Spinal Regions

The human spine is composed of vertebra-disc-vertebra segments for the cervical, thoracic, and lumbar regions, while the sacral (inside your pelvis) and coccygeal (tailbone) regions are fused bone segments with no discs in between. Side note - imagine me explaining how to pronounce ‘coccyx’ over and over to anatomy students every year that I was a teaching assistant. Great fun.

Our spines have natural curvature when viewed from the side (nerds call this the sagittal plane). The cervical region, or your neck, has 7 vertebrae and a lordotic curve, which means that its curve creates a concave space on the back of your neck. The thoracic region has 12 vertebrae and features a kyphotic curve, the opposite of lordosis, which creates a convex curve in your mid-back. Your lumbar region, or lower back, has 5 vertebrae and switches back to a lordotic curve like your neck.

These terms are horrible, I know. If you want to simplify, think hunchback for too much kyphosis, and that person you know who arches their back and sticks their butt out a lot for excessive lordosis. Individuals have varying degrees of kyphosis and lordosis without any health issues, but note that problems can arise from excessive curvature (but we’ll save that for a future newsletter). Speaking of which, scoliosis represents another form of altered curvature. This time in the frontal plane (lateral deviation of the spine visible from the front or back). Like with kyphosis and lordosis, there’s an acceptable amount of lateral deviations that don’t cause problems.

Vertebrae

I mentioned before that the spine is made up of vertebra-disc-vertebra segments. This bone-disc-bone segment is called a functional spinal unit. Each spinal unit is capable of a small amount of bending, but when you add up all of the segments of the spine, we are capable of large bending motions.

The vertebrae within each spinal region differ greatly in many ways, but for our purposes, it is important to know that the vertebrae increase in size as you move down the spine. This is to accompany the increased forces that the lower segments of the spine experience. Vertebrae have many bony prominences and features that I won’t bore you with other than to say that they act as insertion sites for muscles, surfaces for joint articulations, physical barriers for protection, or to restrict motion.

Vertebrae have an anterior body and posterior elements. The body is where intervertebral discs are sandwiched between vertebrae. Between the body and the posterior elements is the spinal canal, comprised of the laminae and pedicles that create an arch to protect the spinal cord. Transverse and spinous processes serve as attachment sites for muscles. The spinous processes, along with the posterior facet joints that connect adjacent vertebrae, serve as physical barriers to limit spinal extension. There are even attachment points called ‘articular facets’ where the back of our ribs attach to our thoracic vertebrae.

Excessive and/or rapid loading can lead to many types of vertebral fractures, depending on posture. Elderly women with osteoporosis are a common phenotype for this injury.

There are several other interesting quirks and variations to vertebrae, but I’ll only mention a few:

1. The first cervical vertebra (C1) is named Atlas, because like the titan of Greek mythology tasked with holding up the world and the heavens, C1 holds up our heads.
2. When we’re born, the sacrum is not yet fused (unlike the image above). There are small intervertebral discs between the sacral vertebrae during childhood and adolescence that later fuse together. After bone growth fuses them, there are only ridges to show the remnants of intervertebral joints - well, actually, some people have S1/S2 joints that never fully fuse. It’s interesting to think that my daughter currently has more spinal joints than I do! Actually, I can hear her crying right now as I write this, while my wife is working hard to put her to sleep. Thanks honey, I love you.
3. When viewed from the sides, thoracic vertebrae look like the head of a giraffe, and lumbar vertebrae look like the head of a moose.

Intervertebral Discs

This is where the bulk of my research experience resides. Some people strive to become athletes, artists, tradesmen, and tradeswomen, but I decided to study the mechanical properties of intervertebral discs. I like to think that I occupied a unique space somewhere between total nerd and cool kid, but the jury is still out.

Anyways, intervertebral discs are the shock-absorbing structures that reside between vertebrae. Discs are largely avascular [1] and aneural [2] (only a small blood supply and nerve innervation to the outer annulus). This makes it easier for damage to build up because of the lack of pain signalling, and harder to repair because of the lack of blood supply - great design, right?? However, as damage to the disc accumulates, blood vessels and nerves begin to grow into the periphery of our discs [3] - leading to an increased sensitivity to pain…seriously, who thought this design was a good idea? Okay, I’m exaggerating. The spine is a masterful piece of engineering, but its peak functional capacity often degrades well before other systems of our bodies.

