Direct contact communication of cells
Synaptic transmission is actually an umbrella term that splits into two entirely different mechanisms: electrical synapses (which use the gap junctions you mentioned) and chemical synapses (which do not use gap junctions). When people say "synaptic transmission" without specifying, they are usually talking about chemical synapses, which are the most common in the human nervous system.
gap junction (electric synapse) = Connexon
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Direct Comparison
| Feature | Contact - Dependent (Juxtacrine) | Gap Junctions (Electrical Synapse) | Classic Synaptic Transmission (Chemical) |
|---|---|---|---|
| What crosses the space? | Nothing. The cells interact entirely on their outer surfaces. | Ions and small molecules pass directly from the inside of one cell to the next. | Neurotransmitters are released, diffusing through the fluid-filled gap. |
| Cytoplasmic Connection? | No. The interiors of the two cells remain completely separated | Yes. Open protein tubes structurally link the two cells' cytoplasm. | No. Cells are separated by a distinct fluid-filled synaptic cleft. |
| Typical Purpose | Cell identity & tissue building. Used to recognize neighbors (e.g., immune cell targeting, embryo development). | Mass synchronization. Used to make an entire tissue act as one uniform unit (e.g., making the heart beat). | Complex computation. Used to send fast, tunable, and highly directed messages across neural circuits. |
How Contact-Dependent Signaling Works
- The Membrane "Handshake": The signaling cell displays a permanent membrane-bound protein ligand on its outer wall. The target cell displays a complementary receptor on its outer wall.
- The Response: When these proteins bind together, the receptor undergoes a shape change that triggers a chain reaction inside the target cell—without anything actually entering the cell.
A classic example of contact-dependent signaling is the Notch signaling pathway, which helps stem cells determine what specific type of cell they should turn into based on the types of cells directly surrounding them.
