Subthreshold components of the cutaneous mechanoreceptive fields of dorsal horn neurons in the rat lumbar spinal cord

1. Intracellular recordings have been made from 76 neurons in the dorsal horn of the fourth and fifth lumbar segments of the spinal cord in decerebrate-spinal rats. The locations of the neurons were identified after horseradish peroxidase (HRP) ionophoresis (n = 18) or calculated from depth readings (n = 58). Sixty-nine of the neurons were found or estimated to lie within the deep dorsal horn (laminae III-V), with the remaining 7 in laminae I and II. 2. Background excitatory activity was present in all the neurons in the absence of peripheral mechanical stimuli. In 22 neurons, this consisted only of subthreshold excitatory postsynaptic potentials (EPSPs), but in 54, a proportion of the EPSPs reached threshold, producing a spontaneous spike discharge (frequency 0.2-50 Hz) that had a rhythmic component in six cells. Spontaneous hyperpolarizations occurred but were uncommon (n = 10). 3. All the neurons had excitatory cutaneous mechanoreceptive fields on the ipsilateral hindlimb. The receptive fields, defined in terms of action-potential discharge, could be subdivided into two areas: a high-probability "firing zone," where skin stimulation elicited an action-potential discharge above the mean + 1 SD of the background activity; and a low-probability firing fringe, where the stimulus elicited a distinct subthreshold depolarization, but the action-potential response fell within the variability of the background discharge. 4. Mechanical stimulation in the middle of the firing zone in all cells generated both supra- and subthreshold excitatory responses, with the former predominating. As the stimuli were applied progressively farther away from the center of the firing zone, the subthreshold component became relatively more prominent. 5. Fifty percent of the 15 neurons that were recorded from for sufficient time (greater than 30 min) to enable the presence, extent, and characteristics of subthreshold responses to be examined in detail were found to have a low-probability firing fringe to their receptive fields. The response to stimulation within this fringe typically consisted of high-frequency, low-amplitude PSPs riding on a sustained depolarization, with an action-potential discharge that could not readily be distinguished from the spontaneous activity. The size of the fringe ranged from a small area adjacent to the firing zone to almost the entire hindlimb. 6. The firing zones of 20 neurons were low-threshold only and in 5 cells were high-threshold only. The majority of neurons were multireceptive, responding both to low- and high-intensity stimuli (n = 51).(ABSTRACT TRUNCATED AT 400 WORDS)

Bench Scale Process Evaluation of Reburning for In-Furnace NOx Reduction

Reburning involves the injection of a secondary fuel above the main firing zone of pulverized coal-fired utility boilers to produce a reducing zone which acts to reduce NOx to molecular nitrogen. Overfire air is added above the reburn reducing zone to complete the combustion. Bench scale evaluations of the process carried out in a plug flow furnace at 23 KW have indicated that NOx reductions of up to 70 percent can be achieved depending on a number of process variables. The dominant variables include the initial NOx level that is to be reduced, the reburning fuel type (pulverized coal type or natural gas), and the residence time and temperature in the reducing zone. The reburning process has been combined with the injection of calcium-based sorbents (limestone) to investigate the potential for combined NOx and SOx reduction.