Permeability of nerve fibres

It has been reported that cholinesterase inhibitors (such as di-isopropyl phosphorofluoridate) increase the permeabihty of squid giant axons towards sodium and potassium. There is also an indication that the erythrocyte requires, among other factors, an adequate acetylchohne-chohnesterase system to prevent a gain of sodium or a loss of potassium. The conclusion that permeabihty is dependent on cholinesterase activity, however, seems to be contested by Strickland and Thompson.  

In addition, they noted that in tissue cultures the growth of the malignant cells (lymphoblasts) of a mouse lymphosarcoma is inhibited by E 600 in a concentration which inhibits the esterase 

These results may well provide a basis for a new approach towards a rational chemotherapy of tuberculosis and neoplastic disease. 

Francis has successfully used D.F.P. in the histological locahzation of cholinesterases in the retina of the eye. 

It may be noted that the retina represents an outlying portion of the brain itself. It arises as a protrusion from the prosencephalon (anterior portion of the cerebrum), and, being a constituent part of the brain, presents the same anatomical and physiological problems as does the central nervous system elsewhere. The more or less complete spatial separation of the synapses from the cell bodies makes the retina eminently suitable for finding out whether a known biochemical constituent is located in the cell bodies, axons, dendrites or at the synapses, and the information so obtained may perhaps be applicable to other parts of the nervous system, where the cell bodies and the synapses are all too intimately mingled for a proper analysis. 

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Hodgkin, A.L. Keynes, R.D. (1955b). The potassium permeability of a giant nerve fibre. J. Physiol. Lond. 128,61-88. 

In addition to stimulating afferent nerve fibre endings in mucosal surfaces and producing a typical PCSI effect, OC also induces the release of tachykinins or neuropeptides like substance P and neurokinin A. This induces neurogenic inflammation of the airway blood vessels, epithelial cells, glands and smooth muscle, leading to vasodilation, increased vascular permeability, neutrophil chemotaxis, mucus secretion and bronchospasm (Smith and Stopford, 1999). 

The most fundamental studies on their mode of action have been those of Narahashi who worked with giant fibre preparations. He proposed that pyrethroids modify axonal conduction within the central nervous system of insects by altering the permeability of the nerve membrane to sodium and potassium ions (Narahashi, 1965 1974 1976 Burt and Goodchild, 1977 Clements and May, 1977). 

Similar voltage-clamp studies have been carried out in myehnated nerve fibres and the permeability sequence determined is given in Table 3 [23]. It should be pointed out here that Hille extended the series of cations to include some organic species. It was pointed out that in the nodal membrane of myelinated nerve fibres the permeabilities to Cs" ", Rb" ", Ca " and were too small to measure. 

The muscle end-plate is thus a chemically excitable membrane, and stands in contrast to the electrically excitable membrane which covers the bulk of every nerve fibre as described above. The conductance (permeability to ions) of a chemically excitable membrane is changed only by the specific chemical messenger (synaptic transmitter). These changes in ionic conductance produce membrane potential changes that are proportional to the concentration of the transmitter. 

Thi ,DPhe BK [Thi ,DPhe ]-bradykinin. bradykinin-potentiating peptide teprotide. bradykinin potentiator B teprotide. BRADYKININ RECEPTOR AGONISTS act at sites recognizing members and derivatives of the bradykinin family of hormone peptides - kinins - of which bradykinin (BK) and kallidin (lysyl-bradykinin Lys-BK KD) are the main mammalian members. The bradykinin family is distinct from the tachykinin family of peptides, though both have profound hypotensive actions and contract many intestinal and other smooth muscles. Historically, it was noted that the former action was relatively slow-developing, hence the name bradykinin. Notable actions of bradykinin and kallidin are to dilate blood vessels and increase their permeability to plasma proteins, and to stimulate sensory nerve C-fibres. These actions are pro-inflammatory, and reflect the fact that the kinin-formation system is activated in inflammation, and enzymes (kallikreins) form the kinins from blood-borne or tissue precursors (kininogens) on injurious insult. 

The basic structural unit of skeletal muscle is a muscle fibre composed of long, cylindrical cells of 10-100 xm in diameter and lengths of up to several tens of mm (usually 20-30 mm). Each muscle fibre is actually a muscle cell containing 100-200 nuclei and normal cell organelles. The muscle fibre is covered by a thin, extensible, semi-permeable membrane, the sarcolemma (or myolemma), which transmits electrical impulses from the nerves. It consists of a cell membrane (plasma membrane) and an outer coat made up of a thin layer of polysaccharide material with numerous thin collagen fibrils. The sarcoplasm of a muscle fibre is comparable to the gel-like cytoplasm of other cells. 

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