CSB332H1 Lecture Notes - Lecture 6: Saltatory Conduction, Schwann Cell, Axoplasm
14 May 2018
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Lecture 6(b): AP Flow in Axon
Current Flow in Axons:
• Distance that the potential spreads depends on:
o Resistance of the membrane vs resistance of the axoplasm
§ High cell membrane resistance = current can spread farther (i.e. laterally)
before it is lost or escapes into the external environment
Myelination:
• Myelin function to:
o Considerably increase the conduction velocity that APs move down the axons
o Prevents current from being lost as Na+ ions drift away from the neuron
ð Ex.
o Thin diameter myelinated axon transmits impulses from 5-30 meters/second
o Thin diameter unmyelinated axon transmits impulses from 0.5-2 meters/second
Myelination Increasing Membrane Resistance:
• Plasma membrane is not permeable to ions; ions can only pass via channels
o Myelination functions in localizing VG channels at
specific leaky junctions
§ Ions only pass at specific regions along the axon
ð Localizing VG channels at specific junctions increases
membrane resistance; only region where ions can pass
ð Unmyelinated axons are slower because VG channels are
found along the entire length of the axon; therefore, ions
are more leaky and have lower membrane resistance
Saltatory Conductance:
• Saltatory conductance is the propagation of action potentials along myelinated axons from
one Node of Ranvier to the next
o Unmyelinated axons have VG channels along the entire length of the axon
o Myelinated axons have VG channels localized to the Nodes of Ranvier
§ Nodes of Ranvier – gap in the myelin sheath
between adjacent Schwann cells
a) Fast:
o VG-Na+ channels are localized to the nodes
§ Maximal current flow happens in the Nodes of Ranvier
b) Efficient:
o Energy (ATP) is only required to maintain the concentration
gradient of ions at the Nodes of Ranvier
