Pyramids are formed by

The pyramidal tracts include both the coicospinal and coicobulbar tracts. These are aggregations of upper motor neuron nerve fibres that travel from the cerebral coex and terminate either in the brainstem (coicobulbar) or spinal cord (coicospinal) and are involved in control of motor functions of the body.The coicospinal tract conducts impulses from the brain to the spinal cord. It is made up of a lateral and anterior tract. The coicospinal tract is involved in voluntary movement. The majority of fibres of the coicospinal tract cross over in the medulla, resulting in muscles being controlled by the opposite side of the brain. The coicospinal tract also contains Betz cells (the largest pyramidal cells), which are not found in any other region of the body.The nerves within the coicospinal tract are involved in movement of muscles of the body. Because of the crossing-over of fibres, muscles are supplied by the side of the brain opposite to that of the muscle. Ref: guyton and hall textbook of medical physiology 12 edition page number:765,766,767