|Function||inhibitory projection neuron|
|Morphology||flat dendritc arbor|
|Presynaptic connections||Parallel fibers and Climbing fibers|
|Postsynaptic connections||Cerebellar deep nuclei|
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These cells are some of the largest neurons in the human brain, with an intricately elaborate dendritic arbor, characterized by a large number of dendritic spines. Purkinje cells are found within the Purkinje layer in the cerebellum. Purkinje cells are aligned like dominos stacked one in front of the other. Their large dendritic arbors form nearly two dimensional layers through which parallel fibers from the deeper-layers pass. These parallel fibers make relatively weaker excitatory (glutamatergic) synapses to spines in the Purkinje cell dendrite, whereas climbing fibers originating from the inferior olivary nucleus in the medulla provide very powerful excitatory input to the 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 synapse with a single Purkinje cell. Alternatively, each Purkinje cell only receives a synapse from a single climbing fiber. Both basket and stellate cells (found in the cerebellar molecular layer) provide inhibitory (GABAergic) input to the Purkinje cell, with basket cells synapsing on the Purkinje cell axon initial segment and stellate cells onto the dendrites.
Purkinje cells send inhibitory projections to the deep cerebellar nuclei, and constitute the sole output of all motor coordination in the cerebellar cortex.
Purkinje cells show two distinct forms of electrophysiological activity:
- Simple spikes occur at rates of 50 - 150 Hz either spontaneously or and when Purkinje cells are activated synaptically by the parallel fibers, the axons of the granule cells.
- Complex spikes are rapid (>300 Hz) bursts of spikes caused by climbing fiber activation, and can involve the generation of calcium-mediated action potentials in the dendrites. Following complex spike activity simple spikes can be suppressed by the powerful complex spike input.
Purkinje cells show spontaneous electrophysiological activity in the form of trains of spikes both sodium as well as calcium dependent (Llinas and Sugimori J. Physiol 1980). P type calcium channels were named after Purjinje cells (Llinas et al PNAS 1989), which are crucial in cerebellar function. It has recently been shown that climbing fiber activation of the Purkinje cell can shift its activity from a quiet state to a spontaneously active state, and vice-versa, serving as a type of toggle switch (Lowenstein et al., 2005, Nature Neuroscience). However, these findings have recently been challenged by a study suggesting that such toggling by climbing fiber inputs occurs predominantly in anaesthetized animals, and that Purkinje cells in awake behaving animals in general operate almost continuously in the upstate (Schonewille et al., 2006, Nature Neuroscience).
Purkinje cell dendrites also release endocannabinoids that can transiently downregulate both excitatory and inhibitory synapses(Kreitzer and Regehr 2001).
In some domestic animals, a condition where the Purkinje cells begin to atrophy shortly after birth, called Cerebellar abiotrophy, can lead to symptoms including ataxia, intention tremors, hyperreactivity, lack of menace reflex, stiff or high-stepping gait, apparent lack of awareness of where the feet are (sometimes standing or walking with a foot knuckled over), and a general inability to determine space and distance. A similar condition known as cerebellar hypoplasia occurs when Purkinje cells either fail to develop in utero or die off in utero prior to birth.
In humans, Purkinje cells are affected in a variety of diseases ranging from toxic exposure (alcohol, lithium), to autoimmune diseases and to genetic mutations (spinocerebellar ataxias, autism) and neurodegenerative diseases that are not thought to have a known genetic basis (cerebellar type of multiple system atrophy, sporadic ataxias).