PNS Nerves

Nerve Structure

What we call nerves are really just bindles of axons. For example, your sciatic nerveThe largest nerve in the body, arising from the sacral plexus. is this huge nerve that carries lots of axons.  These axons descend your leg and one-by-one, they branch off the nerve and innervateTo invade a tissue with nervous tissue is to innervate it. the surrounding somatic tissues.  When we encounter an axon in the peripheral nervous systemPNS All nervous tissue outside the CNS; includes nerves and ganglia., we call it a fiber.  The fibers within a nerve can be motor, sensory, or both.  The sciatic nerve carries motor information to the tissues of the leg. It also carries information from those tissues to the brain.

Nerve Wrappings

Just as bone had its periosteum and the muscles had those three coverings, nerves also have three coverings.  The epineuriumConnective tissue sheath surrounding an entire nerve. covers the entire nerve. This is similar to how the epimysium covered the entire muscle. Again, we have fasciclesBundles of nerve fibers within a nerve.
or
Bundles of nerve fibers within a muscle. that are bundles of axons and surrounded by the perineuriumA connective tissue sheath that surrounds a bundle (fascicle) of nerve fibers..  This microscopic picture you see here is a fascicle surrounded by the dense connective tissue of the perineurium.  These little circles on the top left and bottom right are all axons. They are wrapped in myelin. Then, they are wrapped by the endoneuriumA thin layer of connective tissue surrounding each nerve fiber..  These lines are here, curving in the center of the fascicle. They are also axons. However, they’ve been cut on an oblique angle rather than a straight-up cross section A cut or slice of the body or an organ for study..  The little purple dots in the fascicle are the darkly stained Schwann cell nucleiClusters of neurons in the CNS responsible for processing information.. Each nucleusThe control center of the cell that contains DNA and directs cellular activities. is associated with a myelin wrapping.  These wrappings don’t just separate these axons from nearby tissue.  These wrappings provide routes for blood vessels and lymph vessels.

Spinal Nerves

First, I want to point out that we are going to use terms we’ve used before when talking about vertebrae.  There will be a slight difference here because now we are talking about the nerves that leave the spinal cordThe central nervous system structure that relays signals between the brain and body..  There are 31 pairs of spinal nerves.  Yes, these are pairs. There is one nerve leaving the spinal cord on the left. Another nerve leaves the spinal cord on the right.  In short, there are 8 cervical spinal nerves, 12 thoracic spinal nerves, 5 lumbar, 5 sacral, and 1 coccygeal.  I put coccygeal in parenthesis because my picture here does not represent it.  Ha!  That’s what you get for using opensource materials!  Buyer beware.

The location of a spinal nerve can be really difficult for students to visualize.  This right here and right here are the spinal nerves.  That’s it.  Just that little bit.  AND that little bit of spinal nerve is trapped in a canal between the vertebrae.  This is what makes it so hard to visualize.  Think of another vertebra on top of this one. Consider how the spinal cord is threaded through the vertebral foramenCentral hole in each vertebra where the spinal cord passes through.. Observe the little spinal nerves that exit between the vertebrae.  The next slide will help this visualization.

Herniated Discs

Spinal nerves exit between the vertebrae. Therefore, you need those intervertebral discs to keep the vertebrae spaced apart enough. This allows the nerve to leave. Most people herniate discs while bending forward and lifting something heavy. By bending forward, you exert significant pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. on the dorsalRelating to the back side of the body. side of your intervertebral discs. This pressure is most likely in the lumbar section. Consequently, the gooey center of a disc can be forced into the space where the spinal nerve is. You can see this herniation in the picture on the left.  By leaning forward, you have pushed the disc backward and it impinges on the spinal nerve at that level.  You have pinched a nerve.  A big one, too.  Motor and sensory functions in tissues that branch off this spinal nerve would be affected. 

Yes, we can fix herniations by removing the bulge of the disc.  This simply alleviates the pressure on the nerve.  But, this also reduces the amount of overall tissue this disc has.  Yes, it will heal as pictured here, but the disc itself will be slightly shorter in height.  If your spineProminent ridge on the posterior scapula dividing it into supraspinous and infraspinous fossae. is already suffering from accelerated degenerative disc disease, there might not be enough tissue to do this.  This is where spinal fusion comes into play.  In spinal fusion, we remove the disc and use an artificial spacer to hold the vertebrae apart.