Definitions and concept questions
What is myelin?
an insulating layer, or sheath that forms around nerves, including those in the brain and spinal cord
What is tetrodotoxin (TTX)?
An extremely potent blocker of most voltage gated sodium channels
What causes the initial depolarization that triggers an action potential?
When positively charged sodium ions rush into a neuron with the opening of voltage-gated sodium channels
at resting potential
m gates: closed, h gates: open, n gates: closed
Conduction Velocity
∆ Distance/ ∆ Time
Threshold
The value of the membrane potential which, if reached, leads to the all or nothing initiation of an action potential (aka the voltage needed to trigger an action potential)
Why does it take time for the gates of ion channels to open/close following a change in voltage?
Because conformational changes of the proteins need to occur
What is Saltatory Conduction?
Action potentials jump from node to node down the full length of a myelinated axon, speeding the arrival of the impulse at the nerve terminal
threshold
m gates: a few open, h gates: open, n gates: closed
Total Current
Itotal= INa+ Ik
Itotal= gNa*(Vm-ENa)+gK*(Vm-EK)
Afterhyperpolarization
Where the cell's membrane potential potential falls below the normal resting potential, aka as the undershoot phase
What type of signal is observed in membrane voltage recordings (current clamps)?
All or none spikes after threshold is reached
What do the details about the shape of the passive response indicate?
-where active synapses are located on the dendritic tree
-what part of the neuron the recording was made from
m gates: all open, h gates: about half open, n gates:a few open
Driving Force
Vm−Ex
(The magnitude of the driving force indicates how far an ion is from its electrochemical equilibrium)
Voltage dependent Na and K channels have ____ kinetics and ion selectivity, and ______ voltage dependence
very different, very similar
How does a voltage clamp break the positive feedback cycle of the action potential mechanism?
By using powerful negative feedback
(the current generated by the voltage clamp is equal in magnitude/opposite in sign to whatever currents the opening and closing of channels is generating)
What is the maximum firing rate in neurons?
The reciprocal of the absolute refractory period
ex: absolute refractory period is 4 ms= .004s, so the max firing rate= 1/.004 s= 250 s^-1= 250 Hz
afterhyperpolarization
m gates: closed, h gates: some open, n gates:some open
Capacitor Current
I= C*(dv/dt)
I=current flowing across the capacitor
C =capacitance of the capacitor
dv/dt=derivative of the voltage across the capacitor
Why is action potential a positive feedback
Activating the voltage-dependent Na+ conductance increases Na+ entry into the neuron, which makes the membrane potential depolarize, which leads to more Na+ entry, and further depolarization. This continues until Na+ conductance inactivation and K+ conductance activation restore the membrane potential to the resting level
When obtaining potassium currents, why does using toxins make this process easier?
It allows you to see K currents without needing to go through lots of solution changes
What does the length constant indicate?
How far passive response will spread
length constant= λ
What do m, h, and n gates describe and which channels are they associates with?
m gate: controls Na+ channel opening
h gate: controls Na+ channel closing
n gate: control K+ channel opening
Separation of Currents by ion substitution
gx(t)=(Ix(t)/Vm-Ex)