The spinal cord develops from the embryonic neural tube. The rostral portion of the tube forms the forebrain, midbrain, and hindbrain. The caudal portion will develop into the primitive spinal cord.

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  During the three-vesicle stage of embryonic development, prominent flexures () appear:

  
  
  
  
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    The cephalic flexure is the first to appear and develop at the level of the midbrain.

    
    
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    The cervical flexure develops later between the hindbrain and the spinal cord.

    
    
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    The pontine flexure is the last to appear, developing at the junction of the metencephalon and the myelencephalon.

  
  
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  During the five-vesicle stage of development, the cephalic flexure becomes prominent while the cervical and pontine become less prominent. The persistence of the cephalic flexure results in a curved neuroaxis rather than the previously seen longitudinal axis present in very early embryos and lower vertebrates.

Both the developing and the adult spinal cord are composed of rostrocaudal segments derived from mesodermal somites ( ).

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  Skeletal muscles, dermis, cartilage, tendons, and vertebrae develop from these somites.

For each somite, there is a corresponding vertebra, which is described as a spinal cord segment.

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  In the adult, the spinal cord regions are identified as: cervical, thoracic, lumbar, sacral, and coccyx.

  
  
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  The 31 spinal segments are defined by the 31 spinal nerves exiting the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal ().

Each spinal cord segment provides sensory and motor innervation via spinal nerves for the skin and muscle derived from the same somite ().

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  Spinal nerves enter/exit the spinal cord as dorsal (sensory) and ventral (motor) rootlets ( a, b).

  
  
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  The dorsal and ventral rootlets coalesce to form the dorsal and ventral roots.

  
  
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  The dorsal root is associated with a sensory ganglion known as the dorsal root ganglion (DRG).

  
  
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  The dorsal and ventral roots join just distal to the DRG to form what is known as a mixed spinal nerve.

  
  
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  The mixed spinal nerve emerges from the vertebral column through the intervertebral foramen and then splits into dorsal and ventral primary rami that carry motor and sensory fibers to the posterior, anterior, and lateral aspect of the body. The primary dorsal rami innervate the deep back muscles and the skin on the back. The primary ventral rami innervate the limbs and the remainder of the trunk ().

  
  
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  Each mixed spinal nerve represents one of the 31 spinal segments.

  
  
  
  
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    The cervical spinal nerves/segments innervate the skin and muscle of the head, neck, and arms.

    
    
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    The thoracic spinal nerves/segments innervate the trunk.

    
    
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    The lumbar and sacral spinal nerves/segments innervate the legs and perineal area.

  
  
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  Each spinal nerve maintains a relationship with its somite and the structures that are derived from that somite.

  
  
  
  
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    Each spinal nerve therefore (except C1) innervates a single dermatome ( ).

    
    
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    Each dermatome represents the sensory distribution of a single spinal nerve and forms bands or contours on the body ().

    
    
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    • Developmental rotation of the upper and lower extremities makes the dermatome pattern more complex than that of the chest and abdomen.

    
    
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    Dermatome patterns are clinically significant as they can aid in the diagnosis of nerve injury.