Electrophysiology Flashcards
Describe the mechanisms that generate resting potentials in cells
- Leak channels in membrane (usually open)
- Gated channels in membrane (usually closed; open in response to stimulus)
- Na+-K+-ATPase Pump (pumps 3 Na+ out per 2 K+ in)
Define Membrane Potential
Diff. in voltage (electrical gradient) btwn inside and outside of a cell
Exists in all living cells
Define Resting Potential
Membrane potential of non-excitable or excitable cells in their baseline (unstimulated) state
Define Graded Potential
A change in membrane potential w/ varying amplitude that is proportional to the stimulus
Define Action Potential
A rapid, stereotyped series of changes in membrane potential that is conducted along the axon of a neuron or t he membrane of a muscle fiber
Define Chemical (concentration) Gradient?
Unequal distribution of ions across cell membrane
Define Electrical Gradient (membrane potential)
Unequal distribution of positive and negative charges acros cell membrane
What is the Membrane Potential determined by?
- Concentration gradients of ions (& proteins) across the membrane (Na+, Ca2+, Cl- outside cell > inside)
- Membrane permeability to these ions (at rest, permeability to K+ > Na+ or Ca++)
What is the Equilibrium Potential of an Ion? (Eion)
Membrane potential (Vm) produced if membrane were permeable to only a single ion
Write the Nernst Equation
Eion = 61/z log ([ion]out/[ion]in)
What is the Goldman-Hodgin-Katz (GHK) equation?
- Calculates resting membrane pot. (Vm) produced by all ions to which the membrane is permeable
- Says Vm is determined by (conc. gradient) x (membrane permeability) for each ion
Describe the mechanism of a Graded Potential
Stimulus -> changes in ion permeability of neuron cell membrane (ion channels open or close) -> changes in membrane pot. -> electrical signals
Describe the 3 types of Stimuli involved in a Graded Potential
- Chemical stimuli -> open/close chemically gated ion channels (most neurons)
- Electrical stimuli -> open/close voltage-gated ion channels (all neurons)
- Mechanical stimuli (pressure, stretch, etc.) -> open or close mechanically gated ion channels (some sensory neurons)
What are the features of a Graded Potential?
- Can be hyperpolarization or depolarization
- Usually in dendrites and cell bodies
- Variable in amplitude
- Amplitude is proportional to strength of stimulus
- Decay over time/space
- Travel short distances
What are the features of an Action Potential?
- Large depolarizations (~100mV)
- Constant amplitude
- Do not decay over time/space
- “All or none”
- Used for rapid, long-distance communication within individual neurons
Describe the mechanism of an Action Potential
- Depolarization to threshold -> voltage-gated Na+ channels open rapidly, voltage-gated K+ channels begin to open slowly
- Na+ rushes into cell, down electrical/chemical gradients
- Cell depolarizes, overshoots
- Voltage-gated Na+ channels close, voltage-gated K+ channels open
- K+ rushes out of cell, down its electrical/chemical gradients
- Cell repolarizes, hyperpolarizes
- Voltage-gated K+ channels close
- Cell returns to resting potential
During and Action Potential, why do voltage-gated Na+ channels close while the neuron is depolarized? (describe the mechanism)
- At resting membrane potential, the activation gate closes the channel
- Depolarizing stimulus arrives at the channel. Activation gate opens
- With activation gate open, Na+ enters the cell
- Inactivation gate closes and Na+ entry stops
- During repolarization caused by K+ leaving the cell, the 2 gates reset to their original position
How do neurons return to resting potential after Action Potentials?
- Membrane permeability to Na+ and K+ returns to normal
- Na+/K+ ATPase pumps Na+ out of and K+ into the cell
(but each AP involves movement of a small fraction of Na+ and K+ ions around neuron so it doesn’t significantly change the ion conc. gradients across the membrane –> a neuron can conduct many APs before it must use the Na+/K+ ATPase to reset ion conc. gradients)
Describe Absolute Refractory Period and its significance
Na+ channels cannot be activated; no AP’s possible
- prevents overlap of APs
- prevents backwards propagation of APs
- lasts ~2msec
Describe Relative Refractory Period and its significance
some Na+ channels cannot be activated; K+ channels still open
- APs can be triggered by large graded potentials
- APs are smaller than normal (because K+ channels still open)
- lasts ~2msec
Describe how an Action Potential is propagated down an axon (involving local current flow)
- Depolarization of one segment of membrane -> local current flow -> depolarization of adjacent segments
- Voltage-gated Na+ channels open in adjacent segments of membrane
- AP is re-generated at each segment of axon
- Refractory period prevents backwards propagation of AP
In a small axon, APs conducted continuously travel at only ~5m/sec. How can we speed this up?
- Increase diameter of axon
- increased axon diameter -> decreased membrane resistance to ion flow -> increased speed of AP conduction
- [but neurons would be too big to fit_ - Insulate the axon
- Myelin -> decreased leakage of ions into ECF -> exit cell only at nodes of Ranvier (only nodes contain voltage-gated Na+ channels)
- AP is generated only at nodes (saltatory conduction)
- Increased rate of conduction (~150m/sec)
Describe Saltatory Conduction
- APs jump from one node of Ranvier to the next
- Only nodes have voltage-gated Na+ channels
In demyelinating diseases, why does conduction slow?
current leaks out of the previously insulated regions between the nodes
Where is the Neurotoxin, Tetrodotoxin (TTX) from? What does it do?
- from pufferfish, some newts, toads, octopus, starfish, etc.)
- Blocks some types of voltage-gated Na+ channels
Where is the Neurotoxin, Maurotoxin (MTX) from? What does it do?
- from scorpions
- Blocks some types of voltage-gated K+ channels
Where is the Neurotoxin, Batrachotoxins (BTX) from? What does it do?
- from poison dart frogs
- Activates voltage-gated Na+ channels in the peripheral NS
What do local anesthetics such as novocaine, lidocaine, procaine do?
Block voltage-gated Na+ channels
What is Hyperkalemia? How does it affect the generation of APs?
- increased [K+] in blood
- resting potential is less negative (less polarized)
- APs are initiated more readily
- Decrease K+ concentration gradient -> decrease leakage form neuron
What is Hypokalemia? How does it affect the generation of APs?
- decreased [K+] in blood
- resting potential is more negative (more polarized)
- APs are initiated less readily
- Increase K+ concentration gradient -> increase leakage from neuron
Describe Multiple sclerosis
- Demyelinating disease (destruction of myelin in the CNS)
- Though to be an autoimmune disease
- Leads to slowing or cessation of AP conduction, and nerve damage
- Symptoms may include fatigue, muscle weakness, loss of vision and hearing, inability to walk or speak, partial or complete paralysis, abnormal sensations)
- Irreversible
