Unit 2: Neurophysiology Flashcards
Electrical potential
Voltage, all living cells have it across their membranes (Vm)
How do water molecules bind to ions?
Electrostatically
Hydrated diameter
The water/ion molecule complex has a larger diameter than the ion itself, important limiting factor for ions moving through pores (channels) in membrane
Ohm’s Law
Voltage (V)= I (current) R (resistance)
OR
Current= Voltage*Conductance (G=1/R)
How do non-polar, fat soluble (lipophilic/hydrophobic) molecules cross membrane?
They dissolve in the lip bilayer
How do polar, water soluble (lipophobic/hydrophilic) molecules cross membrane?
They only cross through ion channels or carrier molecule “poles” in membrane
Ion channels/ what they are composed of
They are complex proteins composed of 4-6 polypeptide subunits (has hydrophobic surface that associates with phospholipid bilayer), form an aqueous pore (opening) where ions can pass
Ion channels show 4 levels of protein structure
- Primary: chain of AAs linked by peptide bonds
- Secondary: membrane spanning segments, lipophilic AAs, coil into alpha helices
- Tertiary: folded alpha helices create a subunit
- Quaternary: multiple subunits assemble together
Fundamental properties of channels (3)
- Channels are gated (open –> closed)
- Channels are selective (selectivity based on chemical properties of AA and diameter of pore)
- Ion flow is passive
Channel types (5)
- Random
- Voltage
- Chemical
- Chemical AND voltage-gated
- Mechanical-gated
Random channels
Open or close randomly, leakage channels
Voltage channels
Open or close depending on membrane voltage
Chemical channels
Open or close by binding with a “ligand” often referred to as a “messenger” (extracellular messenger: “neurotransmitter“; intracellular messenger: “second messenger”)
Chemical and voltage-gated channels
Opened by binding transmitter only when membrane voltage is favorable
Mechanical-gated channels (“stretch” channels)
Opened by membrane deformation
Diffusion
Movement of substances down a concentration gradient
Random (Brownian) Motion
Motion of molecules in solution eventually distributes substances uniformly within a compartment
What causes the formation of a chemical gradient?
Lipid bilayer lacking channels resists movement across it
Concentration Gradient
Difference in (ion) concentration within compartment or across a barrier
What does the magnitude of the concentration gradient (Wc) depend on?
Log ration of extracellular to intracellular concentration
Wc=RT (ln[out] -ln[in])= 2.3RTlog10([out]/[in])
Equilibrium
Ion exchange is equal in magnitude, and opposite in direction
What does separation of charge cause?
Voltage difference across membrane
What does a voltage difference cause?
Electrical gradient that drives ion currents through open channels
Can current flow if there is an electrical gradient but there is no concentration gradient?
Yes
Direction of current flow
Left to right (by convention, current flows in the direction of positive charge)
What ion does current go with?
Na+ (Cl- goes opposite way)
What is magnitude (strength) of electrical gradient (We) determined by?
Product of 3 variables (z=valence/charge of ion, F=Faraday’s constant, E=voltage across membrane)
We=-zFE
Initial conditions for equilibrium potential (2)
- K concentration gradient exists across membrane, but no open K ion channels
- Intracellular and extracellular compartments are electrically “balanced” (neutral)
What happens when K channels open?
Allows for outflow (“efflux”) of K+ down its concentration gradient, creating a (net) outward K current
What does K efflux result in?
Separation of charge across membrane that creates a voltage differences and acts as an electrical gradient opposing outward K+ current, electrical gradient flows stronger as K efflux continues
What does net negative charge inside membrane attract?
K+ because there are fewer K+ inside to balance A-
When are ion currents equal?
When efflux of K down its concentration gradient equals influx of K down its electrical gradient (Wc=We, zero net current)
Equilibrium Potential (Eion)
No net current in membrane potential (Vm=Eion)
Ions inside vs Outside
Inside: K+
Outside: Na+ and Ca2+
Nernst Equation
Describes equilibrium potential when electrical potential stops chemical potential from going down gradient
Distribution of ions across membrane
K+: 1:20 (outside/inside)
Na+: 10:1
Cl-: 12:1
Ca2+: 10,000:1
Ions in cytoplasm (inside)
High K, low Na and Cl
Ions in extracellular fluid (outside)
High Na and Cl, low K
***Equilibrium/Reversal Potentials
ENa+: +62 mv
ECa++: +123 mv
ECl-: -65 mv
EK+: -80mv
Ionic driving force
Difference between Vm and Eion, which causes either net inward or net outward current
Large changes in membrane potential are caused by…
Tiny changes in ion concentration
Where does separation of electrical charge exist?
