physiology question...

[letthesunshinein]

I'm stuck... I'm working through a textbook with case scenarios: the case is a guy has a spinal cord injury at such a level that spontaneous respiration is still present.

This means spinal nerves C3,4,5 are still intact, innervating the diaphragm. The most rostral level the injury could be at is between C5 and C6 vertebrae (because nerve C5 exits the spinal column between C4 and C5 vertebrae).

This is my 1st problem: the book tells me the most rostral level for the injury is between C4 and C5?

My second problem is working out what happens to the heart rate and blood pressure. Everything I've found online suggests low BP and low HR, due to loss of sympathetic innervation (neurogenic shock). The answer the book gives has my head spinning:

"Removal of the descending drive from the hindbrain cardiovascular control centre to thoracolumbar sympathetic preganglionic neurons will cause a profound fall in total peripheral resistance and reduce blood pressure. However, both the sensory and vagal motor components of the arterial baroreflex pathway are intact, as they are cranial nerves, so the hypotension will cause tachycardia via reflex reduction in vagal cardiac tone."

Can anyone explain this to me in simple terms, preferably with nice short words for a dumb person to understand?

TIA x

[auden]

I think the spinal level one is because to get spontaneous respiration, he doesn't need his whole diaphragm. OK, it may not be brilliant respiration, but it will be spontaneous and enough to sustain life. He definitely needs some diaphgragm, so can't have knocked out all of C3-C5, but not all of it!

The other stuff. OK, so your heart rate is controlled by two sources: first, sympathetic drive which makes your heart go faster, which is stimulated by both your chemoceptors (ie how much CO2 is in your blood) and also your baroceptor (ie how low your blood pressure is). You're quite right, this whole sympathetic drive will be gone, so you'd expect the heart to go slower.

But also connected to the baroceptor (sensing low BP) is your vagus nerve which is a cranial nerve and so totally unaffected by spinal injury. Your vagus nerve, being parasympathetic, slows the heart rate. And this, when it senses low BP, stops firing. So your heart goes fast, really fast.

so neurogenic shock is just like other shock - hypotensive, and tachycardic.

does that help?