Dendrite

Dendrites ( altgr. δένδρον dendron ' tree ' or dendrites ' belonging to the tree ' ) are called in biology cell extensions of nerve cells that emerge from the cell body and primarily serve the reception of stimuli. A nerve cell typically consists of three parts: the cell body called the soma or perikaryon and cell processes, the dendrites and the axon on the one hand - in Gliahülle the axon - the other. There are also specialized neurons which have no axons (e.g., the amacrine cells of the retina ), or which do not have dendrites (such as the rods and cones of the retina ), or those in which the cell body is not longer between the dendrite and axon lies and the extensions as merge ( pseudouniploare as in the sensitive dorsal root ganglion cells).

Dendrites as units of a cell are not to be confused with dendritic cells of the immune system.

• 3.1 Dendritic spines

• 4.1 signal recording

Dendrite

Despite the importance of the dendrites of the neurons, little is known about how to grow dendrites and orient themselves in vivo and branch. A conjecture about these educational processes represents the synaptotrophe hypothesis according to which the formation of synapses plays a special role in the growth of dendrites.

Otherwise, the dendritic growth is similar to that explained by neurites, through so-called growth cones (English growth cones ). Then both neurites as dendrites have at their tips conical or piston-like outgrowths, which are in a research-based interaction with the surrounding area and of the progress or continuation - the extent and direction of outgrowth of appendages - decide and essentially determine the further behavior of the neurons. Cell culture techniques with time-lapse photography can give a clear view of how axons sprouting their environment absuchend. In the body there are different signals and different processing routes can be controlled via the start, direction and speed, and pauses of the dendrite.

The bulk of the growth of dendrites in the human brain takes place during the late embryonic and neonatal brain development. In this phase grow from the 100 billion nerve cells of our brain dendrites with a total length of several hundred kilometers from. As a key for the growth of the cytoskeleton during dendrite protein, the enzyme Nedd4 -1 is considered to be that essential for normal dendritic growth.

Neurites or axons and dendrites differ in their growth and after growth phases. Considered Cellular requires roughly speaking for the extension growth initially a stabilized support skeleton of microtubules to advance the growth tip. But then it takes for the back-and -forth gambling growth in this region - in unstable equilibrium - and dismantling processes by which individual actin molecules (spherical, globular G- actin) to chains ( fadenförimiges, filamentary F- and can disintegrate again - actin) strung together. In unstable microtubules and / or stable actin filaments no growth is possible. In early stages of development can so about In-/Stabilisation on one side the dendrite temporarily be halted in favor of longitudinal growth of neurites on the other. In principle, these relationships apply even later, for example in regeneration processes after lesions.

Anatomy of dendrites

Form

The shapes and functional diversity of neurons are mainly determined by the different intensity of the dendrites. The figure shows the morphological differentiation of nerve cells, which will be made, inter alia, on whether a neuron not, has one or more dendrites. Some neurons have dendritic trees outright, others the ratio is more balanced Somaoberfläche to dendrite. Finally, there are neurons, which do not have dendrites. Following this morphological classification can be said that dendrites occur only in bipolar neurons and multipolar neurons. In pseudounipolar nerve cells, the distal end of the peripheral extension has typical dendritic character.

Number and shape of dendrites contribute significantly to increase the receptive surface of nerve cells. It has been estimated that at the ends of a single Purkinje cell dendrites 200 000 axons. In general, the dendrites are tree-like branched, ramified extensions of Perikaryons.

Cell components

Structurally, the dendrite is the cell body closer than the axon. Dendrites and perikaryon can be considered in some respects even as a functional unit and are also referred to as somatodendritisches compartment. The composition of the dendritic substantially corresponds to the cytoplasm of Perikaryons. It is therefore impossible to draw a sharp line between the parts of the nerve cell.

Knowledge of cytoplasm, organelles and cytoskeleton allows an in-depth approach to distinguish the extensions ( axons / dendrites ).

The following morphological features can be found:

• In contrast to the axon dendrites are unmyelinated.
• In the larger Stammdendriten to find similar organelles as in perikarya. Especially in sessile origin ( perikaryonnah ) one may even find Nissl bodies ( rough endoplasmic reticulum ), in part. In addition to the smooth and rough endoplasmic reticulum, there are free ribosomes, microfilaments (actin ) and bundles of parallel microtubules. However, their functional orientation is not uniform (as in the axon ) but their polarity is variable, that is, their plus end can be either the periphery or the perikaryon show. While fibrils and Nissl bodies are still light microscopy to identify the other components are only visible in the electron microscope.
• To each branching of the diameter of the dendrites becomes smaller. In very thin dendrites mitochondria lacking. The end portions of the dendrites contain few organelles, and the cytoskeleton is formed only small.
• Compared to the axons (which may be about 1 m long in humans partly) dendrites very small and only reach lengths of several hundred micrometers ( microns ). Neurites grow into the periphery, however, a length of 1 to 1.20 m can reach a diameter of only 2-16 microns.

Distinctions of dendrites

It can be found in the literature, several distinguishing features of dendrites.

