Contents of the Vertebral Canal

After a long break from blogging about anatomy, I figured that I should probably start up again. I'm going to pick up from where I left off last time: talking about the vertebral column. This time, I'm going to talk about the contents of the vertebral canal (basically the space in which the spinal cord etc. sits).

The Vertebral Canal

The vertebral canal, as I just said, is the space in which the spinal cord sits. It's also essentially the "tunnel" made up out of the vertebral foraminae. (Okay, that was two sentences with a whole lot of nothing. I'm sorry.)

The anterior part of the vertebral canal (i.e. what you see if you're sitting in the canal and looking forward) is made up of bodies, discs, and the posterior vertebral ligament (the ligament that runs along the backs of the vertebral bodies- see my post about intrinsic postvertebral muscles).

The posterior side is made up of the laminae, the ligamentum flavum ("yellow" ligament connecting laminae of adjacent vertebrae) and the zygapophyseal joints (a.k.a. Z joints for those of you who can't spell- or be bothered to spell- zygapophyseal). The posterior wall is deficient in the lower part of the sacrum. This part is known as the sacral hiatus.

Finally, the lateral sides of the canal are simply made of intervertebral foraminae (i.e. holes between vertebrae) and the pedicles (which are the bony bits separating foraminae).

The canal is not the same size all the way down. It is larger where the spinal cord is larger (presumably due to plexuses- clusters of nerves- or something?). It is also larger where there is more potential for movement.

Meninges

There are three meninges, which are layers that cover the spinal cord and brain. They are the dura mater, arachnoid mater and pia mater (from outside in). The arachnoid and pia are known collectively as leptomeninges, and are derived from neural crest ectoderm.

The outermost layer is the dura mater. It continues on from the dura mater of the brain, attaching to the circumference of the foramen magnum on its way down. Most of its other attachments are anterior: to the posterior side of the bodies of C2 and C3 and loosely to other areas of the posterior longitudinal ligament. It does, however, also attach to the dorsal side of the coccyx. The dura has "sleeves" which protect nerves as they leave through the intervertebral foraminae.

Although the dura does have some attachments with the posterior longitudinal ligament and whatnot, there is an important space between the dura mater and the inside of the vertebral canal. This space is known as the epidural space, and it's where fat and the internal vertebral venous plexus (basically a crapload of veins) can be found. It also extends a short way into the intervertebral foraminae. The significance of this epidural space is that drugs and so forth injected in this space can essentially traverse the vertebral column.

Another space worth mentioning is the subdural space. The subdural space is between the dura and the arachnoid, and normally doesn't exist because the arachnoid is pressed up against it due to the pressure of CSF (cerebrospinal fluid).

Now on to talking about the leptomeninges (pia and arachnoid mater)! The leptomeninges begin as a single membrane and are separated when CSF begins to flow and push them apart. They don't separate apart that cleanly, however, which is why there are random strands of tissue crossing the gap. The space between them is known as the subarachnoid space, and it's where CSF flows through the canal. The pia adheres closely to the spinal cord, but it also has projections called denticulate ligaments which pierce the arachnoid and attach to the inside of the dura, aiding stability of the spinal cord within the canal. These ligaments continue down to T12.

The subarachnoid space (which, as I just said, is between the arachnoid and pia), continues as far as the dorsal root ganglion. Past here, to my understanding the meninges essentially become continuous with the coverings of the peripheral nerves. From out to in, these are the epineurium, perineurium and endoneurium.

Spinal Cord

Unlike in embryos, the spinal cord in adult humans does not continue down the entire length of the spine. This is because the spinal cord stops growing before the spine does. The spinal cord ends at L1, and this ending is known as the conus medullaris. The conus medullaris is attached to the base of the spine by a thread of fibrous tissue known as the filum terminale. Another important structure here is the cauda equina- these are all of the nerves that are travelling downwards to exit the spine lower down (i.e. through L2-L5 or the sacral region).

The subarachnoid space continues further than the spinal cord. It continues all the way down to S1/S2, which is handy for doing a lumbar puncture: lumbar punctures are usually done between L3 and L4 or between L4 and L5, where the subarachnoid can be accessed without fear of accidentally puncturing the spinal cord. Below this level, the meninges continue and cover the filum terminale. The epidural space, with its fat and veins, is still around too.

Now for a bit more information about the spinal cord itself! Like the brain, the spinal cord has white matter (myelinated axons) and grey matter (nerve cell bodies). However, unlike the brain, the grey matter is on the inside whereas the white matter is on the outside. The closer up the spinal cord you go, there more white matter there is. More grey matter can be found where there are bigger nerves, for example near the brachial plexus (cervical enlargement, C5-T1) and lumbosacral nerves (lumbar enlargement, L2-S2).

Another important point to remember is that sympathetic nerves can be found around T1-L2 and parasympathetic nerves can be found around S2-4 (the pelvic splanchnic nerves). These areas have lateral horns of grey matter in addition to the ventral and dorsal horns found throughout the spinal cord.

You might have noticed that I haven't given C1-C5 much love. Well, they do have their own specialisations too. They give rise to the spinal accessory nerve, which innervates the sternomastoid and trapezius muscles. The roots of the nerve come laterally out of the spinal cord and pass through the foramen magnum, where they join to make the spinal accessory nerve.

Spinal Nerves

First, a quick refresher on spinal nerves. There's a diagram and explanation in one of my earlier posts on embryology. Thoracic spinal nerves (which have the white ramus going to the sympathetic trunk) are considered to be "typical" spinal nerves.

Now for a bit about the sympathetic trunk and referred pain! As I've mentioned before, the white ramus goes to the sympathetic trunk, which branches off into the grey ramus (to the body wall, where it supplies smooth muscle and glands in the skin) and visceral branches. Visceral branches lead to different locations depending on where they start. Sympathetic nerves arising from T1-T5 tend to ultimately innervate the head and neck, as well as thoracic organs. T5-T9 innervates the foregut (essentially all the digestive stuff before the intestines. Thanks, Google Images). T9-T11 innervates the midgut (small intestine, ascending colon, first 2/3 of the transverse colon) and gonads. Finally, T11, T12 and L1/2 innervate the hindgut (the rest of the large intestine) and the pelvic organs. These give rise to segments, or dermatomes, from which pain is "felt" due to sensory nerves from these locations returning with the sympathetic nerves. This is why heart pain is sometimes felt down the arm.