Lecture 3 - Neurophysiology: Neural Signals Flashcards
Membrane potential
The electrical charge across a cell membrane; the difference in electrical potential inside and outside the cell.
Inside the cell is usually more ____ charged than outside the cell, especially at rest
(-)ly
Axons have two basic electrical potentials:
- Resting membrane potential
2. Action potential
Resting membrane potential
The membrane potential of a neuron when it is not being altered by excitatory or inhibitory postsynaptic potentials.
Action potential
The brief electrical impulse that provides the basis for conduction of information along an axon.
How do we know if it is a squid axon?
- huge
- visible by the naked eye
- often used in petri dishs
When we insert our electrode into the axon, we get…
a straight line — the resting potential
How the membrane potential can change:
- Hyperpolarization
- Depolarization
- Threshold of excitation
Hyperpolarization
An increase in the membrane potential of a cell, relative to the normal resting potential. (further from 0)
Depolarization
Reduction (toward zero) of the membrane potential of a cell from its normal resting potential.
Threshold of excitation
The value of the membrane potential that must be reached to produce an action potential.
Changing membrane potential is…
creating unrest!
Neurons – resting and action potentials.
- Resting potential ≈ -70 mV
- Threshold ≈ -55 mV
- Action Potential ≈ +40 mV and Hyperpolarization ≈ -90 mV
Takes approximately 2 msec!
How long does an AP take?
~ 2 msec
AP’s are…
All-or-none & the same everytime
Diffusion gradient
Movement of molecules from a region of high concentration to regions of low concentration. (e.g. sugar dissolving in water)
Electrostatic gradient
Molecules can carry charge (ions)
Cations (+ charge)
Anions (- charge)
Move towards areas of unlike charge (opposites attract)
Dynamic equilibrium
Gradients can balance one another.
Where are these ions?
- Intracellular fluid
- Extracellular fluid
Intracellular fluid
The fluid contained within a cell.
Extracellular fluid
Body fluids located outside the cell.
Membrane Equilibrium
when there’s certains ions inside & outside the cell
A– = protein K+ = potassium Na+ = sodium Cl– = chloride
What is happening inside of cell?
- anions cannot leave cell
K+ is achieving dynamic equilibrium as the force of diffusion and electrostatic pressure balance each other out
- normally can free float inside & outside the cell b/c those 2 pressures are working against each other forming an equil.
What is happening outside of cell?
- low concentration of K+
High concentration of:
- Cl- is achieving dynamic equilibrium as force of diffusion and electrostatic pressure balance each other out
- Na+ is being forced into the cell as force of diffusion and electrostatic pressure are both pushing that direction
- but Na+ has to wait for a Na+ channel to open & the opening of that channel is the basis & start of an A
Dynamic equilibrium
- dynamic equilibrium maintained by sodium-potassium pump. (imp. for cleaning up after an AP)
- electrostatic pressure and diffusion work to get Na+ inside the cell. (so you need a mechanical pump)
- the sodium-potassium pump pushes it outside again to maintain equilibrium.
Describe the sodium-potassium pump
- A- ions & K+ ions have HIGHER concentration INSIDE axon relative to outside…
- whereas Cl- ions & Na+ ions are more concentrated outside the axon
- Na+ channels are ordinarily closed to prevent entry of Na+
- Na+/K+ pump exchanges 3 Na+ for 2 K+. The HIGH concentration of extracellular Na+ is due to this pump. 10x as much Na+ is outside the cell as inside, contributing to the membrane’s RP of -70
- K+ is free to ENTER & LEAVE the cell but Na+ CANNOT reenter once pumped out
Describe Action Potentials -70mV
- every neuron has a resting charge or RESTING POTENTIAL ~=-70mV
- maintained by SODIUM-POTASSIUM PUMP - continually pumping Na+ out & K+ in
- when an ion channel opens Na+ rushes into the cell & K+ goes out changing the potential
- with enough stimulation of this kind the resting potential passes a threshold (~= -55 mV) & the cell ‘FIRES’
- this reverses the polarity of the cell for a brief period - known as the cell’s ‘ACTION POTENTIAL’
- this is all generated at the AXON HILLOCK
- sodium channels only open briefly & then cannot open for some period of time (absolute refractory period)
The pufferfish as a neuroscientific weapon
contains TETRODOTOXIN – sodium channel blocker prevents action potentials
Fugu - Japanese preparation of a pufferfish that has to be prepared carefully
“About three o’clock in the morning we found ourselves seized with an extraordinary weakness and numbness all over our limbs. I had almost lost the sense of feeling; nor could I distinguish between light and heavy bodies of such as I had strength to move, a quart pot full of water and a feather being the same in my hand….”
Captain James Cook
Laws of conduction
- all-or-none – once triggered an action potential can’t be stopped.
- variable information, representing the strength of a response to a stimulus (or the strength of a command to act), is conveyed by FIRING RATE.
Increased firing rate…
produces AP’s faster
Axon Hillock
where the action potential begins.
Terminal Buttons
the end point for the action potential.
What direction do AP’s flow? & can AP’s reverse?
Action potential flows toward the terminal.
- does not reverse direction because area where the action potential came from is still in refractory.
Describe the direction of travel of AP
a. In response to a signal, the soma end of the axon becomes depolarized
b. The depolarization spreads down the axon. Meanwhile, the first part of the membrane repolarizes. Because Na+ channels are inactivated & additional K+ channels have opened the membrane cannot depolarize again
c. The AP continues to travel down the axon
