Can you tell me what is the main nerve in your back?

There is no main . Nerve in your back but there is the spinal cord which begins at the base of your skull and extends down to on averge to the lumbar 1 vertebral level on average in adults with the cauda equina extendingbthe restbof the way down which is a group of nerves.
Spinal cord. "your back "to me means the whole spine did it mean something else to your spine contains/protects all the nerves that control your arms and legs the spinal cord would be my main nerve in my back L5 and s1 are the most symptomatic nerves of the low back cause siatica.