268 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



A new technique has been proposed for extending 

 direct \ision to structures within the central nervous 

 system. This is accomplished by the use of a long thin 

 solid cone of glass mounted in front of the microscope 

 objective which extends to the focal plane. Incident 

 illumination is supplied through the objective, and 

 the cone is moved through the tissue until the desired 

 object appears in the small field of view bounded by 

 the flat end of the cone. Using this technique, moto- 

 neurons have been seen in unfixed tissue, and their 

 nuclei and dendrites are clearly visible. It is hoped 

 that this technique can be used for determining micro- 

 electrode position. 



Marking techniques have been used extensively for 

 locating gro.ss microelectrodes. One method produces 

 a lesion by passing radio-frecjucncy current through 

 an insulated metal electrode bared at its tip. The 

 lesion is subsequently made visiijle by staining the 

 fixed and sectioned material. Another method using 

 steel electrodes plates off iron by passing a current 

 through the electrode. The region where the iron 

 has been deposited is then stained blue by ferrocy- 

 anide and prepared by frozen section technique, Mar- 

 shall's modification of Hess' method (45). The resolu- 

 tion of about half a millimeter made possible by these 

 two techniques is not yet great enough to permit 

 identification of single cells or parts of cells. Success 

 in marking a single cortical pyramidal cell has been 

 claimed recently by Rayport (48) who passed a cur- 

 rent through a micropipette filled with a 3 n 

 FeNH4S04 solution. Iron ions moved by electrophore- 

 sis into the penetrated cell were later stained blue !)>• 

 the ferrocyanide reaction of Hess (34). This is the 

 only case known to the author of a single nerve cell 

 penetrated blindly and sub.sequently identified 

 visually. 



The identification of nervous structures by infer- 

 ences made from the potentials recorded from micro- 

 electrodes is uncertain and .subject to revision when- 

 ever further information modifies the assumptions on 

 which such inferences are based. However, pending 

 the development of a more direct method, the study 

 of single unit activity in the central nervous system 

 is limited to such inferences as can be made based on 

 comparisons of potentials recorded from microelec- 

 trodes in the central nervous system with those re- 

 corded from cells under direct vision or otherwise 

 identified. 



Axons 



The potentials to be expected from axons in the 

 central ner\ous system can be predicted from meas- 





FIG. 5. Effect of \olume conductor on potentials recorded 

 by intracellular electrodes. The microclectrode is inserted in a 

 fiber of a ventral root. In A and B the root is surrounded by 

 paraffin oil, in C and D the oil is replaced by Ringer's fluid. 

 A and C, stimulus just subthreshold for penetrated fiber; B 

 and D, maximal stimulus. Calibration : 50 mv. Time : i msec. 

 [From Frank & Fuortes (26).] 



urements made on peripheral nerve (see Chapter 

 III). If a nerve surrounded by an insulating medium is 

 made to conduct a synchronous volley of impulses, a 

 fairly large action potential can be recorded mono- 

 polarly from a gross electrode in contact with the 

 ner\e. As seen in figure 5^4, a microclectrode simi- 

 larly situated records the same large external action 

 potential with respect to a distant electrode on inac- 

 tive tissue. If the microclectrode then passes through 

 the membrane of a fiber participating in the volley, 

 the action potential it records will be the algebraic 

 sum of the outside potential previously recorded and 

 the action potential produced across the fiber mem- 

 brane (fig. 5/}). The effect of a volume conductor, 

 such as the spinal cord or brain surrounding the active 

 fibers, can be simulated b>' replacing the insulating 

 medium with a conductor such as saline. Figure 5C 

 shows that the external recording is then markedly re- 

 duced and the action potential developed across the 

 penetrated fiber membrane is recorded by an internal 

 microelectrode with little distortion (fig. jD). When 

 the intpulses in the fibers are not synchronous, the al- 

 ready small external potential field becomes negligi- 

 ble; but, whatever the external field may be, it will 

 be approximately recorded by an electrode inside or 

 outside a resting fiber. Thus it can be anticipated that 



