The double consonant rule applies when you add <span>a suffix beginning with a vowel to a base word ending with consonant + vowel + consonant. For example: the verb <em>to plan - </em>when you add a suffix beginning with a vowel (for example, -ing), to a base word ending with consonant + vowel + consonant (pLAN), the final consonant is doubled = planning the verb <em>to refer </em>+ -ed = referred </span>
"The peripheral nervous system—which is a network of nerves extending from your brain and spinal cord to all parts of your body—carries signals back and forth." is correct.