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The pyramidal tracts include both the corticospinal and corticobulbar tracts. These are aggregations of upper motor neuron nerve fibres that travel from the cerebral cortex and terminate either in the brainstem (''corticobulbar'') or spinal cord (''corticospinal'') and are involved in control of motor functions of the body. The corticobulbar tract conducts impulses from the brain to the cranial nerves.〔Chapter 9 of "Principles of Physiology" (3rd edition) by Robert M. Berne and Mathew N. Levy. Published by Mosby, Inc. (2000) ISBN 0-323-00813-5.〕 These nerves control the muscles of the face and neck and are involved in facial expression, mastication, swallowing, and other functions. The corticospinal tract conducts impulses from the brain to the spinal cord. It is made up of a lateral and anterior tract. The corticospinal tract is involved in voluntary movement. The majority of fibres of the corticospinal tract cross over in the medulla, resulting in muscles being controlled by the opposite side of the brain. The corticospinal tract also contains Betz cells (the largest pyramidal cells), which are not found in any other region of the body. The pyramidal tracts are named because they pass through the pyramids of the medulla. The myelination of the pyramidal fibres is incomplete at birth and gradually progresses in caudo-cranial direction and thereby progressively gaining functionality. Most of the myelination is complete by two years of age and thereafter it progresses very slowly in cranio-caudal direction up to twelve years of age. ==Structure== The term ''pyramidal tracts'' refers to upper motor neurons that originate in the cerebral cortex and terminate in the spinal cord (''corticospinal'') or brainstem (''corticobulbar''). Nerves emerge in the cerebral cortex, pass down and may cross sides in the medulla, and travel as part of the spinal cord until they synapse with interneurons in the middle part of the spinal cord, known as grey matter. There is some variation in terminology. The ''pyramidal tracts'' definitively encompass the corticospinal tracts. Many, but not all authors also consider the tracts to include the corticobulbar tracts. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Pyramidal tracts」の詳細全文を読む スポンサード リンク
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