Typical Spinal Nerve

How Many Spinal Nerves are There?

There are 31 pairs of spinal nerves, each pair  is attached to a part of spinal cord called spinal segment. The 31 pairs of spinal nerves are as follows:

• Eight pairs of cervical spinal nerves (C1-C8)
• Twelve pairs of thoracic spinal nerves (T1-T12)
• Five pairs of lumbar spinal nerves (L1-L5)
• Five pairs of sacral spinal nerves (S1-S5)
• One pair of coccygeal nerve  (Co1)

Describe Briefly Typical Spinal Nerve.

• Thoracic spinal nerves are the examples of typical spinal nerves. Unlike cervical, lumbar and sacral nerves , they do not form plexus. 
•  Each spinal nerve  is attached to  a part of spinal cord called spinal segment via anterior and posterior roots  which in turn have 1-8 rootlets.
• Anterior root is motor and comprises of axons of neuron located in the anterior horn (motor neurons) and lateral horn ( in  T1-L1 spinal segments ( preganglionic  sympathetic neurons) and  in S2-S4 spinal segments – preganglionic parasympathetic neurons)
• Posterior root is sensory and has a swelling called dorsal root ganglion consisting of neuronal cell bodies of sensory fibers.
• The two roots join together to form the spinal nerve trunk which in turn divides into large anterior ramus and small posterior ramus ( ramii contain both sensory and motor fibers).

 Name the Branches of Typical Spinal Nerve.

• Dorsal ramus curves backwards to supply the muscles of back and skin over them. Its branches are:
  • Lateral and medial: one of them supplies the muscles , the other branch is called posterior cutaneous branch which in turn divides into medial and lateral branches to supply the skin.
• Ventral ramus (is also called internal costal nerve in thoracic region) runs  just above the inferior border of the rib and supplies intercosatal muscles and skin on anterolateral aspect of trunk. Its branches are:
  • White and Gray ramii communicans:  They connect the ventral ramus to sympathetic ganglion. White rams communicans carry the preganglionic fibers to sympathetic ganglion where  most of them synapse with neurons in the sympathetic ganglion and gray ramus conveys postganglionic sympathetic fibers to ventral ramus.
  • Muscular branches: They supply intercostal muscles
  • Branches to periosteum of ribs and parietal pleura.
  • Lateral cutaneous branch:   They arise along the midaxillary line, pierce the overlying muscles and fascia to reach skin. it divides into anterior and posterior branches.
  • Anterior cutaneous branch:  They arise  little lateral to  median plane and divides into  lateral and medial cutaneous branches.

Define Dermatome.

The area of skin supplied by a spinal nerve via cutaneous branches of dorsal and ventral ramii is called a dermatome. Dermotomes as seen in quadruped position: