Fire action mechanisms

The situation improved with the introduction of the wheel-lock (Fig 1), which produced its own fire by mechanical action, in the early 16th century. This name was used because of the prominent wheel which was rotated to produce fire. In addition to the wheel, the mechanism consisted essentially of the cock, the mainspring, and the sear. The wheel was serrated to provide a rough surface. The cock was a clamp which held a small piece of iron pyrites. Iron pyrites is a mineral which, when rubbed against a roughened piece of metal, will produce sparks... 

Their fire-retardant mechanism is predominantly due to condensed phase action involving a combination of endothermic decomposition, water release, and oxide residue formation. 

Expandable interlayers provide an alternative way for rapid delivery. Migration, even if promoted by an incompatible interlayer, is sometimes not rapid enough to deliver active substances to the surface therefore, a new adaptive concept has been developed recently [53,54], A material utilizing this mechanism contains a phosphorylated polyol (PPOL) interlayer of relatively low decomposition temperature intercalated between layers of nanoparticle. At the early stage of fire action the gaseous degradation products of the interlayer separate the nanolayers and drive them to the surface in the most rapid way. 

A low hydrogen ion concentration occurs as a result of hyperventilation, in which carbon dioxide is blown off in the lungs. The condition occurs in certain otherwise normal people who hyperventilate as a reaction to stress. A frequent result is a reduced concentration of ionized calcium in the extracellular fluid, the condition of hypocalcaemia. The effect is hypocalcaemic tetany, which consists of involuntary uncoordinated contractions of skeletal muscles and of bizarre subjective sensations and numbness. These symptoms and signs are due to hyperexcitability of peripheral nerve fibres, which fire action potentials spontaneously. The mechanism is that the reduced hydrogen ion concentration results in dissociation of complexes between plasma albumin and hydrogen ions, with a binding of plasma calcium to the albumin sites which are vacated by the hydrogen ions. Hence there is a fall in the concentration of ionized calcium in the extracellular fluid and this results in the nervous hyperexcitability. 

The major inhibitory neurotransmitter in the cerebral cortex is y-aminobutyric acid (GABA). It attaches to neuronal membranes and opens chloride channels. When chloride flows into the neuron, it becomes hyperpolarized and less excitable. This mechanism is probably critical for shutting off seizure activity by controlling the excessive neuronal firing. Some antiepileptic drugs, primarily barbiturates and benzodiazepines, work by enhancing the action of GABA. 

Injured axons may develop spontaneous and repetitive firing known as ectopic activity. Injured primary afferents develop spontaneous action potential discharges that occur uncoupled from receptor stimulation in the periphery (Fig. 57-5). These ectopic discharges can be evoked by protons, cytokines or mechanical stimulation of the injured nerve. Another source of ectopic activity are pacemaker currents produced in the plasma membrane... 

Indirect mechanisms Nicotine has indirect effects on monoamine systems. A considerable amount of research has examined the relationships between nicotine and dopamine activity in the brain, in light of dopamine s role in reinforcement and nicotine s addictive properties. Nicotine increases dopamine turnover in the striatum and cerebral cortex (Clarke and Reuben 1996 Tani et al. 1997 Nanri et al. 1998). It also increases burst activity in dopamine neurons of the ventral tegmental area (VTA), a primary source of dopamine to the forebrain (Nisell et al. 1995 Fisher et al. 1998). Such a firing pattern in the VTA is associated with processes of reinforcement, learning, and cognitive activity. Nicotine actions on dopaminergic neurons occur at both somatodendritic sites and synaptic terminals. Further, both systemic nicotine and direct administration into the VTA increase dopamine release in the nucleus ac-... 

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