Bordercells are found throughout the spinal cord. They are similar in size and appearance to alpha motor neurons and were previously classified as alpha motor neurons. They are located on the fringes of the front horns (also deeper in the gray matter) and especially in the swellings of the throat and lumbar.
The border cells are considered to receive, in principle, the same information that adjacent alpha motor neurons receive. But instead of sending their axons out to skeletal muscle, the border cells send their axons across the midline. The axons then travel via the spinal cord side cord up to and into the cerebellum.
The axons emanating from border cells (and from the interneuron in the intermediate gray matter of the spinal cord) in the lower half of the spinal cord form the anterior cerebellar pathway of the spinal cord (tractus spino-cerebellaris anterior ). Those from the upper half of the spinal cord form the upper cerebellar pathway of the spinal cord (tractus spinocerebellaris rostralis).
The signals from the border cells and the interneuron constitute the internal feed-back information from the spinal cord to the cerebellum and can be said to reflect the behavior of the alpha motor neuron and connected interneurons at the moment when movement commands to the muscles are delivered. How the movement command is then executed, yes, the cerebellum is informed via the external feed-back information that is conveyed to the spinal cord from muscles and joints in the form of proprioceptive signals and then on from the spinal cord to the cerebellum (tractus spino-cerbellaris posterior and tractus cuneo-cerebellaris).
Whether the movement command then led to the intended pre-programmed movement or not, the cerbellum in collaboration with the olivainferior complex can determine by comparing the internal feed-back with the external one. Should the result not match the plan, it is the cerebellum's task to correct the signal flow to the alpha motor neuron.