Flashcards in Module B-03 Deck (34)
occur in the primary afferent sensory neuron that experiences the stimulus
occur in postsynaptic neurons after the primary afferent neuron
local change in potential due to stimulus which increases in amplitude with increase in amplitude of stimulus
how do graded potentials spread?
They depolarize adjacent regions without membrane receptors by decrementally spreading positive charge
Distance over which grade potential declines to 37% or 1/e of the original
Length constant increases with ___________ and decreases with ________
electrical resistance of the membrane ; electrical resistance of cytoplasm
Larger diameter axons have _______ length constants
The larger the area of axon the _________ the cytoplasmic resistance
The greater the length constant the ________ the decrement, the _______ the intracellular resistance and the ________the membrane resistance
slower; lower ; greater
Depolarizations that are all or none and show no decrement
Trigger zone aka
impulse initiation zone (IIZ)
dense population of voltage sensitive Na+ channels which open when sufficient depolarization has been reached
IIZ or trigger zone in Multipolar neurons
Axon hillock ( btw cell body and first myelinating cell)
IIZ or trigger zone in pseudounipolar and bipolar neurons
near sensory nerve ending
What occurs during rising phase of Action potential
- opening of enough Na+ channels that exceeds K+ efflux
- neuron depolarizes
- more Na+ channels open
-Na+ moves down its electrochemical gradient into the cell
- delayed rectifying K+ channels are also induced but are very slow
What occurs during falling phase
- efflux of K+ causes repolarization
- often hyperpolarizes
- depolarization induced Na channel inactivation
General structure of superfamily of voltage gated ion (K+,Na+, Ca2+) channels
- 4 identical subunits
- each subunit has 6 membrane spanning domains
- charged voltage sensor in segment 4
- pore loop
What confers ion specificity to the voltage gated ion channel
Absolute refractory period
- all Na+ channels are inactivated
- no impulse can be generated
Relative refractory period
incomplete return of inactivated Na+ channels to normal closed state plus residual K+ currents
- lasts longer (several ms)
- new impulse can be generated if stimulus is large enough
Where are voltage gated Na+ channels found in myelinated neurons of PNS and CNS?
2) Trigger zone (IIZ)
Where are voltage gated K+ channels found in myelinated neurons of PNS and CNS?
Where are voltage gated Ca2+ channels found in myelinated neurons of PNS and CNS?
Where do impulses occur in myelinated axons
at nodes of Ranvier
What is the effect of demyelination on impulse conduction
large uninsulated internodes diminish the axonal length constant
passive flow of charge within the cell
How is impulse conducted along unmyelinated axon?
- electrotonic conduction depolarizes the adjacent Na+ channels beyond threshold and this continues along the axon length
Continuous conduction occurs in
Saltatory conduction occurs in
How is impulse conducted along myelinated axon?
- positive charge enters node of Ranvier then moves down its electric gradient to the next node which then reaches thershold
Why are conduction velocities in unmyelinated axons slower?
1) they have smaller diameters (smaller length constants)
2) requires sequential driving of ALL axonal segments
Why are conduction velocities in myelinated axons faster?
1)They have larger diameters (larger length constant)
2) only need to drive current at the nodes
What effect do Lidocaine and Procaine have on membrane channels?
Block Voltage gated Na+ channels