Overview
This lecture focused on comparing dendrites and axons, highlighting their structural and functional differences, similarities, and the types of axons found in the nervous system.
Dendrite vs. Axon: Structural & Functional Differences
- Dendrites are generally the input region of a neuron; axons are generally the output.
- Dendrites branch at acute angles (<90°); axons branch at right angles (90°).
- Dendrites contain all organelles (except the nucleus); axons lack rough ER and Golgi apparatus.
- Dendrites primarily generate graded (local) potentials; axons generate action potentials.
- Dendrites typically signal locally over short distances; axons conduct signals over long distances.
- Dendrites are generally unmyelinated; axons are generally myelinated (but some exceptions exist).
- Dendritic end plates have receptors for neurotransmitters; axon terminals (buttons) are output structures with various shapes.
Types of Axons
- Type A axons are heavily myelinated, have large diameters, and conduct signals ~300 mph (found in sensory and motor neurons).
- Type B axons are less myelinated, smaller diameter, and conduct ~30 mph (found in autonomic fibers).
- Type C axons are unmyelinated, very thin, and conduct ~3 mph (also found in autonomic fibers).
Limitations & Clarifications in Definitions
- Not all dendrites are strictly input and not all axons are strictly output due to some signaling exceptions.
- Dendrites usually signal toward the soma, while axons usually signal away—but sensory neurons can be exceptions.
- The primary distinguishing feature: only axons can generate action potentials.
Similarities
- Both dendrites and axons lack a nucleus.
- Both are integral neuron parts and capable of signaling.
- Both contain microtubules, but axons have more.
Key Terms & Definitions
- Dendrite — Neuron extension specialized for receiving input, creating graded potentials, and usually signaling locally.
- Axon — Neuron extension specialized for transmitting output, capable of generating action potentials, usually signaling over distances.
- Graded Potential — Local change in membrane potential, typically in dendrites.
- Action Potential — Rapid, long-distance electrical signal generated by axons.
- Myelination — Insulating cover on some axons that increases signal speed.
- Axon Hillock/Trigger Zone — Area at the start of the axon where action potentials are initiated.
Action Items / Next Steps
- Review textbook or lecture material for more dendrite/axon examples and exceptions.
- Prepare a comparison list of dendrite vs. axon characteristics for exam practice.
- Investigate real-world exceptions to myelination in dendrites as discussed.