<unk>  is based on tungsten needle microelectrodes which are inserted through the skin and into a nerve. Anaesthetics are not required because the procedure induces only minimal discomfort. The tungsten microelectrodes have a shaft diameter of 100-200 μm, a tip diameter of 1-5 μm, and they are insulated to the tip with an epoxy resin. Electrode impedance varies between 0.3 and 5 MΩ at 1 kHz as measured initially. However, the impedance tends to decrease during experiment and is usually below 1 MΩ while impulses are recorded. Nerve discharges are determined by voltage differences between the intra-neural electrode and a reference needle electrode in the vicinity. The 2 electrodes are connected to a differential amplifier with a high input impedance and an appropriate band-pass filtering, often 500 to 5000 Hz. Signals are monitored on a computer screen and stored on a hard disc for off-line analysis. Any peripheral nerve that can be reached may be a target for<unk>  recordings, typically in the arm or leg, although recording from facial nerves and the vagus nerve have also been achieved. In order to locate deep nerves, electrical stimulation through a needle electrode or ultrasonic monitoring is often used. This is rarely needed when recording from cutaneous superficial nerves, such as the superficial peroneal or superficial radial nerves, that can easily be located visually and by palpation. When electrical localization is needed, weak electrical shocks are delivered either through the recording electrode or through a separate stimulation needle while adjusting the electrode tip until a neural response is observed, either a muscle twitch or a cutaneous sensation reported by the subject. In ultrasonic monitoring, a linear, high frequency ultrasound probe is used. The microelectrode is then inserted 1–2 cm from the probe, ideally in a 90° angle to the ultrasonic beam. This generates the best wave reflection and image. Ultrasonic approach accurately locates the depth of the nerve and identifies surrounding anatomical structures of interest, such as blood vessels and bony structures, which may affect the placement of a microelectrode. A particular advantage is that the ultrasonic approach visualizes the electrode and the nerve at the same time, thereby facilitating electrode manipulation to reach the nerve. Once the electrode tip is in the nerve, small adjustments are required, first, to penetrate the sheath of an individual fascicle and, second, to take the tip close to the nerve fibers of the kind you are interested to explore, be it multi-unit sympathetic activity or single unit activity of either a myelinated afferent or a small unmyelinated fibres. The marking technique Recording of single afferent impulses from C-fibers was greatly improved by the development of the so-called 'marking technique'. This technique is based on a unique property of many kinds of C-fibres, i.e. a decrease of conduction velocity in the wake of preceding impulses. By combining repetitive electrical stimulation and physical stimulation, e.g. mechanical or thermal stimuli, units responding to the physical stimulation will display sudden slowing of their conduction velocity which can be easily visualized in raster plots of latency. This allows identification and characterization of nerve fiber responsiveness to natural stimulation. The marking technique is very efficient as it allows simultaneous recordings of several fibers. However, it generates only semi-quantitative information about unitary activity, whereas recordings of impulse trains allow more comprehensive description of functional properties of sense organs. Micro-stimulation The<unk>  electrode may be used not only for recording of nerve impulses but for stimulation of individual fibers as well. An interesting application is to combine successive recording and stimulation of the same afferent. Once the functional properties of an afferent have been defined, e.g. with regard to sensitivity, receptive field structure, and adaptation, the electrode may be reconnected to a stimulator to give trains of electrical pulses of controlled strength, rate, and duration. It has been found that the percept elicited from a single tactile afferent in the glabrous skin of the hand, may be remarkably detailed and closely matching the properties of the afferent, indicating a high degree of specificity. Although this approach to bridge the gap between biophysical events in a single afferent and mental phenomena within the mind is simple and straight forward in principle it is demanding in practice for a number of reasons. Micro-stimulation has also been used to characterize individual motor units with regard to contraction properties.
Microneurography
Electropalatography
Urinometer
Rhinomanometry
Doppler optical coherence tomography
Pyelogram
Videokymography
Microperimetry