Distal Dendrites

The dendrite branches farther from the cell body are called distal dendrites. In the diagram some of the distal dendrites are marked with blue lines.

Distal dendrites are thinner than proximal dendrites. They connect to other dendrites at branches in the dendritic tree and do not connect directly to the cell body. These differences give distal dendrites unique electrical and chemical properties. When a single synapse is activated on a distal dendrite, it has a minimal effect at the cell body. The depolarization that occurs locally to the synapse weakens by the time it reaches the cell body. For many years this was viewed as a mystery. It seemed the distal synapses, which are the majority of synapses on a neuron, couldn’t do much.

We now know that sections of distal dendrites act as semi-independent processing regions. If enough synapses become active at the same time within a short distance along the dendrite, they can generate a dendritic spike that can travel to the cell body with a large effect. For example, twenty active synapses within 40 um of each other will generate a dendritic spike.

Therefore, we can say that the distal dendrites act like a set of threshold coincidence detectors.

The synapses formed on distal dendrites are predominantly from other cells nearby in the region.

The image shows a large dendrite branch extending upwards which is called the apical dendrite. One theory says that this structure allows the neuron to locate several distal dendrites in an area where they can more easily make connections to passing axons. In this interpretation, the apical dendrite acts as an extension of the cell.