Know Your Spine - Parts of the Spine and the Spinal Cord

In a normal person, the spine starts from the base of the head to the buttocks. It runs in the midline with muscles and rib cage supporting on the sides. The spine is divided into four parts for easier understanding. All these parts show something in common. The names of these parts from head to lower down are:

1. Cervical spine.

2. Thoracic spine.

3. Lumbar spine.

4. Sacrum spine.

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So normally, when we tell the doctor that “I have cervical,” then we must understand that it is not a problem, but the name of a normal part of the spine.

Cervical Spine:

This is the uppermost part of the spine. It has 7 vertebrae. In every spine, there are a certain number of vertebrae, which are bones and they are arranged like a stack. So, in the cervical spine, we have 7 vertebrae. This part of the spine has lordosis, that is, bent backward. The 1st vertebra is connected to the skull and is responsible for nodding or “yes” movement. The 2nd vertebra is connected to the first vertebra and is responsible for “no” or side to side movement. The unique feature of these two vertebrae is that these are very mobile and there is no disc between them. Almost 50 % of our head movements are because of them.

Rest 3rd, 4th, 5th, 6th vertebrae make the rest of the 50 % of movements. They all have a disc between them. Now the last, that is, the 7th vertebra is actually a transitional vertebra. That means, it is a cervical vertebra but behaves like thoracic vertebrae (explained below).

Thoracic Spine:

The thoracic spine has 12 vertebrae. It is present at the opposite of the chest wall. This part of the spine has kyphosis, that is, bent forward. This is the least mobile and most stable part of the spine. The reason for this is the ribs. We have 12 ribs. So, each vertebra is attached to one rib on the right and left side and in front, these ribs are attached with the sternum bone. So, it forms a strong cage, hence the mobility is very less. Only mobility happening at the thoracic spine is rotation because of which we can rotate our trunk backward while moving our lower limbs.

There are disc between each vertebra. Since this part of the spine is not much mobile, the incidences of disc prolapse or disc herniation are not very common in this part.

Lumbar Spine:

This part of spine is present in the lower back. It has a lot of similarities with the cervical spine. Like the cervical spine, this part of the spine has lordosis, that is, bent backward. This is again a very mobile part of the spine. Collective movements of this lumbar spine help us to bend forward and backward. The lumbar spine has 5 vertebrae. These 5 vertebrae have discs between them.

Since this is again the mobile segment of the spine, the incidence of disc prolapse is very common. The incidence is more common than the cervical spine because this is the most weight-bearing part of the spine. The cervical spine has to bear the weight of only the head, but the lumbar spine has to bear the weight of the entire upper trunk. The maximum movement is present between L4-5 and L5-S1, hence 90 % of the incidences of disc prolapse are common here.

Sacrum and Coccyx:

The sacrum and coccyx are the last part of the spine. They are actually fused to form a single part with no disc space in between. The sacrum is formed from 5 fused vertebrae and coccyx is formed from 4 fused vertebrae. They have no mobility in them. The main function is to connect the spine with our pelvis. So, they form a joint known as sacroiliac joint on right and left side. In this joint, the sacrum is the part of the spine and iliac is the part of the pelvis. Since there is no mobility, the disorders of these are very less and not commonly seen.

Spinal Cord:

Now, all these vertebrae are arranged to form a canal, running from top to bottom. This canal is known as the spinal canal through which the spinal cord runs. So broadly, we can say that the function of these vertebrae or spine is to protect the spinal cord. This spinal cord is covered by bone all around. The spinal cord is the continuation of the brain itself. The actual spinal cord starts from the base of the head and runs down and finishes at the L1 vertebrae. From there, it is a continuation of the spinal cord known as cauda equina. It is not the spinal cord, but it is the collection of nerve roots. These nerve roots are responsible for nerve supply in our lower limbs and also for normal bladder and bowel functions.

The structure of the spine is like an “S,” that is, the cervical region has lordosis, the thoracic region has kyphosis, and again the lumbar region has lordosis. So, this “S” arrangement helps us to stand straight and look forward. Otherwise in animals, they have similar parts of the spine, but the curve in the overall spine is kyphosis, which does not allow them to stand straight. Due to this arrangement of the spinal cord, whenever there is prolapse of the disc backward (which is in maximum number of cases because that is the path of least resistance), then there is compression of that particular part of spinal cord and there could be pain in the supply of that nerve and numbness or weakness in motor power.

The important thing to know here is that the most common site of disc prolapse is the lower lumbar region and we have cauda equina part of spinal cord carrying nerve roots. Also, we have nerve roots in it responsible for bladder and bowel dysfunction. Hence, prolapse in that region can cause dysfunction of bladder and bowel. This is known as “red flag symptoms” and whenever you notice such symptoms then this is known as “cauda equina syndrome.” This is an emergency and it needs to be managed surgically. There is no alternative option available. Hence report to the doctor immediately.

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