Dermatomes and Spinal Cord Injury Levels
Created: Feb 2022
Serious spinal cord injuries, e.g. due to broken back, can result in partial or complete paralysis of the body. The extent of this paralysis (level of disability) varies depending on the part of the spinal cord that suffered damage.
The place of spinal cord damage determining the disability level is related to the existence of dermatomes- the areas of skin which are connected to the same spinal nerve.
Spinal nerves are connected to the spinal cord along its length via the dorsal (sensory) and ventral (motor) nerve roots. Spinal nerves branch away from the spine and run towards the skin surface. They pass the signal arising from e.g. touching an object back to the spinal cord, which then travels to the brain and which we perceive as a sensation of touching that object. As shown in Figure 1, the spinal nerves running from different vertebrae (bones) of the spine connect to different parts of our bodies (different dermatomes). The 8 cervical nerves (C2-C8, except C1) supply the neck and arms, the 12 thoracic nerves (T1-T12) supply the chest and stomach areas, the 5 lumbar nerves (L1-L5) connect to the lower back and legs, while the 5 sacral nerves (S1-S5) connect to the (mostly back of the) legs, buttocks, and genitals.
Figure 1. Spinal nerves branch from the spinal cord (top) and connect to and relay signals from different areas of the skin called dermatomes.
If the spinal cord is severely damaged (a complete spinal cord injury), for example due to a fall or sports accident, the signals relayed by the spinal nerves from the specific dermatomes do not reach the brain and vice versa. In other words, we loose sensation and cannot move certain part of our bodies (loss of sensory and motor function). However, the percentage of our bodies that becomes paralyzed is closely tied to the exact position where the complete spinal cord injury took place, and the lowest part of the spinal cord that remained undamaged. This is because only the signals from the spinal nerves whose roots are located at and below the site of the damage will not be relayed, while the ones from the (undamaged) nerves above will still reach our brain. For example, if the spinal cord was badly damaged at the site connecting to lumbar nerves’ roots, a person might not walk, but they still retain arm movement. On the other hand, if the complete spinal cord injury resulted in damage at the nerve root of the C3 nerve, a person is likely to be fully paralyzed from neck down and have some difficulty speaking, as most of the spinal cord becomes nonfunctional. Overall, the higher (closer to the neck) the spinal cord injury occurs, the more severe the resulting disability. Also, since the complete injuries that occurred at any given position render at least the lowest sacral nerves impaired, all injured persons could suffer from loss of bladder or bowel control.
Complete spinal cord injuries are currently considered incurable, though the intensive rehabilitation can help to improve daily life quality. Also, some spinal cord injuries are incomplete, meaning that part of the signals can travel past the damaged site towards the brain, so a person retains some level of sensory and motor function in the nerves below the damaged part of the spinal cord.
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