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Transcript
HOW THE CEREBELLUM IS WIRED THE BASIC FEATURES of cerebellar circuitry have been known since the seminal work of Spanish neuroanatomist Santiago Ramón y Cajal in the late 1800s. The central neuron is the Purkinje cell, named for Czech physiologist Johannes E. Purkinje, who identified it in 1837. The Purkinje cell provides the sole output of the cerebellar cortex and is one of the largest neurons in the nervous system, receiving an extraordinary 150,000 to 200,000 inputs (synapses)â an order of magnitude more than any single neuron in the cerebral cortex. These inputs spring principally from one of the smallest vertebrate neurons, the cerebellar granule cell. Granule cells are packed together at a density of six million per square millimeter, making them the most numerous type of neuron in the brain. The axon, or main trunk line carrying the outgoing signal, of every granule cell rises vertically out of the granule cell layer, making multiple inputs with its overlying Purkinje cell. The axon then splits into two segments that stretch away in opposite directions. These segments align into parallel fibers that run through the arms, or dendrites, of a Purkinje cell like wires through an electrical pole, providing a single input to many hundreds of Purkinje cells. Granule cells also communicate with three other types of neuronsâ the stellate, basket and Golgi cellsâ which help to modulate the signals emitted by both the granule and Purkinje cells. This basic pattern occurs in every cerebellum, indicating that it must be â J.M.B. and L.M.P. integral to its function. Parallel fibers of granule cells Stellate cell Purkinje cell dendrites Basket cell Ascending granule cell axon Granule cells Golgi cell Purkinje cell ALICE Y. CHEN Area of detail Granule cells www.sciam.com SCIENTIFIC AMERICAN COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. 53
