First look at the neurovasculature
Blood leaves the heart and distributes throughout the body under high pressure. Blood vessels are a closed network of tubes that transport blood. They are of three types: arteries, veins, and capillaries.
Most blood vessels of the circulatory system consist of three layers or tunics. The tunica externa (adventitia) is a layer of outer connective tissue or sheath. Tunica media is the smooth middle layer formed primarily of smooth muscles. It contains various amounts of elastic fibers in medium and large arteries. The tunica intima is the inner endothelial lining of the blood vessels. Capillaries consist only of this tunic.
Arteries transport blood away from the heart and to the body under relatively high pressure compared to veins. Blood passes through arteries of declining caliber. These vessels are further subdivided into three classes depending on overall vessel size, amount of elastic tissue or muscle in the tunica media, wall thickness compared to lumen, and function (large elastic arteries, medium muscular arteries, and arterioles).
Veins generally transport low oxygenated blood (large pulmonary veins are the exception) from capillary beds toward the heart. This gives the veins their dark blue appearance. In the venous system, the blood pressure is low. Therefore, the walls of the veins (the tunica media in particular) are thin compared to the arteries, while their luminal diameter is large.
Valves are often present as well. They serve to facilitate the flow of blood to the heart. Normally, veins do not spurt blood when severed, nor pulsate. And they are also subdivided into three classes (large veins, medium veins, and venules)
Capillaries are simple tubes that connect between the arterial and venous sides of the circulation. They permit the exchange of materials with the extracellular fluid surrounding the cells. They carry out oxygen and nutrients from the arteries to reach the cells that make up the body tissue. Capillaries are arranged in the form of a capillary bed network that connects the arterioles and venules together. The blood enters the capillaries from the arterioles, and then drains from them by the venules.
A nerve is a bundle of nerve fibers outside the central nervous system (CNS: Brain and spinal cord). Its function is to use electrical and chemical signals to transmit sensory and motor impulses between the brain or spinal cord and other parts of the body. Nerves are surrounded by a connective tissue covering that binds the fibers together. Vasa nervorum are the blood vessels that supply the nerve fibers and their covering. It should be noted that nerves and nerve fibers are two different structures, and it is important to distinguish between them.
Nerves are supported and protected by three connective tissue coverings, which makes them a fairly strong and resilient structure. The epineurium is a thick connective sheath that forms the outermost covering of the nerve. It includes blood vessels, lymphatics and fatty tissue. The perineurium is a dense connective tissue layer that encloses a fascicle of nerve fibers. It provides effective protection against penetration by foreign substances. The endoneurium is the innermost layer surrounding the neurilemma cells and axons. It is formed by delicate connective tissue.
Types of nerves
All nerves are cranial nerves, spinal (segmental) nerves or derivatives of them. Cranial nerves are 12 pairs in total, 11 of which arise from the brain. The 12th pair arises from the superior part of the spinal cord. These nerves exit the cranial cavity and are given a descriptive name.
Spinal nerves, on the other hand, are 31 pairs in total. They arise in bilateral pairs from a specific segment of the spinal cord, and are identified by a letter and a number that designates their region in the spinal cord and their superior-to-inferior order.