Dendrite: Structure, Function & Diseases

The branch-like and multiply branched cytoplasmic processes of a nerve cell (neuron), through which information is received and impulses are transmitted to the body, is referred to in technical language as the dendrite. This serves to receive electrical stimuli and transmits them to the cell body (soma) of the nerve cell.

What is a dendrite?

In medicine, this area is classified as histology, cytology, neuroscience, and physiology. The synonym is protoplasmic process. Dendrites serve as the primary receptor of stimuli. Action potentials within dendrites can travel in either direction. If a nerve cell is depolarized, the electrical excitation state does not propagate exclusively in the axon (nerve cell process, also axis cylinder, neuraxon), but also as a retrograde action potential in the dendrites. This process, known as feedback, alters the reception requirements of the protoplasmic processes and affects the subsequently arriving synaptic signal. The feedback leads to a more pronounced connection between the two neurons. If the impulse is initiated before the synaptic signal, this mechanism weakens the neural connection. This process is important for neuronal plasticity.

Anatomy and structure

The term dendrite is derived from the Greek language and stands for “tree-like.” This term gives a clue to the anatomy and structure of dendrites in the form of highly branched cytoplasmic projections that arise from the cell body (perikaryon) of neurons. A nerve cell is composed of an average of 1 to 12 dendrites, most of which have a smooth surface. However, there are also nerve cells whose protoplasmic process has spines or spinous processes. Often, these act as input regions for the recording of synaptically transmitted information, which is subsequently evaluated in the perikaryon and summed up and transmitted to the other nerve cells via the axon. However, this stimulus transmission occurs only in the case of a potential excess, in order to prevent stimulus overload. The neuraxon is surrounded by lipid-rich cells that electrically insulate it from the environment. These cells are also called Schwann cells, which are composed of lipid-rich myelin. These are interrupted in regular sections by Ranvier’s cord rings. Excitation flowing across the axon is transmitted by differential voltage across the uninsulated Ranvier’s laced rings within each laced ring. By means of the dendro-dendritic contact, electrical signals can also be transmitted from one dendrite to another. The dendro-axonic contact transmits signals from dendrite to axon and the dendro-somatic contact further transmits signals from dendrite to perikaryon. Dendrites have a shorter and more branched anatomy than axons. Their origin is broadly formed, with tapering with each branch, whereas nerve cell processes have a constant diameter along their complete length. The branching pattern depends on the type of nerve cell. Consequently, the branching of individual nerve cells can be so diverse that dendrites and axons cannot be readily distinguished. Under the light microscope, neurofibrils can be seen in the plasma of the dendrites and Nissl clods up to the first branch. With the aid of the electron microscope, actin filaments, microtubules, ribosomes, endoplasmic reticulum (protein synthesis), and possibly Golgi apparatuses can be seen. Axons, on the other hand, occur without endoplasmic reticulum and Golgi apparatuses. The outgrowth of dendrites from the cell body (dendritogenesis) often occurs after axogenesis. Physicians distinguish between six different nerve cell types: Pyramidal cell, Purkinje cell, amacrine cell, stellate cell, granule cell, and primary sensory neuron in the spinal ganglion.

Function and tasks

The main function of dendrites is to receive stimuli and transmit them to the cell bodies. The transmission of electrical excitation formation is referred to in technical language as afferent, since it always occurs in the direction of the nerve cell. However, it is quite possible that the transmission within the dendrites also proceeds in another direction.This reverse directional guidance occurs whenever an action potential is formed in the axial cylinder, which is distributed backward to the individual dendrites in the form of a feedback loop. This mechanism causes the synapse and the signals transmitted to this site to be affected and the two neurons involved become tightly coupled. This process is important for “neuronal plasticity”, which reflects the fact that nerve cells can adapt and remodel themselves depending on how often they are used. The nerve cells serve as a sophisticated network and information carrier. This exchange of information occurs through the synapses on the basis of chemical messengers (neurotransmitters) by means of presynaptic terminal buttons. These transmit the information to the nerve cells. The number of synapses plays a more important role than the number of nerve cells. Not all neurons are the same, however, because neurons differ in the way they function. By exposing the neurons to a stimulus, for example a touch or a taste sensation, the excitation state occurs, which transmits the received information.

Diseases

Every day we are exposed to a large number of stimuli. These stimuli must be relayed to the brain. The human brain is the “control center” for all automatically running processes of sensory perception (sight, hearing, smell, taste) as well as independent and perceptual processes, for example, the purposeful movement of the body. The task of transmitting stimuli is performed by the cells (neuron) found throughout the body. The human brain alone has a trillion nerve cells and is able to store an infinite amount of information by recombining the connections between the individual nerve cells. Without this perfectly functioning network of nerve cells, which daily filters the overload of stimuli coming in from outside, humans would hardly be able to live due to too many sensory impressions, as they would not be able to process them. For example, we react to a touch. The dendrites receive the stimulus of this touch through a widely ramified branch system and pass it on to the cell body (soma) of the nerve cells. On the soma is the axon hillock, which merges into the axon cylinder. The excitation states received by the dendrites accumulate in the axon hillock. However, these are only transmitted in the case of a potential excess in order to prevent a stimulus overload. The dendrites act as a filter that allows us to perceive the senses in an orderly fashion without the discomfort of sensory overload. If this “filtering system” did not function properly, we would not be able to perceive the aforementioned touch and respond to our environment after processing the signals relayed through the dendrites.

Typical and common nervous disorders

  • Nerve pain
  • Nerve inflammation
  • Polyneuropathy
  • Epilepsy