Only across the membrane
Capacitance
Membrane stores electrical charge
Are intra- and extracellular fluids charged or neutral?
Electrically neutral: equal number of + and - charges in each compartment
What is membrane potential (Vm) determined by?
Weighted sum of all ion channel currents
Ion current contribution to Vm is proportional to…
Relative density of its channels (i.e. total conductance) in a patch of membrane
Godman-Hodgkin-Katz equation
Extension of the Nernst equation, expresses the contribution of each ion to Vm as the weighted sum of each ion’s concentration gradient
Density of K+ channels at rest
Much higher than Na+ or Cl- channels
Permeability of K to Na
pK: pNa= 1 : 0.05 (Vm is strongly dependent on EK+***)
Increasing [K+]out….
Depolarizes the membrane (makes it less negative than at rest)
Depolarization
Membrane potential becomes less negative (positive ions in)
What 2 things play a role in regulation of [K+]out?
Blood-brain barrier and astrocytes
“Leakage” currents
At rest, Vm=-65 mv so Cl- is close to equilibrium. Ions not in equilibrium give rise to leakage currents and could cause concentration gradients to decline if unchecked
Ion pumps
Transport proteins, they establish and maintain concentration gradients, critical for long-term maintenance of resting potential
What do ion pumps do?
Pumps expend energy from breakdown of ATP to exchange ions against concentration gradients (“active transport”)
What is the neuronal membrane highly permeable to at rest?
Potassium because of open membrane potassium channels
Movement of potassium ions across membrane, down concentration gradient, leaves inside of neuron…
Negatively charged
Polarized
Negative membrane potential
Other names for action potentials (3)
Spikes, discharges, impulses
Action Potential
Brief reversal of the resting membrane potential (inside briefly becomes positive relative to outside)
Excitable Membranes
Membranes that generate APs are called “excitable” (mostly found in neurons and muscle fibers)
What underlies AP activity?
Voltage-gated ion channels
Characteristics of APs (3)
- All-or-nothing event
- Duration is very brief (0.5-2.0ms)
- Does not require energy (PE–>KE)
Phases of a “spike”
Resting potential –> Threshold –> Rising phase –> Overshoot –> Falling phase –> Undershoot
Nav channels
Voltage-gated Na+ channels
Threshold
Membrane at rest, only resting K+ channels and Na+ channels open, Nav and voltage gated K+ channels closed
Rising Phase
Threshold to open Nav channels is more positive than Vrest, caused by inward Nav channel current
When depolarized to Nav threshold, wha happens to inward vs outward current?
Inward current through Nav channels just exceeds outward “leakage” current via open resting K channels
Positive feedback process
Inward current depolarizes membrane further and opens more Nav channels–> explosive amplification of inward depolarizing current
Falling Phase/AP is terminated by (2)
- Nav “inactivation”
2. Falling phase: gK»gNa, so Vm rapidly repolarizes toward Vrest
Delayed Rectifier
Group of slow opening and closing voltage-gated K+ channels compared to Nav
What accelerates repolarization of membrane?
Outward (+) current moving through Kv channels (persists because it is delayed=”undershoot”)
Refractory Period
Brief period (1-2ms) during and after generation of the AP when membrane is unable, or less able (inhibited, not impossible), to generate another AP
Importance of refractory period (3)
- Affects duration of AP
- Sets upper limit of firing rate
- Prevents AP from re-invading membrane that has just discharged so that AP propagates away from site of intiation
Absolute Refractory Period
Time during AP overshoot when all available Nav channels are either open or inactivated, threshold for generating another Ap is effectively infinite
When does the relative refractory period take place?
