Where is endoneurium located

Peripheral nerve fibers are grouped based on the diameter, signal conduction velocity, and myelination state of the axons. These classifications apply to both sensory and motor fibers. Fibers of the A group have a large diameter, high conduction velocity, and are myelinated.

The A group is further subdivided into four types A-alpha, A-beta, A-delta, and A-gamma fibers based on the information carried by the fibers and the tissues they innervate. Fibers of the B group are myelinated with a small diameter and have a low conduction velocity. The primary role of B fibers is to transmit autonomic information.

Fibers of the C group are unmyelinated, have a small diameter, and low conduction velocity. The lack of myelination in the C group is the primary cause of their slow conduction velocity. Saltatory conduction : Demonstrates the faster propagation of an action potential in myelinated neurons than that of unmyelinated neurons.

C fiber axons are grouped together into what is known as Remak bundles. These occur when an unmyelinated Schwann cell bundles the axons close together by surrounding them. The Schwann cell keeps them from touching each other by squeezing its cytoplasm between the axons. C fibers are considered polymodal because they can often respond to combinations of thermal, mechanical, and chemical stimuli.

A-delta and C fibers both contribute to the detection of diverse painful stimuli. Because of their higher conduction velocity, A-delta fibers are responsible for the sensation of a sharp, initial pain and respond to a weaker intensity of stimulus.

These nerve fibers are associated with acute pain and therefore constitute the afferent portion of the reflex arc that results in pulling away from noxious stimuli. An example is the retraction or your hand from a hot stove. Slowly conducting, unmyelinated C fibers, by contrast, carry slow, longer-lasting pain sensations.

Privacy Policy. Skip to main content. Peripheral Nervous System. Search for:. Structure of a Nerve A nerve is the primary structure of the peripheral nervous system and is composed of bundles of axons. Learning Objectives Describe the structure of nerves.

The amount of epineurial tissue varies along a nerve and is more abundant around joints. The thickness of the epineurium varies in different nerves and in different locations of the same nerve. The proportion of epineurium is higher in larger nerves with increasing numbers of nerve fascicles.

However, the epineurium is absent around monofascicular nerves and at nerve endings. The epineurium contains adipocytes, fibroblasts, connective tissue fibers, mast cells, small blood and lymph vessels, and small nerve fibers innervating the vessels. Epineurium is a permeable structure, and its fibroblasts are ultrastructurally identical to fibroblasts elsewhere in the body.

Scattered throughout the epineurium, fibroblasts form the epineurial collagen, the most prominent component of this layer. As collagen is a protein stained by most acid dyes, collagen fibers turn weak pink wiht eosin in preparations stained with hematoxylin-eosin.

Under the electron microscope, fibers of mature collagen have frecuent cross bandings. Elastic fibers are also present, and these are considerably more compact than collagen fibers. They stain weak pink in sections stained with hematoxylin and eosin, brown with orcein, and blue-purple with resorcin-fuchsin. In electron micrographs, elastin fibers typically appear more stained darker at the periphery and are embedded in a sub-stance containing thinner elastin filaments.

The epineurium of some nerves contains a considerable amount of fat, as is the case with the sciatic nerve. The common peroneal and tibial nerves, however, contain less fat than the sciatic nerve, and usually the former contains less fat than the latter. Seen under the microscope, intraneural adipose cells resemble honeycombs, with empty vacuoles due to fat dissolution, during the fixation process Figure Mast cells are distributed throughout connective tissue and are often located in the proximity of small blood vessels.

Vasa nervorum supplying peripheral nerves arise from branches of regional arteries. Branches from these arteries enter the epineurium to form an vascular plexus Figures 22 and From the plexus, vessels penetrate the perineurium and enter the endoneurium as arterioles and capillaries.

In nerves consisting of several fascicles, arteries, veins, and lymphatics run longitudinally and parallel to nerve fascicles. Whereas gross anatomy descriptions of distal peripheral nerves accurately identify each connective layer enclosing axons endoneurium , nerve fascicles or bundles of axons perineurium , and single peripheral nerves epineurium , this becomes more complex where the connective tissue binds more than one nerve.

An example of this is the sciatic nerve at the popliteal fossa. Various terms, such as paraneurium, paraneural sheaths, common epineural sheath, conjuctiva nervorum, or adventitia, are interchangeably used to refer to the same connective tissue.

Small nerves formed by a single group of fascicles feature a layer of perineurium enclosing each fascicle along with scarce amounts of adipose tissue. A connective tissue known as epineurium composed of collagenous fibers encloses the nerve. Specific techniques enable identification of perineurium using staining methods positive for EMA epithelial membrane antigen and of collagen with Masson trichrome and EMA-negative stain.

Similarly, staining techniques aid in the identification of structures in more complex nerves, such as the sciatic nerve, where groups of variable numbers of fascicles are present. In these types of nerves, EMA staining reveals that perineurium encloses each nerve fascicle, as opposed to connective tissue made up by collagen fibers typically present in epineurium that encloses groups of fascicles detected with Masson trichromic stain.

Microscopic analysis of complex nerve structures such as the sciatic nerve at increasingly proximal locations shows that nerve branches within these nerve structures appear divided by their respective epineurial layers even before physical branch division materializes.

