Roots and Rami

Ventral Roots & Rami

Before we go digging into the main picture and labeling anatomyThe study of the structure of the human body., let’s pause for a moment. We should remind ourselves where we are.  This is a superiorAbove or toward the upper part of the body. view of a transverse section A cut or slice of the body or an organ for study. of the spinal cordThe central nervous system structure that relays signals between the brain and body..  This is the spinal cord itself. It is represented by the circular blue area in the middle. It is threaded through all the vertebral foramenCentral hole in each vertebra where the spinal cord passes through. of the vertebrae.  This little teeny tiny length right here is the spinal nerve.  Just that.  No more.  Just that very little length of nerve is THE spinal nerve.  Remember that all this stuff here, coming off of the spinal cord, is still obscured by the vertebrae. It is threaded through this transverse canal before it leaves the encasement of the vertebral column.  So, this yellow still is technically PNS nervous tissue whereas the blue is technically CNSComposed of the brain and spinal cord; integrates and processes information.  material. 

What are these lines I’ve drawn?  This is a simple pathway that a reflex might take.  Sensory info moves through this blue neuron this way, right? Because it’s incoming, sensory.  And motor information moves out.  Notice how the spinal nerve is the area where the sensory and motor neuronsNeurons that carry outgoing information from the CNS to effector organs. create a two-way highway. 

Roots are structures that come off the spinal cord and then combine to form the spinal nerve.  These are the ventralRelating to the front or belly side of the body. roots.  Roots are medialToward the midline of the body to the spinal nerve.  Rami come off the spinal nerve and split into large networks.  These the ventral rami.  Rami are lateralAway from the midline of the body. to the spinal nerve.

Nerve Plexuses

All of your ventral rami coming off of a spinal nerve split into networks called plexuses.  These are all motor plexuses.  There are no sensory plexuses and there are no dorsalRelating to the back side of the body. plexuses.  Take note!!!  Only outgoing, motor info is carried into these plexuses.  Furthermore, these plexuses are NOT named for the regions of the spinal cord.  Look.  Brachial.  That is not a spinal cord section, but it is a plexus.  I have a picture of the brachial plexusA network of nerves in the neck and shoulder that control the upper limbs. here. You are going to say, “Amy, that looks like it’s coming off the cervical vertebraeSmallest vertebrae; have transverse foramina for vertebral arteries; often bifid spinous processes..”  Yeah, I don’t disagree, but this cluster of neuronsThe functional cells of the nervous system that transmit signals. forms a plexus that innervates your arm.  This is the brachial plexus.  No arguments. The plexuses are named for the regional tissue they serve. They are not named for the region of the spinal cord from which they derive.

Each of these plexuses have important axons innervating somatic and visceral tissue.  But, it’s all motor.  So every axon destined for a somatic location is headed to the neuromuscular junctionThe connection between a motor neuron and a muscle fiber. and carries voluntary information.  Every axon headed for a visceral organ will innervateTo invade a tissue with nervous tissue is to innervate it. smooth muscle and carry involuntary information.  That’s a gross generalization, but let’s keep it at that right now. 

Notable in the cervical plexusA network of nerves that supplies the neck, shoulders, and diaphragm. is the nerve that innervates the diaphragm and controlsVariables that remain constant to ensure a fair test. breathing: the phrenic nerveA nerve from the cervical plexus that controls the diaphragm..  Christopher Reeves played Superman back in the 80s. In the 90s, he fell from a house and sustained a neckNarrow region just below the head; common fracture site. injury.  This injury left him paralyzed, affecting both voluntary muscles and involuntary functions. One such function is breathing, which is controlled by the phrenic nerve arising from the cervical plexus.  Even after his injury, he and his wife continued their philanthropic efforts.  Inspiring.

The only nerve from the brachial plexus I know, I’m kidding, I know more, is the ulnar nervenerve from the brachial plexus that controls hand and forearm muscles..  I’ve been using this nerve as an example this semester because it is the “funny bone” nerve.  The ulnaMedial forearm bone (pinky side); forms the elbow hinge with humerus. nerve runs between the olecranon processLarge, curved posterior projection—forms the bony point of the elbow. of the ulnar bone and your skinThe body’s largest organ, providing protection and regulation..  There is no cushioning there. When you tap the elbow, the sensory axons in the ulnar nerve send info to your spinal cord.  Your spinal cord immediately interprets it and send an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. back saying, “withdraw your arm, dummy!”  The nerve that would control the muscles would arise from the brachial plexus.