Discs are anchored to vertebrae by hard, collagenous structures called endplates and have two main regions:

1. the outer annulus fibrosus, and
2. inner nucleus pulposus.

The annulus is primarily type I collagen [4] and functions as a tensile wrap around the nucleus. The nucleus is made of mostly type II collagen [4] and is the jelly-like center of our discs that, when surrounded by the tensile wrap of the annulus, gives them their shock-absorbing capacity. My PhD supervisor, Dr. Diane Gregory, liked to describe the disc as a jelly-filled onion. Like onions and Shrek, the annulus has many layers (about 20-25). A herniation is when the nucleus finds its way out of the layered annulus. You may have heard other terms like “bulging disc” or even “slipped disc”, but they all generally refer to the same thing. These injuries can be incredibly painful because the nucleus material that comes out of the disc (or the bulging disc itself) can compress spinal nerves. Further, since the nucleus has never been exposed to our immune systems (avascular and aneural - remember), we have a massive inflammatory response to herniations. This has large implications for how we recover from herniations and was the topic of one of my scholarship research proposals on spontaneous herniation regression. Maybe one day I’ll get around to telling you all about that, but we have a lot of ground to cover before we get that specialized.

Things that surround the spine: ligaments, muscles, and nerves

Ligaments connect bones to other bones, and muscles also…connect bones to other bones, albeit through tendons. While ligaments provide passive support and resistance to motion, muscles span across joints and allow for movement via contractions (shortening). Name a major vertebral structure - there’s probably a ligament that attaches to it or near it. We have ligaments that connect the spinous processes, transverse processes, laminae, and many more. The most prominent ligaments run along the anterior and posterior sides of our vertebral bodies, providing resistance to both flexion (bending forward) and extension (leaning back). The important concept with ligaments is that they are passive structures (they don’t contract actively like muscles) and they create an additional layer of stability.

Several layers of muscle stabilize and move the spine. The general premise is that muscles closer/deeper to the spine produce less force and primarily add stability, while the larger muscles on top produce much more force and thus movement. A whole host of muscles are involved in this process:

• anterior core musculature - “core” muscles
  • rectus abdominis
  • transverse abdominis
  • internal and external obliques
• erector spinae muscles
  • IIiocostalis
  • Longissimus
  • Spinalis
  • other smaller muscles like multifidus, semispinalis, rotatores, and more
• larger muscles (just a couple larger ones, there’s more)
  • Latissimus dorsi (lats)
  • trapezius (traps)
  • rhomboids (major and minor)
  • quadratus lumborum
  • glutes (maximus and medius)
  • hamstrings

It’s quite the symphony of structures all working together. Our muscles work hard to keep our spines in advantageous positions during loading. Without them, our spines would buckle and snap under the forces of our everyday lives. Thank your muscles, and keep using them.

Last on the anatomy list is nerves. Nerves exit the spine at every spinal level and function as the electrical conductors of our bodies. Nerves deliver electrical signals to the tissues they innervate (connect with) and allow for sensation and action of a given area. In this way, the spinal cord is like a tree trunk, and the nerves that exit at each level are its branches, carrying signals to and from the nervous system. When these branches, or even the trunk itself, are compressed or damaged, we may experience pain, weakness, numbness/tingling, and even a loss of sensation. Severe injuries to the tree can result in a full loss of function. A common pathology is sciatica, which is technically the compression of the sciatic nerve. Since the roots that feed into the sciatic nerve originate from L4 all the way to S3, compression of any of those nerve roots can produce similar symptoms of pain/numbness/weakness in our butt cheeks and radiating down our legs. Similarly, any other pathology that infringes on the territory of a nerve root can produce similar symptoms to the affected area (think herniation, bulging disc, inflammation from facet joint arthritis etc.).

Thank you for your attention and time. I hope I didn’t scare you away with all of the Latin and anatomical terms. I appreciate you and hope you have a great weekend.

Best,

Mitch

References:

1. Urban et al., 2004: https://pubmed.ncbi.nlm.nih.gov/15564919/
2. Edgar, 2007: https://pubmed.ncbi.nlm.nih.gov/17905946/
3. Vincent et al., 2019: https://www.sciencedirect.com/science/article/abs/pii/S8756328219301243?via%3Dihub
4. Eyre & Muir, 1977: https://pubmed.ncbi.nlm.nih.gov/577186/