constant
What parameter do you set for a voltage clamp?
Vm, the starting membrane potential
Driving Force (I)
Vm-Ex
Describe how ion channel gating can be modeled as having 'states'.
Voltage sensor senses 3 states: closed, open, and inactivated
What are the advantages/disadvantages of various techniques of recording ionic currents?
Voltage-clamp: Can't measure mammalian neurons with it, but is easier Patch-clamp: need to measure the macroscopic current, can sample everything from single ion channels to every ion channel in a cell
Determine the direction of the net ion flux when we change the potential away from the equilibrium potential.
Watch for the flow of ions; when a side goes more negative, the flow of positive ions (K+) is going to be toward that negative side. Vice versa if the side goes positive.
NEUROPHYSIOLOGY
a branch of physiology concerned with the function of the nervous system, specifically, electrical signaling by neurons
List the forces that are 'in play' at electrochemical equilibrium.
concentration gradients, active /passive transport,
What parameter do you calculate for a voltage clamp?
conductance
How are conductance and driving force related to ionic current?
g=I/(Vm-E)
What are typical values of equilibrium potentials for Ca ions in neurons?
in: 0.0001 out: 1-2
What are typical values of equilibrium potentials for K+ ions in neurons?
in: 140 out: 5
What are typical values of equilibrium potentials for Cl ions in neurons?
in: 4-30 out: 110
What parameter do you measure for a voltage clamp?
ionic currents
Ionotropic receptors
ligand-gated ion channels, rapid changes in postsynaptic membrane potential
cell body (soma)
nerve center of the neuron. Contains nucleus and major organelles for function
dendritic spines
sites of excitatory synaptic inputs from other neurons, spines confer unique signaling properties to these neurons
What is the evidence for, and significance of quantal release of neurotransmitter?
stimulus-evoked subthreshold EEP amplitudes approximated MEPP amplitude
Nernst z
valence (charge, ie. -1, +1, +2, etc.)
Describe the differences between voltage-gated and ligand-gated ion channels.
voltage channels are structures that change in conformation in response to changes in Vm, and ligant-gated channels are activated by neurotransmitters.
What are the major classes of channels? How can they be distinguished experimentally?
voltage-gated ion channels (via voltage sensors) & ligand-gated ion channels (neurotransmitter measuring)
