Electrophysiology Flashcards
- integrated proteins that span the cell membrane
* When open, permit the passage of certain ions
Ion Channels
• Ion channels permit the passage of some ions, but not others
• Selectivity is based:
o channel size
o distribution of charges that line it
o size and charge of ions
o how much water the ion attracts and holds around it
Ion Channel Selectivity
- ion channels have gates controlled by voltage (differences in membrane potential)
- location: axon hillock, unmyelinated axons, along the nodes of Ranvier in myelinated axons
- responsible for generation and propagation of action potentials (outgoing signals from neurons)
Voltage-gated
- ion channels (chemically-gated channels) are opened or closed by hormones, second messengers, or neurotransmitters
- location: dendrites, cell body
- responsible for synaptic potentials (incoming signals to neurons)
Ligand-gated
- ion channels (leakage channels) are always open
- location: cell membrane on dendrites, cell body and axon
- responsible for resting membrane potential
Non-Gated
- Potential difference generated across a membrane because of a concentration difference of an ion
- Can be generated only if the membrane is permeable to the ion
- SIZE depends on the size of the concentration gradient
- SIGN depends on whether the diffusing ion is positively or negatively charged
- Created by the diffusion of very few ions
- Do not result in changes in concentration of the diffusing ions
Diffusion Potential
• Diffusion potential that exactly balances (opposes) the tendency for diffusion caused by a concentration difference
Equilibrium Potential
- Chemical and electrical driving forces that act on an ion are equal and opposite
- No net diffusion of the ion occurs
Electrochemical Equilibrium
• In the presence of a nondiffusible ion, diffusible ions distribute themselves so that at equilibrium their concentration ratios are equal
Gibbs-Donnan Equilibrium
• Used to calculate the equilibrium potential at a given concentration difference of a permeable ion across a cell membrane
Nernst Equation
• Calculates membrane potential on the inside of a membrane when a membrane is permeable to several different ions
Goldman-Hodgkin-Katz Equation
- Measured potential difference across the cell membrane in millivolts (mV)
- Expressed as the intracellular potential relative to the extracellular potential
- A resting membrane potential of -70 mV means 70 mV, cell negative
Resting Membrane Potential (RMP)
• Any change in which membrane voltage shifts to a less negative value
Depolarization
generation of a nonpropagated response
Local potential
generation of a propagated response
Action potential
- Vary in magnitude (voltage) according to the strength of the stimulus
- A more intense or prolonged stimulus opens more ion gates than a weaker stimulus
Local potentials are GRADED
- Get weaker as they spread from the point of stimulation
- As Na spreads out under the plasma membrane and depolarizes it, K flows out
- Prevents local potentials from having any long-distance effects
Local potentials are DECREMENTAL
• if stimulation ceases, K diffusion out of the cell quickly returns the membrane voltage to its resting potential
Local potentials are REVERSIBLE
- Alteration in the membrane potential of a cell resulting from activation at the synapse
- If the intracellular voltage increases, it is called an excitatory post-synaptic potential (EPSP)
- If the intracellular voltage decreases, it is called an inhibitory post-synaptic potential (EPSP)
SYNAPTIC POTENTIALS
- Transmembrane potential difference produced in sensory receptors
- Occurs generally as depolarization resulting from inward current flow
- Influx of current will often bring the membrane potential of the sensory receptor towards threshold for triggering an action potential
GENERATOR/RECEPTOR POTENTIALS
- Passive spread of charge inside a neuron due to local changes in ionic conductance
- Ionic charge enters in one location and dissipates to others, losing intensity as it spreads (graded response)
ELECTROTONIC POTENTIALS
- Rapid changes in the membrane potential that spread along the nerve fiber
- To conduct a nerve signal, the action potential moves along the nerve fiber until it comes to the fiber’s end
Action Potential
- If a stimulus depolarizes the neuron to threshold, the neuron fires at its maximum voltage
- If threshold is not reached, the neuron does not fire at all
- Above threshold, stronger stimuli do not produce stronger action potentials (not graded)
Action potentials follow ALL-OR-NONE LAW.
- Action potentials do not get weaker with distance
- An action potential at the end of a nerve fiber will be just as strong as an action potential in the trigger zone up to a meter away
Action potentials are NON DECREMENTAL.
- If a neuron reaches threshold, the action potential goes to completion
- Action potentials cannot be stopped once it begins
Action potentials are IRREVERSIBLE.
- Resting membrane potential before the action potential begins
- Membrane is polarized because of the -90 mV membrane potential
Resting Stage
- When the threshold potential (-65 mV) is reached, the voltage-gated Na+ channels overwhelm K+ and other channels
- Membrane suddenly becomes permeable to sodium ions
Depolarization
• in large nerve fibers, the great excess of positive sodium ions causes the membrane potential to overshoot beyond the zero level
Overshoot
o Necessary actor in both depolarization and repolarization of the nerve membrane
Voltage-Gated Sodium Channel
- Sodium channels begin to close and the potassium channels open more than normal
- Rapid diffusion of potassium ions to the exterior re-establishes resting membrane potential
Repolarization
o also plays an important role in increasing the rapidity of repolarization of the membrane
Voltage-Gated Potassium Channel
- K+ conductance remains higher than at rest after closure of the Na+ channels
- Membrane potential is driven very close to the K+ equilibrium potential
After hyperpolarization (undershoot)
• Depolarization process travels over the entire membrane if conditions are right, or it does not travel at all if conditions are not right
All-or-Nothing Principle
Factors Affecting Conduction Velocity
- Fiber Size
- Increasing the diameter of a nerve fiber results in decreased internal resistance and faster conduction velocity - Myelination
• Myelin acts as an insulator around nerve axons and increases conduction velocity
• Myelinated nerves exhibit saltatory conduction
• Time periods after an action potential, during which a new stimulus cannot be readily elicited
Refractory Periods
- Another action potential cannot be elicited, no matter how large the stimulus
- Coincides with almost the entire duration of the action potential
Absolute Refractory Period
- inactivation gates of the Na+ channel are closed when the membrane potential is depolarized and remain closed until repolarization occurs
- No action potential can occur until the inactivation gates open
Ionic Basis of Absolute Refractory Period
- Begins at the end of the absolute refractory period and continues until the membrane potential returns to the resting level
- Action potential can be elicited only if a larger than usual inward current is provided
Relative Refractory Period
- K+ conductance is higher than at rest
- Membrane potential is closer to the K+ equilibrium potential and farther from threshold
- More inward current is required to bring the membrane to threshold
Ionic Basis of Relative Refractory Period
- Cell membrane is held at a depolarized level such that the threshold potential is passed without firing an action potential
- Occurs because depolarization closes inactivation gates on the Na+ channels
Accommodation
