Purkinje cell

Purkinje cells are multipolar (numerous dendrites and an axon ), inhibitory (GABA as a neurotransmitter ) neurons in the cortex of the cerebellum. This outward-facing, nerve cell- containing region is - also called ( cerebellar ) cortex - analogous to the cerebrum. They are the largest cells in the cerebellum and some of the largest nerve cells at all, and the only efferent cells of the cortex, that is, their axons constitute the sole output of the cerebellar cortex

Purkinje cells are characterized by a relatively large cell bodies (50-70 microns ) and are arranged in a single layer at a distance of 50-100 microns. They also have a much branched dendritic tree; this is almost entirely in the sagittal plane of the cerebellum. The human cerebellum has about 15 million Purkinje cells.

The cortex of the cerebellum is divided into three layers (gray colored area in the illustration ). ( Stratum moleculare ) very outside in the molecular layer is the dendritic tree of the Purkinje cells, subsequently the Purkinje cell layer ( stratum also purkinjense ) that of their somata ( cell bodies ) is formed. Finally, granule cell layer (stratum granulosum ), in which are the axons of Purkinje cells.

The Purkinje cells and the cerebellar nuclei are strictly somatotopically organized. That is, the muscles of the head is in the rear, which represents the legs in the front portions of the cores; similar to the homunculus of the cerebral cortex.

History

They were named after their discoverer, the Czech physiologist January Evangelista Purkyně (1787-1869), who in 1837 first described.

Synaptic connectivity of the Purkinje cells

Excitatory inputs

His afferents receives the cerebellum from the periphery ( inter alia from muscle spindles ), the brain stem and the cerebral cortex; which ultimately converge on the Purkinje cells.

The Purkinje cells receive two excitatory inputs and that of the climbing fiber system and the mossy fiber - parallel fiber system; these are excitatory and glutamatergic.

• The climbing fiber arises from the inferior olive, a core area in the medulla oblongata. It derives its name from the fact that they umschlängelt the dendritic tree of the Purkinje cell in ascending order: Especially at primary dendrites in the proximal, ie closer to the soma located part, makes it particularly strong synapses with the Purkinje cell.

In contrast to the parallel fiber input - requires a significant spatial summation to trigger an action potential (AK) - is the probability of transmitter release at the presynaptic terminals at the arrival of a AK very high ( even a single climbing fiber action potential excites the Purkinje cell thresholds).

A Purkinje cell has synaptic contacts with only one climbing fiber; however, they converge to some 100 000 parallel fibers. Shortly after birth, most Purkinje cells are initially innervated by multiple climbing fibers. By elimination of supernumerary synapses typical Monoinnervation of the Purkinje cells formed during development out by one climbing fiber.

• The mossy fibers ( axons of neurons in brainstem nuclei and spinal cord) and parallel fibers ( axons of granule cells in the granule cell layer)

The mossy fibers (about 50 million ) are axons of neurons in brainstem nuclei and spinal cord. They convey information from the cerebral cortex and activate the granule cells, whose axons, the parallel fibers, which in turn excite the Purkinje cells. A parallel fiber has only one synapse per Purkinje cell, respectively.

This advance to the molecular layer and bifurcate there at right angles. The part of the Körnerzellaxons after the bifurcation is referred to as a parallel fiber. These fibers pass through in a parallel arrangement ( hence the name), the molecular layer and meet at a right angle to the dendritic tree of the Purkinje cells. Each parallel fiber innervates many Purkinje cells, but forms rarely more than one, never more than two synapses. That is, the axon does not end at the point of junction. This type of innervation is also called en passant synapse. Each Purkinje cell has more than 100, 000, according to sources up to 200 000 parallel fiber synapses. The individual parallel fiber synapse is " weak " compared to the climbing fiber synapse. The probability of transmitter release in the presynaptic part is so low.

Inhibitory inputs

The Purkinje cell is innervated significantly in the molecular layer of two inter -locking Euro types: the basket cells and stellate cells.

In the proximal part of the dendritic tree of the Purkinje cells which are especially the basket cells, distally more particularly the stellate cells. In the granule cell layer to find the Golgi cells ( multipolar ganglion cells of type Golgi, which appear as large granule cells of the cerebellar cortex ) as a further inter Euro type. The synapses of these three interneurons are GABAergic, that is, they use GABA as a transmitter.

