Glass-Coated Tungsten Microelectrodes

Multifunctional Freestanding Microprobes for Potential Biological Applications

Deep-level sensors for detecting the local temperatures of inner organs and tissues of an animal are rarely reported. In this paper, we present a method to fabricate multifunctional micro-probes with standard cleanroom procedures, using a piece of stainless-steel foil as the substrate. On each of the as-fabricated micro-probes, arrays of thermocouples made of Pd-Cr thin-film stripes with reliable thermal sensing functions were built, together with Pd electrode openings for detecting electrical signals. The as-fabricated sword-shaped freestanding microprobes with length up to 30 mm showed excellent mechanical strength and elastic properties when they were inserted into the brain and muscle tissues of live rats, as well as suitable electrochemical properties and, therefore, are promising for potential biological applications.

A simple method for reconditioning epoxy-coated microelectrodes for extracellular single neuron recording

Epoxy-insulated tungsten microelectrodes can be used once or twice in our lab before the impedance becomes too low. Dipping the electrodes in epoxy followed by curing restores their initial high impedance which is associated with good isolation of single neurons. It is a cost effective and simple procedure.

Contributions of intrinsic and synaptic activities to the generation of neuronal discharges in in vitro hippocampus

1. Extracellular and intracellular records were made from guinea-pig hippocampal slices to examine the contributions of intrinsic cellular properties and synaptic events to the generation of neuronal activity. Extracellular signals were filtered to pass action potentials, which could be detected within a distance of about 80 microm from a discharging cell. 2. Spontaneous action potentials were invariably detected in records from the stratum pyramidale of CA3 region. Blocking excitatory synaptic transmission with NBQX and APV reduced their frequency by 23 +/- 35 %. Suppressing synaptic inhibition, while excitation was already blocked, increased the rate of spike discharge to 177 +/- 71 % of its control value. 3. Most action potentials recorded intracellularly from CA3 pyramidal cells were initiated in the absence of a detectable synaptic event. In contrast, most action potentials generated by inhibitory cells located close to stratum pyramidale were preceded by an EPSP. 4. In 31 simultaneous recordings, intracellular pyramidal cell action potentials appeared consistently to initiate extracellular spikes with a mean latency of 2.2 +/- 1.0 ms. Single inhibitory cell action potentials could initiate a reduction in the frequency of extracellular spikes of duration 10-30 ms. 5. Some identified extracellular spikes (n = 9) consistently preceded intracellularly recorded IPSPs. IPSPs were initiated monosynaptically with latencies of 0.9-1.5 ms. In reciprocal interactions, single pyramidal cell action potentials could trigger the discharge of an identified unit that in turn appeared to initiate an IPSP in the same pyramidal cell. 6. These data suggest that intrinsic cellular mechanisms underly much of the spontaneous activity of pyramidal cells of the CA3 region of the hippocampus in vitro. Both synaptic inhibition and a strong excitation of inhibitory cells by pyramidal cells act to reduce population activity.

A simplified method for manufacturing glass-insulated metal microelectrodes

A simplified method to manufacture durable, glass-insulated, tungsten microelectrodes with sufficient control of the final electrode impedance is described. This method requires only two instruments, an electrolytic etcher for wires and pipette puller, for manufacturing these electrodes. The manufacture of these electrodes involves 3 steps: (1) etching tungsten wire to sharpen the tip, (2) insulating the electrode by pulling a glass pipette over the sharpened tungsten wire and (3) assessing and adjusting the tip exposure and impedance of the electrode to meet recording requirements. Control over the electrode impedance is easily accomplished by varying the distance between the uppermost portion of the heating coil and the sharpened wire tip before a glass pipette is pulled over the wire tip. This distance determines the area of tip exposure and also the location where the glass insulation ends and the exposed electrode tip begins. A performance test of these electrodes in a chronically prepared monkey showed that they were strong enough to repeatedly penetrate thickened dura mater without significant changes in impedance and to isolate cortical neuronal activity after these multiple penetrations. Furthermore, the strength of these microelectrodes eliminated the need to remove reactive granular tissue from the dura overlying the recording site.

