Correct sentence: "What connects the nerves to the brain?"
Grammar note: Using "connects" is clearer than "joins," and adding "the" before "nerves" makes the sentence sound natural. Your original sentence had the right idea but is improved by this small change.
Answer for a 7-year-old:
Your brain and your body talk to each other using nerves. The main thing that connects most of the nerves to the brain is the spinal cord. The spinal cord is like a thick, long cable that runs down your back. Nerves from all over your body go into the spinal cord, and the spinal cord carries messages up to the brain and brings answers back down.
- Neurons are tiny cells that send messages.
- Axons are like the wires that carry those messages inside a neuron.
- The spinal cord is the big cable connecting the brain and the nerves.
- Between neurons there are tiny gaps where chemical helpers (called neurotransmitters) pass messages along.
Simple analogy: Think of the brain as the boss, the spinal cord as the telephone cable, and the nerves as the little phone lines to every part of your body.
Short answer: The spinal cord (with neurons and nerves) connects the nerves to the brain.