Learn About the Location, Structure, and Functions of the Endoneurium

Endoneurium structure
Endoneurium is a layer of connective tissue that encloses individual peripheral nerve fibers within a nerve. This Buzzle post provides information on this connective tissue, focusing on endoneurium function, structure, and location.
The central nervous system (CNS), which comprises the brain and spinal cord, doesn't contain Schwann cells and endoneurium. While Schwann cells are responsible for the formation of myelin, glial cells called oligodendroglia cells are responsible for the production of myelin in the CNS.

The nervous system of the human body consists of the central nervous system and the peripheral nervous system. The peripheral nervous system consists of nerves, which are bundles of nerve fibers that connect the central nervous system to other parts of the body. Nerve fibers are thread-like extensions of nerve cells that perform the vital function of carrying information from the receptor cells or organs to the nerve centers that are present in the brain and spinal cord. A peripheral nerve fiber comprises an axon (a long nerve fiber that conducts away from the cell body of the neuron), axolemma (membrane covering the axon), myelin sheath, neurolemma/Schwann's sheath, and endoneurium. In case of the peripheral nerves, Schwann cells that are wrapped around the nerve fibers produce a fatty substance called myelin.

Myelin insulates the axons. Endoneurium, perineurium, and epineurium are layers of connective tissue that cover the individual axons, fascicle, and the entire nerve, respectively. It must be noted that both myelinated and unmyelinated axons have an endoneurium.
Neurons, Myelin Sheath, and the Endoneurium
Nerves can be sensory, motor, or sensory-motor (mixed). The major structures of a nerve cell include cell body (soma), axon, dendrite, synaptic terminals, etc. Dendrites are short branches of a nerve cell that extend from the cell body. These act as receptors, perform the function of receiving impulses from the nearby neurons at the synapses, and transmit them to the cell body. The cell body contains specialized organelles that produce proteins for the axons, dendrites, and the synaptic terminals. Unlike dendrites, an axon is a long cytoplasmic process that conducts nerve impulses away from the cell body.

Neurons can be divided into sensory neurons, motor neurons, or interneurons. Sensory neurons contain afferent fibers, and conduct information in the form of nerve impulse from the receptor cell or sensory organs such as eyes, ears, nose, tongue, and skin to the central nervous system. Interneurons carry impulses from sensory neurons to motor neurons. Motor neurons contain efferent fibers, and perform the task of relaying impulses from a cell body located in the central nervous system to an axon that leads to an effector such as a muscle or gland.
Brain Neuron Cell Diagram
Motor neuron function
Motor Neuron
Nerves that contain both sensory and motor neurons are called mixed nerves. Spinal nerves are an example of mixed nerves. Once the impulse is received at a dendrite, they move along the axon to reach the terminal branches. At the end of the terminal branches is a synapse, from where the signal is carried across to the dendrites of the nearby neurons. Electrical signals as well as chemical messages are transmitted from one cell to the other. Axons can carry nerve impulses for distances ranging from 0.1 millimeter to 2 meters. There are structures that protect the axons, and help in speeding up the movement of action potentials in the axon.

Many nerve cells are coated with several concentric layers of myelin. The myelin sheath is composed of the membranes of interstitial glial cells that wrap themselves around the axons to form several concentric layers.
Myelin Sheath Formation
These concentric layers form an insulating sheath that allows the nerve impulses to travel along the axon at a rapid rate. Besides insulating the axon, this sheath prevents the interference from nearby neurons and increases the impulse speed. The layers of myelin sheath around axons reduce the number of action potentials that are needed for the signal to reach the terminal region of the axon. The axons and dendrites of sensory and motor neurons are myelinated. The outer membrane of an axon remains uncovered only at the nodes of Ranvier, which are gaps between the myelin sheath that are located between each pair of Schwann cells. Endoneurium, perineurium, and epineurium are layers of connective tissue that protect the axons within a nerve or bundle of nerves.
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Endoneurium Function and Structure
Peripheral nerves consist of bundles of nerve fibers that contain myelinated and unmyelinated axons. When a small bundle of nerve fibers is wrapped in a tubular sheath, it is referred to as a funiculus. Small nerves might comprise a single funiculus, but in case of large nerves, the funiculi join to form large bundles of nerve fibers called fasciculi. These are kept together by membranous sheaths or layers of connective tissue called endoneurium, perineurium, and epineurium.

The epineurium is a sense layer of connective tissue that envelops the entire nerve trunk. It surrounds the fascicles and the blood vessels that supply the nerve. From this layer arise partitions of vascular connective tissue that move across the nerve and separate fascicles from one another. It is a tough tissue that cannot be easily penetrated by a needle.

Each bundle of nerve fibers or fascicle is covered by a layer of connective tissue called perineurium. It is a sheath made of special cells that hold the axons of a fascicle together. Unlike epineurium, the perineurium is a smooth membrane that can be separated from the nerve fibers that it envelops.
Connective tissue around peripheral nerve fibers
Layers of Connective tissue around Peripheral Nerve, Fascicles, and Individual Axons
Within each bundle of nerve fibers, a connective tissue called endoneurium surrounds each individual nerve fiber, along with its myelin coating and neurolemma, which is a thin layer containing Schwann cell cytoplasm. Even blood vessels could be enclosed within the endoneurium. The endoneurium contains collagen fibers, Schwann cells, endothelial cells, fibroblasts, and macrophages.

The endoneurial cells form bundles of collagen fibers, thereby forming a supportive extracellular matrix for axons. It holds the nerve fibers together, allowing them to form bundles or fasciculi. It also contains endoneurial fluid. In the event of an injury to the peripheral nerves, this fluid is released into the nearby tissues. Imaging studies such as MRI can help detect the presence of this fluid, thereby making it easier to diagnose such injuries.
On a concluding note, the main function of the endoneurium is to protect the axons. To add to that, this connective tissue layer also helps increase the speed at which axons communicate within the body.