PY 105 Chapter Notes - Chapter 10: Saltatory Conduction, Schwann Cell, Ependyma
Saltatory Conduction
• The axons of many neurons are wrapped in an insulating sheath called myelin
• Myelin is created by Schwann cells, a type of glial cell, that exist in conjunction with neurons,
wrapping layers of specialized membrane wrapped around the axons
• No axon can enter or exit a neuron where the axonal membrane is covered with myelin
• There is no membrane depolarization and no voltage-gated sodium channels in regions of the
axonal plasma membrane that are wrapped in myelin
• There are periodic gaps in the myelin, Nodes of Ranvier
• Voltage-gated sodium and potassium channels are concentrated in the nodes of Ranvier in
myelinated axons
• Myelin sheath dramatically speeds the movement of action potentials by forcing the action
potential to jump from node to node
o This is called saltatory conduction
Glial Cells - specialized non-neuronal cells that provide structural and metabolic support to neurons
Cell types
Location
Primary function
Schwann cells
PNS
Form myelin - increase speed of conduction of APs along axon
Oligodendrocytes
CNS
Form myelin - increase speed of conduction of APs along axon
Astrocytes
CNS
Guide neuronal development. Regulate synaptic communication via
regulation of neurotransmitter levels
Microglia
CNS
Remove dead cells and debris
Ependymal cells
CNS
Produce and circulate cerebrospinal fluid
Equilibrium Potentials
• During the action potential, the movement of Na+ and K+ ions across the membrane through the
voltage gated channels is passive; driven by gradients
• The equilibrium potential is the membrane potential at which this driving force does not exist
o There would be no net movement of ions across the membrane
• Na+ has a positive equilibrium potential, ions are driven inward by their concentration gradient.
o If the interior of the cell is too positive, the positively-charged ions are repelled, the
electrical gradient would drive sodium out
• K+ has a negative equilibrium potential
o Ions are driven outward by their concentration gradient
o If the interior of the cell is too negative, the positively-charged ions cannot escape the
attraction; the electrical gradient drives potassium in
• Nernst equation: Eion = RT/zF(ln)X(outside)/X(inside)
Refractory Period
• The passage of one action potential makes the neuron nonresponsive to membrane
depolarization and unable to transmit another action potential, or refractory
• Absolute refractory period - a neuron will not fire another action potential no matter how strong a
membrane depolarization is induced
o Voltage-gated sodium channels are inactivated
o Will not open again until the membrane reaches the resting potential and the Na+ channels
have returned to their "closed" state
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