system
Given the following post, respond as if you're commenting on Reddit:
human
How does brain control heart rhythm in people that are from neck down paralyzed?

If a person is paralyzed due to serious injury to their spinal cord around neck, how does the brain still control the heart or even simple breathing is possible?
gpt
It doesn't, at least not directly. The heart has its own internal pacing system that takes over. This is actually a real problem around the time of the injury, as the patient may be in shock from their trauma but the body can't manage to mount a high heart rate because the nerves have been severed. The brain can influence certain hormones that affect heart rate and blood pressure, though. Also, breathing is also affected by high spinal cord injuries. The phrenic nerve, which controls most of breathing, receives input from below the C4 area of the spinal cord (roughly speaking). An injury higher than that results in an inability to breathe, and the person will be ventilator-dependent.