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27 Cards in this Set

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name phases of AP
1.depolarization
2. repolarization
3. hyperpolarization
4. back to normal RMP
background info AP
- during AP, memb, pot. reverses rapidly and transiently due to voltage gated Na and K channels.
- a graded pot., if large enough, can initiate an AP before the graded pot. dies out.
Depolarization
- membrane becomes less polarized (less neg)
- occurs when a stimulus causes voltage-gated Na channels to open (Na+ enters through gates, amt. depends on strength of depolarization
Threshold AP
- is trigger point for AP (depolarize 10-20mv to -50 to -60 mv [threshold value])
- stronger depolarization
- influx of Na+ balances efflux of K+
Weak Depolarization
Na+ enters axon but so little that it diffuses away from membrane and RMP is changed; K+ efflux dominates
Above threshold
-influx of Na+ dominates (more Na+ enters than can diffuse away)
- have regenerative, positive feedback cycle (as more Na+ enters, get more depolarization which in turn causes more v-gated Na+ channels to open
- membrane voltage rises rapidly to +35mv (membrane voltage above 0 to +35 called overshoot)
During time Na+ gates are open....
K gates are activated but are slow to respond so remain closed.
Repolarization
- Na+ channels close after 1 msec. (time gated- Na+ influx ceases)
- K+ channels open (slow to respond- K+ outflow begins)
- Membrane voltage drops rapidly and goes below original RMP
Hyperpolarization
- K+ gates slow to close (continuous outflow of K+)
- Membrane voltage becomes a few mv more negative than original RMP
- also called afterpolarization or undershoot
- Na++K pumps reestablish orginal ionic distributions
Channel Gating: Na+ (Sodium Activation)
- two gates: activation(m) & inactivation (h)
- at rest, m gate closed, h gate open (m gate prevents Na+ from entering)
- depolarization: m gate opens rapidly, h gate closes slowly
- pNa and gNa increase
- 1 msec later(time-gated), h gate closes and channel is non-conductive, m still open; channels close even though membrane is still polarized
- stops influx of Na+
- after repolarization, m gate closes and h gate opens
channel gating: K+
- potassium has n gates
- n gates are closed at rest (channel is closed)
- n gates are also closed immediately after depolarization
- after 1 ms delay, depolarization causes n gates to open
- open later than Na+ gates so flow of K+ out of cell occurs later than Na+ in
- K+ flow high at time when Na+ influx reaches peak
- K+ channels slow to close so have small afterpotential(hyperpolarization)
polarization
-initially has polarizaed state
- charges are seperated acr. plasma membrane (membrane has capacitance)
What is trigger zone?
- axon hillock (closer to cell body)
- initial segment of axon
- density of voltage sensitive channels is highest here (initial segment has lowest threshold for generation of AP)
- initial AP starts with trigger zone
What is trigger zone?
- axon hillock (closer to cell body)
- initial segment of axon
- density of voltage sensitive channels is highest here (initial segment has lowest threshold for generation of AP)
- initial AP starts with trigger zone
refractory period: absolute
- lasts about 1 msec, sets limit of 1000 APs/sec
- when h gates close, m gates cannot conduct Na+ no matter what the state of the m gate
- completely insensitive: no amount of depolarization can cause the cell to fire AP
refractory period: relative
- with time, h gates begin to reopen
- stimulus above threshold can trigger AP
Propagation of the AP
- triggered by depolarization, series of APs travel along axon
- depolarization must exceed threshold
- propagated w/o decrement
- occurs w/a certain conduction velocity
- permits long distance communication
- neurons (the ICF) are not particularily good conductors (high resistance compared to a copper wire)
- the plasma membrane is a poor insulator (has low resistance compared to insulator surrounding household wire)
- circumstances favor a rapid decay of current with distance
The AP is..
a depolarization in excess of threshold.
strong depolarization brings the neighboring patch of membrane above threshold to set up a depolarization and subsequent AP in that region.
each segment responds to depolarization of preceeding segment (dominos)
speed of AP propagation
- range from 0.1 m/s to 100 m/s
- want to decrease rate at which depolarization decr. w/distance
- if region above threshold is larger, AP @ one location can set up another AP @ greater distance & rate will be faster
- voltage decr. depends on relative resistance of 1-2 paths: fiber interior or transmembrane
how to enhance speed of AP?
- want to decr. interior resistance or incr. membrane resistance.
- decr. resistnace of longitudinal path down axon interior: incr. diameter of axon, resistance less bc current has many paralell paths to chose from., dominant feature in invertebrates (1mm in squid, vert: 1 um to 30-50 um)
- fastest vert. axons faster than fastest invert. axons
-much larger size in invert. axons
- vert. adopted stategy of incr. memb. resistance to current by wrapping axon w/extra layers of insulating cell memb. called myelin.
what does myelin sheath do?
- speeds conduction
- froms resistnace, high density Na+ channels at node
-forces a larger portion of current flow as a result of pot. diff. to move down int. of fiber.
- incr. spacial spread of depolarization along axon and incr. rate of AP propagation
- to set up new AP @ distant point along axon, Na+ influx must have access to axon membrane: NOdes of Ranvier! depolarization jumps from node to node: saltatory conduction
what is saltatory conduction?
depolarization jumps from node to node
direction of AP propagation?
Orthodromic- goes from trigger to terminal knobs in only one direction.
moves in orthodromic direction bc neuron is refractory in the other direction.
molecular structure of the v-gated sodium channels..
- large protein of 2000 aa
- 4 domains each w/6 transmembrane segments s1-s6 in circ. structure
- ionic selectivity: extracellular loop connecting s5 & s6
- voltage sensitivity: positively charged arginine & lysine residues on s4
- sodium inactivation: intracellular loop btw. domains 3 & 4
molecular structure of v-gated potassium channel...
- similar to sodium but channel made w/ 4 smaller polypeptides instead of one large one; one is subunit of 6 transmembrane segments
-channel formed by aggregation of four subunits.
Calcium Dependent APs
- found in non-neuronal excitable cells such as (cardiac) muscle and in paramecium
- voltage dependant Ca++ channels found in most neurons.
- AP has rapid upstroke caused by opening of Na+ channels: depolarization produced by Ca++ is slower & more sustained than Na+ bc Ca++ channels inactivate more slowly
- during repolarization, Ca++ influx produces a plateau which prolongs the depolarization
Ca++ influx can have several functions in addition to contributing to the AP...
- trigger for relaese of neurotransmitter from presynaptic terminal
- activates other kinds of ion channels: K+
---Ca++ activated K+ channels responsible for repolarization & hyperpolarization (lasts longer, afterpolarization, makes it more diff. to produce action potential)