<unk>  The term<unk>  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 oblongata, and travel as part of the spinal cord until they synapse with interneurons in the grey column of the spinal cord. There is some variation in terminology. The<unk>  definitively encompass the corticospinal<unk> , and many authors also include the corticobulbar<unk> . Corticospinal<unk> Nerve fibres in the corticospinal<unk> originate from pyramidal cells in layer V of the cerebral cortex. Fibres arise from the primary motor cortex (about 30%), supplementary motor area and the premotor cortex (together also about 30%), and the somatosensory cortex, parietal lobe, and cingulate gyrus supplies the rest. The cells have their bodies in the cerebral cortex, and the axons form the bulk of the<unk> . The nerve axons travel from the cortex through the posterior limb of internal capsule, through the cerebral peduncle and into the brainstem and anterior medulla oblongata. Here they form two prominences called the medulla oblongatary pyramids. Below the prominences, the majority of axons cross over to the opposite side from which they originated, known as decussation. The axons that cross over move to the outer part of the medulla oblongata and form the lateral corticospinal<unk>  whereas the fibres that remain form the anterior corticospinal<unk>  About 80% of axons cross over and form the lateral corticospinal<unk>  10% do not cross over and join the<unk>  and 10% of fibres travel in the anterior corticospinal<unk>  The nerve axons traveling down the<unk> are the efferent nerve fibers of the upper motor neurons. These axons travel down the<unk>  in the white matter of the spinal cord until they reach the vertebral level of the muscle that they will innervate. At this point, the axons synapse with lower motor neurons. The majority of axons do not directly synapse with lower motor neurons, but instead synapse with an interneuron that then synapses with a lower motor neuron. This generally occurs in the anterior grey column. Nerve axons of the lateral corticospinal<unk> that did not cross over in the medulla oblongata do so at the level of the spinal cord they terminate in. These<unk>  contain more than 1 million axons and the majority of the axons are myelinated. The corticospinal<unk>  myelinate largely during the first and second years after birth. The majority of nerve axons are small (<4μm) in diameter. About 3% of nerve axons have a much larger diameter (16μm) and arise from Betz cells, mostly in the leg area of the primary motor cortex. These cells are notable because of their rapid conduction rate, over 70m/sec, the fastest conduction of any signals from the brain to the spinal cord. Horizontal section through the lower part of the pons, showing the fibers of the corticospinal<unk> (#19) passing through the pontine nuclei Corticobulbar<unk> Fibres from the ventral motor cortex travel with the corticospinal<unk> through the internal capsule, but terminate in a number of locations in the midbrain (cortico-mesencephalic<unk> , pons (Corticopontine<unk> , and medulla oblongata (cortico-bulbar<unk> . The upper motor neurons of the corticobulbar<unk> synapse with interneurons or directly with the lower motor neurons located in the motor cranial nerve nuclei, namely oculomotor, trochlear, motor nucleus of the trigeminal nerve, abducens, facial nerve and accessory and in the nucleus ambiguus to the hypoglossal, vagus and accessory nerves. These nuclei are supplied by nerves from both sides of the brain, with the exception of the parts of the facial nerve that control muscles of the lower face. These muscles are only innervated by nerves from the contralateral (opposite) side of the cortex.
Pyramidal tracts
Anterior corticospinal tract
Frontopontine fibers
Cerebrospinal fibers
Geniculate fibers
Extrapyramidal system
Striatopallidal fibres
Hypothalamotegmental tract