Spinal Nerves USMLE, Limited Edition > Neuroscience > Neuroscience

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:

1 / 7 • 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

2 / 7 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.

RELATED TUTORIALS

Related Tutorials

• The Spine

• Vertebral Column

• Spinal Meninges

• Autonomics Overview

3 / 7 FULL TEXT

OVERVIEW

• To start, let's place the spinal nerves in the general context of the spinal cord and spinal canal to understand them.

To begin, start a table.

• Denote that we'll distinguish the nerve roots vs nerve rami.

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.

Innervation:

• Dorsal (posterior) – innervates the back.

• Ventral (anterior) - innervates the limbs and anterior torso.

ANATOMY IN CROSS-SECTION

• Show that we'll draw a superior cross-section.

• Label posterior and anterior.

• To start, draw an outline of a hemi-cord axial section.

• Include the center gray matter (lamina 10).

Next, draw the:

• Ventral horn (which produces motor innervation)

• Dorsal horn (which receives sensory innervation)

• The intermediate zone (aka intermediate gray), which contains both autonomic and cerebellar pathway neurons.

- Here, indicate that we'll focus on the intermediolateral cell column (the autonomic pathway neurons).

- As a tricky nomenclature point, consider that the term "intermediate zone," is also used in the embryology of the neural tube.

4 / 7 Bony Landmarks

Next, draw the bony landmarks that enclose the spinal canal:

• The vertebral body.

• And vertebral arch.

Ventral Root

• Now, draw the ventral root, which carries motor fibers.

Dorsal Root

• Then, the dorsal root, which receives sensory fibers.

Dorsal Root Ganglion

• Specify the dorsal root ganglion, which houses the primary sensory neuron, which is pseudounipolar: it sends fibers both towards the spinal cord and also toward the periphery.

Mixed Spinal Nerve

Show that 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.

We'll further specify their innervation patterns and targets in a bit. Again, remind ourselves that the rami are not the roots, so we don't confuse them!* Also be aware that there is substantial intertextual variation in the actual location of the posterior ramus.

Sympathetic Ganglion

• Off the anterior ramus, draw the sympathetic ganglion.

• And a representative projection fiber.

• Then, draw a couple of representative ganglia at lower levels of the sympathetic chain.

- We'll see that nerve fibers ascend and descend the chain.

Show that the sympathetic ganglion attaches to the anterior ramus via the:

5 / 7 • 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

Overview

Then, draw two human figures: one facing posteriorly and the other anteriorly.

• Show that the anterior ramus (ventral raumus) innervates the limbs and anterior torso.

• Posterior ramus (dorsal ramus) innervates the posterior head and posterior trunk.

Anterior Ramus

• First, let's address the anterior ramus.

• Draw a muscle section.

• Indicate that 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.

- Write that they innervate the limb muscles and the anterolateral trunk muscles.

• Then, draw a skin cube and show that the anterior ramus carries fibers from it to the join 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, connect the fiber to a sensory neuron, forming a pseudounipolar neuron, which learn about in detail, elsewhere.

Anterior Ramus Innervation

• Write that the anterior ramus provides:

• Sensory reception (coverage) from the skin, vasculature, and musculoskeletal regions of the limbs and anterolateral trunk.

Posterior Ramus

• Next, let's address the posterior ramus.

• Draw a muscle section.

• Indicate that the posterior ramus carries motor fibers.

- Write that they innervate the paraspinal muscles.

6 / 7 • Draw a skin cube and show that the posterior ramus carries fibers from it to the join the anterior ramus to form the mixed spinal nerve.

Posterior Ramus Innervation

• Write that the posterior ramus provides:

• Sensory reception (coverage) from the – skin of the posterior head and trunk and related musculoskeletal regions.

• Notably, it innervates the vertebrae, notably the articular facet joints, which we'll take a moment to specifically address.

Facet Joints

• Draw a sagittal view of two levels of vertebrae and indicate the intervening articular facet joints.

• Then, as a clinical correlation, write that 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

Heart Innervation

• Next, draw the heart, as a representative sympathetic target organ.

• Show a sympathetic motor fiber 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.

• Then, show the post-synaptic fiber sympathetic 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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