How is the somatic nervous system controlled?

The somatic nervous system is in charge of VOLUNTARY movement - usually associated with skeletal muscles.
Movement from the body is often driven by the Cerebral Cortex - It is broken up into association areas. (if you wish to look into a more specific explanation)
Sensory Neurons will carry information from the PNS to the CNS where interneurons will act to interpret the information within the Primary Motor Association area of the Cerebral Cortex and an appropriate movement-related response will carry forth from there.