Purkinje cells or Purkyně cells are the characteristic large multipolar neurons with strong verästeltem dendritic tree in the cortex of the cerebellum ( cortex cerebelli ) whose axons the efferents represent the cerebellar cortex.
Analogous to the cerebrum a is in the cerebellum also bark ( cortex ) is formed with only three-layered structure. The cell bodies (somata) of Purkinje cells are located in the middle layer of the cerebellar cortex, the stratum purkinjense. These nerve cells, arranged in a single layer at a distance of 50–100 µm, are the largest cells in the cerebellum with a perikaryon of around 50–70 µm. They are characterized by an extremely branched dendrite tree , which is aligned almost flat like a trellis in the outer layer of the cerebellar cortex, the stratum molecularulare . The human cerebellum has around 15 million Purkinje cells, each with one axon.
Axons of the Purkinje cells, the only exits of the cerebellar cortex, project mainly onto the cerebellar nuclei , and also onto the lateral vestibular nuclei . They each form inhibitory synapses and use γ-aminobutyric acid (GABA) as a neurotransmitter . The projections of the Purkinje efferents onto the cerebellar nuclei are somatotopically organized: the head area is represented in the rear parts, the area of the rear extremities or tail in the front parts of the nuclei.
These neurons are named after their discoverer, the Bohemian physiologist Jan Evangelista Purkyně (1787–1869), who first described them in 1837, as did the cortical layering of the cerebellum.
Synaptic interconnection of the Purkinje cells
The cerebellum cortex is divided into three layers (see figure above). The dendritic tree of the Purkinje cells branches out in the molecular layer ( stratum molecularulare ), followed by the Purkinje cell layer ( stratum purkinjense ) with their perikarya (cell bodies). Inward, the granular cell layer ( stratum granulosum ) connects with the numerous granular cells between which axons from Purkinje cells to the cerebellar nuclei located in the cerebellar marrow run.
The cerebellum receives its afferents from the periphery (including muscle spindles), the brain stem and the cerebral cortex; which ultimately converge on the Purkinje cells.
The Purkinje cells receive two exciting inputs, namely from the climbing fiber system and the moss fiber parallel fiber system ; these are excitatory and glutamatergic.
- The climbing fiber arises from the lower olive , a core area in the medulla oblongata . It gets its name from the fact that it winds its way around the dendritic tree of the Purkinje cell, which is located in the stratum molecular.
In contrast to the parallel fiber input - which requires a considerable spatial summation to trigger an action potential (AK) - the probability of transmitter release at the presynaptic endings when an AK arrives is very high (even a single climbing fiber action potential excites the Purkinje cell above threshold).
A Purkinje cell has synaptic contacts with only one climbing fiber; on the other hand, about 100,000 parallel fibers converge on them. Shortly after birth, most of the Purkinje cells are initially still innervated by several climbing fibers. By eliminating redundant synapses, the typical mono-innervation of the Purkinje cells is formed during development through a climbing fiber .
- The moss fibers (axons of neurons in brainstem nuclei and spinal cord) and parallel fibers (axons of the granule cells in the granule cell layer)
The moss fibers (approx. 50 million) are axons of neurons in brain stem nuclei and spinal cord. They convey information from the cerebral cortex and activate the granule cells, the axons of which are the parallel fibers, which in turn excite the Purkinje cells. A parallel fiber has only one synapse per Purkinje cell.
These rise up into the molecular layer and fork there at right angles. The part of the granule cell axon after the fork is called the parallel fiber. These fibers run through the molecular layer in a parallel arrangement (hence the name) and meet the dendrite tree of the Purkinje cells at a right angle. Each parallel fiber innervates many Purkinje cells, but rarely forms more than one, never more than two synapses. That is, the axon does not end at the site of the synapse. This type of innervation is also known as the en-passant synapse. Each Purkinje cell has more than 100,000, depending on the source up to 200,000, parallel fiber synapses. The single parallel fiber synapse is "weak" compared to the climbing fiber synapse. The probability of transmitter release in the presynaptic part is therefore low.
The Purkinje cell is innervated in the molecular layer essentially by two types of interneuron : the basket cells and the stellate cells.
In the proximal part of the dendrite tree of the Purkinje cells these are especially the basket cells , more distally mainly the stellate cells . The Golgi cells (multipolar ganglion cells of the Golgi type, which appear as large granule cells of the cerebellar cortex ) are another interneuron type in the granule cell layer . The synapses of these three interneurons are GABAerg , that is, they use GABA as a transmitter .
It was recently discovered that, contrary to previous assumptions, the Lugaro cells also have synapses with the Purkinje cell in the granular layer. However, these are only active under certain conditions. For example, they need the presence of the neurotransmitter serotonin .
Exit of the Purkinje cell
Only the axons of Purkinje cells represent the outlets of the cerebellar cortex. Action potentials of a Purkinje cell are generated in the axon on the first Ranvier's ring , approx. 75 µm from the soma. The axon terminals release GABA as a transmitter and thus have an inhibitory effect on the neurons downstream from them ( inhibitory PSP ). These lie predominantly in the complex of the cerebellar nuclei in the medulla of the cerebellar hemisphere on each side
and beyond that in the nucleus vestibularis laleralis.
The dendritic tree of the Purkinje cells is, like that of some other central neurons, covered with thorny processes - in English spines - and this is about five times denser than in pyramidal cells . The postsynaptic membrane region of an excitatory synapse is located on a thorny process on the dendrites of Purkinje cells . The GABAergic inhibitory synapses do not terminate in thorns.
