Efferent axons arise from either the somatic or the autonomic nervous system. Somatic efferent motor neurons innervate skeletal muscle and have cell bodies located in somatic motor nuclei of the brainstem cranial nerves or in the ventral horns of the spinal cord spinal nerves.
Preganglionic visceral efferent neurons of the sympathetic part of the autonomic nervous system arise from the intermediolateral column of the spinal cord between levels T1 and L2 and synapse on paravertebral or prevertebral preaortic ganglia.
Peripheral nerves thus contain both preganglionic and postganglionic sympathetic fibers. Preganglionic visceral efferent neurons of the parasympathetic part of the autonomic nervous system arise from the parasympathetic nuclei within the brainstem cranial part of parasympathetic nervous system or sacral spinal cord between the S2 and S4 segments sacral part of the parasympathetic nervous system.
Only preganglionic parasympathetic fibers travel along peripheral nerves to synapse on intramural ganglia in the wall of target organs. Afferent axons are either somatic or visceral and have cell bodies either in the dorsal root ganglia of the spinal nerves or in the sensory ganglia of the cranial nerves.
Somatic afferent sensory neurons transmit impulses from the receptors for touch, temperature, or pain nociceptors located in the body wall skin and from the proprioceptors in the skeletal muscles and joints. Visceral afferent neurons transmit information from viscera interoceptors and nociceptors.
The visceral afferent axons travel along the visceral efferent fibers and pass through the communicating branches and dorsal roots of the spinal nerves or along the vagus nerve to enter the CNS. Axons of peripheral nerves are ensheathed by Schwann cells. Their myelin sheath modified plasmalemma separates the axons from the endoneurium.
Schwann cells are distributed along the axons in longitudinal chains depending on myelination along the axon. The coordinated differentiation of the axons and their myelinating cells requires close communication between neurons and glia. Signals provided by the axons regulate the proliferation, survival, and differentiation of glial cells. On the other hand, reciprocal glial signals affect axonal cytoskeleton and transport and are required for axonal survival and regeneration.
Schwann cells also have a guiding function for growing axons, indicating that glia do more than provide support to the axon. Schwann cell phenotypes are characterized by distinct morphologies and differential expression of myelin proteins, cell adhesion molecules, receptors, enzymes, intermediate filament proteins, ion channels, and extracellular matrix proteins.
All Schwann cells are surrounded by basal lamina, whose extracellular matrix molecules, such as laminin, regulate key aspects of Schwann cell development. Nerve fibers are classified according to axonal diameter, conduction velocity, type of receptor, and myelin sheath thickness Table 1.
Conduction velocity is related to axonal diameter; that is, the larger the fiber is, the faster the conduction will be. The larger the fiber, the more concentrated the local anesthetic must be to affect neural blockade. Myelinated nerve fibers are ensheathed by myelin, greatly extended and modified plasmalemma of the Schwann cells Figures 4 and 5. Myelin formation begins with extension of the Schwann cell cytoplasm and development of the inner mesaxon, which wraps around the axon several times.
During the wrapping process, the cytoplasm is nearly extruded between the plasmalemma. The proposed molecular structure of myelin fits the concept of plasmalemma as a lipid bilayer with integral and peripheral membrane proteins attached to the extracellular or to the cytoplasmic side of plasmalemma.
Myelin sheath wraps the axon in segments. Interruptions, which occur in the myelin sheath at regular intervals along the length of axons and expose the axon, are called nodes of Ranvier Figure 6. Each node indicates an interface between the myelin sheaths of two different Schwann cells located along the axon. The nodal region and its surroundings can be further subdivided into several domains Figure 6 that contain a unique set of ion channels, cell adhesion molecules, and cytoplasmic adaptor proteins.
In the PNS, the node is in contact with Schwann cell microvilli and covered by its basal lamina Figure 6. When the membrane at the node is excited, the local circuit that is generated cannot flow through the high-resistance myelin sheath. It therefore flows out and depolarizes the membrane at the next node, which may be 1 mm or farther away. The low capacitance of the sheath means that little energy is required to depolarize the remaining membrane between the nodes, resulting in increased speed of local circuit spreading.
Myelination is an example of cell-to-cell communication in which axons interact with Schwann cells. The number of myelin layers is determined by the axon and not by the Schwann cell. Myelin sheath thickness is regulated by a growth factor called neuregulin 1 Nrg1.
The compaction of myelin sheath is associated with the expression of transmembrane myelin-specific proteins such as protein 0 P0 , a peripheral myelin protein of 22 kilodaltons PMP22 , and a myelin basic protein MBP.
The absence of proteins that regulate myelin sheath formation might result in severe hypomyelination or dismyelination in humans and experimental animals. Unmyelinated axons are also enveloped by Schwann cells and their basal lamina. An individual Schwann cell can ensheath a single or several unmyelinated axons Figures 8 and 9. Unmyelinated fibers predominate in human cutaneous spinal nerves, where the average ratio of unmyelinated to myelinated fiber density is 3.
In unmyelinated fibers, conduction velocity is proportional to the square root of fiber diameter and is much slower compared to saltatory conduction in myelinated fibers Table 1. In a peripheral nerve, nerve fibers and their supporting Schwann cells are held together by connective tissue organized into three distinctive components that have specific morphological and functional characteristics.
