CJH332H1 Lecture Notes - Lecture 7: Axon Hillock, Electric Current, Membrane Potential
Lecture 7: Story of Myelin
Where e’e been preiously
• Neurons cheat all the time
• The initial rapid rise in sodium conductance due to voltage
dependent sodium channels
• Voltage dependent potassium take a bit longer to turn on
- Conductance of K out of the cell
- Delay in turning off also
• Influx of Na and efflux of K
Current flow during AP
• Positive current from the influx of sodium spreads ahead of
the impulse to depolarize the adjacent segment of the membrane toward threshold
• There is no inherent directionality of propagation (if AP starts at the middle of axon)
• Yet APs orall do’t reerse their diretio
Regeneration action potentials
1) AP begins at axon hillock (happens normally)
2) Electrical current spreads
3) Current spread is the electrical event that triggers voltage gated channels (and thus the action potential) a tiny
bit down the axon
4) This is repeated until it reaches the end of the axon
• The refractory period is responsible for what appears to be the uni-directional nature of action potentials
1. Na+ channels locally open is response to stimulus (positive influx - depolarization), generating an action
potential
2. Some depolarizing current passively flows down axon
3. Local depolarization causes neighboring Na+ channels to open
and generates an action potential
4. Upstream Na+ channels inactive while K+ channels open.
Membrane potential repolarizes and axon is refractory
5. Process is repeated, propagating action potential along the
axon
• Voltage gated sodium channels inactivated and cannot be opened
again/reactivated unless the channel is reset
• Passive property is the basic property of all membranes (esp nervous system)
• Length constant (λ) – distance where voltage response is 1/e (37%) of the initial voltage (how long it takes 100%
injection for 1/e to die away to leave 37% of the initial voltage)
• For electrical signals, it is better to have big as length constant as possible
Improving propagation
• Increase the length constant (λ) = slowing decay by:
- Increasing the diameter (decreases Raxial = axial resistance)
- Decreasing the leakiness of membrane (increases Rmembrane = membrane resistance)
find more resources at oneclass.com
find more resources at oneclass.com
