Nerve fibres

M.p. 103°C. Noradrenaline is released in the adrenal medulla with adrenaline, and also at the sympathetic nerve endings. Its release from a nerve fibre is followed by binding to a receptor molecule on the next nerve or muscle fibre, probably causing a change in the electrical charge of the receptor-cell membrane. Biosynthetically it normally serves as a precursor for adrenaline. 

C27H45OH, crystallising in the form of acicular crystals and found in all animal fats and oils, in bile, blood, brain tissue, milk, yolk of egg, the medullated sheaths of nerve fibres, the liver, kidneys and adrenal glands. 

The axonal membrane is a lipid bilayer in the nerve fibre. Ionic channels and other proteins are located in the membrane to achieve electrical activity. Action potentials are generated and conducted along the membrane. 

The putative binding site for local anaesthetic molecules at the sodium channel has been identified as two amino acids in the sixth membrane-spanning segment of domain IV [2]. This binding site is located directly underneath the channel pore and can only be reached from the internal side of the membrane. Because local anaesthetics are applied exterior to the nerve fibre, they have to penetrate the axonal membrane before they can bind to the channel. 

A substance which is released at the end of a nerve fibre by the arrival of a nerve impulse and by diffusing across the synapse or junction effects the transfer of the impulse to another nerve fibre (or muscle fibre or some receptor). 

In smooth muscle, by contrast, one sympathetic nerve fibre can influence a number of muscle fibres by releasing noradrenaline from varicosities along its length without there being any defined end-plate junctions. The result of receptor activation is a slow change in potential and inactivation of the NT is initially by uptake and then metabolism. In other words, the NT function is geared to the slower phasic changes in tone characteristic of smooth muscle. 

The afferent fibres differ in their conduction velocity and degree of myelination, and can be distinguished by their diameter. The large diameter A S-fibres are myelinated by Schwann cells and hence have a fast conduction velocity. This group of nerve fibres innervates receptors in the dermis and is involved in the transmission of low-threshold, non-noxious information, such as touch. The A5-fibre is less densely myelinated and conveys both non-noxious and noxious sensory information. The unmyelinated C-fibre conveys high-threshold noxious inputs and has the slowest conduction velocity of all three fibre types. 

In addition to changes within the nerve, sympathetic afferents become able to activate sensory afferents via as yet poorly characterised a-adrenoceptors. These interactions between adjacent sensory and autonomic nerve axons and between ganglion cells result in excitation spreading between different nerve fibres. These peripheral ectopic impulses can cause spontaneous pain and prime the spinal cord to exhibit enhanced evoked responses to stimuli, which themselves have greater effects due to increased sensitivity of the peripheral nerves. 

In diabetic rats, TRPVl is enhanced on myelinated fibres and is hyperphos-phorylated by PKC [127]. In accordance with these findings, anti-TRPVl antiserum was shown to ameliorate pain in a murine model of diabetic neuropathy [128]. In humans, the density of TRPVl-positive nerve fibres is increased in women with chronic breast pain [129] and with vulvodynia [19]. Disruption of TRPV 1 gene causes attenuation of bone cancer pain in mice [130]. Pharmacological blockade of TRPVl by agonists relieved pain in AIDS patients [131]. 

An additional and widespread neuroactive (transmitter-like) compound is nitric oxide (NO). This gaseous secretion is a product of the action of the enzyme NO-synthase on arginine. It is implicated in at least two roles within the non-sensory tissues of the organ, and at particular synapses in the AOB. One nitric oxidergic effect is initiated by the nerve fibres supplying the smooth muscle component of the vasomotor tissues. The other effect is the expected action of NO on the output... 

Fig. 6.18 Experimental arrangement for measurement of the membrane potential of a nerve fibre (axon) excited by means of current pulses (1) excitation electrodes, (2) potential probes. (According to B. 

Corpus callosum A bridge of nerve fibres between the two cerebral hemispheres. 

It has been known for some time that the enteric nervous system does not simply regulate smooth muscle contraction, but is intimately involved in the control of transport processes in enterocytes. Nerve fibres in the mucosa terminate subjacent to the basement membrane of epithelial and entero-endocrine cells, on which muscarinic acetylcholine receptors (mAChRs)... 

Tetzlaff, W. Tight junction contact events and temporary gap junctions in the sciatic nerve fibres of the chicken during Wallerian degeneration and subsequent regeneration. /. Neurocytol. 11 839-858,1982. 

Forshaw PJ, Ray DE (1990) A novel action of deltamethrin on membrane resistance in mammalian skeletal muscle and non-myelinated nerve fibres. Neuropharmacology 29 71-81... 

The brain does not send a continuous current through the nerve, but short spurts . We call them impulses, which transfer between nerve fibres within the synapses of cells (see Figure 7.16). The cell floats within an ionic solution called plasma. The membrane separating the synapse from the solution with which the nerve fibre is in contact surrounding the cell is the axon, and is essential to the nerve s operation. 

Mercuric chloride may induce catecholamine release from adrenals. The initial phase may be due to amine displacement by the mercury ion but the secondary phase probably involves alteration of membrane structures [95]. Mercury compounds have also been shown to increase the efflux of monoamines from mouse striated slices [96] and from adrenergic nerve fibre terminals [97], the effect being attributed to inhibition of Na /K+-ATPase activity and(or) disruption of intracellular Ca2+ regulatory mechanisms [96]. 

Evans EF, Klinke R. 1982. The effects of intraocochlear cyanide and tetrodotoxin on the properties of single cochlear nerve fibres in the cat. J Physiol 331 385-408. 

On the surface of a resting living excitable cell such as muscle or nerve fibre there is a potential of the order of 100 mV., i.e. the outside is electropositive compared with the inside. Thus the surface of the intact unstimulated nerve is everywhere equi-potential (fig. 1 a). It may be worth while for a moment to try... 

Fig. 1. Nerve fibre, (a) Before stimulation, (b) Initial electrical disturbance at point of stimulation, (c) The passage of electrical disturbance is shown in both directions. In the body, however, the fibres are stimulated at only one end and hence conduction is in only one direction.

We must now consider what happens when a nerve impulse reaches the end of a nerve fibre. The single nerve cell, together... 

In Chapter in we drew attention to the fact that some nerve fibres are myelinated. It has been suggested that the pseudocholinesterase of the central nervous system may be concerned in myelin metabolism and that inhibition of pseudo-cholinesterase... 

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