A simple glass-coated, fire-polished tungsten electrode with conductance adjustment using hydrofluoridic acid

A method is described to produce glass-coated tungsten microelectrodes in 4 simple steps: (1) etching of the wire, (2) coating with glass, (3) fire-polishing, and (4) reopening with hydrofluoric acid to adjust the conductance to a final value. Continuous conductance control is provided during the reopening process by means of an admittance meter to guarantee an exact final adjustment of the conductance required. The complete process yields electrodes of high reliability within a few minutes and the quality of the electrodes remains largely unaffected by any of the manufacturing parameters involved, so that high-performance electrodes are produced without sophisticated procedures. The electrodes have been tested successfully over several years recording from cells in the striate visual pathway of the cat.

Organization of postcranial kinesthetic projections to the ventrobasal thalamus in raccoons

To determine the presence and organization of kinesthetic, as compared with other mechanosensory projection zones in the thalamus of raccoons, unit-cluster responses to mechanical stimulation of the postcranial body were mapped electrophysiologically in the thalami of 14 raccoons anesthetized with Dial-urethane. A distinct zone of kinesthetic projections (from receptive fields in muscles, tendons, and joints) was found in the rostral and dorsal aspects of the mechanosensory projection zone. These projections are somatotopically organized: those from axial structures lie dorsalmost and those from successively more distal limb regions are successively more caudoventral. The kinesthetic forelimb representation is large and lies rostrodorsal to a large central core of cutaneous projections from the forepaw digits. A few scattered kinesthetic projections were found at the caudal edge of the sensory thalamic region. The large, spatially and somatotopically distinct kinesthetic projection zone in the thalamus parallels those seen in the cortex and medulla of raccoons. Similar findings in monkeys, and suggestions from data in cats and humans support the hypothesis of a distinct pathway to the cortex for kinesthetic information in all mammals.

Fiber microelectrodes for electrophysiological recordings

Methods for the fabrication of tungsten-glass and platinum-rhodium-quartz fiber microelectrodes and of fiber pipettes are described and the electrical and mechanical properties of fiber electrodes are discussed. These properties (minimal tissue damage, good single unit isolation and temporal stability) make them particularly suited for multielectrode recordings from the central nervous system.

Neuronal organization of the rabbit cochlear nucleus: some anatomical and electrophysiological observations

The cochlear nucleus of the young adult rabbit was studied using both anatomical and electrophysiological techniques. The cytoarchitecture of the cochlear nucleus, as revealed by Nissl-staining and Bodian's Protargol method, was quite similar to that of the cat. The cell types observed by the Golgi-Cox method were very similar to those observed in the cat cochlear nucleus. Bushy and stellate cells were predominant in the anterior ventral cochlear nucleus, and multipolar and globular cells were observed throughout the posterior ventral cochlear nucleus, surrounding a region containing mainly octopus cells. The dorsal cochlear nucleus was revealed as a limited structure with a prominent band of fusiform cells. The polymorphic layers continued varieties of giant neurons similar to those observed in the cat. Units in the cochlear nucleus were classified electrophysiologically, in terms of their discharge pattern, and located histologically. The ventral cochlear nucleus contained mainly "primarylike," "chopper" and "onset" units, but the dorsal cochlear nucleus contained a greater variety of response patterns. "Buildup," "pauser" and "chopper" response patterns were predominant, and the dorsal cochlear nucleus also obtained more inhibitory units than the ventral cochlear nucleus. Both the types of units observed and their location in each nucleus were very similar to those reported for the cat. Both divisions of the ventral cochlear nucleus and the dorsal cochlear nucleus were found to be tonotopically organized in a dorsal (high frequency) to ventral (low frequency) direction. A trend for an organization in a medial to lateral direction was also apparent, particularly in the dorsal cochlear nucleus.

A simple method for glass insulating tungsten microelectrodes

A method for insulating tungsten wires with glass by collapsing a glass capillary tube around the wire is described. A common laboratory electrode puller can be used. A new method for removing the glass from the tip using hydrofluoric acid is also described.

