action potential II Flashcards
The distance that current will propagate depends on:
Ri – internal resistance Higher Ri -> smaller spread of current Rm – membrane resistance Higher Rm -> larger spread of current Cm - membrane capacitance Higher Cm -> slower spread of current
Axons are poor cables because
- Rm is low, Ri is high, and Cm is high
- Therefore, for a typical axon, λ is small: ~ 1mm
- However, axons are well-built for active propagation
active propagate using local current flow depends on
depends on voltage gated Na+ channels
_____ increases conduction velocity
myelin
myelin increases conduction velocity by
- increases membrane resistance (and therefore length constant)
- decreases membrane capacitance
- allows for salatory conduction
conduction velocity ranges from
0.5- 120 m/s
conduction velocity depends on
myelination
axon width
axons carrying the most time sensitive intro are
larger and more myelinated
demyelinating disease
MS
Guillain Barre
MS
damages myelin in CNS
Guillain Barre is
immune response against myelin in axons innervating muscles
In demyelinating diseases, myelin sheaths are
damaged, which slows and/or blocks conduction (therefore nervous system function)
demyelination may play a role in
chronic pain
how does demyelination playa role in chronic pain?
- Nodes of Ranvier are disrupted
* Isolated clusters of voltage-gated Na+ channels
Safety factor
Density of voltage-gated Na+ channels is 5-10 times higher than necessary for AP
propagation (providing additional “safety” that propagation will occur)
Why the redundancy? Allows for:
1) Branching of axons
2) Shorter refractory period
Inhibitors of AP
- Action potential generation and conduction depends on voltage-gated Na+ channels
- If these channels are blocked, current can only be propagated passively
Neurotoxins
• Extracellular blockers of voltage-gated Na+ channels – abolish action potentials
– Tetrodotoxin (e.g., from pufferfish) – Saxitoxin (e.g., from clams)
• Modulators of voltage-gated Na+ channel function – may shift voltage dependence; alter ion selectivity
– Batrochotoxin (e.g., from frogs)
Local anesthetics
• Example: Lidocaine
• Binds to intracellular portion voltage-gated
Na+ channels, blocking Na+ entry – Requires channel to be open to enter the cell
• Greatest effect on axons that are: – Smaller (lower safety factor)
– More active (channels must open)
• Preferentially affect pain over touch axons
Axons are poor cables for
passive current propagation
Axons are well built for fast active propagation because
– Myelination, Nodes of Ranvier
– High safety-factor
AP’s are critical for survival
– Blocking APs can be fatal (although local blockade is
useful)
– Demyelinating diseases reduce conduction velocity and degrade information transmission
Important APs
- Nervous system: Representation of sensory information and motor plans
- Muscle: Excitation-contraction coupling
- Heart: Pacemaker potential and control of contraction
To “boost” the signal,
a fresh injection of charges is required.
This boost in positive charges comes from
Na+ flowing into the cell through voltage gated sodium channels.
The sodium channels sense the voltage of the propagating electrical signal and the activation gate _____, allowing an influx of _____.
opens
sodium into the cell
This is the signal “boost” which allows another electrical signal to be propagated along an axon.
Know why action potential propagation is much slower than the velocity of light.
The system of action potential relies on the diffusion of ions through channels that are gated. The process of diffusion, while fast, is still slower than the speed of light.
More importantly, there are millisecond delays in the gating mechanisms of the voltage gated channels.
The activation gate of the Na gate has a relatively fast response, but it is still a mechanical, conformational change in a protein that requires time to occur.
This yields conduction speeds up to 200 m/s, far, far slower than the speed of light.
Addition of layers of myelin increases ______ and decreases _______, allowing for a faster conduction speed.
membrane resistance
membrane capacitance
After producing an action potential an axon cannot generate another one for a few ms, because of
because the delay in the opening of the sodium inactivation (h) gates.
Myelin increases the speed of conduction of nerve fibers, when it is lost, ____.
fibers that were previously fast can become slow or lose the ability to conduct action potentials
Increasing axon diameter increases conduction velocity by increasing_______.
cross-sectional area and thus the number of Na channels
Hyperkalemia can be caused by
redistribution of K ions out of cells by insulin deficiency or resistance, acidosis or impaired movement of K into cells (Beta receptor blockade).
or by impaired ability for the kidneys to secrete K, a side affect of drugs. Meals very high in K, combined with renal disease and limb ischemia also cause increased K in the ECF.
Cardiac arrhythmias due to hyperkalemia are a result of
the membrane potential being shifted closer to threshold (i.e. slightly depolarized).
myelin reduces the leak of charges out of the axon, so ________.
very little current is lost between the nodes
Since little charge is lost, the next node is depolarized to threshold very rapidly (fewer negative charges are needed to maintain the membrane potential, so few Na ions required), resulting in action potentials “jumping” from node to node.
Activation gates may swing open, but with the h gates closed,
Na will still be unable to flow through the channel.
The potassium channels also contribute to the refractory period because after a cell repolarizes, there is a
delay in the closing of K channels.
All cardiac muscle cells are capable of ______, but the cells of SA node fire fastest and set the tempo of heart beats. When other cells begin firing out of sync, or cease firing when they should, cardiac arrhythmias result.
spontaneous depol
The higher K conductance makes it more difficult for a stimulus to depolarize the axon because ______.
K is still flowing out of the cell
Myelin prevents ions from ______ which in turn increases membrane resistance and decreases capacitance.
leaking out of cells,
If axons become demyelinated, they can slow action potentials due to decreased ______ or lose the ability to conduct because too many of the charges (ie current) are lost between nodes, preventing another action potential.
resistance
Decreasing axon diameter results in ____.
slower conduction
Large axons reach threshold at _____ strengths than small axons.
lower stimulus
Safety factor is also increased for
large diameter axons compared to smaller axons.