Follows the ARP, associated with AP falling phase and undershoot
Cause of relative refractory period
Prolonged outward K currents through Kv channels (high threshold, slow inactivating)
Threshold during RRP
Elevated but not infinite, initiation requires stronger supra threshold depolarization that can open more Nav channels to overcome outward K currents causing afterpotential
Sub-threshold inward currents
“Graded” i.e. proportional to the strength of the stimulus (different amplitudes?)
Supra-threshold inward currents
(via resting ion channels) may result in repetitive generation of APs
Supra-threshold depolarization
Elicits AP due to activation of enough voltage-gated channels to exceed outward currents through resting channels
When can APs repeat?
If depolarization is sustained above threshold because channels repeatedly cycle between open and closed states
Stimulus strength causes…
Stronger stimulus –> greater depolarizing current –> faster membrane reaches threshold to initiate another AP
What is firing rate proportional to?
Stimulus strength
What is firing rate limited by?
Duration of the absolute refractory period
Patch Clamp technique
Sakmann & Neher, electrode that can form a tight electrical seal with membrane, resolves minute currents carried by single ion channels
Structure of voltage-gated channels
Proteins with subunits, has a selectivity filter and voltage sensor
Selectivity filter in voltage-gated channel
Loop segments located in the pore restrict channel to specific ion(s)
Voltage sensor in voltage-gated channel
Membrane-spanning segments of each domain are charged, making them voltage-sensitive
What causes voltage-gated channels to open?
When membrane is depolarized to threshold, electrostatic migration of charge in voltage sensor causes conformational change
“Ball and chain” model
Pore becomes plugged by + charged internal loop of channel protein complex, has to close/plug channel or it will die after it fires once
How to unplug pore (aka “deinactivate”)
Vm must become more negative than threshold, has to “rearm” it to open again if depolarized to threshold
Voltage-gated Potassium channels (subunits and pore)
Subunits: KCNA gene, determine channel subtypes
Pore: selectivity for K ions by stripping water off K but not sodium
Where does current flow during AP?
Flows in through voltage-gated sodium channels
Electrotonic flow
Passive flow, depolarization spreads from site of AP initiation to adjacent membrane in both directions
Where does AP begin?
“Spike initiation zone” in axon initial segment (AIS)
Why do APs begin in AIS? (2)
- Nav channels are clustered at high density in AIS
2. AIS has lowest threshold for generating an AP
Membrane Cable Properties (3)
- Cytoplasmic Conductance: gi( =1/Ri)
- Membrane Conductance: gm(=1/Rm)
- Membrane Capacitance
Cytoplasmic Conductance
Proportional to fiber diameter/volume, limits current flow along the membrane
Membrane Conductance
Proportional to resting ion channel density and ion channel conductance, limits current flow across membrane
Membrane Capacitance
Created by the phospholipid bilayer, which separates and stores charge, affects how fast membrane potential changes when ion channels open
Length Constant (λ)
Distance over which a change in membrane potential falls to 37% (1/ε) of its original magnitude
λ = (Gi/Gm)^½
Longer length constant=
Further potential will travel (faster conductance)
Time Constant (τ)
Rate of membrane potential change, expresses time it takes membrane to reach 63% of a new steady-state potential in response to an instantaneous change in membrane potential
What is time constant directly & indirectly proportional to?
Membrane capacitance (Cm) and indirectly proportional to conductance (Gm)
The shorter/faster/lower the time constant, the…
Shorter the latency to generate APs
Nerve conduction velocity
Speed at which APs propagate down their axon
What optimizes propagation of APs? (2)
- Increased fiber diameter (has a higher internal conductance)
- Myelination (ex: security line at airport, speed up so you can’t look back at loved ones)
Myelin
Increases λ by decreasing Gm (conductance), decreases τ by lowering Cm (capacitance-inversely proportional to membrane thickkness)
Where are Nav channels restricted to?
