Unmyelinated nerve fiber

R = / -C H ), in low doses, exhibits the former behavior and is used primarily as an extradural agent in obstetrics. The lowest effective extradural concentration of etidocaine (21, X = CH, R = R = 2H, R = / -C H ), however, shows both adequate sensory and profound motor blockade so that it is useful in surgical situations where maximum neuromuscular blockade is necessary. In an isolated nerve preparation, bupivacaine blocks unmyelinated C fibers which are mainly responsible for pain perception at a much greater extent than the myelinated A fibers which carry motor impulses. It is postulated that absorption of bupivacaine by the vasculature at the site of injection, combined with the slow diffusion of this agent, results in an insufficient amount of the drug penetrating the large A fibers to cause motor conduction blockade. Clinically, motor block can be observed in some procedures. 

The gray matter is composed of nerve cell bodies and unmyelinated intemeuron fibers. The location of the gray matter in the spinal cord is opposite to that of the brain. In the brain, the gray matter of the cerebrum and the cerebellum is found externally forming a cortex, or covering, over the internally located white matter. In the spinal cord, the gray matter is found internally and is surrounded by the white matter. 

Walker JJ, Philips DE. 1987. An electron microscopic study of endrin induced alterations in unmyelinated fibers of mouse sciatic nerve. Neurotoxicology 8 55-64. 

Local anesthetics preferentially block small fibers because the distance over which such fibers can passively propagate an electrical impulse is shorter. During the onset of local anesthesia, when short sections of a nerve are blocked, the small-diameter fibers are the first to fail to conduct electrical impulses. For myelinated nerves, at least two and preferably three successive nodes of Ranvier must be blocked by the local anesthetic to halt impulse propagation. Therefore, myelinated nerves tend to become blocked before unmyelinated nerves of the same diameter. For this reason, the preganglionic fibers are blocked before the smaller unmyelinated C fibers involved in pain transmission. 

Tire anatomy of the brain is quite complex, and only a few terms will be defined here. Tire cerebrum, which is made up of two hemispheres, accounts for the largest part of the brain. Tire deeply folded outermost layer, the cerebral cortex, consists of gray matter, a mass of cell bodies, and fine unmyelinated nerve fibers. Beneath this lies a layer of white matter made up of myelin-covered axons connecting the cerebral cortex with other parts of the brain. Tire two cerebral hemispheres are connected by the corpus callosum, a band of 2 x 108 nerve fibers. Remarkably, these fibers can be completely severed with a relatively minimal disruption of the nervous system. In the past the corpus callosum was sometimes cut to control almost incessant epileptic seizures that could not be prevented by drugs. The "split-brain" patients suffered relatively little disability as long as both eyes functioned normally. Studies of these patients provided some insights into the differing functions of the two hemispheres of the cerebrum.395... 

The exact reason for the differential susceptibility of nerve fibers based on their axonal diameter is not known. One possible explanation is that the anesthetic is able to affect a critical length of the axon more quickly in unmyelinated fibers, or small myelinated neurons with nodes of Ranvier that are spaced closely together compared to larger fibers where the nodes are farther apart.17 As indicated earlier, a specific length of the axon must be affected by the anesthetic so that action potentials cannot be transmitted past the point of blockade. Other factors such as the firing rate of each axon or the position of the axon in the nerve bundle (e.g., in the outer part of the bundle versus buried toward the center of the nerve) may also affect susceptibility to local anesthesia.62 In any event, from a clinical perspective the smaller-diameter fibers appear to be affected first, although the exact reasons for this phenomenon remain to be determined. 

The Schwann cells that myelinate PNS behave differently. Once they reach their final destination, each Schwann cell surrounds several small-diameter axons in a polyaxonal pocket. As axons mature, the Schwann cell segregates a single axon from the polyaxonal pocket. Thus Schwann cells in peripheral nerves have two major phenotypes those that ensheath multiple axons (unmyelinated fibers) and those that myelinate single axons. The production of a basal lamina is a prerequisite for myelination. 

Local anesthetics are used in the clinical situation to interrupt the nociceptive process at one or more points between the peripheral, high-threshold nociceptor and the cerebral cortex, by blocking transduction by infiltration at the site of injury or incision by preventing transmission in afferent myelinated A 8 and unmyelinated C fibers by blockade of peripheral nerves or nerve plexuses or by epidural injection. 

A nerve consi.sts of myelinated or unmyelinated nerve fibers (Fig. 20-5a). These nerve fibers consist of chains" of neurons. The junction between adjacent neurons in the chain consists of the synaptic knob of the transmitting neuron separated by a gap of about 30 to 50 nm from either the dendrite, axon hilhx k. or cell body of the other neuron. This... 

Pain information is carried by the largely unmyelinated C fibers, while sharp pain is tramimitted by myelinated A3 fibers. The sen.sitivity of nerve fibers to local anesthetic appears to vary according to the size, anatomical type, and degree of conductance of the nerve fiber. In general, the... 

The nerve fibers in the cochlea can be classified as afferent fibers (toward the brain) or efferent fibers (toward the periphery). The afferent fibers that terminate on IHCs constitute approximately 90-95% of all afferent fibers in the cochlea. These afferent fibers originate from type I ganglion cells and are coated with a thick myelin sheath. Each fiber is connected to only one IHC, but each IHC is innervated by 2-10 individual afferent fibers. The principal neurotransmitter released by IHCs, glutamate, activates the afferent fibers in a quantal manner. The remaining 5-10% of afferent fibers are connected to OHCs and are unmyelinated. They originate from type II ganglion cells. Each afferent fiber is highly branched and is connected to 6-100 OHCs. OHCs are innervated mainly by efferent fibers, which are derived from the medial olivocochlear bundle. Efferent fibers release acetylcholine (ACh) as their principal neurotransmitter. Each OHC is in contact with 2-5 efferent synaptic boutons at its base. IHCs receive axodendritic efferent innervation onto afferent fibers from the lateral olivocochlear bundle (reviewed by Raphael and Altschuler, 2003). 

The term nociceptor refers to sensory receptors that respond to noxious stimuli (see Kruger et al. 2003 for review). A variety of cutaneous primary afferent nociceptors have been described, primary among them are the unmyelinated C fibers that are characterized by free nerve endings. The C-polymodal nociceptor responds to... 

Nociceptive transmission takes place in A5 and C-afferent nerve fibers. Stimulation of large-diameter, sparsely myelinated A5 fibers evokes sharp, well-localized pain, whereas stimulation of unmyelinated, small-diameter C fibers produces dull, aching, and poorly localized pain. These afferent, nociceptive pain fibers synapse in various layers (laminae) of the spinal cord s dorsal hom, releasing a... 

PD pathology is characterized by threadlike proteinaceous neurite inclusions and intracellular Lewy bodies (Fig. 14.1) occurring exclusively in unmyelinated fibers. In pathological stage I and II of the disease, first Lewy neurites, then Lewy bodies appear almost simultaneously in the olfactory bulb and the dorsal visceromotor nucleus of the vagal nerve. Degeneration in the olfactory bulb seems to be very slow. 

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