The Spinal Cord and Spinal Nerves

The Spinal Cord

• There are 31 spinal nerves and 33 vertebrae.
  • 8 Cervical nerves (7 cervical vertebrae)
  • 12 Thoracic nerves (12 vertebrae)
  • 5 lumbar nerves (5 vertebrae)
  • 5 sacral nerves (5 fused vertebrae)
  • 1 coccygeal nerve (4 vertebrae)

• The cord finishes at L1 (the conus medullaris)
  • Dangling nerve roots are called the cauda equina.

The spinal nerves and spinal vertebrae.

The pathway of spinal nerves

It is important to know the pathway of nerves exiting and entering the spine to be able to localise nerve lesions

Pathway Summary:

• • Spinal cord  rootlets come from 2 types of nerve roots
    • The ventral root which comes from the ventral horn
      • Contains motor fibres(visceral and somatic) and so is leaving the SC
    • Dorsal root which comes from the dorsal horn
      • Contains sensory fibres and so is entering the SC
      • Cell bodies in the DRG
      • NB sensory fibres travel in reverse to motor fibres, but for the purposes of simplifying the description the verbs I will use describe the anatomy as though we are leaving the spinal cord and going distally as we would with motor impulses  (rather than e.g. bringing sensation back from the limbs)

• These two roots unite at the intervertebral foramen to form a mixed spinal nerve (contains both sensory and motor fibres)
  • This then divides to form the 4 types of rami
    1. Ventral ramus => Supplies tissues via nerve plexuses and the sympathetic chain (more on these later)
    2. Dorsal ramus => Supplies dorsal tissues directly
    3. Rami communicantes connect the spinal nerve with the sympathetic ganglion to allow autonomic input
       • White rami communicantes contain fibres going to the sympathetic chain and onto the nerve plexuses/other levels
       • Gray rami contain fibres going from the chain to the ventral ramus
    4. There are also small meningeal rami 

Nerve Roots

• Ventral – Efferent fibres
• Dorsal – Afferent fibres
  • Dorsal root ganglion is where cell bodies of these nerves lie
• N.B. Spinal nerves with more efferent fibres will have a relatively larger ventral horn and vice versa
  • E.g. C2 has larger ventral horns than L5
  • Due to more efferent fibres for more muscle

Rami

• N.B. These contain mixed fibres as the spinal nerve forms first
• Meningeal ramus –
  •  Very small and purely sensory to dura mater
  • Used in meningitis
• Posterior/dorsal ramus
  • Supplies myotomes and dermatomes of the back
  • Do not form plexuses – connect to tissues directly
• Anterior/ventral ramus
  • Supply a larger area therefore biggest.
    • Anterior and lateral regions of the trunk
    • Upper and lower limbs
  • Form major somatic nerve plexuses.
  • Connect to the paravertebral ganglia of the sympathetic chain via the rami communicantes.
    • White Rami Communicantes: 
      • Carry preganglionic SNS nerve fibres from the SC to the PV ganglia where they synapse
      • Only exist from T1-L2 (i.e. where the sympathetic fibres originate from the intermediolateral cell column)
    • Gray Rami Communicantes:
      • Carry post-ganglionic fibres from the PV ganglia to the ventral rami (where they continue to the relevant tissues)

Protection

• Dura, Arachnoid, Pia maters.
• Dura mater transitions to epineurium in spinal nerves.
  • Continues to also form terminal filum at S2 which acts as anchorage for the spinal canal.
• Pia mater wraps around individual rootlets.