Now the study's getting a bit more panicky, because the test is tomorrow :( On the upside, they did say that we'd only be tested on limited vertebral column stuff. On the downside, EMBRYO SLIDES. Ugh.
Evolution of regions
To examine the evolution of different regions of the spine, we're going to have a look at living creatures.
First we'll start off with fish. Fish have thoracic vertebrae (and therefore ribs), as well as caudal vertebrae (vertebrae past the anus). That's it. They don't have necks, because they can just swim onto their food.
Next we'll look at amphibians, because they spend some time in the water (like fish) and some time on the land (unlike fish). Amphibians also have thoracic and caudal vertebrae, but they also have sacral vertebrae because their hindlimbs need a firm attachment. And yes, they still have no neck. Instead, they have other specialisations for gaining food (maybe this is why frogs have long tongues? Who knows).
Reptiles are the next group of interest. They're a bit like amphibians, but they spend more time on the land. This is the first group to have a neck! Yay! This cervical (neck) region seems pretty important for land animals, but not for aquatic animals. Mammals that have returned to the sea, such as dolphins and whales, retain their cervical vertebrae, but they're flattened and quite rudimentary. Although reptiles are more advanced with their fancy-pants necks and all, they still move a lot like fish, with side-to-side undulations of their bodies.
Movement starts to become more developed with the addition of the lumbar region in mammals. This is kinda important because the alignment of the limbs of mammals (rotated under the body) isn't very conducive to the whole side-to-side undulations thing. Instead their lumbar regions allow them to have flex-extend locomotion, which allows for galloping and so forth. Mammals that return to the sea also have lumbar vertebrae, and also move by flex-extend locomotion.
The relative size of the thoracic and lumbar regions can also give a clue as to the preferred locomotion of the animal. You see, while the lumbar region is good for flexion and extension, the thoracic region is for axial rotation. Hence, terrestrial animals that need to run and gallop a lot have a more developed lumbar region, whereas arboreal (i.e. tree-dwelling) animals have a more developed thoracic spine, allowing them to swing from branch to branch.
Oh, and one last quick note before moving on. Birds have all of the main spinal regions (cervical, thoracic, lumbar, sacral and caudal). However, all of these regions, with the exception of the neck, are fused to some degree.
The motion segment
The motion segment, to my understanding, is a collective name for all the bits and pieces in the spine that allow for motion. Another way of looking at it is that the motion segment refers to two vertebrae plus the joints in between. The motion segment can be divided up into two sections: anterior elements and posterior elements. Anterior elements are more load-bearing (carrying 80% of the weight) and are comprised of the intervertebral discs and the vertebral bodies themselves. Posterior elements, on the other hand, are more responsible for the movement. Posterior elements include the bony bits (spinous processes, transverse processes and laminae) along with associated ligaments and zygapophyseal joints. (Yup, that's a longish word- I'll cover more on this later, I promise.)
The Intervertebral Disc (IVD)
In my previous post, I alluded to the development of the intervertebral disc. Time to go back into that topic with more gusto!
So, a recap plus a bit of expansion on what I talked about before. In my previous post, I mentioned that the notochord gets "pinched off" into the region between somites. The significance of this is that they begin to form the nucleus pulposus, which makes up the middle of the intervertebral discs. The notochordal cells themselves begin to degenerate at around 6 months gestation as they are replaced by acellular material derived from the annulus fibrosus (yes, yes, I'll get to that later), but some cells do remain until adulthood. The nucleus pulposus is eventually made up of a hydrophilic mixture made up of collagen fibrils, hyaluronic acid and proteoglycans, allowing it to retain water easily. (This water can be squeezed out during our everyday activities, which is why we're generally slightly shorter at night than in the morning.) As we age, the nucleus is gradually replaced with fibrous tissue and contains less water.
Alright, so you might be wondering what the annulus fibrosus is. It is made up of many layers of fibrocartilage that run in rings around the nucleus pulposus. Well, actually, pretty much only the inner layers run in rings around the nucleus pulposus- the outer layers just join the two endplates of adjacent vertebrae to each other.
This overall structure of a hydrophilic core surrounded by fibrocartilage is quite useful. Intervertebral discs are capable of transmitting loads and allowing movement. The "envelope" formed by the annulus fibrosus provides tensile strength which maintains the pressure in the nucleus pulposus, keeps the vertebrae apart and allows the vertebrae to move. However, this can go wrong: if the envelope is weak in any way, the nucleus can escape or the annulus can bulge and press on other structures.
Control of movement – Zygapophysial joints
The zygapophysial joints exist mainly between the inferior articular facet of one vertebra and the superior articular facet of the next. The angles of motion available depends to a large extent on the angles of the facets. In the thoracic region, the facets are set on the arc of a circle- very conducive to rotation. In the lumbar region, however, the facets are "radial" which means that rotation is not possible. Cervical region facets are oblique which allows for a wider range of movement.
The intervertebral discs also play roles in movement. In the lumbar and thoracic vertebrae, the joints are perpendicular to the plane of the disc so that some rotation occurs in the disc. In the cervical vertebrae, however, some translation occurs instead.
Regions and Ribs
Finally, I'm going to give a quick guide to how to tell the different vertebral regions apart.
Cervical vertebrae are somewhat easy, as they have transverse foraminae (i.e. little holes in the transverse processes).
Thoracic vertebrae can either be easy or hard to tell apart, depending on what you're provided with. Thoracic vertebrae are attached to ribs, so if there are ribs present, it's really easy. However, if there aren't, then you have to look for the rib facets (little bumps where the ribs attach). Sometimes the rib facets are really subtle and hard to see. Also, one point of interest is that most thoracic vertebrae actually have two demifacets- one at the top and one at the bottom. (Most ribs attach to two demifacets from adjacent vertebrae.) The exception is T12 which only has one rib facet.
I'm going to skip lumbar and go straight to sacral. Sacral are really easy, as they're all fused together.
Finally, lumbar is basically the "none of the above" category.
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