Dendrites

Dendrites are the parts of the neuron that receive stimulation in order for the cell to become active. They occupy a large portion of a neuron’s surface area and are responsible for receiving signals from other neurons.

Dendrite Anatomy

Each dendrite is approximately 2 μm (micrometers), with between 5-7 sets of dendrites per neuron. They project out directly from the soma and can branch extensively.

Dendritic Trees

Oftentimes neurons may be represented by a tree, where the axons are the roots, the soma is the trunk and dendrites are the branches. As such, the term dendritic tree describes how dendrites branch out and form dense arborizations like the branches of a tree. These neurological trees surround the neuron and are enormous in size compared to the soma (cell body). To illustrate, the size of the soma is like a tennis ball in the middle of a room. The size of the dendritic tree is the room itself.

Dendritic spines

On the dendritic tree are what can be described as “leaves.” These leaves are known as the dendritic spines and synapse with axon terminal bulbs to relay information between neurons. Dendritic spines can change their forms and synaptic connections (along with the passage of every signal to them). They may also disappear with each given stimulus.

Dendritic Function

The main function of dendrites is to receive and process incoming communication from other neurons. Each individual neuron’s dendrites can receive thousands of signals. These signals can be either excitatory (causes the neuron to fire) or inhibitory (keeps the neuron from firing). Based on the sum of these signals, it is decided whether a neuron will be excited into firing an impulse. In the event that the neuron does fire, an action potential is sent down the axon (you can read more about action potential here). In addition to deciding whether or not a neuron will fire, dendrites also play a vital role in determining the degree or strength of the impulse fired.

Since this is such a key role, malfunction in the dendrites can lead to several neurological disorders. Studies have linked dendrite malfunction to disorders such as schizophrenia, autism, depression, anxiety, Alzheimer’s syndrome, and so on.

Special Features of Dendrites

Dendritic Arborizations

Dendrites often develop after the formation of the axon. Additionally, every type of neuron has an unique pattern of dendrites and spines that develop synaptic surfaces on them. Regardless of the stage of axon development, dendrites start to form after the neuron approaches its final mature stage, whether heterotopic (in an abnormal place) or normal.

Dendrite patterns

There are many classifications regarding dendritic arborization used in the field. For instance, arborization patterns can be classified based on:

1. Randomness vs. Regularity in response to branching of segments

2. Causes a continuous spectrum from “radiate” to “tufted” types of arborization

3. The Degree of Deviation of individual branches from a radiated or rectilinear (straight) course (Szentágothai and Arbib, 1974)

Pruning of Dendrites

Much like how a gardener might prune a bush to get rid of unwanted parts, pruning of dendrite trees is the process of trimming useless dendrites during the development of the nervous system. Dendritic pruning is essential to eliminate unused dendrites and accelerate the growth of new dendrites. Hence pruning is also necessary for obtaining new memory and facilitating learning.

Facts

• Every neuron has 128 basal dendritic segments, and each dendritic segment has up to 40 synapses.

• Approximately 75% of the dendritic membrane of a regular neuron is tested to participate in synaptic transmission.

• A single dendrite has tens of thousands of dendritic spines (on average, 200 000 dendritic spines per neuron.)

Summary

• 5-7 per cell, approximately 2 μm in length each

• Project directly from the soma and branch extensively

• Generally produce tree-like arborizations around the neuron, called the dendritic tree

• Have protrusions known as dendritic spines which synapse with axons

• Responsible for receiving and processing electrical messages from other neurons

• Malfunction of dendrites have been linked to several neurological disorders

• Special features of dendrites include arborization, various patterns, and pruning

References

Dendrites: Definition & Function. (2015, December 17). Retrieved November 22, 2021, from https://study.com/academy/lesson/dendrites-definition-function-quiz.html.

Aminoff, M. J., Daroff, R. B., & Shah, M. M. (2014). Dendrites. In Encyclopedia of the Neurological Sciences (2, pp. 970–970). Academic Press/Elsevier. Retrieved from https://www.sciencedirect.com/science/article/pii/B9780123851574000567.

Biologydictionary.net Editors. (2017, June 03). Dendrite. Retrieved November 28, 2021, from https://biologydictionary.net/dendrite/.

Dharani, K. (2015). Chapter 6 - Dendrites and Primary Thoughts. In The Biology of Thought (pp. 109–122). Academic Press. Retrieved from https://www.sciencedirect.com/science/article/pii/B9780128009000000063.

Lumen. Neurons. (n.d) Lumen. Retrieved November 22, 2021, from https://courses.lumenlearning.com/waymaker-psychology/chapter/cells-of-the-nervous-system/.

Khan Academy. (n.d.). Overview of neuron structure and function (article). Khan Academy. Retrieved November 28, 2021, from https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/a/overview-of-neuron-structure-and-function.