The epineurium forms the outermost connective tissue of the peripheral nerve, the perineurium surrounds each nerve fascicle separately, while the individual nerve fibers are embedded in the endoneurium Figures 10 to The epineurium is a condensation of a loose areolar connective tissue that surrounds a peripheral nerve and binds its fascicles into a common bundle Figure 10 and Figure It is the thickest where continuous with the dura covering the CNS and more abundant in nerves adjacent to the joints, where nerves are subject to pressure.
Susceptibility to compression injury is therefore likely to be greater in unifascicular than in multifascicular nerves because the latter have greater amount of epineurium. As the peripheral nerve divides and the number of fascicles is reduced, the epineurium becomes progressively thinner and eventually disappears around monofascicular nerves. The epineurium contains collagen, fibroblasts, mast cells, and fat cells.
Elastic fibers are also present, particularly adjacent to the perineurium, which are mainly oriented longitudinally. Collagen and elastic fibers are aligned and oriented to prevent damage by overstretching of the nerve bundle, suggesting that the epineurium is designed to accommodate the stretch.
Human epineurium is constructed predominantly of type I and type III collagen, with the type I predominating. The diameter of the collagen fibrils averages 60— nm. Adipose tissue inside a nerve surrounds the fascicles and forms adipose sheaths that separate the fascicles from each other.
The thickness of adipose sheaths varies from one fascicle to another and is greater in larger nerve trunks, highlighting its protective function in cushioning the fascicles against damage by compression. Loss of epineural fat may present a risk factor for pressure-caused palsies in emaciated, bedridden patients.
In contrast, excessive adipose tissue can also delay the diffusion of local anesthetic injected near a nerve, thus interfering with the anesthetic blockade. Epineurium is continuous with the connective tissue called adventitia or mesoneurium that surrounds the nerve when passing through, underneath, or between the muscle fascia, serving as 1 a conduit for the injected local anesthetic, 2 a path allowing for nerve gliding, and 3 a layer of protection against nerve trauma.
Because their attachment is loose, nerves are relatively mobile except where tethered by entering vessels or exiting nerve branches. The perineurium is a specialized connective tissue surrounding individual nerve fascicles Figures 10 and This protective cellular layer is thinner than the epineurium and separates the endoneurium from the epineurium.
The perineurium consists of alternating layers of flattened polygonal cells, which are thought to be derived from fibroblasts, and collagenous connective tissue, the formation of which is controlled by the Schwann cells. The flattened polygonal cells, which constitute the lamellae, are specialized to function as a diffusion barrier. The number of lamellae varies, depending mainly on the diameter of the fascicle; the larger the fascicle is, the greater the number of lamellae.
In mammalian nerve trunks, the perineurium contains 15—20 cell layers. Contiguous cells in each layer interdigitate along extensive tight junctions.
The cells may branch and give rise to processes and contribute to the adjacent lamellae. Each layer of cells, enclosed by basal lamina, can reach a thickness of up to 0. Collagen fibers originate in a lattice-like arrangement, in which bundles are circular, longitudinal, and obliquely arranged. The innermost perineural cell layer adheres to a distinct boundary layer of densely woven collagen fibers and subperineurial fibroblasts that mechanically links the perineurium to the endoneurial contents.
Collagen fibers are predominantly type III, although type I collagen fibers are also present. The diameter of the collagen fibrils is substantially smaller than that of the epineural fibrils, with an average of 52 nm in the rat sural nerve. The basal lamina of polygonal cells is composed of collagens IV and V, fibronectin, heparan sulfate proteoglycan, and laminin.
The ubiquitous presence of pinocytotic vesicles rich in phosphorylating enzymes underlie the assumption that the perineurium functions as a metabolically active diffusion barrier, playing an essential role in maintaining the osmotic milieu and fluid pressure within the endoneurium.
For instance, in one of our studies, inflammatory cells accumulated between the nerve fascicles in piglets after exposure of the nerve to ultrasound gel did not penetrate the perineurium. Because of its tightly adherent cellular structure and more longitudinally oriented collagen, the perineurium is less tolerant to elongation than the epineurium.
In the rabbit, mechanical failure during elongation coincided with a disruption of the perineurium while the epineurium remained intact. On the other hand, SF penetrated very rapidly into peripheral ganglia and into the epineurium and perineurium of large peripheral nerves. The penetration of SF into the endoneurium of large nerves was, however, much more restricted with tracer detectable within the endoneurium only at high doses and long survival times.
Even in such cases, the level of SF fluorescence was much lower within nerve fascicles than in the epineurium and the perineurium, and a sharp gradient in fluorescence intensity persisted at the inner border of the perineurium. The extent of extravasation into the endoneurium varied markedly betwen different fascicles of the same nerve and between different nerves in the same animal. Experiments involving injection of high doses of SF adjacent to the nerve indicated relatively little movement of SF across the perineurium, which indicates that the observed accumulation of tracer within the endoneurium was the result of direct extravasation of SF from the endoneural blood vessels.
This may be due to a diffusion of SF along the extracellular space of the endoneurium from nerve terminals where the perineurial barrier is open-ended. Nerves can be further categorized based on where they connect to the central nervous system. Spinal nerves innervate much of the body and connect through the spinal column to the spinal cord.
Spinal nerves are assigned letter-number designations according to the vertebra where they connect to the spinal column. Cranial nerves innervate parts of the head and connect directly to the brain. Cranial nerves are typically assigned Roman numerals from 0 to 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.
0コメント