Definition
Dendrites are the cytoplasmic extensions of a nerve cell, which usually branch off from the nerve cell body (soma) in a knot-like manner and become more and more finely branched in two parts. They serve to receive electrical stimuli from upstream nerve cells via synapses and transmit them to the soma. The dendrites also help to nourish the nerve cell.
On average, a nerve cell has about 1 to 12 dendrites. Most dendrites have a smooth surface (smooth dendrites). However, there are also nerve cells whose dendrites have so-called spinous processes or spines (spiny dendrites). These spines allow the formation of a special type of synpase, because the spines allow a very exact adjustment of the plasma composition of a small area.
Structure of dendrites
Research on the exact structure and growth of dendrites continues. Dendrite growth usually begins at the end of the embryonic phase after axon growth and continues in early childhood. It is assumed that newly developing dendrites, similar to newly sprouting axons, form a structure with which they orient themselves and find their way to the next target cell.
This structure is called a growth cone and follows a chemically defined path to a target cell. This growth cone is mobile and scans the environment for suitable signals. If an attraction exists, the growth of the dendrite is prolonged.
If rejection occurs, its growth period is shortened or it comes to a standstill. Various enzymes are very important for the growth of dendrites. If one of these enzymes is missing, growth can be stopped and nerve cell activity can be reduced.
Whether, how fast and in which direction a dendrite grows is probably regulated by chemical and physical processes and reactions in the body. Growth pauses are also initiated by these signals. The principle of growth is found in development, as well as, for example, after damage.
The term dendrite is derived from the ancient Greek dendron or dendrites, which means “tree” or “belonging to the tree”. Accordingly, the dendrites sprout “tree-like” branched out of the nerve cell bodies. As a rule, they have a total length of over 100 kilometres.
Compared to the axons, they are much shorter, with a length of about a few hundred micrometers. Unlike the axon, the diameter of the dendrite changes. It tapers towards the tip of the dendrite.
The dendritic trunk contains a cell organelle for protein production, which is also called rough endoplasmic reticulum. In the nerve cells these protein factories are called Nissl plaques. In the dendrite tips is the so-called Golgi apparatus, in which substances are “addressed” and sent on, similar to a mail room.
Most, but not all dendrites have mitochondria, which are known as the “power plant of the cell”. In very thin dendrites these are missing. Furthermore, the dendrite tips contain microtubules, structures that have a transport function.
The microtubules also ensure that the growth cones are “pushed” during the growth phase. Some authors regard nerve cell bodies and dendrites as one unit. The dendrite pattern and the number of dendrites mainly determine the diversity and functions of the nerve cell.
Multipolar nerve cells characteristically have several dendrites. They occur most frequently in the body, for example in the motoneurons in the spinal cord. Here you can learn more about motoneurons Bipolar nerve cells have only one dendrite.
Its structure is similar to that of an axon, except that it does not have a specific connection end point, a so-called synaptic end bulb. These nerve cells are found in the retina of the eye and in the ear. Unipolar nerve cells are very rare and have no dendrites.
They are found in the first neuron in the retina. Usually the dendrites do not have a coating, the so-called medullary sheaths. Pseudounipolar nerve cells are an exception. These are located in the spinal nerves and cranial nerves.
