To raise an arm upward, the muscle on one side of it contracts. What happens to the muscle on the opposite side of the arm? contracts relaxes both contracts and relaxes neither contracts nor relaxes

What is the main difference between the central nervous system (cns) and the peripheral nervous system (pns)? .the cns involves the brain and spinal cord, and the pns involves the body nerves. the cns involves the body nerves, and the pns involves the brain and spinal cord. the cns involves sensory neurons, and the pns involves motor neurons. the cns involves motor neurons, and the pns involves sensory neurons.