Progress in the area of MHD stability and disruptions, since the publication of the 1999 ITER Physics Basis document Nucl. Fusion 39 2137-2664, is reviewed. Recent theoretical and experimental research has made important advances in both understanding and control of MHD stability in tokamak plasmas. Sawteeth are anticipated in the ITER baseline ELMy H-mode scenario, but the tools exist to avoid or control them through localized current drive or fast ion generation. Active control of other MHD instabilities will most likely be also required in ITER. Extrapolation from existing experiments indicates that stabilization of neoclassical tearing modes by highly localized feedback-controlled current drive should be possible in ITER. Resistive wall modes are a key issue for S128 Chapter 3: MHD stability, operational limits and disruptions advanced scenarios, but again, existing experiments indicate that these modes can be stabilized by a combination of plasma rotation and direct feedback control with non-axisymmetric coils. Reduction of error fields is a requirement for avoiding non-rotating magnetic island formation and for maintaining plasma rotation to help stabilize resistive wall modes. Recent experiments have shown the feasibility of reducing error fields to an acceptable level by means of non-axisymmetric coils, possibly controlled by feedback. The MHD stability limits associated with advanced scenarios are becoming well understood theoretically, and can be extended by tailoring of the pressure and current density profiles as well as by other techniques mentioned here. There have been significant advances also in the control of disruptions, most notably by injection of massive quantities of gas, leading to reduced halo current fractions and a larger fraction of the total thermal and magnetic energy dissipated by radiation. These advances in disruption control are supported by the development of means to predict impending disruption, most notably using neural networks. In addition to these advances in means to control or ameliorate the consequences of MHD instabilities, there has been significant progress in improving physics understanding and modelling. This progress has been in areas including the mechanisms governing NTM growth and seeding, in understanding the damping controlling RWM stability and in modelling RWM feedback schemes. For disruptions there has been continued progress on the instability mechanisms that underlie various classes of disruption, on the detailed modelling of halo currents and forces and in refining predictions of quench rates and disruption power loads. Overall the studies reviewed in this chapter demonstrate that MHD instabilities can be controlled, avoided or ameliorated to the extent that they should not compromise ITER operation, though they will necessarily impose a range of constraints.

SUMMARY The predilection sites of cerebrovascular lesions (cerebral hemorrhage and/or softening) were studied in 1,278 strokeprone spontaneously hypertensive rats (SHRSP). The precise supply to the main cerebral arteries was determined by trypan blue injections and microangiography. The three major territories were the anteromedial cortex, the occipital cortex, and the basal ganglia. A common angioarchitectural characteristic of these three areas was the blood supply through "recurrent branching" from the main stream. In the basal ganglia, where there is a preponderance of lesions, the arteries responsible for these lesions belonged to the lateral group of lenticulostriate arteries. The primary pre-stroke arterial lesions were further studied microangiographically in SHRSP killed at the time the initial symptoms of stroke were detected. These points were located at the "boundary zone" of the main cerebral arteries. Our findings indicated the importance of these two angioarchitectural minor loci as the basis for functional or organic circulatory disturbances that may cause stroke. Since these local factors of stroke are common in the cortex and basal ganglia of rats and basal ganglia of humans, these SHRSP are regarded as good pathogenetic models for studies on stroke in humans.

There is now much evidence that the mechanical excitation of certain sensory receptors sets up a potential change in the receptor and that this potential change initiates impulses. The microphonic potential of the ear has been known for a long time, but recently its relation to impulse initiation has been clarified by Davis, Tasaki & Goldstein (1952). This situation, however, differs from that at touch and stretch receptors, because the microphonic potential is generated outside the sensory axon, while it is probable that the potential changes that follow mechanical stimulation in peripheral receptors are generated across the axonal membrane. Gray & Malcolm (1950) showed that during the interval between a mechanical pulse and the initiation of an impulse at a Pacinian corpuscle the excitability of the receptor rose steadily, and that if no impulse materialized the excitability then fell again; it was concluded that the time course of this excitability change represented the time course of a potential change at the axon terminal. Katz (1950) demonstrated a potential change occurring in muscle spindles during stretch, which he showed was responsible for the initiation of impulses and called the 'spindle potential'. Recently, Alvarez-Buylla & Ramirez de Arellano (1953) observed a potential in Pacinian corpuscles, which is clearly the immediate precursor of the impulse.These potential changes in receptors are the earliest known signs of activity in the process of impulse initiation and further knowledge of the fundamental properties of receptor mechanisms demands a greater knowledge of their properties. These experiments were carried out on the Pacinian corpuscle for a number of reasons; the most important being that it seemed practicable to prepare a single receptor in such a way that records of the current flow along the last internode could be obtained. Such a technique can clearly distinguish potentials generated in the short (c. 0 5 mm) non-myelinated terminal from those in the rest of the axon. It is also an advantage to have a stable preparation. Pacinian corpuscles, although extremely sensitive, are not spontaneously * British Council Scholar.

SUMMARY1. Impulse discharges in single chorda tympani fibres of rats and hamsters in response to gustatory stimuli representing the four basic qualities of taste as well as to cooling and warming of the tongue were recorded.2. In both rats and hamsters many fibres responded to more than one kind of gustatory stimulus as well as to thermal ones. The thermal sensitivity of these fibres was found to be about -05 impulses/sec.°C for cooling and 0 5 impulses/sec .°C for warming.3. Statistical calculations of the frequency of responses of chorda tympani fibres to single gustatory stimuli or combinations of stimuli were made, assuming that responsiveness to any one stimulus is independent of that to other stimuli. Evidence for relatively specific sensitivity to a particular stimulus or a particular combination of stimuli was obtained in the rat and the hamster.4. Across-fibre correlation coefficients between the amounts of responses to a pair of stimuli were calculated. In both the rat and the hamster, significant positive correlations were obtained with HCI, quinine and cooling. In addition, a positive correlation between sucrose and warming and a negative correlation between sucrose and NaCl were obtained in the hamster. The results are discussed in relation to the interaction of gustatory and thermal sensations of the human tongue.

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