The thorny processes of a Purkinje cell differ in whether they are innervated by the climbing fiber or the parallel fibers. Climbing fiber spines are mainly located in the proximal (closer to the soma) part of the dendrite tree, while parallel fiber spines mainly occupy the thin distal (further away from the soma) areas of the dendrite tree.
Thorn processes for climbing fibers develop differently than those for parallel fibers. The creation of the parallel fiber spines does not require any synaptic activity. It is probably intrinsically given in the development of the neuron . The growth of climbing fiber spines, on the other hand, requires the presence and activity of functional parallel fiber synapses and, on the other hand, is inhibited by the activity of the climbing fiber.
Receptors and ion channels in Purkinje cells
Purkinje cells express glutamate receptors on the exciting glutamatergic climbing and parallel fiber synapses . Remarkably, mature Purkinje cells lack NMDA receptors . The only ionotropic glutamate receptors in Purkinje cells are AMPA receptors . Since the latter on the GluR2 subunit have their is calcium - permeability low.
Purkinje cells are the only neurons in the CNS to express the GluRδ2 receptor . The amino acid sequence of this receptor suggests that it is an ionotropic glutamate receptor. Nevertheless, neither a direct binding of glutamate to this subunit nor its incorporation into a known glutamate receptor has been demonstrated. GluRδ2 receptors are mainly located on the parallel fiber synapses of the Purkinje cells. In the absence of the receptor, synaptic plasticity is disrupted at the cellular level and motor control and motor learning are disrupted at the behavioral level. Many other postsynaptic proteins interact with the GluRδ2 subunit. The GluRδ2 receptor thus plays a key role in the function of the cerebellum, even if its function and its mechanism of action are still unclear.
Metabotropic glutamate receptors, predominantly the subtype mGluR1, are localized perisynaptically ( i.e. at the edge, not in the center of the synapse) in both parallel fiber synapses and climbing fiber synapses in Purkinje cells .
As in all neurons, voltage-activated sodium channels in Purkinje cells are responsible for the creation and transmission of action potentials. They are primarily expressed in the soma and axon of the Purkinje cell. In the dendrite tree, their density decreases rapidly with increasing distance from the soma. For this reason, in contrast to other types of neurons such as pyramidal cells in the hippocampus, the action potential in Purkinje cells does not penetrate the dendrite tree strongly. The sodium channels Na v 1.1, Na v 1.2 and Na v 1.6 are mainly expressed in the Purkinje cells of mammals .
In the dendrite tree and soma of the Purkinje cells there are voltage-dependent calcium channels , most of which belong to the P / Q type . With strong depolarization (such as an action potential or synaptic activity of climbing and / or parallel fibers), these cause calcium ions to flow into the cell.
The membrane of the endoplasmic reticulum (ER) of Purkinje cells contains ligand-activated calcium channels, both IP3 receptors and ryanodine receptors . When activated, both release calcium ions from the ER calcium store into the cytosol , where they increase the concentration of so-called free calcium ions. Of the various IP 3 receptors, subtype 1 (IP 3 R1) is primarily expressed in Purkinje cells , and in fact in at least 10 times the amount compared to other cell types.
Spontaneous activity of the Purkinje cells
Purkinje cells are characterized by a high level of spontaneous activity. This means that they generate action potentials regardless of whether they are excited by climbing or parallel fibers. They fire at a frequency of approx. 50–150 Hz. Calcium-activated potassium channels , the so-called BK channels , serve as rhythm generators .
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- Stanley Finger: Origins of Neuroscience: A History of Explorations Into Brain Function . Reprint. Oxford University Press, 2001, ISBN 0-19-514694-8 , p. 43.
- Walter cherry : Jan Evangelista Purkyně 1787–1868. A contribution to the 200th anniversary of his birthday. Akademie-Verlag, Berlin 1989 (= session reports of the Academy of Sciences of the GDR. Year 1988, No. 5 / N), ISBN 3-055-00520-1 , p. 28 f.
- Functional architecture of the rat cerebellar nuclei. (PDF; 1.7 MB) Dissertation . University of Tübingen.
- The Neuroscientist - The Lugaro Cell Is a Major Effector for Serotonin in the Cerebellar Cortex (PDF; 1.2 MB) - Article hosted at princeton.edu
- BK Channels Control Cerebellar Purkinje and Golgi Cell Rhythmicity In Vivo - Article at plosone.org
- Theodor H. Schiebler, Horst-W. Korf: Anatomy: histology, history of development, macroscopic and microscopic anatomy, topography . 10., completely revised. Edition. Steinkopff, 2007, ISBN 978-3-7985-1770-7 , pp. 786-791.
- Rainer Klinke , Hans-Christian Pape, Armin Kurtz, Stefan Silbernagl: Physiology: Textbook . 6th, completely revised edition. Thieme, Stuttgart 2009, ISBN 978-3-13-796006-5 , pp. 769/770.
- Entry on Purkinje cell in Flexikon , a wiki of the DocCheck company
- THE DENDRITEN DEVELOPMENT OF PURKINJE CELLS THROUGH GLUTAMATE RECEPTOR-CONTROLLED SIGNAL PATHS (PDF; 2.1 MB) - Dissertation at db-thueringen.de
- Organization of spines on the dendrites of Purkinje cells - Articles about the organization of dendritic spines in fish and mammals in pnas.org (Engl.)
- Dendritic Spines of Rat Cerebellar Purkinje Cells: Serial Electron Microscopy with Reference to Their Biophysical Characteristics (PDF, 2.3 MB) - article on dendritic spines in rats at jneurosci.org (Engl.)