By Elizabeth A. Murray, Mount St. Joseph University
The spinal cord, a spinal nerve, is—by definition—considered a mixed nerve, both sensory and motor in nature. How? So, then, what makes spinal nerves different? They control skeletal muscles and receive sensations from the trunk and limbs. Let’s learn more about them.
How are Spinal Nerves Formed?
Each spinal nerve offshoot comes from the convergence of an anterior or ventral root with a posterior or dorsal root. Each anterior root is entirely motor, meaning all of the neural pathways in it are outgoing efferent pathways from the central nervous system (CNS) such as to smooth muscle or skeletal muscle. Each posterior root is entirely sensory, in that all of the neurons in this structure are carrying incoming sensory afferent information from receptors in the periphery, such as touch receptors in the skin or proprioception monitors in the muscles or joints that help us balance.
When these two roots come together, they form what is called a spinal nerve, making it a mixed nerve, both sensory and motor in nature. This convergence of the anterior and posterior roots happens just before the resulting spinal nerve exits the intervertebral foramen. Then, almost immediately, each of the spinal nerves divides into a pair of distributional branches, known as rami; the singular form is ramus.
Ventral and Dorsal Ramus
The larger of the pair is called the ventral ramus, and the smaller of the pair is known as the dorsal ramus of that given spinal nerve. One could also call these anterior and posterior, respectively, to avoid confusion.
The two roots of a spinal nerve are functionally distinct, either sensory or motor. They extend out from the spinal cord and merge to form a spinal nerve just before it exits the intervertebral foramen. That spinal nerve is then mixed in function—it carries motor components from its anterior root and sensory components to its posterior root.
Once this spinal nerve exits between adjacent vertebrae, it shortly divides into distributional branches called rami, which stay mixed—carrying both efferent and afferent neurons.
Functions of the Rami
The dorsal rami of spinal nerves supply what we will call the true back muscles—those muscles whose job it is to support and move the spine. So, the motor axons or motor fibers that start in the spinal cord travel out the anterior root, become part of a mixed spinal nerve, and then—if they are headed to the true back muscles that operate the spine—they will travel through the dorsal ramus of the spinal nerve. True back muscles are the motor job of a dorsal ramus. The sensory job of a spinal nerve’s dorsal ramus is to relay sensations from the skin of the middle third of the back.
The term ‘middle third’ might sound odd, but consider taking the back and longitudinally dividing it into thirds. That central third is the approximate region of skin governed by the dorsal rami.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
The Role of Dorsal Rami
Each spinal nerve is responsible for sensation from a given patch of skin, known as a dermatome. The territory from the back of the head, down to a taper that ends at the so-called natal cleft, is the sensory turf that belongs to the dorsal rami of the spinal nerves. As we move from that middle third, out to the more lateral aspects of the back, we begin to run into the ventral ramus territory of that same spinal nerve.
Yet, it is the dorsal rami that relay the sensations from the skin down the middle of the back and carry the motor control over the true back muscles. They assign all other functions attributed to spinal nerves to the ventral rami. But remember, cranial nerves govern most of the sensory and motor functions in the head and neck.
So, if dorsal rami are concerned with the skin down the middle third of the back and the back muscles that actually operate the spine, then ventral rami are concerned with everything else that isn’t supplied by cranial nerves.
The Ventral Rami
In terms of their sensory function, ventral rami transmit the sensations for all other skin below the head: This means the skin of the two lateral thirds of the back, the entire skin of the anterior and lateral trunk, as well as 100% of the skin of the upper and lower limbs, whether on the front of the limb or the back of the limb—but not the skin of the face. Sensations from there travel by a cranial nerve. That’s the extensive sensory or afferent function of a ventral ramus of a spinal nerve.
Ventral rami have essentially take two main forms. First, in the thorax, the ventral rami assume a relatively simple pattern, by hugging a rib as it proceeds around the chest wall. These ventral rami are called intercostal nerves as they travel between ribs. The second pattern that a ventral ramus can assume is to become part of what’s called a nerve plexus.
Plexus
The term plexus is Latin for braid or plait, and the ventral rami of spinal nerves, other than those in the thorax, braid together into what’s called a plexus.
Plexuses are braids of ventral rami that overlap to supply a given body region. For example, the brachial plexus is a braid of ventral rami that come together to supply the skin and muscles of the upper limb. The lumbar and sacral plexuses are groups of ventral rami that braid together and supply the skin and muscles of the lower limbs. These nerve plexuses are entirely composed of ventral rami originating from either superior to or inferior to the intercostal nerves that assume the simple rib-hugging pattern.
Understandably, no dorsal ramus has any role in any of those plexuses, as they supply the skin of the back and true back muscles.
Common Questions about Spinal Nerves
This convergence of the anterior and posterior roots happens just before the resulting spinal nerve exits the intervertebral foramen. Then, almost immediately, each of the spinal nerves divides into a pair of distributional branches, known as rami.
The territory from the back of the head, down to a taper that ends at the so-called natal cleft, is the sensory turf that belongs to the dorsal rami of the spinal nerves.
Plexuses are braids of ventral rami that overlap to supply a given body region. For example, the brachial plexus is a braid of ventral rami that come together to supply the skin and muscles of the upper limb.
