Thorny process

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Thorns of a dendrite of a nerve cell from the striatum (part of the basal ganglia).

Spinous process or shortly Dorn - English spine - is a fine, often mushroom-shaped protrusion of the surface of a nerve cell called the mainly on dendrites takes place from different neurons of the brain. In most cases, the cell membrane at the bulged projection tip has a postsynaptic region (postsynaptic) highlighted represents the an upstream neuron with a presynaptic axon terminal (presynaptic) excitations transmits , here in excitatory signals transferred are.

Thorn processes occur in the central nervous system in all vertebrates. In the human brain, excitatory synapses usually end on dendritic thorns , anatomically spinula [s. Gemmula] called dendritica . Some nerve cells carry thousands of such extension courses, each about 0.2 to 2 microns long, which (Engl. A different thickness head head ) from the narrower neck (Engl. Neck ) may be discontinued. However, thorny processes are not only found on dendrites. The shape, size and biochemical characteristics of the spines influence the signal transmission at synapses.

A thorn can be formed, change depending on the synaptic activity and take on different shapes (morphological plasticity), and also regress. These structural changes influence the functional conditions of a synapse ( synaptic plasticity ) and can lead to strengthening of synaptic connections (possible correlates of long-term memory ) for a short, long or long-term period (late long-term potentiation LTP ).

Mushroom spines often also contain a so-called thorn apparatus , which is believed to be important as a calcium store for intracellular Ca 2+ signaling pathways, but is not yet understood.

Appearance and occurrence

Types of thorny processes

Spinous processes were already in 1888 by R. Cajal in the cerebellum described by birds and humans both Purkinje cells of the cerebellar cortex as on pyramidal cells of the cerebral cortex - archikortikal particularly in the hippocampus - to find and yet in other brain regions, such as subcortical (see illustration above) or in Thalamus . Thorns usually grow out of the dendrites of a nerve cell, but they can also be formed on its soma or in the area of ​​the axon mound .

Dendritic thorns occur with different densities in different shapes and sizes of protrusions. Often they grow up with a narrow neck and end with a more or less voluminous head (" terminal head "). This carries a postsynaptic dense (PSD) membrane region with transmitter receptors , ion channels and signal-transmitting systems. The opposite is the presynaptic region of another neuron, mostly on a " ENDK n Intelligent and sustainable plant ". Only the appearance of the appendages can roughly distinguish between different types, but the transitions are fluid:

  • Filopodia : Very long, thread-like protuberances without a head. Such filopodia are also viewed as a preliminary stage of dendritic branches and can carry several synapses.
  • Thin spines (. Engl thin spines ): Thorns with narrow, long neck and clear Remote Head
  • Mushroom-shaped spikes (Engl. Mushroom spines ): thorns with a narrow neck and a bulky, spherical head
  • Seated thorns (. English sessile spines ): thorns without a clearly definable neck
  • Stubby spines : Short, indistinguishable from neck and head

Little is known in detail about the shape-determining factors of a thorn for the flexible cytoskeleton made of F-actin . It is assumed that the signal transmission of the associated synapse is influenced by the form ( → function), that a subspace of the nerve cell can be delimited and designed as a subcompartment with special biochemical conditions. In addition, it has been proven that dendritic thorns do not permanently belong to a certain type, but change their shape ( morphological plasticity ), possibly also over time in the sense of a life cycle of thorns. The larger the thorn process, the higher the number of receptor molecules for the neurotransmitter in the postsynaptic membrane region (PSD).

Function and form differentiation

Thorns are adaptable structures that specialize in synaptic transmission and the transformation of postsynaptic signals into special changes in shape, depending on synaptic activity. They can influence the transmission of excitation and signal transmission in several ways:

  • Surface enlargement: Dendritic thorns enlarge the surface of dendrites and thus ensure that more synapses can find space on them. They also shorten the path that axons have to travel.
  • Electrical resistance: The narrow "neck" of dendritic thorns may represent electrical resistance, as ions cannot easily pass through this bottleneck. This could strengthen the electrical signal at synapses. However, this hypothesis is controversial.
  • Biochemical compartmentalization: As protuberances on dendrite surfaces, they form separate units that are only connected to the rest of the dendrite via a more or less narrow "bridge". They thus hinder the diffusion of molecules into or out of an extension and thus enable changes to remain initially limited to individual postsynapses.

With the thorn process, a postsynaptic element is emphasized and set off as a subspace that can be designed differently depending on its synaptic activity. Due to the shaping cytoskeleton made of actin filaments, thorn processes can be formed in different shapes depending on the width of the base, length of the neck and size of the head. While the respective spatial shape of the membrane envelope has an influence on the transmission of electrical potential changes, the partitioned space can be understood as a compartment for biochemical signaling processes - for example, intracellular Ca 2+ levels that increase rapidly for a short time . In particular, mushroom-shaped spines on neurons of the cerebrum - but not of the cerebellum - often contain a thorn apparatus in the plasmatic interior as a specific organelle , which consists of a few lamellae of smooth endoplasmic reticulum, presumably serving as a calcium store and in different ways Can influence different forms of synaptic plasticity.

literature

Individual evidence

  1. ↑ a b c d e f E.A. Nimchinsky et al. a .: Structure and function of dendritic spines in Annual Review of Physiology . Harvard 2002, 64, pp. 313–353, as PDF (English) ( Memento of the original from September 27, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. . @1@ 2Template: Webachiv / IABot / www.neurox.us
  2. ^ Isabell Lockard: Desk Reference for Neuroscience . Springer S. & BM, 2012, ISBN 9781461228028 , p. 247.
  3. ↑ Ramón y Cajal, S. Estructura de los centros nerviosos de las aves. Rev. Trim. Histol. Standard. Pat. 1, 1-10 (1888).
  4. ↑ KE Sorra et al. a., in: The international journal of neuroscience. London 1998, 18, p. 658. ISSN  0020-7454
  5. ↑ Z. Nusser et al., In: Neuron. Cambridge 1998, 21, p. 545. ISSN  0896-6273
  6. ^ T. Bonhoeffer , R. Yuste, in: Neuron. Cambridge 2002, 35, p. 1019. ISSN  0896-6273
  7. ↑ BL Sabatini et al., In: Current opinion in neurobiology. Oxford 2001, 11, p. 349. ISSN  0959-4388
  8. ↑ M. Segal, E. Korkotian: Synaptopodin regulates release of calcium from stores in dendritic spines of cultured hippocampal neurons . In: The Journal of Physiology . tape 589 , no. 24 , 2011, p. 5987-5995 , doi : 10.1113 / jphysiol.2011.217315 , PMID 22025667 .
  9. ↑ P. Jedlicka, A. Vlachos, SW Schwarzacher, T. Deller: A role for the spine apparatus in LTP and spatial learning . In: Behavioral Brain Research . tape 192 , no. 1 , 2008, p. 12-19 , doi : 10.1016 / j.bbr.2008.02.033 , PMID 18395274 .