Recently it has been discovered that, contrary to previous assumptions, also the Lugarozellen in the granule cell layer, with the Purkinje cell synapses. However, these are only active under certain conditions. For example, they need the presence of the neurotransmitter serotonin.

Output of the Purkinje cell

The axons of the Purkinje cell represent the sole output of the cerebellar cortex and innervate the cerebellar nuclei. As they pour out GABA, they have an inhibitory effect. In the Purkinje cell axon, the action potentials are generated, namely away from the soma on the first Ranvier constriction, about 75 microns.

Spines

The dendritic tree of the Purkinje cells is similar to that of many central neurons, dense spinous processes - the so-called spines - occupied. On the spines of the Purkinje cells are excitatory synapses. The GABAergic inhibitory synapses do not have spines.

The spines of Purkinje cell differ in whether they are innervated by the climbing fiber and the parallel fibers. Climbing fiber spines are located mainly in the proximal (closer to the soma located ) part of the dendritic tree, whereas parallel fiber spines occupy mainly the thin distal ( further away from the soma ) regions of the dendritic tree.

Climbing and parallel fiber spines develop differently. The system of parallel fiber spines does not need synaptic activity. It is intrinsically likely set in the development of the neuron. The growth of climbing fiber spines, however, requires the presence and functional activity of parallel fiber synapses and on the other hand by inhibiting the activity of climbing fiber.

Receptors and ion channels in Purkinje cells

On the excitatory glutamatergic climbing fiber and parallel synapses expressing Purkinje cell glutamate receptors. Remarkably, do not have mature Purkinje cells via NMDA receptors. The only ionotropic glutamate receptors in Purkinje cells are AMPA receptors. Since the latter have the GluR2 subunit their calcium permeability is low.

Purkinje cells expressing the only neurons in the CNS the GluRδ2 receptor. The amino acid sequence of this receptor suggests that it is an ionotropic glutamate receptor. Nevertheless, so far neither a direct binding of glutamate to this subunit, nor their installation in a known Glutamtarezeptor was detected. GluRδ2 receptors are mainly located to the parallel fiber synapses of the Purkinje cells. In the absence of the receptor results in impairments of synaptic plasticity at the cellular level and disorders of motor control and motor learning on a behavioral level. Many other postsynaptic proteins interact with the GluRδ2 subunit. So the GluRδ2 receptor plays a key role in the function of the cerebellum, although its function and mechanism of action are still unclear.

Both of parallel fiber and climbing fiber synapses on Purkinje cells are in perisynaptisch (ie, at the edge, not the center of the synapse ) located metabotropic glutamate receptors, predominantly of subtype mGluR1.

As in all neurons voltage-gated sodium channels are responsible for the origin and transmission of action potentials in Purkinje cells. They are expressed primarily in the soma and axon of the Purkinje cell. In the dendritic tree takes its density with increasing distance from the soma rapidly. For this reason, the action potential in Purkinje cells penetrate in contrast to other types of neurons, such as pyramidal cells in the hippocampus is not strong in the dendritic tree. In the Purkinje cells of mammals, especially the sodium channels Nav1.1, Nav1.2, and Nav1.6 are expressed.

In the dendritic tree and soma of the Purkinje cells, there are voltage-dependent calcium channels that belong for the most part to the P / Q-type. This effect at high depolarization ( such as an action potential, or synaptic activity of climbing and / or parallel fibers) an influx of calcium ions into the cell.

In the membrane of the endoplasmic reticulum (ER) of Purkinje cells are ligand- activated calcium channels, both IP3 receptors and ryanodine receptors. Both set when activated calcium ions from the ER calcium storage in the cytosol free and there increasing the concentration of so-called free calcium ions. Of the various receptors of the subtype 1 IP3 ( IP3R1 ) in Purkinje cells is primarily expressed as compared to other cell types in at least a 10-fold amount.

Spontaneous activity of Purkinje cells

Purkinje cells are characterized by a high level of spontaneous activity. By this is meant that they generate action potentials regardless of whether they are excited by climbing or parallel fibers. They fire at a frequency of about 50-150 Hz as rhythm generators commonly used are calcium - activated potassium channels, the so-called BK channels.