We’ve been using the sciatic nerveThe largest nerve in the body, arising from the sacral plexus. as an example a lot.  It arises from BOTH the lumbar and sacral plexuses.  In factA statement based on direct observation that is repeatedly confirmed., many rami of the sciatic nerve exit from various levels in these two plexuses.  This is why lower back issues are hard to diagnose by symptom.  I’m sure my doctor knows that my sciatic nerve is suffering. But does she actually know WHICH level is causing the most pain?  No.  She doesn’t and she doesn’t need to.  She will give me a cortisone injection into the epidural spaceThe space between the dura mater and vertebrae, filled with fat and blood vessels. at L4. Then, she’ll let gravity and diffusionPassive movement of molecules from areas of high to low concentration. do the rest. 

Dorsal Roots & Rami

The previous two slides focused on the structures that leave the spinal cord on the ventral side. They also covered the anteriorThe front of the body or toward the front when standing in the anatomical position. side.  We know that nerve plexuses are only associated with the ventral side of the spinal nerve. They are the outgoing routes, located lateral to the spinal nerve.  Now, let’s shift our attention to the structures on the dorsal slide of the spinal cord. 

Anything dorsal should be associated with incoming sensory information.  These here are the dorsal roots coming off the spinal cord.  Of course, we have left and right. The same definition still applies to these roots. They are between the spinal cord and the spinal nerve. This means that they are medial to the spinal nerve.  But, this time they are dorsal.  Ignore the big bump for now.  Take a look at this diagram up top. You will notice there is no plexus on these dorsal nerves. They are coming into the spinal nerve.  Correct, we do not have sensory plexuses.  There is only a dorsal ramus.  ONE dorsal ramus, singular, which there are many ventral rami plural. 

The Dorsal Root Ganglion

The distinctive structure on dorsal roots is this bump right here.  This bump is called the dorsal root ganglionA cluster of neuron cell bodies located in the peripheral nervous system (PNS)..  A ganglion is a bunch of cell bodies.  For example, unipolar, sensory cell bodies can aggregate here. Their axons form a whole bunch in the dorsal root ganglion.  In a previous chapter, we designated the neurons with the cell bodies coming off the axon as unipolar.  Most sensory neuronsNeurons that carry information from sensory receptors to the CNS. are unipolar, whereas most motor neurons are multipolar.  So, follow me as I draw this here.  Information from a unipolar sensory neuron would enter from the dorsal ramus, one dorsal ramus.  It would then pass through the spinal nerve, this tiny tiny little length.  Then, the axon technically becomes part of the dorsal root. It is medial to the spinal nerve. It enters the gray matterThe inner portion of the spinal cord composed mostly of neuron cell bodies and synapses. of the spinal cord.

The dorsal root ganglion is the cell bodyThe central part of a neuron containing the nucleus and organelles. of the unipolar neuronA neuron with one process that splits into peripheral and central branches, common in sensory neuron.  In fact, it’s a collection of cell bodies of a whole bunch of sensory neurons. 

All together now

Let’s once again look at the superior view of a transverse section of the spinal cord.  Let’s label posterior and anterior.  And, let’s highlight the short short spinal nerves.  Labeling the spinal nerves allows us to immediately differentiate between roots and rami.  Forcing both of my decisions to label anterior and posterior and the spinal nerve is this dorsal root ganglion. I know that the dorsal root ganglion should be toward the posterior side of the spinal cord. The dorsal root ganglion is medial to the spinal nerve. It is located between the spinal nerve and the spinal cord. Labeling these two things first allows me to clearly labeled the dorsal and ventral rami.

Many lectures will follow this one. The lectures, especially those discussing the Gray matter of the spinal cord, will use this characteristic diagram on the left. We will never again look at a transverse section of the spinal cord. It is encased in all of the meninges(singular: meninx) – Protective membranes surrounding the spinal cord and brain.. It is also viewed against the background of a vertebra. I recommend that you take the picture on the left. Print out many of them. This way, you can fill them out quickly. You can do so easily when we discuss how information moves into the spinal cord and then back out again. That is what we have established here in this mini lecture: the route through which information enters the spinal cord. Another route is how information leaves the spinal cord.