Dermatomas: Definition, Function, Types, Quantity and Location

The skin area of ​​the human anatomy 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 body’s periphery.

Function

The sensory nerves in the periphery of the body are nerves that transmit 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 essential to understand the anatomy of the spine. The spine is divided into 31 segments 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 thoracics, five lumbar, five sacral, and one coaxial spinal nerve. There are specific dermatomes for each spinal nerve, 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 has to do with any part of the neck, including the neck on which the headrests and the cervix.

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

Twelve 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 inner 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 midsection 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.

Five lumbar dermatomes (L1-L5) that provide a 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.

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

  • S1 – Fingers of the feet and 4 and 5, and the outer part of the ankle bone (lateral malleolus).
  • S2 – The heel bone’s outer side (lateral portion) (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 (with S4) and immediately on and next to the anus.
  • One coccygeal nerve 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.