Purkinje cells or Purkinje neurons, named for Czech physiologist Jan Evangelista Purkyně who identified them in 1837, are a unique type of prominent, large neuron located in the cerebellar cortex of the brain. With their flask-shaped cell bodies, many branching dendrites, and a single long axon, these cells are essential for controlling motor activity. Purkinje cells mainly release GABA (gamma-aminobutyric acid), a neurotransmitter which inhibits some neurons to reduce nerve impulse transmission. Purkinje cells efficiently control and coordinate the body's motor motions through these inhibitory actions.
Structure
Purkinje cells are some of the largest neurons in the human brain (Betz cells being the largest), with an intricate and elaborate branching structure (known as a dendritic arbour) characterized by a large number of dendritic spines. Found within the Purkinje layer of the cerebellum, Purkinje cells align like dominos - stacked one in front of the other - with their expansive dendritic arbours forming almost-two-dimensional layers, through which parallel fibers from deeper-layers pass. These parallel fibers form relatively-weak excitatory synapses - specifically, glutamatergic synapses - connected to the spines of Purkinje cell dendrites, while climbing fibers - originating from the inferior olivary nucleus of the medulla - provide very powerful excitatory input to proximal dendrites and cell soma. Parallel fibers pass orthogonally through the Purkinje neuron's dendritic arbor, with up to 200,000 parallel fibers forming a granule-cell-Purkinje-cell synapse within a single Purkinje cell. Each adult Purkinje cell receives approximately 500 climbing fiber synapses, collectively originating from a single climbing fiber in the inferior olive, a phenomena which has led to the observation that a "highly conserved one-to-one relationship renders Purkinje dendrites into a single computational compartment". However, observations of murine Purjinke cells have found multi-innervation among a subset of cells comprising multiple primary dendrites - a dendritic motif uncommon in rodents but "predominant" in humans. Both basket and stellate cells (found in the cerebellar molecular layer) provide inhibitory (GABA-ergic) input to the Purkinje cell. Basket cells synapse on the initial segment of the Purkinje cell axon, while stellate cells synapse to the dendrites. Purkinje cells send inhibitory projections to the deep cerebellar nuclei, constituting the sole output of all motor coordination in the cerebellar cortex. Purkinje cells also receive feedback signals associated with efference copies from deep cerebellar nuclei, forming an internal feedback loop within the cerebellum that enables recurrent processing.
Molecular The Purkinje layer of the cerebellum, which contains the cell bodies of the Purkinje cells and Bergmann glia, express a large number of unique genes. Purkinje-specific gene markers were also proposed by comparing the transcriptome of Purkinje-deficient mice with that of wild-type mice. One illustrative example is the Purkinje cell protein 4 (PCP4) in knockout mice, which exhibit impaired locomotor learning and markedly altered synaptic plasticity in Purkinje neurons. PCP4 accelerates both the association and dissociation of calcium (Ca2+) with calmodulin (CaM) in the cytoplasm of Purkinje cells, and its absence impairs the physiology of these neurons.
Development Mammalian embryonic research has detailed the neurogenic origins of Purkinje cells. During early development Purkinje cells arise in the ventricular zone in the neural tube, the nervous system´s precursor in the embryo. All cerebellar neurons derive from germinal neuroepithelia from the ventricular zone. Purkinje cells are specifically generated from progenitors in the ventricular neuroepithelium of the embryonic cerebellar primordium. The first cells generated from the cerebellar primordium form a cap over a diamond-shaped cavity of the developing brain called the fourth ventricle forming the two cerebellar hemispheres. The Purkinje cells that develop later are those of the cerebellum's center-lying section called the vermis. They develop in the cerebellar primordium that covers the fourth ventricle and below a fissure-like region called the isthmus of the developing brain. Purkinje cells migrate toward the outer surface of the cerebellar cortex and form the Purkinje cell layer. Purkinje cells are born during the earliest stages of cerebellar neurogenesis. Neurogenin2, together with neurogenin1, are transiently expressed in restricted domains of the ventricular neuroepithelium during the time-window of Purkinje cell genesis. This spatio-temporal distribution pattern suggests that neurogenins are involved in the specification of phenotypically heterogeneous Purkinje cell subsets, ultimately responsible for constructing the framework of the cerebellar topography. There is evidence in mice and humans that bone marrow cells either fuse with or generate cerebellar Purkinje cells, and it is possible that bone marrow cells, either by direct generation or by cell fusion, could play a role in repair of central nervous system damage. Further evidence points yet towards the possibility of a common stem cell ancestor among Purkinje neurons, B-lymphocytes and aldosterone-producing cells of the human adrenal cortex.
Function
Purkinje cells show two distinct forms of electrophysiological activity:
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