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Purkinje fibers are a unique cardiac end-organ. Further histologic examination reveals that these fibers are split in ventricles walls. The electrical origin of atrial Purkinje fibers arrives from the sinoatrial node. Given no aberrant channels, the Purkinje fibers are distinctly shielded from each other by collagen or the cardiac skeleton.
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.
Each climbing fiber will form synapses with 1-10 Purkinje cells. Early in development, Purkinje cells are innervated by multiple climbing fibers, but as the cerebellum matures, these inputs gradually become eliminated resulting in a single climbing fiber input per Purkinje cell.
This delay allows the ventricles to fully fill with blood before contraction. The signal then passes down through a bundle of fibres called the bundle of His, located between the ventricles, and then to the Purkinje fibers at the bottom (apex) of the heart, causing ventricular contraction. [citation needed]
The only excitatory neurons present in the cerebellar cortex are granule cells. [10] Plasticity of the synapse between a parallel fiber and a Purkinje cell is believed to be important for motor learning. [11] The function of cerebellar circuits is entirely dependent on processes carried out by the granular layer.
Muscle cells are grouped into muscle fascicles by enveloping perimysium connective tissue. [1] Fascicles are bundled together by epimysium connective tissue. [1] Muscle fascicles typically only contain one type of muscle cell (either type I fibres or type II fibres), but can contain a mixture of both types.
The bundle branches were separately described by Retzer and Braeunig as early as 1904, but their physiological function remained unclear and their role in the electrical conduction system of the heart remained unknown until Sunao Tawara published his monograph on Das Reizleitungssystem des Säugetierherzens (English: The Conduction System of the Mammalian Heart) in 1906. [4]
Sensory information relayed from the pons through the mossy fibers to the granule cells is then sent along the parallel fibers to the Purkinje cells for processing. Extensive branching in white matter and synapses to granular cells ensures that input from a single mossy fiber axon will influence processing in a very large number of Purkinje cells.