Reflex question

[copperwindow]

If I have a spinal cord injury and cannot feel anything beneath my waist, will my knee jerk if a doctor taps it with a hammer?

[WoodenTaco]

I think the answer to this one is going to be a definite maybe. Depends on the spinal injury, I would guess.

[Broomstick]

If the nerves involved in the reflex are intact, yes, your leg will jerk even if you're paralyzed and can't feel it. It all depends on where the damage occurred on the spine. A higher injury - where you're paralyzed from the chest down, for example - is more likely to leave those nerves intact than an injury lower down the spine. I don't know exactly where the critical location(s) is - you'd need a medical expert for that.

How the damage occurred might also be a factor. A crushing or severing type injury at a discrete location is more likely to leave nerves below the injury intact, whereas an injury due to an interruption in blood flow to the spinal cord might actually kill off a lot of nerves below a certain point.

[Chief Pedant]

Originally Posted by copperwindow

If I have a spinal cord injury and cannot feel anything beneath my waist, will my knee jerk if a doctor taps it with a hammer?

Yes.

If one considers the "waist" to be around the belly button somewhere, that's in the vicinity of the 10th thoracic vertebra (T-10).

The patellar reflex needs an intact spinal cord in the lumbar region (around L2-4 or so).

When the patellar tendon is tapped, stretch receptors in the quadriceps fire off and go to the spinal column. They trigger an efferent (outgoing) response that only requires that single point of connection (it's a "monosynaptic" loop) before returning a signal to contract the quadriceps muscle. No higher functions are required, so it doesn't matter if you are paralyzed at the waist.

Many times these sorts of reflexes are significantly exaggerated or blunted in a spinal injury patient, due to an assortment of reasons. The quads might be very atrophied; other inhibitory mechanisms might be absent and the response might be more jerky; an absence of activity might lead to a build-up of transmitter chemicals so the response is increased--and so on.

[Napier]

>an intact spinal cord in the lumbar region (around L2-4 or so)

The spinal cord ends by L2, at the conus.

[Chief Pedant]

Originally Posted by Napier

>an intact spinal cord in the lumbar region (around L2-4 or so)

The spinal cord ends by L2, at the conus.

Thanks...I should have clarified T-10 and L2-4 as the functional levels; these levels are named for where the nerves enter, and not for the anatomy of the physical spinal cord at that exact vertebral level...a point of some significance when I need CSF.

[jtgain]

This reminds me of Dirty Rotten Scoundrels where Michael Caine kept hitting Steve Martin in the leg with a switch (while he was only pretending to be paralyzed)...LOL

[Bearflag70]

The Reflex is a lonely child just waiting by the park.