262 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



under study or by restricting the sensitivity of the 

 recording device to the electrical activity of that unit 

 alone. 



Single Fiber Isolatian 



Bv carefully dissecting trunks of peripheral nerve 

 fibers or spinal roots into smaller and smaller bun- 

 dles, it is often possible to reach a size of filament 

 within which only a few or even only one nerve fiber 

 remains functional (see Chapter III). Such a fiber 

 can then be stimulated or its action potential recorded 

 in isolation. This technique has been used to study 

 the patterns of activity of indi\"idual motoneurons in 

 response to various types of excitation and inhibition 

 (i, 6, 21, 31, 37, 38, 44). EssentialK' the same tech- 

 nique has been used by Fessard & Matthews (24), by 

 Kato et al. (40) and by others to limit afferent im- 

 pulses to those carried by a single fiber. Such single 

 unit aff^erent impulses have been shown to produce 

 long-lasting potential changes on nearby afferent 

 fibers (the dorsal root potential) and reflex excitation, 

 provided other excitatory pathways had pre\iously 

 brought the reflex nearly to threshold. The same tech- 

 nique has enabled Adrian & Zotterman (2) and 

 Cohen et al. (13) to study activity patterns of single 

 sensory receptors. Finally, Fatt (22) has similarly iso- 

 lated a single motor fiber from its peripheral nerve. A 

 nerve impulse was conducted antidromically by this 

 fiber, and its invasion of the motoneuron cell body 

 and dendrites was followed by mapping the electrical 

 potential field produced in the surrounding volume 

 conductor of the spinal cord. 



MiCROELECTRODES. In the experiment just described, 

 isolation of the unit was achie\-ed b\' the i^eripheral 

 nerve dissection which permitted only one fiber to be 

 stimulated. An alternative method of isolating 

 responses from single units is to use a microelectrode 

 which is small enough to be selectively sensitive to the 

 activity of a single cell. With such a method it is not 

 necessary to interfere with the patterns of activity of 

 other cells in the nervous system since the potentials 

 they generate in the microelectrode are small com- 

 pared with the signals being studied. This method of 

 isolation requires that the microelectrode be of small 

 enough dimensions to permit it to be placed closer to 

 the unit to be studied than to other acti\e units. A 

 wire or needle sharpened to a diameter of about o.oi 

 mm (10 m) and insulated except at its tip satisfies this 

 requirement for many nerve cells in the spinal cord 

 and brain (7). Much larger electrodes (50 to too ^) 

 appear to damage individual units (50) while still far 



enough away from them so that their potential fields 

 are masked by the background activity of other cells. 

 Smaller metal recording electrodes require special 

 techniques. A number of these ha\e Ijeen de\Tloped 

 and are described below. 



MET.AL ELECTRODES. .Svactichin (52) dcscribes a tech- 

 nique for filling fine glass pipettes with silver solder 

 (fig. i.-l), thus providing a very small metallic record- 

 ing surface down to less than i /i, well insulated by a 

 smooth tapering glass shaft. The tips of these electrodes 

 are plated with rhodium and then coated with 

 platinum black to prevent oxidation and to increase 

 the surface area. 



Howlancl et al. (35) has also used a glass insulated 

 metal wire prepared by drawing a glass pipette down 

 onto a I 2 ;u nichrome wire which had previously been 

 passed through the tube. While single unit acti\ity 

 has been recorded with these electrodes in the cat's 

 spinal cord, they are not very satisfactory for this 

 piu"pose and ha\e been u.sed mostly to record the 

 composite responses of fiber tracts and cell groups. 



Dowben & Rose (16^ have devised a inore satis- 

 factory metal microelectrode which they have used 

 with consideraljle success in studving unitary activity 

 of the thalamus. These workers have made use of the 

 low melting point of the metal indium which permits 

 them to fill predrawn glass pipettes of 3 to 5 ^i tip 

 diameter with the metal (fig. iB). The metal surface 

 at the tip is coated with gold and then platinum 

 black which probably reduces the electrical resistance 

 of the metal-to-electroK te surface due to the porous 

 nature of the platinum black and reduces the rate 

 at which the surface becomes polarized during the 

 passage of electrical currents. 



Perhaps the ultimate in fine tipped metal micro- 

 electrodes is produced by the electroetching and 

 polishing technique (32). Hubel (36), applying this 

 technique to a tungsten wire which he then insulates 

 with a clear lacquer down to the tip, has produced an 

 electrode of 0.4 /j tip diameter with which he has re- 

 corded the intracellular potentials of motor horn 

 cells in the spinal cord of the cat (fig. iC). 



A metal-electrolyte interface or junction behaves 

 somewhat like a condenser due to polarization 

 eflPects. In general the greater the current density at 

 the junction the more rapidly it becomes polarized. 

 Thus, as the tip of a metal electrode becomes smaller 

 the difficulty with polarization increases. Successful at- 

 tempts have been made to reduce polarization by 

 coating the microelectrode tip with platinum black 

 (16, 52) and by using amplifiers which draw very 

 small currents (Bak, A. F., manuscript in prepara- 



