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  • Rob

The Lumbar Spine Masterclass, Part 5

We have covered a lot of material along the way. Feel free to go back and review to help pull everything together. Part 1, we talked about the skeletal structure of the lumbar spine. Part 2, was all about the disc. Part 3, ligaments of the lumbar spine took our attention. Part 4, the muscles were the highlights.

There is enough information in those previous blog posts to make my brain sweat!

Here in Part 5, the nerves of the lumbar spine, what do they do, where to they go and what do the innervate (supply).

There are a few noteworthy nerves. We are going have a gander at the ventral and dorsal rami and see where the different branches go. The nerve root comes off the spinal cord and splits into two different nerve, the ventral (front) and the dorsal (back) rami. I remember this by thinking about sharks. Sharks have a dorsal fin that comes out of the water and scares people. The dorsal fin is on the sharks back. So dorsal in anatomical terms means back, just one of the weird ways I remember certain things. I almost forgot about the sympathetic nerve trunks. They are on the anterolateral aspect of the vertebral bodies. These are important as you will see there are some branches better the sympathetic and regular nerves.

Taken from:

The ventral ramus divides into two different branches, skeletal and muscular. So what does this nerve supply? The ventral rami pierce the intertransverse ligaments and lie between the two layers. The ventral rami of the lumbar spine help to form the lumbar plexus, L1-4 and the L4 and L5 ventral rami join to form the lumbosacral trunk. The lumbosacral trunk joins the lumbosacral plexus.

The muscular branches innervate QL, psoas and the small intertransversarii muscles.

The skeletal branch connects with the grey rami communicans, from the sympathetic nerves, and supplies the anterior longitudinal ligament, anterior and lateral aspects of the annulus.

Another branch of the ventral ramus attaches with the gray ramus communicans to form the sinuvertebral nerve. This nerve is formed after the ventral ramus has left the spinal canal and it turns around and heads back into the intervertebral foramen. The sinuvertebral nerve innervates the posterior longitudinal ligament, the posterior and posterolateral annulus as well as the ventral dural sac and the basivertebral and epidural veins.

The dorsal ramus divides into three branches, medial, intermediate and lateral.

The lateral branch of the dorsal ramus goes to iliocostalis, lumbar and thoracic portions, the thoracolumbar fascia and the L1-3 cutaneous branches. These cutaneous branches can travel as far as the greater trochanter of the femur.

The intermediate branch is simple. It supplies the longissimus muscle. Interesting note, sometimes the intermediate branch is missing. Well instead of being its own branch what it does it becomes a branch off the lateral branch. It still supplies longissimus, no matter where it originates from.

The medial branch of the dorsal ramus is a good one to remember. It supplies goes to the z joints, the level it is at and the level below. So each z joint has two medial branches supplying it. In addition, basically any muscle that comes off the spinous process, is innervated by this nerve. The interspinous muscles and multifidus are both innervated by the medial branch of the dorsal ramus. Interestingly enough, this nerve also only innervates those muscles at the level in which it comes out at. For example, the L3 branch will only innervate the L3 multifidus and interspinous muscles. This, in fact, creates one of the pure myotomes of the body, muscles purely innervated by one segmental level of the spine. Too bad the muscles are too small to actually test for fatigable weakness or decreased reflexes.

I think the most important things to take away from the above is that the disc is innervated by the sinuvertebral nerve and the z joints are innervated by the medial branch of the dorsal ramus.

The next blog post will be the final in our series. It is one of the least talked about aspects of the lumbar spine...

If you have any questions or comments about the Lumbar spine, please click here to contact New Leaf Physio, a mobile Kelowna physiotherapy clinic.

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