Action potential travels down axon of presynaptic neuron
Vesicle fuses with plasma membrane and releases neurotransmitter into the synaptic cleft
Neurotransmitters bind to ligand-gated ion channels, opening them
Ions entering through the open channels cause a depolarization of the membrane, opening voltage-gated ion channels
Ions pass through these voltage-gated ion channels, causing further depolarization of the membrane
This depolarization (action potential) travels along the membrane as more voltage-gated ion channels are opened
Cytoplasms of adjacent neurons are connected, allowing ions (and action potentials) to travel directly to the next cell
Degree of myelination affects action potential conduction speed
Bare plasma membrane – After stimulus, voltage quickly degrades as ions leak across the membrane
Non-myelinated axon – While voltage still quickly degrades, the action potential is regenerated as new voltage-gated ion channels are opened; conduction speed is slow because it takes time to open new channels
Myelinated axon – Due to myelin insulating the membrane, voltage does not degrade as quickly; new voltage-gated ion channels are only opened at the Nodes of Ranvier so the conduction speed is faster (because the action potential appears to "jump" from Node to Node, it is called saltatory conduction)
Gasser Classification of Nerve Fibers
Group A – Largest diameter, thick myelin sheaths, conduction speeds of up to 120m/s
Group B – Intermediate diameter, lightly myelinated, conduction speeds of around 10m/s
Group C – Smallest diameters, not myelinated, conduction speeds of 1m/s or less
