Title: Dendrite: function, structure & meaning

dendrites are short, branching processes of nerve cells. They are primarily responsible for picking up incoming electrical stimuli and sending them to the cell body (soma) forward.

Dendrites – structure

Each neuron usually has one to twelve dendrites. These go from soma of nerve cells as appendages mostly on the opposite side from where the axon originates.

Overall differ dendrites and axons in some aspects:

AspectDendriteAxonprimary functionStimulus absorption, transmission to the cell body, conduction of excitation to another neuron, formation of action potentialslocalizationBranches off the perikaryonBranches off from the soma, followed by the axon hillock and initial segment.lengthRather short In humans up to 1.5 m possiblemyelinationNo myelinationMyelination possible: Begins behind the initial segment, where action potentials arise.line speedRather slow (due to lack of myelination)High conduction speeds possible through myelinationCytoplasmic composition

• Strong resemblance to the cytoplasm of the soma: cell organelles (rER, ribosomes, Golgi apparatus) reach into the dendrite.

• Greater difference to the soma: cell organelles such as the Golgi apparatus and rER are missing, but mitochondria are, Vesicles and lysosomes are present.
• High density of sodium channels in the membrane

Also, dendrites are mostly more branched as axons. Their origin on the cell body is broad and narrows with each of these branches:

Figure 1: Structure of a neuron

Because of the similarity in composition of the cytoplasm of soma and dendrite, the two sections appear to be almost fluid. Your unit is therefore also called somatodendritic compartment designated.

dendritic spines

Some dendrites have so-called thorns. It refers to membrane protuberances, which get their name from their prickly appearance. On these thorns are synapsesthrough which the dendrite receives information from other nerve cells.

Figure 2: Dendritic spinous processes. Source: wikipedia.org

occurrence of dendrites

There are different types of neuronswhich can be distinguished by their structure. Criteria for the distinction are va Number and arrangement of dendrites and axons.

Overall, a distinction is made between unipolar, pseudounipolar, bipolar and multipolar neurons.

Unipolar Neurons, such as B. photoreceptors, have no dendrites: only one axon arises from their soma.

Figure 3: Types of neurons.

The following nerve cells, on the other hand, have dendrites:

Bipolar Neurons

In the case of bipolar neurons a dendrite opposite an axon. Between them is the perikaryon.

A classic example of the occurrence of bipolar neurons in the body are the bipolar cells the retina. They transmit the excitation of the photoreceptors to ganglion cells, whose fibers continue to the brain as the optic nerve.

Pseudounipolar Neurons

Viewed from the cell body, pseudounipolar neurons act almost like a normal unipolar cell. Namely, the perikaryon itself has only a direct exit.

However, this departure branches out T-shaped in a axon and one Axon-like dendrites on.

Pseudounipolar nerve cells can be found e.g. Tie spinal ganglia. There, sensory perceptions are wired before being transmitted in the spinal cord.

Multipolar Neurons

Multipolar neurons are the most common neuron type in the body. They have one axon and one wide branched dendritic tree.

Common subtypes of multipolar neurons are the pyramidal cells and the Purkinje cells.

pyramidal cells

Pyramidal cells have a large, almost pyramidal perikaryon. They have an axon, but numerous dendrites with dendritic spines are occupied.Pyramid cells possess apical dendrites and basal dendrites. Apical dendrites are longer and leave the soma opposite the axon. Pyramidal cells are mainly in the cerebral cortex represented where they are lined up close together. They are able to form connections over long distances. such as Belly Betzsche giant pyramid cells, found in the primary motor cortex. The nerve fibers formed by their axons are collectively referred to as the pyramidal tract. They conduct signals to move our muscles.

Figure 4: Pyramid cell (Betzsche giant cell). Source: wikipedia.org

Purkinje cells

Purkinje cells are a key cell type cerebellar cortex. You can easily recognize them by their conspicuous heavily branched dendritic tree. The connection to Purkinje cells serves to modify movement processes and motor learning.Figure 5: Dendrite tree of Purkinje cells. Source: DocCheck Flexikon

The primary function of dendrites is the Recording electrical stimuli and the forwarding to the soma.These stimuli are conveyed using synapses transfer. The most common are chemical synapses represented, which can exist between different components of nerve cells.

A dendrite does not necessarily have to be involved in every synapse. Likewise, there are synapses between the axon and soma (axo-somatic synapse) and between two axons (axo-axonal synapse).

Dendrites are part of following synapse types:

• Axo-dendritic synapses: Synapses between a dendrite and a presynaptic terminal button
  • Most common synapse type
  • Insertion either at the shaft of the dendrite or at the spine (spine synapse)
• Dendro-dendritic synapses: Connections between dendrites of different neurons
• Dendro-somatic synapses: Connection between soma and dendrite

Each of these synapses can, depending on neurotransmitters and receptor population of the postsynapse, excitatory (= exciting) or inhibitory (= inhibitory). First the total of the various excitatory and inhibitory influences decides in the soma whether an excitation is passed on via the axon. If it is too low or predominantly inhibitory, the action potential will not occur.