Midterm 1 - Neurotransmission Flashcards
Membrane Potential
Electrical charge across a membrane. The difference between the inside and the outside.
Resting Potential
The membrane potential when the axon is neither being excited nor inhibited.
- 70 mV
Depolarization
- Towards 0
- A rise in the membrane potential from the normal resting potential.
- EPSP
Hyperpolarization
- In the negative direction
- A decrease in the membrane potential relative to the resting potential.
- IPSP
Action Potential
A brief electrical impulse that’s responsible for the conduction of information down an axon.
AKA, “spike,” “the neuron fires”
Critical Firing Level
- The threshold of excitation.
- The value of the membrane potential that must be reached in order to produce an action potential.
- Change = +15 mV (-55 mV)
Where Do EPSPs and IPSPs Occur?
Dendrite
How long is an AP?
1 mSec
Summation
When 2 EPSPs build on each other.
2 Types.
2 Types of Summation
- Temporal - Same neuron sends a signal 2x in a row in a very short time period.
- Spatial - When 2 different neurons send a NT to a neuron at about the same time.
Does Summation Apply to IPSPs?
NO.
2 IPSPs building on each other don’t yield a reverse spike - the most that happens is that the membrane potential just drops to a certain level and stays there.
Concentrations of Ions In and Out of Cell At Rest
- More sodium (Na+) outside than inside
- More potassium (K+) inside than outside
- More chloride (Cl-) outside than inside
- More calcium (Ca2+) outside than inside
Resting Membrane Potential
(Visual)
Action Potential Graph
Driving Forces on an Ion
(2)
- Diffusional Force - An ion will want to go from an area of high concentration to an area of low concentration.
- Electrostatic Force - Opposites attract.
Which Ion Has The Largest Driving Force Acting On It?
Na+
(Both diffusional and electrostatic forces want to push it inside)
Driving Force on Cl- inside Cell?
Because it’s negative, it wants to leave the cell to go to the positive environment (electrostatic force)
Driving Force on K+ inside Cell
Because K+ is in high concentration on the inside, the diffusional force wants to push it out of the cell.
Driving Force on Na+ Inside Cell
Na+ is happy where it is b/c the inside of the cell is negative and it’s positive.
Driving Force on Cl- Outside of Cell
Diffusional force wants to push inside because it is in higher concentration outside.
Driving force on K+ Outside Cell?
Electrostatic force wants to push it in to a negative environment because it’s positive (opposites attract).
Diffusional Force on Sodium Outside The Cell?
Both the diffusional force and the electrostatic force want to push Na+ into the cell b/c
- There is less concentration of it inside the cell (diffusional force)
- It is positive and is attracted to a negative environment (electrostatic force)
Equilibrium Potential
&
Equation
At some point, an ion will reach a balance where the driving force is zero (the diffusional and electrostatic forces are balanced).
Nernst Equation (need to know concentration of ion inside and outside of cell)
E(Na+) = +45 mV
E(K+) = - 80 mV
Driving Force on Na+?
- -70 at rest
- EP = +40
- Therefore, 110 mV of driving force on Na+ (diffusional + electrostatic)
Na+ will rush in until the diffusional and electrostatic forces balance (driving force = 0). Once its EP is reached, Na+ channels will close.
***All the K+ will leave the cell until it reaches it’s equilibrium potential of -80mV.
Permeability of Membrane to K+ vs Na+
- Very permeable to K+ (leak channels), therefore resting membrane potential is closer to K+ equilibrium potential
- Whereas the membrane is not permeable to Na+ unless the ion channels are open.
Na+, K+ Pump
Carries 3 Na+ out for every 2 K+ back in
Ensures that Na+ remains in greatest concentration in extracellular fluid despite both the forces of diffusion and electrostatic pressure.
All-or-None Law
- An action potential is always the same value (e.g., +45).
- It either fires or it doesn’t.
- The only thing that changes is the frequency of firing.
Rate Law
- An action potential can vary in frequency
- Ex. The difference between a small pain and a big pain is the frequency with which a neuron fires.
Where do neurons synapse?
Dendritic Spine
How Is A Message Conveyed From The Dendrite Down the Axon?
- If there are more EPSPs than IPSPs at the initial segment/axon hillock, there will be an AP and the neuron will fire.
- As AP sweeps down axon and if at NOR the critical firing level is reached, the spike will happen again (nothing happens in the internode space).
- At the end, there are Ca2+ voltage gated channels and if they open, the NT will be released.
NOT Saltatory Conduction but rather a WAVE OF DEPOLARIZATION. Na+ will leak out as goes along but so long as the charge is +15mV at NOR, the AP will be re-propagated.
Speed of Neurocommunication
Avg. = 120 meters/second
What travels faster to brain: Messages About Temperature or Messages About Pain?
Temperature.
Pain fibers are small and unmyelinated, whereas temperature axons are myelinated and wider around.
To Make An Axon Faster …
- Myelinate (provides insulation, AKA decreases resistance)
- Make axon giant (the larger it is around, the less resistance = more current)
Ohm’s Law
Cable Properties
- Speed of transmission down an axon.
- Decrease resistance by insulating with myelin and making axon bigger
- Voltage = Current (Amps) x Resistance (Ohm)
Multiple Sclerosis
Disease of demyelination. Because myelin is lost, there’s a loss of voltage and a lack of re-propagation at the subsequent NOR.
Picture of Forces and Concentrations of Ions Inside and Outside of Cell @ Rest
