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Spinal Nerve Anatomy & Physiology

Spinal Nerve Anatomy & Physiology

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Spinal Nerves
Summary
Anatomy
Roots
Divide into:
  • Dorsal (posterior), which carries sensory fibers.
  • Ventral (anterior), which carries motor fibers.
Rami
Originate from mixed spinal nerve, so they carry motor and sensory fibers.
  • Dorsal (posterior) – innervates the back.
  • Ventral (anterior) - innervates the limbs and anterior torso.
Spinal cord
  • Ventral horn (produces motor innervation)
  • Dorsal horn (receives sensory innervation)
  • The intermediate zone (aka intermediate gray) contains both autonomic and cerebellar pathway neurons.
    • Here, we'll focus on the intermediolateral cell column (the autonomic pathway neurons).
    • Note that the term "intermediate zone" is also used in the embryology of the neural tube.
Bony landmarks
The following bony landmarks enclose the spinal canal:
  • Vertebral body.
  • Vertebral arch.
Physiology
  • The ventral root carries motor fibers.
  • The dorsal root receives sensory fibers.
  • The dorsal root ganglion houses the primary sensory neuron, which is pseudounipolar.
    • It sends fibers both towards the spinal cord and also toward the periphery.
  • Where the roots merge, they form the mixed spinal nerve, which divides into the:
    • Anterior ramus (ventral ramus), which innervates the limbs and anterior torso
(we leave out a section of its outline, proximally, because this is the region of the sympathetic chain, which we'll introduce momentarily).
    • Posterior ramus (dorsal ramus), which innervates the back.
Reminder: The rami are not the roots!*
Autonomic Anatomy
Sympathetic Ganglion
  • Off the anterior ramus, lies the sympathetic ganglion.
  • The sympathetic ganglion attaches to the anterior ramus via the:
    • Gray ramus communicans, proximally, which is unmyelinated.
    • White ramus communicans, distally, which is myelinated.
  • We specify their myelination status because it helps us remember the sympathetic motor nerve course, if we keep in mind that the anterior root fibers are myelinated.
Innervation
  • The anterior ramus (ventral ramus) innervates the limbs and anterior torso.
  • Posterior ramus (dorsal ramus) innervates the posterior head and posterior trunk.
Anterior ramus
  • The anterior ramus carries motor fibers that emerge from an anterior horn neuron, pass along the ventral (anterior) root, join the mixed spinal nerve and then pass down the anterior ramus.
  • They innervate the limb muscles and the anterolateral trunk muscles.
  • The anterior ramus carries fibers from the skin that join the mixed spinal nerve, pass along the dorsal root and synapse in the dorsal horn (or ascend directly to the brainstem, which is not shown).
At the dorsal root ganglion, lies a pseudounipolar neuron, which learn about in detail, elsewhere.
  • The anterior ramus provides sensory reception (coverage) from the skin, vasculature, and musculoskeletal regions of the limbs and anterolateral trunk.
Posterior ramus
  • The posterior ramus carries motor fibers that innervate the paraspinal muscles.
  • The posterior ramus carries fibers from the skin to the join the anterior ramus to form the mixed spinal nerve.
  • The posterior ramus provides sensory reception (coverage) from the – skin of the posterior head and trunk and related musculoskeletal regions.
  • It innervates the vertebrae, notably the articular facet joints.
Posterior Ramus Denervation (Clinical Correlation)
  • As a clinical correlation, to achieve facet block, clinicians purposefully disrupt the posterior ramus to treat painful facet joints. Treatments include blocking the posterior ramus via denervation, anesthesia, and electrical stimulation.
Sympathetic Innervation of the Heart
  • Sympathetic motor fibers emerge from the intermediolateral cell column, pass down the ventral root, along the anterior ramus, enter the sympathetic chain via the white ramus (white because it's myelinated), then down the chain a couple of representative segments to synapse in an inferior sympathetic ganglion.
  • Post-synaptic fibers pass via a cardiopulmonary splanchnic nerve to innervate the heart.
Be aware that some texts reserve the term "splanchnic" for those nerves that originate from prevertebral ganglia.
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