Neuronal Signaling

ACTION POTENTIALS

Overview

Action Potentials

Action potentials are all-or-nothing events
They are self-propagating
The conduction speed is determined by axon diameter (thicker = faster) and amount of myelination (more = faster).

Signaling Steps

Chemical

Chemical Synapse Steps

1) Action potential travels down axon of presynaptic neuron.
2) Vesicle fuses with plasma membrane and releases neurotransmitter into the synaptic cleft.
3) Neurotransmitters bind to ligand-gated ion channels, opening them.
4) Ions entering through the open channels cause a depolarization of the membrane, opening voltage-gated ion channels.
5) Ions pass through these voltage-gated ion channels, causing further depolarization of the membrane.
6) This depolarization (action potential) travels along the membrane as more voltage-gated ion channels are opened.

Electrical

Electrical Synapse

Cytoplasms of adjacent neurons are connected, allowing ions (and action potentials) to travel directly to the next cell.

Myelin

Myelin Thickness

Degree of myelination affects 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)

Nerve Classification

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