The development of some peripheral and central auditory responses in the neonatal cat

The development of cochlear nerve action potential thresholds at different frequencies (AP audiograms) and inferior colliculus (IC) single unit thresholds and tuning was examined in barbiturate-anaesthetized kittens. AP thresholds decreased over the whole frequency spectrum during the first 5 weeks of life. Thresholds to high-frequency stimulation remained higher in 7-week-old animals than in adults. These results are in contrast to previous reports which have suggested that the AP response and gross cochlear anatomy are mature by the end of the second week. These differences may be due to the fact that the AP audiogram technique provides a measure of the activity of discrete regions along the cochlear partition. IC units in animals younger than 3.5 weeks had significantly elevated thresholds and broader tuning than those of the adult cat. Comparison of AP audiogram and IC unit thresholds in the adult revealed that these indices show similar frequency-dependent sensitivity. The slower maturation revealed by the AP audiogram may be due to the greater number and/or synchrony of cochlear nerve discharges needed to produce the gross AP. If this were the case, perception of suprathreshold sounds might not develop as quickly as thresholds for sound detection.

Microelectrode tip in five seconds. A new simple, rapid, inexpensive method

A method using a oxyacetylene torch flame is described; it makes it possible to obtain easily and rapidly, without skill or special equipment, a tungsten micro-electrode tip in a very few seconds.

Organization and development of brain stem auditory nuclei of the chicken: tonotopic organization of n. magnocellularis and n. laminaris

Extracellular recordings of responses to tone-burst stimulation were used to determine the tonotopic organization of n. magnocellularis (NM) and n. laminaris (NL) in hatching chickens. NM cells show "primary-like" response patterns to ipsilateral stimulation, and are arranged in dorso-ventral isofrequency columns. Units responding to the highest frequency tones (about 4,100 Hz) are situated at the rostromedial pole of the medial division. Units with lower characteristic frequencies (CF's) are found at successively caudal and lateral sites, until extremely low CF's ( less than 500 Hz) are represented dorsoventrally in the daudolateral tail of the lateral division. No evidence was found of auditory input to the region which receives projections from the macula lagena. NL receives polarized, binaural, excitatory input. Units have similar CF's and thresholds to tones presented to either ear. The tonotopic organization in NL matches that found in NM--high CF's rostromedially and low CF's caudal and lateral. Quantitative procedures were developed for relating CF to the position of a unit within either nucleus. These analyses account for 79% and 89% of the frequency variance found within NM and NL, respectively, and predict the CF of a neuron by its position within each nucleus.

Species differences in mechanosensory projections from the mouth to the ventrobasal thalamus

To determine whether the largely ipsilateral, inverted representation of mouth parts in the ventrobasal thalamus of sheep was unique to that species or an expansion of a general mammalian pattern, the corresponding thalamic projections were mapped electrophysiologically in a selected series of mammals (oppossums, agoutis, squirrel monkeys, cats, raccoons, and sheep) representing major branches of evolution among therian mammals. In mapping, tungsten microelectrodes were used to record multi-unit discharges in the thalamus in response to mechanical stimulation of oral surfaces. The pattern of projections seen in sheep is not a general mammalian pattern; there is extensive variability among mammals in the laterality and internal orgainzation of the projections from the mouth. In spite of the great variability, the results suggest an hypothesis concerning phylogenetic trends: descendants of palaeoryctoid insectivores (cats, raccoons, and sheep in our sample) have extensive ipsilateral projections from the mouth, in other therian mammals (opossums, agoutis, and squirrel monkeys in our sample) the ipsilateral component is small or absent.

Unit Responses from Commissural Fibers of Optic Lobes of Fish

The paired optic lobes of teleost fish are connected by two commissures. One of these, the tectal commissure, was studied with metal microelectrodes. Fibers are rhythmically active for prolonged periods in the dark and respond to light by a decrease in the rate of discharge. There is a rebound acceleration when the light is turned of. Each fiber is influenced by light in one eye only, and there is no response when light is projected into the opposite eye. This behavior resembles the "off" response recorded from the optic lobes and the optic nerve of fish. Unlike most units from the visual pathways of lower animals, single commissural fibers do not seem to give any recognizable response to patterned input such as small light or dark objects or small light sources stationary or moving anywhere in the visual field, nor do they respond to a vertical black bar moved over a white background.