Neuron Signal Generation and Propogation Flashcards
What decides whether or not to fire an AP?
voltage gated channels
excitability
the likelihood that a neuron will fire an AP depending on the strength of the signal
What method is used to analyze a neuron’s excitability?
Current clamp - uses an intracellular electrode to inject current commands across the surface membrane and record the neuronal electrical response. Used to determine minimal signal to fire APs
Threshold voltage
Point at which voltage-gated channels trigger a switch to accelerated voltage change. (Fire AP!)
What two parameters are used to measure excitability?
Rheobase - minimum current needed to reach threshold
Chronaxie - time needed to fire at a current that is 2x Rheobase
Voltage clamp
Way to study proteins involved in making cell membrane properties…gives user more control over membrane potential. Provides direct control of channel gating stimulus, allowing the sensitivity of the voltage-dependence to be studied in great detail
Total conductance for a channel type =?
Conductance of a single channel of that type times the number of channels of that type in the membrane times the probability that the channels are in open state and thus able to conduct ions
How is voltage gated channel function determined?
- number of gating charges
- intrinsic stability of closed and open in the absence of membrane potential
- kinetics that determine how rapidly the transitions between these states occur
- selectivity properties that determine which ions can pass through open channel
- inactivation will occur in many channels during sustained depolarization
Describe AP sequence
- Depolarization cause voltage gated Na channels to open
- Further depolarization
- THRESHOLD! = positive feedback to open more Na channels = more depolarization
- short delay, K channels start to open and Na channels inactivate
- Cell returned to negative potential
- Refractory period (cannot fire another AP)
Why does great care need to be taken particularly when changing extracellular Ca2+ levels?
Surface charge.
Close to membrane, fixed charges exist on lipids/proteins/glycosylations and ions can bind to these charges which would seem like you have made a change in membrane potential, alltering how a channel responds to environment/stability of the channel. This effect is especially apparent with divalent ions (i.e. Ca2+)
High serum Ca2+ levels (hypercalcemia) = ?
symptoms resulting from DECREASED excitability of neurons: fatigue, depression, confusion, and cardiac arrhythmias
Low serum Ca2+ (hypocalcemia) = ?
symptoms resulting from INCREASED excitability: arrhythmias, cramps, tingling in the extremities, interruption of breathing
What wraps around myelinated axons?
oligodendrocytes or Schwann cells (repetitive wrapping of a glial cell around axon)
What does the myelinated wrap do?
Reduces amount of current needed to depolarize the membrane by reducing the membrane conductance (increasing resistance) and reducing capacitance (greater charge separation) –> allows conduction velocity to increase linearly with diameter (also increases thickness of axon)
What are the effects of axon diameter on propagation speed?
OPPOSING
when axon becomes more narrow:
- internal resistance goes up (slows conduction)
- membrane resistance goes up (accelerates conduction)
balancing effects mean that conduction velocity increases as the square root of diameter