Node of Ranvier
Kv1.1 Shaker channels
Low-threshold/rapid activation, segregated from Nav channels, reducing effect on rate of AP initiation
Kv3.1 Shaw channels
High threshold/rapid activation, co-localized with Nav channels in the node, enhancing speed of AP reolarization
Types of communication in the NS (4)
- Ephatic
- “Gas-transmission” (nitric oxide)
- Electrical
- Chemical
Ephatic communnication
Intercellular current spread due to membrane apposition (positioning), known to occur in the cerebellum
“Gas-transmission” comunication
NO is a free-radical gaseous signaling molecule, short half-life (few sec), powerful neuromodulator, vasodilator
Electrical communication
Direct communication between neurons
Chemical communication
Communication via chemical messengers
Electrical synapses
Low-resistance junctions that conduct electrical potentials directly from one cell to another, neuron-to-neuron, neuron-to-glial cell
Bi-directional current flow
Non-rectifying
Uni-directional current flow
Rectifying, can only go in one direction
Gap junction structure
Narrow cleft with membrane-bridging channels (connexons)
What do connexons do?
Permit ions and other small polar molecules to pass from one cell to another
Advantages of Electrical Transmission (4)
- Reliable
- Fast (no delay)
- Efficient
- Tough
Chemical synapse structure (3 parts)
- Presynaptic endings (terminals): synaptic vesicles, secretory granules, active zones
- Synaptic cleft
- Postsynaptic membrane: receptors, enzymes, signaling molecules, structural proteins
Synaptic Transmission
Chemical messenger (NT) is released by terminal in response to depolarization and binds to selective receptors
Synaptic efficiency (chemical** vs electrical)
Chemical: activity in terminals can have graded (excitatory and inhibitory effects) on postsynaptic neuron activity
Electrical: response in postsynaptic neurons in less than or equal to that in pre-synaptic neuron
Plasticity (chemical vs electrical)
Chemical: effectiveness can be modified by “experience”, trophic factors, hormones, etc.
Electrical: little to none
Is chemical or electrical transmission favored more? Why?
Chemical because it is more flexible and plastic
Small neurotransmitters (3)
- Acetylcholine
- Amino Acids: Glutamate, GABA, Glycine
- Biogenic amines: Dopamine, Epinephrine, Norepinephrine, Serotonin, Histamine
Large NTs
Neuropeptides
Neuropeptide characteristics
Over 30 identified, co-released with classical transmitter, cleaved from larger peptides, referred to as neuromodulators (mediate long-term changes in excitability)
Coexistence
Many neurons contain 2 NTs: one small in small vesicles and one large packaged in larger vesicles
Transmitter Release
See slides/textbook
Activation of Receptors
“Lock and Key”
1. Transmitter is released and diffuses across cleft 2. Binds to receptors which are specific for given NT
Ionotropic Receptors
Associated with ligand-activated ion channels, direct, fast and shot-acting, result is post-synaptic potential
EPSP: Na+ or Na+/K+
IPSP: K+, Cl-
Metabotropic Receptors
Associated with signal proteins and G proteins, indirect, slow and longer-lasting, result: varied, modulatory
Where are metabotropic receptors located?
Pre-synaptic membrane, autoreceptor function is to maintain appropriate level of (subsequent) NT release
Termination of Transmittion (4)
- Re-uptake
- Enzymatic degradation
- Diffusion
- Scavenging
Re-uptake
Free NT directly taken up by terminal, re-packaging into vesicles of enzymatically destroyed
Enzymatic degradation
Breakdown of NT by enzymes, followed by breakdown products, re-synthesis of NT in nerve terminal (Ach)
Diffusion
NT concentration in cleft declines
Scavenging
Free NT taken up by astrocytes via specialized membrane transporters
Steps in synaptic transmission (7)
See slides
Postsynaptic Potentials (PSPs)
Membrane potential changes caused by synaptic activity
Functional Classes of PSPs (3)
- Excitatory (EPSP)
- Inhibitory (IPSP)
- Shunting
EPSP
Net inward cation (e.g. Na+) current (depolarizing), increases probability of reaching AP threshold
IPSP
Net outward current (hyperpolarizing), decreases likelihood of reaching AP threshold
Shunting
Open channels prevent/reduce depolarization by “clamping” Vm near Eion
Location of ionotropic receptor
Dendrite
Nicotinic acetylcholine (nAchR) receptors
Permeable to cations (both Na and K), reversal potential Er~0mv
Negative Vm=
Net inward current (depolarizing, INa>Ik)
Positive Vm=
Net outward current (hyperpolarizing, Ik>INa
