Spinal Cord – Segments and Organization
If you thought that the majority of the central nervous system (CNS) was found in the brain, think again. The spinal cord houses two-thirds (2/3) of the CNS.
Let’s Start at the top.
The Foramen Magnum
Imagine you are a tiny vessel traveling the inner space of the brain. You decide to start your descent and explore the spinal cord. As you get closer to the base of the brainstem, you start to see an opening. This is the foramen magnum of the occipital bone of the skull (# 13 on the picture). Through it passes the medulla oblongata. It also marks the beginning of the spinal cord.
The Cervical Level
Right below the foramen magnum the spinal cord starts off with segment C1 of the cervical level. As you will soon see in your exploration, the spinal cord has a slightly different organization depending on which level you are at. From top to bottom these levels are:
- The cervical level
- The thoracic level
- The lumbar level
- The sacral level
- The coccygeal level
The cervical level is composed of 8 segments that appear to bulge out (get bigger) as you travel down. If you were to take a “satellite” picture of the C5 level, you would see a greyish area in the form of a butterfly in the middle, surrounded by a whiter landscape. These are the gray matter (unmyelinated) and white matter (myelinated axons) of the spinal cord.
At the C5 level you can see that the anterior horns (the anterior part of the grey butterfly) are much more developed than the posterior horns. This reflects the great number of motor neurons from the upper limbs that synapse in this area.
Indeed, we require extensive motor control and sensitivity to move our fingers with the dexterity that is unique to us humans.
The large amount of white matter at this level is a testament to all the ascending and descending nerve fibers that pass through this section (all the fibers connecting the upper limbs -of course, but also all the fibers that connect every part of your body down to the lower limbs).
The Thoracic Level
Let’s continue our descent down the next level. Here we will travel through the longest level of the spinal cord: the thoracic level.
The thoracic level is composed of 12 segments. Contrary to the cervical level that gets larger as you travel down, the thoracic level actually gets thinner as you travel from the top (T1) segment down to the bottom (T12) segment.
Here you will take another “satellite” snapshot of the landscape. In the middle you still find the grey butterfly area, but it’s as if the butterfly had undergone a major diet. The grey anterior and posterior horns of the spinal cord appear to be very thin. Contrastingly, the white matter occupies a more important area.
If you pay close attention you will also see that the gray matter now as two little “spikes”, one on each side. These are named the Lateral Horns. (They will disappear once you reach the L3 lumber segment).
- Can you think a reason why the grey butterfly appears to be starving at this level?
Less gray matter means less effector and receptor neurons. Indeed we don’t have as much sensitivity and motor control over our chest, mid back, and abdominal walls area (it’s not like we can type, grasp, walk, take, or do anything much with our backs).
The Lumbar Level
If the thoracic butterfly appeared to be starving, the lumbar level appears to have gained a lot of weight.
Here, both the anterior and posterior horns appear quite “round” and full. There is less white matter, as all the fibers connecting the upper limbs and thoracic levels do not travel this far down. The gray matter is quite important because of the number of effector and receptor neurons that are necessary to allow us to walk, run, jump, and travel.
The Sacral and Coccygeal Level
We are nearing the end of our descent through the spinal cord. Here you find that the sacral level is surprisingly large. While composed of very little white matter, for the same reasons as above, the grey matter is quite important. This is probably due to the increased sensitivity and neuron affluence required by the genitals and pelvic area.
The sacral level has 5 segments.
The last level, the coccygeal level/segment (Co1), is not as interesting (in my opinion) as it only relates to the area around the coccyx.
However, if you decide to completely travel all the way down to the coccygeal segment, you will find another small opening. This is the conus medullaris. A few nerve fibers travel through this opening. All the fibers that travel out of the conus medullaris are called cauda equina, but the cauda equina is not part of the spinal cord itself.
Important To Remember:
- The spinal cord starts at the top with the foramen magnum.
- The spinal cord is composed of different levels each with a different number of segments:
- The Cervical level has 8 segments
- The Thoracic level has 12 segments
- The Lumbar level had 5 segments
- The Sacral level has 5 segments
- The coccygeal level
- The shape and amount of gray and white matter reflect the number of fibers and neurons traveling through a particular area.
If you want more articles and videos about the Nervous System, you can find them here. More resources are available to help make Biology fun. I invite you to absorb all the content you can find here at Interactive-Biology.com.