The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all parts of the bodies of bilaterally symmetric and triploblastic animals—that is, all multicellular animals except sponges and diploblasts. It is a structure composed of nervous tissue positioned along the rostral to caudal axis of the body and may have an enlarged section at the rostral end which is a brain. Only arthropods, cephalopods and vertebrates have a true brain.
Myelin is a lipid-rich material that surrounds nerve cell axons to insulate them and increase the rate at which electrical impulses are passed along the axon. The myelinated axon can be likened to an electrical wire with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Rather, myelin sheaths the nerve in segments: in general, each axon is encased with multiple long myelinated sections with short gaps in between called nodes of Ranvier.
A nerve is an enclosed, cable-like bundle of nerve fibers in the peripheral nervous system.
Schwann cells or neurolemmocytes are the principal glia of the peripheral nervous system (PNS). Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings, such as the Pacinian corpuscle. The two types of Schwann cells are myelinating and nonmyelinating. Myelinating Schwann cells wrap around axons of motor and sensory neurons to form the myelin sheath. The Schwann cell promoter is present in the downstream region of the human dystrophin gene that gives shortened transcript that are again synthesized in a tissue-specific manner.
Neurilemma is the outermost nucleated cytoplasmic layer of Schwann cells that surrounds the axon of the neuron. It forms the outermost layer of the nerve fiber in the peripheral nervous system.
Nervous tissue, also called neural tissue, is the main tissue component of the nervous system. The nervous system regulates and controls body functions and activity. It consists of two parts: the central nervous system (CNS) comprising the brain and spinal cord, and the peripheral nervous system (PNS) comprising the branching peripheral nerves. It is composed of neurons, also known as nerve cells, which receive and transmit impulses, and neuroglia, also known as glial cells or glia, which assist the propagation of the nerve impulse as well as provide nutrients to the neurons.
A motor nerve is a nerve located in the central nervous system (CNS), usually the spinal cord, that sends motor signals from the CNS to the muscles of the body. This is different from the motor neuron, which includes a cell body and branching of dendrites, while the nerve is made up of a bundle of axons. Motor nerves act as efferent nerves which carry information out from the CNS to muscles, as opposed to afferent nerves, which send signals from sensory receptors in the periphery to the CNS. Efferent nerves can also connect to glands or other organs/issues instead of muscles. In addition, there are nerves that serve as both sensory and motor nerves called mixed nerves.
In neuroscience and anatomy, Nodes of Ranvier, also known as myelin-sheath gaps, occur along a myelinated axon where the axolemma is exposed to the extracellular space. Nodes of Ranvier are uninsulated and highly enriched in ion channels, allowing them to participate in the exchange of ions required to regenerate the action potential. Nerve conduction in myelinated axons is referred to as saltatory conduction due to the manner in which the action potential seems to "jump" from one node to the next along the axon. This results in faster conduction of the action potential.
Axonotmesis is an injury to the peripheral nerve of one of the extremities of the body. The axons and their myelin sheath are damaged in this kind of injury, but the endoneurium, perineurium and epineurium remain intact. Motor and sensory functions distal to the point of injury are completely lost over time leading to Wallerian degeneration due to ischemia, or loss of blood supply. Axonotmesis is usually the result of a more severe crush or contusion than neurapraxia.
The epineurium is the outermost layer of dense irregular connective tissue surrounding a peripheral nerve. It usually surrounds multiple nerve fascicles as well as blood vessels which supply the nerve. Smaller branches of these blood vessels penetrate into the perineurium. In addition to blood vessels which supply the nerve, lymphocytes and fibroblasts are also present and contribute to the production of collagen fibers that form the backbone of the epineurium. In addition to providing structural support, lymphocytes and fibroblasts also play a vital role in maintenance and repair of the surrounding tissues.
The supratrochlear nerve is a branch of the frontal nerve, itself a branch of the ophthalmic nerve (CN V1) from the trigeminal nerve (CN V). It provides sensory innervation to the skin of the forehead and the upper eyelid.
Myelin incisures, are small pockets of cytoplasm left behind during the Schwann cell myelination process.
A muscle fascicle is a bundle of skeletal muscle fibers surrounded by perimysium, a type of connective tissue.
The perineurium is a protective sheath that surrounds a nerve fascicle. This bundles together axons targeting the same anatomical location. The perineurium is composed from fibroblasts.
A nerve fascicle, is a bundle of nerve fibers belonging to a nerve in the peripheral nervous system. A nerve fascicle is also called a fasciculus. A nerve fascicle is enclosed by perineurium, a layer of fascial connective tissue. Each enclosed nerve fiber in the fascicle is enclosed by a connective tissue layer of endoneurium. Bundles of nerve fascicles are called fasciculi and are constituents of a nerve trunk. A main nerve trunk may contain a great many fascicles enclosing many thousands of axons.
Nerve injury is an injury to nervous tissue. There is no single classification system that can describe all the many variations of nerve injuries. In 1941, Seddon introduced a classification of nerve injuries based on three main types of nerve fiber injury and whether there is continuity of the nerve. Usually, however, peripheral nerve injuries are classified in five stages, based on the extent of damage to both the nerve and the surrounding connective tissue, since supporting glial cells may be involved.
Classification of peripheral nerve injury assists in prognosis and determination of treatment strategy. Classification of nerve injury was described by Seddon in 1943 and by Sunderland in 1951. The lowest degree of nerve injury in which the nerve remains intact but signaling ability is damaged is called neurapraxia. The second degree in which the axon is damaged but the surrounding connecting tissue remains intact is called axonotmesis. The last degree in which both the axon and connective tissue are damaged is called neurotmesis.
Epineurial repair is a common surgical procedure to repair a nerve laceration via the epineurium, the connective tissue surrounding nerve fibers originating from the spinal cord. It is intended to allow the restoration of sensory function. When a nerve is lacerated or cut, repair is done by sewing the cut ends together through the epineurium to increase the potential of the proximal part growing correctly along the route the degrading distal part leaves behind. Usual sensation and mobility will not be an immediate result because nerves grow at a rate of approximately 1 millimeter per day, so it will take a few months to notice the final outcome. Research in use of nerve grafts and nerve growth factors is being done to speed recovery time.
Nerve allotransplantation is the transplantation of a nerve to a receiver from a donor of the same species. For example, nerve tissue is transplanted from one person to another. Allotransplantation is a commonly used type of transplantation of which nerve repair is one specific aspect.
Cryoneurolysis, also referred to as cryoanalgesia, is a medical procedure that temporarily blocks nerve conduction along peripheral nerve pathways. The procedure, which inserts a small probe to freeze the target nerve, can facilitate complete regeneration of the structure and function of the affected nerve. Cryoneurolysis has been used to treat a variety of painful conditions.
