Action Potentials Flashcards
Action Potentials
Brief, rapid, large (100mV) changes in
membrane potential
-potential actually reverses (when depolarizes past 0)
- Na+ and K+ gates involved
Do not decrease in strength as they travel from their site of initiation
- don’t send signal for everything (AP threshold)
-AP are all one size, all or nothing
Na and K gates
Sodium gates need time to reset, has to reset before another action potential can be sent. Fast and open/close quickly
Potassium gates are slower to close/open. They are more spread out.
After peak, electrical and concentration gradient want potassium out because more positive inside
Process of action potential
Resting period (-70mv)
- Graded potential reach threshold of -55mv which triggers AP
-Triggers release of Na gates which allow sodium to rushing, depolarization. Turning from -55mv to +30 mv
- At peak, Na gates close and K gates open. Potassium rushes out. Repolarization
-K slow to close, overshoot. Hyperpolarization of -80mv
After hyper polarization
After hyperpolarization Na and K pump gradually restores the concentration gradients disrupted by action potentials
Sodium back into ECF from ICF
Potassium back into ICF from ECF
this resets gradient
AP characteristics
All or nothing principle
Refractory Periods
Self Propagating
Unidirectional movement
All or nothing principle
Neurons either reach threshold and
produce a full-sized AP, or no AP is
produced at all (-55mv or nothing)
Refractory periods
Na+ gates need time to reset
- Ions need time to reset
Overshoot of K+ gates causes the cell to hyperpolarize – requires greater GP to reach threshold
Two types of refractory periods
Absolute Refractory Period
-When a second AP is not possible even with a large stimulus
- Resting Na gates, getting ions back
Relative Refractory Period
-A second AP is possible when a greater than normal stimulus
-Hyperpolarization
Self Propagating
Self-propagating AP’s in unmyelinated neuron
An impulse in one region is enough of a
disturbance to cause the neighbouring
regions to reach threshold and trigger an AP
Uni directional movement
Refractory period causes the impulse to
move in one direction only
Summary of AP
Ion changes produce the phases of the
action potential
- Resting potential, -70 mv, if GP gets up to -55mv threshold then AP
-Depolarization, N gates open, sodium moves in, makes inside more positive. from -55mv to +30mv
-Repolarization, at peak sodium gates close and potassium gates open. Potassium rushes out of cell while sodiums stays in, inside more negative
- hyperpolarization, leak channels, trying to reach -89mv resting state of potassium. K+ gates slow to close
Movement in AP
Resting
- Na+ out and K+ in
Depolarization
-Na+ moves in
- K+ stays in
(makes more positive)
Repolarization
-K+ moves out
-Na+ still in
(switched, needs rest, Na K pump)
Refractory period
-Ions reset
* Na moves out/K+
moves in
Overshoot of K+
- refractory period, need bigger AP
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Two types of propagation
Two types of propagation
Contiguous/continuous conduction
* Conduction in unmyelinated fibres
* Action potential spreads along every portion of the membrane
- moves a to b to c to d
Saltatory conduction
* Rapid conduction in myelinated fibres
- much faster
-jump from node to node, impulse jumps of insulated regions
- around 50x faster
-Na and K gates at nodes of ranvier
Continuous is slow and at pH receptors
Salutary is fast and in pain/msucle fibres
Myelin
Fatty insulator
-Primarily composed of lipids
- Formed by oligodendrocytes in CNS
- Formed by Schwann cells in PNS
Leaves exposed nodes (nodes of ranvier)
movement of ions in Nodes of ranvier. Jump from node to node (exposed) because insulated means can’t get movement of charge
Multiple Sclerosis
-Loss of myelin
-Decreased speed of impulses
Loss of coordination in muscles and nerves
- cant get impulse down fast enough
- from poor coordination or no muscle contraction
Nerve conduction
Depends on:
Neuron diameter (bigger/fatter=faster)
Myelination (Faster)
Temperature (higher=faster)
Eg. Frog nerves (lower temp and smaller diameter) vs human (faster)
A-delta fibres (big, myelinated, pain and muscle) vs C fibres ( not myelinated, small)
