Ventral and dorsal spinal nerve root, dorsal root ganglion, draw general structure of the spinal nerve and its branches, autonomic fibers of spinal nerve

General Description of the Spinal Cord[edit | edit source]

The spinal cord, along with the brain, make up the central nervous system (CNS).
Nerves leaving (efferent) and entering (afferent) the spinal cord and brain make up the peripheral nervous system (PNS).
The spinal cord extends from the foramen magnum to about vertebra L1 or L2, and is enclosed by the vertebra.
It is surrounded by the meninges and cerebrospinal fluid.
Nerves leaving the spinal cord are called spinal nerves.
The spinal cord is divided into cervical, thoracic, and lumbosacral regions.
White matter (contains axons to transfer stimuli) is on the outside of spinal cord and grey matter (containing cell bodies and nuclei) is on the inside.
The grey matter is shaped like a butterfly, with ventral and dorsal horns.
The ventral horns contain cell bodies of the motor neurons that innervate skeletal muscles.
The dorsal horns contain cell bodies of sensory neurons (receive somatosensory information from periphery).
The lateral horns (found in thoracolumbar spinal cord) contain cell bodies of autonomic neurons that innervate visceral organs.
Ventral spinal nerves ——> motor innervation.
Dorsal spinal nerves ——> sensory innervation.

Spinal Cord - Snell Clinical Neuroanatomy

The axons of the ventral horn of the spinal cord exit through from the ventral side as 6-10 thin filaments known as the rootlets of the ventral root. After a short distance, those rootlets merge and form the ventral roots of the spinal nerve, containing efferent somatic motor fibers.
Sensory information reach the spinal cord through the dorsal root ganglion (containing cell bodies for sensory neurons + involved in initial assessment of impulse and can generate reflex responses), then dorsal roots (containing afferent sensory fibers), then rootlets, then into dorsal horns (where we have interneurons).
The ventral and dorsal roots merge together laterally to form the spinal nerve.
So, the spinal nerve contains both, motor fibers from the ventral root and sensory fibers from the dorsal root.
There are 31 pairs of spinal nerves: 8 pairs of cervical spinal nerves, 12 pairs of thoracic spinal nerves, 5 pairs of lumbar spinal nerves, 5 pairs of sacral spinal nerves, and 1 pair of coccygeal spinal nerve.
Spinal nerves leave through the vertebral canal through the intervertebral foramina.
Shortly after leaving the intervertebral foramina, the spinal nerve splits into a ventral ramus (thickest) and dorsal ramus. The 2 rami have both motor AND sensory fibers.
Adjacent ventral rami of spinal nerve communicate to form plexuses:

In the thoracic region, the ventral rami remain distinct from each other and become continuous with the intercostal nerves.

So basically, the ventral rami innervate (sensory and motor) upper and lower limbs, ventral thoracic region, and ventral abdominal region.

The dorsal rami of the spinal nerve also carries motor and sensory fibers. Motor fibers innervate the autochthonous/deep muscles of the back, whereas the sensory fibers innervate the skin of the back on both sides of spinal cord.

The ventral rami of the spinal nerves give off 2 branches called rami communicates (communicating rami) —> white and grey ramus communicans. They are responsible for the communication of the spinal nerves with the autonomic nervous system.

Spinal Nerve - Wikipedia

In the thoracolumbar region of the spinal cord, we have lateral horns (another name is intermediolateral column) of the grey matter which contain the autonomic neurons (intermediolateral neurons).
The axons of those neurons travel to the ventral root and then to the spinal nerve.
Immediately after the splitting of spinal nerve into ventral and dorsal rami, the axons of the autonomic neurons leave the spinal nerve to reach the sympathetic trunk, especially the sympathetic ganglion. This thin branch that leaves the spinal nerve is called white ramus communicans (Preganglionic sympathetic fibers). It has a white color due to myelinated axons.
The grey ramus communicans (postganglionic sympathetic fibers) return into the spinal nerve. It has a grey color due to unmyelinated axons.

Somatomotor fibers: pass from anterior horn cell bodies to anterior root.
Somatosensory and viscerosensory fibers: originate from pseudounipolar neurons located in the dorsal root ganglion and extend into the dorsal horn of the spinal cord through the dorsal root. The sensory endings (where stimulus is created and sent) reach the dorsal ganglion through ventral and dorsal rami for somatosensory and ventral and dorsal rami, and grey rami communicans for viscerosensory.
Visceromotor fibers: originate from the lateral horn of the grey matter and then the axons travel through the white ramus communicans into the sympathetic ganglion (paravertebral ganglion), where synapsis occurs (presynaptic fibers). Then the post-synaptic fibers leave the ganglion and go through the grey ramus communicans.

Anterior Branch (ventral ramus): Supplies motor fibers to the muscles of the anterior and lateral walls of the trunk and to the muscles of the limb. Also it conveys the corresponding (somatic) sensory fibers to corresponding skin areas and muscle spindles. Also the post-synaptic visceromotor neuron passes through this branch.
Posterior Branch (dorsal ramus): It contains the same fibers as the anterior branch, but in this case they innervate the deep muscles of the back and also supply with sensory fibers each side of the vertebral column.
Meningeal Branch: It is a recurrent sensory branch that branches out just after the dorsal root ganglion and contains somatosensory fibers that extend up to the meninges of the spinal cord.
Communicating Branch (ramus communicans): Connects the sympathetic (paravertebral) ganglion with the spinal nerve. It consists of the white and grey rami. Through the white ramus enter the pre-synaptic fibers from the intermediolateral column (lateral horn) and synapse in the sympathetic chain ganglion with the post-synaptic fiber, which exits the ganglion through the grey ramus and continues towards the anterior and posterior branches of the spinal nerve for somatovisceral innervation. Post-synaptic fibers do not always exit via the grey ramus, but can ascend/descend to superior/inferior sympathetic ganglia or enter prevertebral ganglia prior to reaching the target organ.

Neuroanatomy: An Illustrated Colour Text, 4th (fourth) Edition by Crossman PhD DSc, 2010.
Snell, Richard S. Clinical Neuroanatomy. 7th ed., Wolters Kluwer Health/Lippincott Williams & Wilkins, 2010.
https://en.wikipedia.org/wiki/Spinal_nerve.