Soma (Cell Body)
The soma contains the nucleus and most organelles. It integrates signals
from thousands of synaptic inputs on the dendrites. When the membrane
potential reaches threshold (~-55mV), an action potential is triggered.
Dendrites
Branching extensions that receive signals from other neurons. Dendritic
spines are small protrusions where most excitatory synapses form. A single
neuron can have 10,000+ dendritic spines receiving input.
Axon & Myelin
The axon transmits signals away from the soma. Myelin sheaths (from
oligodendrocytes in CNS, Schwann cells in PNS) insulate the axon,
dramatically increasing conduction speed from ~1 m/s to 100+ m/s.
Saltatory Conduction
Nodes of Ranvier
Gaps between myelin where ion channels cluster
Signal Jumping
Action potential "jumps" node to node
Energy Efficient
Fewer ions need to be pumped back
Speed Boost
100x faster than unmyelinated axons
Synaptic Terminals
Axon terminals contain vesicles filled with neurotransmitters. When an
action potential arrives, Ca2+ influx triggers vesicle fusion and
neurotransmitter release into the synaptic cleft.
Key Numbers
- Resting potential: -70 mV
- Action potential peak: +40 mV
- Duration: ~1-2 milliseconds
- Refractory period: ~2 ms absolute, ~4 ms relative
- Axon diameter: 0.1-20 micrometers
- Myelin thickness: ~1 micrometer