Considering pyramidal cells ( quite a large nerve cell ), two types of dendrites can be distinguished: apical and basal dendrites. Both originate at the top of the pyramid cells but apical dendrites are longer than basal dendrites. The apical dendrite pointing to the opposite direction and the axon extending vertically transversely through the layers of the cerebral cortex. Both apical and basal dendrites have spines. While there are many basal dendrites, rises to the surface of the cortex on only one long, strong Apikaldendrit.

Sometimes the apical dendrites are still divided into distal and proximal dendrites. The distal apical dendrites are longer and the axon projected into the opposite direction. Because of their length they form non-local synapses, which are far away from the nerve cell. Proximal apical dendrites are shorter and receive pulses from nearer neurons, such as interneurons.

Furthermore, one can distinguish dendrites according to whether they possess dendritic spines or not. One speaks therefore of smooth ( "smooth dendrites " ) or thorny ( " spiny dendrites " ) dendrites. With smooth dendrites of the nerve impulse is absorbed directly. In spiny dendrites both dendrite and spines take on the pulse.

In general, the spiny dendrites excitatory signals, inhibitory synapses received can be found, however rather on smooth dendrites ( sections ).

Dendritic spines

The small spike-like projections on the surfaces branched dendritic trees are called dendritic spines (English spines ) or spinous processes. Here are often, most of the synaptic contacts localized.

In general, a spinous process receives input from exactly one synapse of an axon. These fine extensions ( dendrites on a nerve cell process ) support the afferent transmission of electrical signals toward the cell body of the neuron. The thorns can take different shape, distinctly developed and often have a bulbous head and a thin neck connecting the head to the dendrite. The dendrites of a single neuron can carry hundreds or thousands of thorns. In addition to its function as a postsynaptic region ( postsynaptic ) - some with a mandrel apparatus as calcium storage - and allowed reinforcibility of synaptic transmission ( long-term potentiation LTP) dendrites also can serve to increase the potential number of contacts between neurons.

Spinous processes represent a kind Subkompartimentierung the dendritic membrane represents the possible thereby fine-tuning the individual spinous process with its particular ionic environment and its specific cAMP level may be important for the selectivity and storage of information.

Functions

The largest contributors to the supply of the neurons take over the glial cells, a type of support tissue. As well as the dendrites are involved in the diet of the nerve cell. However, their main task is to receive stimuli or signals mostly from other nerve cells and forwarding the subsequently formed pulses to the perikaryon (nerve cell bodies ) towards ( afferent or zellulipetal ) - in contrast to neurites or the axon, through which signals of this neuron at the axon hillock continuously starting ( efferent ) and other cells are fed.

Signal recording

A nerve cell may be a sensory cell - such as olfactory neurons ( olfactory receptors ) or photoreceptor cells ( photoreceptors ) - or receive signals from other cells - for example, from other nerve cells by dock neurotransmitters to specific receptors in the postsynaptic membrane regions of the nerve cell. In most cases, these postsynapses are not in the range of axon axon hillock or soma (body) of the nerve cell, but on their dendrites. Contact points between neurons called inter Euro tional synapses, where several types are distinguished (see also classifications of synapses ). Dendrites are involved in the following types:

• Dendro - dendritic synapses connecting different dendrites together. Some dendrites show presynaptic specializations on which they come into contact with other ( postsynaptic ) and dendrites can form so dendrodendritische synapses. Than chemical synapses this may be formed with pre-synaptic vesicles and post-synaptic membrane regions. Sometimes synapses have also called gap junctions neither vesicles nor the usual membrane compaction and can not signals in only one direction (unidirectional) transfer, but bi-directional. Also dendrodendritische chemical synapses with both sides mirror image synaptic vesicles and membrane deposits do occur as so-called reciprocal synapses, where in one direction an excitatory transmitter (eg, glycine) and in the other an inhibitory (eg GABA) distributed. Examples of dendrodendritischen synaptic contact in the animal world are bidirectional synapses in the stomatogastric ganglion of the lobster, which connects the oral cavity nerval with the stomach, or the Reichardt motion detectors in the fly eye. In humans, for example come reciprocal synapses in the olfactory bulb in front of the olfactory tract.
• Axodendritische synapses: Usually axons and their branches ( axon collaterals ) within the nervous system as a presynaptic to a dendrite to form synapses axodendritische.
• Axospinale dendritic synapses: In this special case axodendritischer synapses, the axon reaches around the spinous process of a dendrite.

The one of the many different synapses of a nerve cell incoming ( afferent ) pulse changes the membrane potential in this region ( postsynaptic potential ). This potential change rapidly spreads throughout the neighboring areas of membrane, with increasing distance growing weaker, and can either depolarisiernd (EPSP ) or hyperpolarizing ( IPSP ) to be. By hyperpolarized regions forwarding depolarizing potentials can be lifted. Running at any given time sufficiently strong depolarization at the axon hillock together, so that a certain threshold is exceeded - then an action potential is initiated, the neuron is excited. Almost simultaneously incoming stimuli can will add on in their effects and build an excitation potential by summing the axon hillock. In general, the closer the axon hillock is a synapse, the stronger their impact on the excitation of these nerve cell, the formation of action potentials - because the farther postsynaptic potential changes ( elektrotonisch ) spread, the more they are weakened. Studies on the Dendritenpotential were made very early.