Lets say you want to perform flexion of the thigh. Doing so would stretch the extensors of the thigh, which would initiate the myotatic (stretch) reflex, rendering you unable to flex your thigh. This is obviously not how things work. When alpha motor neurons to the flexor muscles are excited, the extensor muscles that should act against it are inhibited by means of inhibitory interneurons alpha motor neurons gamma motor neurons delta alpha neurons( interneurons, alpha motor neurons, gamma motor neurons, delta alpha neurons ) that branch off of said alpha motor neuron, thus allowing flexion to occur.

The Flexor Reflex and the Crossed Extensor Reflex are reflexes that cause a limb to flex/extend (respectively) in response to harmful stimuli. Such reflexes are mediated by excitatory inhibitory( excitatory, inhibitory ) interneurones.

In terms of the Corticospinal tract, most fibres decussate at the decussation of the pyramids in the medulla pons midbrain( medulla, pons, midbrain ), to form the lateral medial dorsal ventral( lateral, medial, dorsal, ventral ) corticospinal tract.