Dermatomas: Definition, Function, Types, Quantity and Location

It is the skin area of ​​the human anatomy that is mainly supplied by branches of a single spinal sensory nerve root.

These spinal sensory nerves enter the nerve root in the spinal cord and their branches reach the periphery of the body.

Function

The sensory nerves in the periphery of the body are a type of nerve that transmits signals of sensations (for example, symptoms of pain, touch, temperature) to the spinal cord from specific areas of our anatomy.

To understand dermatomes, it is important to understand the anatomy of the spine. The spine is divided into 31 segments, each with a pair (right and left) of the anterior and posterior nerve roots.

Types

The types of nerves in the anterior and posterior roots are different. The anterior nerve roots are responsible for the motor signals of the body and the posterior nerve roots receive sensory signals such as pain or other sensory symptoms.

The anterior and posterior nerve roots are combined on each side to form the spinal nerves as they exit the vertebral canal (the bones of the spinal column, or spinal column).

The 31 segments of the spine on each side give rise to 31 spinal nerves, which are composed of 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coxigeal spinal nerves. There are specific dermatomes for each of these spinal nerves, except the first cervical spinal nerve.

Dermatomes are used to represent sensory nerve patterns that cover various parts of the body, including:

Head and neck:

  • Upper limbs (arms, hands, torso, etc.).
  • Lower extremities (hip, leg, foot, buttocks, feet, etc.).

How many dermatomes are there and where are they located in the body?

There are 30 dermatomes in the body. They are numbered according to the corresponding vertebral level from which they emanate. For example, there are:

7 cervical dermatomes (C2-C8, since the C1 nerve has no dermatome). The “C” means cervical, which means it has to do with any part of the neck, including the neck on which the head rests, and the cervix.

  • C2 – Head and neck.
  • C3 – Head and neck.
  • C4 – Neck.
  • C5 – Neck.
  • C6 – Thumb.
  • C7 – Middle finger.
  • C8 – Little finger.

12 thoracic dermatomes (T1-T12) – “T” refers to the thoracic or upper thoracic area of ​​the body or thorax (frontal and dorsal).

  • T1- Internal forearm.
  • T2 – Upper internal arm.
  • T3 – Middle of the back (dorsal).
  • T4 – Level of the nipples.
  • T5 – Halfway from the level of the nipples to the xiphoid process.
  • T6 – Bottom of the foot.
  • T7 – Upper section between the xiphoid process and the navel.
  • T8 – Halfway from the level of the xiphoid process to the level of the umbilicus.
  • T9 – From the mid section of the xiphoid process to the navel.
  • T10 – Navel level.
  • T11 – Between the level of the navel and the groin (inguinal ligament).
  • T12 – The midpoint of the groin.

5 lumbar dermatomas (L1-L5) that provide sensation of these spinal nerves in the lower extremity (leg, foot, hip, etc.) – “L” refers to the five lumbar vertebrae, the discs below them, and the area corresponding to the lower part of the back.

  • L1 – The area of ​​the hip and groin.
  • L2 – The inside of the thigh.
  • L3 – Knee.
  • L4 – The inner part of the ankle bone (medial malleolus).
  • L5 – Bottom of the foot and toes 1-3.

5 sacral dermatomes (S1-5) – “S” refers to the sacrum or sacrum, which lie below L5.

  • S1 – Fingers of the feet and 4 and 5, and the external part of the ankle bone (lateral malleolus).
  • S2 – The external side (lateral portion) of the heel bone (calcaneus).
  • S3 – The middle portion of the buttocks, the perianal area, the penis and the scrotum.
  • S4 – The skin over the perianal area (in addition to S5); Perianal and genital areas.
  • S5 – The skin over the perianal region (together with S4) and the skin immediately on and next to the anus.
  • 1 coccygeal nerve that originates in the spinal cord and emerges at the level of the tail bone (coccyx).

The C1-C7 nerve roots emerge above their respective vertebrae; the C8 nerve root emerges between the C7 and T1 vertebrae, while the remaining nerve roots emerge below their respective vertebrae.