Student Number : 9803381J - MSc Dissertation - Faculty of Engineering and the Built Environment / Although pulsed power has been used in many parts of the world over the last few decades to initiate high-voltage discharges through rock, no systematic test work on South African ore bodies and related rock types has been done so far. As part of CSIR Miningtek’s integrated approach of combining underground comminution with a novel Tore© hydrotransport system, which has been shown to operate well with coarse particles up to 10 mm, various rock types were fragmented in single discharge mode under laboratory conditions. The work was conducted at the University of the Witwatersrand’s high-voltage laboratory with a custom-designed test rig. The rig configuration was based on a critical review and analysis of the literature and on assessments of existing test facilities elsewhere. Core samples with diameters ranging from 16 to 48 mm were cut from test specimens with thicknesses ranging from 8 to 48 mm. Rock types included Ventersdorp Contact Reef, Carbon Leader, Elsburg Formation, UG2 and Merensky, as well as pure quartz, shales, lava and dykes. A six-stage Marx generator provided a voltage rise time of 2 000 kV/μs to create a discharge through the rock, in preference to a discharge through the surrounding water, which acts as an insulator at ramp-up times faster than 0,5 μs. High-speed photography, and an analysis of the voltage and current signals for various rock types and for water alone, were used to quantify the potential benefits of rock breaking by electric discharge. It was found that some Kimberlite specimens and mineralised gold-bearing reefs were much easier to fragment than hanging wall or footwall material. Merensky reef appeared to be more susceptible than the less brittle UG2 material. A correlation was derived between the dynamic resistivity of various rock types, measured at 16 MHz excitation frequency, and the electrical breakdown strength at which discharge took place. The fragments created had a more cubical shape than would be created by conventional impact crushing. However, the high voltage requirements of about 30 to 35 kV per millimetre of rock thickness would necessitate not only efficient mechanical and electrical contact between the electrodes and the rock, but also considerable safety features for underground installations. The clearly identified, preferential fracturing of reef rock types, compared with the hanging or footwall materials, suggests that the greater benefit of electric rock breaking may lie in primary rock breaking as a mining method, rather than in secondary comminution of broken rock to enable hydraulic transportation by pipeline to surface.

The overall aim of the present thesis is to clarify the control of intracellular chloride homeostasis in central neurons, because of the critical role of chloride ions (Cl–) for neuronal function. Normal function of the central nervous system (CNS) depends on a delicate balance between neuronal excitation and inhibition. Inhibition is, in the adult brain, most often mediated by the neurotransmitter γ-aminobutyric acid (GABA). GABA may, however, in some cases cause excitation. GABA acts by activating GABA type A receptors (GABAARs), which are ion channels largely permeable to Cl–. The effect of GABAAR-mediated neuronal signaling - inhibitory or excitatory - is therefore mainly determined by the Cl– gradient across the membrane. This gradient varies with neuronal activity and may be altered in pathological conditions. Thus, understanding Cl– regulation is important to comprehend neuronal function. This thesis is an attempt to clarify several unknown aspects of neuronal Cl– regulation. For such clarification, a sufficiently sensitive method for measuring the intracellular Cl– concentration, [Cl–]i, is necessary. In the first study of this thesis, we examined two electrophysiological methods commonly used to estimate [Cl–]i. Both methods, here called the interpolation and the voltage-ramp method, depend on an estimate of the Cl– equilibrium potential from the current-voltage relation of GABA- or glycine-evoked Cl– currents. Both methods also provide an estimate of the membrane Cl– conductance, gCl. With a combination of computational and electrophysiological techniques, we showed that the most common (interpolation) method failed to detect changes in [Cl–]i and gCl during prolonged GABA application, whereas the voltage-ramp method accurately detected such changes. Our analysis also provided an explanation as to why the two methods differ. In a second study, we clarified the role of the extracellular matrix (ECM) for the distribution of Cl– across the cell membrane of neurons from rat brain. It was recently proposed that immobile charges located within the ECM, rather than as previously thought cation-chloride transporter proteins, determine the low [Cl–]i which is critical to GABAAR-mediated inhibition. By using electrophysiological techniques to measure [Cl–]i, we showed that digestion of the ECM decreases the expression and function of the neuron-specific K+ Cl– cotransporter 2 (KCC2), which normally extrudes Cl- from the neuron, thus causing an increase in resting [Cl–]i. As a result of ECM degradation, the action of GABA may be transformed from inhibitory to excitatory. In a third study, we developed a method for quantifying the largely unknown resting Cl– (leak) conductance, gCl, and examined the role of gCl for the neuronal Cl– homeostasis. In isolated preoptic neurons from rat, resting gCl was about 6 % of total resting conductance, to a major part due to spontaneously open GABAARs and played an important role for recovery after a high Cl– load. We also showed that spontaneous, impulse-independent GABA release can significantly enhance recovery when the GABA responses are potentiated by the neurosteroid allopregnanolone. In a final commentary, we formulated the mathematical relation between Cl– conductance, KCC2-mediated Cl– extrusion capacity and steady-state [Cl–]i. In summary, the present thesis (i) clarifies how well common electrophysiological methods describe [Cl–]i and gCl, (ii) provides a novel method for quantifying gCl in cell membranes and (iii) clarifies the roles of the ECM, ion channels and ion transporters in the control of [Cl–]i homeostasis and GABAAR-mediated signaling in central neurons.