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5.3 & 5.4
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Elaina Alcalde
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Cards (16)
Action Potentials:
-move by continuous regeneration (re-triggering)
-move in 1 direction (due to refractory periods)
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Once initiated, an action potential moves along the membrane of a (1.) until it reaches the (2.)
Neuron's axon
2.
Synaptic end bulbs
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Three Phases of an Action Potential
Depolarizing
phase
Repolarizing
phase
After-hyperpolarizing
phase
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Propagation of an Action Potential
1. Entry of Na+ produces a local
current
→ spreads laterally to depolarize adjacent areas of membrane
2. Adjacent membrane area is depolarized to threshold
3.
Na+ channels open in 'new' region of membrane
4.
Entry of Na+ in 'new' region of membrane produces another local current → spreads to adjacent areas of membrane
5.
The 'new' adjacent membrane area is depolarized to 'threshold' → Depolarization Phase of action potential in this 'new new' region of membrane
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1st Factor for Velocity of Propagation of an Action Potential
Size (axon diameter): bigger = faster as diameter increases
→
resistance to current flow In axoplasm decreases
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2nd Factor for Velocity of Propagation of an Action Potential
2.
Myelination (insulation): insulation increases speed of action potential propagation
-glial cells form an insulating sheath around axons
*oligodendrocytes (CNS)
*Schwann cells (PNS)
-sheath is not continuous (has gaps)
→
saltatory conduction
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Saltatory vs. continuous conduction
Saltatory: in myelinated axons (faster)
Continuous: in unmyelinated axons
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Importance of refractory periods
Establishes maximum rate (frequency) of action potentials
Ensures forward propagation (axon hillock
→
axon terminal)
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Saltatory Conduction
-Action potentials only occur at nodes of Ranvier
-Ions can only be exchanged at the nodes of Ranvier
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Demyelination:
absence of myelin 'insulation'
→
action potential fails
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Demyelination
Disruption of myelin sheath
Multiple Sclerosis (central)
2.
Guillan-Barre (peripheral)
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Fugu (pufferfish) contains
tetrodotoxin
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How is an electrical signal (action potential) transmitted from one excitable cell to another?
Synaptic transmission
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Spatial summation
Signals from MANY pre-synaptic neurons add up at the trigger zone (axon hillock)
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Temporal summation
Increased FREQUENCY of action potentials from pre-synaptic neurons add up at the trigger zone
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Graded potentials (post-synaptic electrical events) are either...
1.
Depolarizing (if Na+ enters): Excitatory Post-Synaptic Potential - EPSP
2.
Hyperpolarizing (if K+ leaves or Cl- enters): Inhibitory Post-Synaptic Potential - IPSP
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