Each peripheral nerve at both plexus and terminal sites is encircled by concentric clusters of fat tissue, which appear just prior to the division of the nerve Figure 24 and The adipose tissue extends alongside its collateral or terminal branches. The amount and shape of fat tissue vary along the nerve, progressively losing their concentric contour and becoming unevenly distributed. The collagen layer, similar to the epineurium, that wrapps together the nerve components and the adipose tissue in between has been refered to as paraneurium, paraneural sheath, common epineural sheath, conjuctiva nervorum or adventitia by different authors.

In clinical practice, ultrasound-guided injection of local anesthetics enables the indirect identification of paraneurium as the space between this layer and the nerve expands displaying a concentric shape. Neural layers surrounding fascicles, groups of fascicles, nerves, as well as more complex nerve structures have similar morphology, and they are mainly composed of collagen fibers.

Therefore, it may seem reasonable to unify terminology noting that present denominations based on the anatomic location of each neural fascia seems rather confusing. However, epineurium and paraneurium may best to avitted terms to avoid the present confusion. Both epineurium and paraneurium have similar functions, which include insulation and protection of nerves from injury.

Paraneural compartments facilitate longitudinal displacement of nerves controlling body movement. This movement is necessary to neutralize lateral compression by changing their shape. If the tissue is exposed to external irritation, it reacts, leading to interfascicular fibrosis.

In relation to the anatomic features of the sciatic nerves, Andersen et al found the sheath surrounding the sciatic nerve to be a thin transparent structure that was clearly distinctive, both macroscopically and microscopically different from the epineurium.

The sheath facilitated the spread of the injectate along the nerve. However, its projections did not completely encircle the nerve. The internal layers of the paraneurium around the sciatic nerve had a similar structure of that sheath. Vloka et al used the term common epineural sheath. Orebaugh et al reported that needle-tip placement in the interscalene region and injection of anesthetic solution took place frequently inside the epineurium.

When Spinner injected the inner epineurium, the dye expanded within the same compartment but did not cross over or extend to the common external epineurial space. A peripheral nerve at both plexus and terminal sites is encircled by concentric clusters of adipose tissue.

This explains why perineural injections result in low opening injection pressure. Diffusion of anesthetic into the axons is influenced by the presence and characteristics of the connective tissue sheaths eg, perineurium, myelin and the size and location of the axons inside fascicles.

During intravenous peripheral anesthesia Bier block , the local anesthetic most likely reaches the peripheral nerve endings through the intraneural capillary network. Perineurium and endoneural capillary endothelium protect the axons from foreign substances thanks to their tight junctions between endothelial cells and among perineural cells. Local anesthetic injected outside the epineurium of a nerve must traverse both the epineurium and perineurium to reach the axons.

Subsequently, only a small proportion of the injected anesthetic comes in direct contact with axons leading to delayed onset, incomplete or failed neural blockade. The composition and arrangement of the connective tissue of peripheral nerves play a major role in protection of the peripheral nerves and in the practice of regional anesthesia. Characteristics and variability of the connective tissue may substantially influence the spread of the local anesthetic during nerve block injection and therefore the dynamics and quality of the neural blockade.

Supplementary video related to this subject can be found at Anatomy of Nerve Blockade Video. This section outlines spinal sonography techniques, relevant sonoanatomy, and practical considerations for ultrasound-guided Central neuraxial blocks.

Reviews new and traditional concepts regarding the dura mater, arachnoid layer, trabecular arachnoid, pia mater, and nerve root cuffs, and NYSORA Tips Fascicles within the nerve are surrounded by perineurium which confers structural protection against penetration and overstretching injury. A blood-nerve barrier is formed by tight junctions in the inner layers of the perineurium and tight junctions in endoneurial capillaries.

Epineurium is permeable and consists of moderately dense connective tissue that binds nerve fascicles. The epineurium contains adipocytes, fibroblasts, connective tissue fibers, mast cells, small lymphatics, as well as blood vessels and small nerve fibers innervating the vessels. Rev Esp Anestesiol Reanim ;— Panamericana Ed, , pp 71— The endoneurium has properties analogous to the blood—brain barrier.

It prevents certain molecules from crossing from the blood into the endoneurial fluid. In this respect, endoneurial fluid is similar to cerebrospinal fluid in the central nervous system. During nerve irritation or injury, the amount of endoneurial fluid may increase at the site of damage. This increase in fluid can be visualized using magnetic resonance neurography to diagnose nerve damage.

An illustration of a cross-section of a nerve highlighting the epineurium and perineurium. Individual axons can also be seen as tiny circles within each perineurium. A nerve conveys information in the form of electrochemical impulses known as nerve impulses or action potentials carried by the individual neurons that make up the nerve.

The impulses travel from one neuron to another by crossing a synapse, and the message is converted from electrical to chemical and then back to electrical. Neurologists usually diagnose disorders of the nerves by a physical examination, including the testing of reflexes, walking and other directed movements, muscle weakness, proprioception, and the sense of touch. This initial exam can be followed with tests such as nerve conduction study, electromyography, or computed tomography.

Learning Objectives Describe the structure of nerves.