Dendritic spine
Spinous process or short spike (English spine ) is a fine, often mushroom-shaped protrusion of the surface of a nerve cell called that is found predominantly on dendrites of different neurons of the brain. In most cases, the cell membrane of the everted extension peak postsynaptic region ( postsynaptic ) is highlighted, on the one upstream neuron with a presynaptic axon terminal ( presynaptic ) transfers excitations that are transferred here in excitatory signals.
Spinous processes occur in all vertebrates in the central nervous system in the human brain forming excitatory synapses mostly on dendritic spines and some nerve cells carry thousands of these, each about 0.2 to 2 microns long with a narrow neck ( engl. neck ) and various sizes of head ( engl. head). Both shape, size as biochemical peculiarities of thorns affect signal transmission at synapses.
A mandrel may be formed, vary depending on the synaptic activity and various forms ( morphological plasticity ), and are formed again. These structural changes influence the functional conditions of a synapse (synaptic plasticity ) and can briefly, longer lasting (early ) or long-term persistent (late long-term potentiation LTP) lead to gains of synaptic connections (possible correlates of long-term memory ). Mushroom -shaped spines developed (german mushroom spines ) often contain a so-called spine apparatus, probably as calcium storage for intracellular Ca2 - signaling pathways has significance, however, is not yet understood.
Appearance and occurrence
Spinous processes were described in 1888 by R. Cajal in the cerebellum of birds and in humans are both on Purkinje cells of the cerebellar cortex as on pyramidal cells of the cerebral cortex - archikortikal particularly in the hippocampus - how to find even in other brain regions, such as subcortical (see figure) or in the thalamus. In most cases, grow thorns from dendrites, but they can also be formed on the soma of a nerve cell, or in the area of Axonhügels.
Dendritic spines appear differently on densely distributed in different sizes and shapes of the protrusions. They often grow with a narrow neck upwards and end with a more or less bulky head ( " Endköpfchen "). This carries a postsynaptic density ( PSD ) membrane area with transmitter receptors, ion channels and signal transmitting systems. ( The opposite is the pre-synaptic region of the other neuron, usually on a " Endknöpfchen ". ) Solo after the appearance of the projections grossly different types are different, but the transitions are fluid:
- Filopodia: Very long, thread-like protrusions without head. Filopodia also be regarded as a precursor of dendritic branching and multiple synapses wear.
- Thin spines ( engl. thin spines ): Thorns with a narrow, long neck and clearly Remote Head
- Mushroom-shaped spines (English mushroom spines ): Thorns with a narrow neck and a bulky, spherical head
- Sessile spines (English sessile spines ): Thorns without a clearly discernible neck
- Stub -like spines (English stubby spines ): In short, without distinction of head and neck
About the shape-determining for the moving cytoskeleton of F -actin factors of a mandrel is described in detail as yet little known. It is believed that is exerted influence on the shape of the signal transmission of the associated synapse (→ function), defined a subspace of the nerve cell and can be designed as a subcompartment with specific biochemical conditions. In addition, it was demonstrated that dendritic spines not permanently belong to a particular type, but change their shape (morphological plasticity ), and possibly also over time in terms of a life cycle of thorns. The larger a spinous process, the higher is commonly the number of receptor molecules for the neurotransmitter in the postsynaptic membrane region (PSD ).
Function and form differentiation
Mandrels are adaptable structures, specializing in the synaptic transmission and the transformation of postsynaptic signals in particular changes in the form, depending on synaptic activity. You can influence synaptic transmission and signal transduction in several respects:
- Increase in surface area: Dendritic spines increase the surface area of dendrites, thus ensuring that more synapses can find place on them. They also shorten the path length, which must travel axons.
- Electrical resistance: the narrow " neck " of dendritic spines possibly represents an electrical resistance, since ions this bottleneck can not happen so easily. This could be the electrical signal to be amplified at synapses. This hypothesis is controversial.
- Biochemical compartmentalization: As protuberances of Dendritenoberflächen they form separate units that each communicating only through a more or less narrow " bridge " with the remaining dendrites. Thus hinder the diffusion of molecules into or out of an extension and thus allow changes initially remain confined to individual postsynapses.
With the spinous process of a postsynaptic element is highlighted and deposed as a subspace, which can be configured differently depending on its synaptic activity. By forming the cytoskeleton of actin filaments spinous processes can be formed in various shapes according to the width of the base of the neck length and size of the head. While the respective spatial shape of the membrane envelope has an influence on the propagation of electrical potential changes, the partitioned space as compartmentalization of biochemical signaling processes - for example, rapidly briefly increased intracellular Ca2 levels - to be understood. In particular mushroom-shaped developed spines (English mushroom spines ) to neurons of the brain - but not cerebellar - contained in the cytoplasmic space often additionally as a specific organelle a mandrel apparatus, which consists of several blades smooth endoplasmic reticulum, probably acts as a calcium - memory and in different ways, affect various forms of synaptic plasticity.