like any Nerf of the peripheral nervous system spinal nerves are made up of parallel bundles of axons wrapped in several layers of connective tissue in this learning objective we will look at the anatomy of a spinal nerve as well as the path that a spinal nerve takes following its exit from the vertebral column out the intervertebral fora so the 31 pairs of spinal nerves are named and numbered according to the region and the level of the vertebral column from which they emerge however not all spinal cord segments are aligned with the corresponding vertebrae given that the spinal cord ends at roughly the second lumb vertebrae so the roots of the lower Lumber the sacral and the cagal nerves actually descend from the bottom of the spinal cord at an angle to reach the corresponding vertebrae and into veral foramina before they then emerge from that vertebral column so once the spinal nerve or cranial nerve for that matter exits the spinal cord or brain stem it's made up of many individual axons surrounded by layers of connective tissue individual axons within a nerve whether they are melinated or un melinated are wrapped in a enduum the innermost layer of the connective tissue the enduum consists of a mesh of collagen fibers fibr blasts and macres groups of axons and their surrounding endoneurium are then held together in bundles called fyles each fasle is then wrapped in a perineurium which is our middle layer of connective tissue so the per is a bit of a thicker layer of connective tissue which consists of up to 15 layers of fibro blasts within a network of collagen fibers the outermost layer of connective tissue surrounding the entire nerve is then called a epineurium it consists of fiber blast and thick collagen fibers so from innermost to outermost almost deep to superficial we have the endon ium which surrounds each individual axon we have the perum which surrounds our facies and our epineurium which surrounds the entire nerve so just to help with you remembering these terms Endo means within so it's inside of that spinal nerve wrapped around the single axon Perry means around so the perum wraps around the fasle and then our epineurium so Epi means on top of so it's sitting on top of the entire nerve a short distance after that spinal nerve has then passed through the intervertebral Forin or foramina a spinal nerve will divide into several branches and these branches are known as Rami or Ramis as a singular the posterior or dors MUSC ramas serves the Deep muscles of the skin and the posterior surface of the trunk the anterior or the ventral ramas serves the muscles and structures of the upper and lower Limbs and the skin on the lateral and anterior surfaces of the trunk and then in addition to the posterior and anterior Rami spinal nerves off also give off a men andal branch and this men men andal Branch actually re-enters the vertebral cavity through that intervertebral Forin and supplies the vertebra the vertebral ligaments and the blood vessels of the spinal cord and the meninges so if we take a look at this image here we have obviously our spinal cord in the middle we have our spinal cord exiting here and we have our anterior and our posterior Roots here so the convergence where they join becomes our spinal cord and and then it branches off into those different branches so we have our posterior Branch here which goes off to serve the Deep muscles the muscles CL closer to the middle of the body and also the posterior or the back surface of our trunk we have this anterior Branch here which serves our upper and our lower Limbs and then also the skin on the side and the front of our trunk and then we have this tiny little meno Branch here which goes out and then back in through that intervertebral Forin and serves all of the structures within this space here so the spinal cord the meninges the ligaments the vessels now if we continue to move further away from the spinal cord then the axons of the anterior Rami of our spinal nerves don't actually go directly to the structures that they Supply or that they innovate instead they form a network on both the left and the right sides of the body by joining with several other axons from anterior Rami of adjacent nerves this interconnected network is what we call a plexus with the principal plexuses of the body our survical plexus brachial plexus Lumber plexus and sacral plexus and we also have a smaller cockal plexus now this is true of all of the spinal nerves except for the thoracic nerves 2 to 12 we also call these nerves the intercostal nerves and giving the proximity or the close location of these nerves to the structures that they actually innovate these ones do actually go directly to those structures so again if we take a look at this image here this is an image of our brachial plexus and we have spinal nerves C5 C6 C7 C8 8 and T1 so these are the anterior Rami of those spinal nerves in a plexus we will also call these The Roots the Roots of several spinal nerves then unite to form trunks so in the brachial plexus we have a superior trunk a middle trunk and an inferior trunk so it's just some of these root uh yeah Roots sorry coming together to form a trunk these trunks then diverge or they split into divisions so we either have the anterior or the posterior divisions and you can see those represented by the slightly different color green here these divisions then unite again to form a cord called the lateral the posterior or the medial cord and these cords are named for their relationship to the auxillary artery so the auxilary artery is a blood vessel that runs kind of in your underarm region these cords are named based on how close they are to this particular blood vessel from there the cords then split or reunite again to form the nerves which Supply the muscles of the shoulders and the upper limbs from here name the nerves are named generally for the structures the muscles or the regions that they innovate so for example we have the auxiliary nerve the radial nerve and the oler nerve so auxiliary is the underarm region radius and oner of bones in our lower arms now I do not expect you to remember all of this we do not actually learn about individual nerves or nerve plexuses in regional anatomy and physiology however many of you will go on to study further anatomy where you will be required to know the trunks the towards the branches of all of these different practices so I just wanted to I guess introduce you to that concept now what I do expect you to know or understand for this unit is that nerve plexuses enhance the efficiency and the flexibility of the nerve system by allowing nerve fibers from different spinal nerves to mix and redistribute so this Arrangement helps ensure that even if one nerve is damaged that the function it serves can still be maintained to some extent due to that kind of overlapping of the pathways so this organization of nerve plexuses is crucial for coordinating movements maintaining sensory perception and regulating various physiological processes throughout the body it kind of serves a little bit like The Circle of Willis in the brain now lastly the skin over the entire body is supplied by sematic Sensory neurons that carry nerve impulses from the skin to the spinal cord and the Brain each spinal nerve contains sensory neurons that serve a particular region of the body the area of skin that provides sensory input to the central nervous system via one pair of cranial nerves is called a dermatome so d e r m a t o m e now the nerve Supply in adjacent dermatomes or dermatomes that are the next to each other will overlap somewhat however knowing which spinal cord segment supplies each dermatome makes it possible to locate the damaged regions of the spinal cord if the skin in a particular region is stimulated but sensation is not perceived then the nerve supplying that dermatome is likely damaged similarly given that the plexus or network of nerves that a single nerve can pass through to reach a dermatome when undertaking let's say a surgery or a particular part of the body needs to be antiz for complete anesthesia to occur several adjacent spinal nerves may need to be blocked by an anesthetic drug so lastly a written summary of all of those structures for you so the endon neurum is the connective tissue surrounding an individual neuron axon the perum is a connective tissue surrounding the facies which just is a bundle of axons within a nerve the epineurium is the connective tissue that sits on top surrounding the entire nerve a plexus is a network of nerves arising from the anterior ramus or Branch within the body our major plexuses are our cervical brachial Lal Lumber sorry and sacral plexuses and lastly a dermatome is an area of skin which is