IDENTIFICATION AND ANALYSIS OF SINGLE UNIT ACTIVITY IN CENTRAL NERVOUS SYSTEM 



'^73 



lieve that they supply inhibition to the motoneurons, 

 the activity of which excites them. 



That both fibers and somata of interneurons are 

 penetrated is indicated by figure 12 which shows that, 

 after known primary afferents (^A) and motoneuron 

 somata (5) are eliminated, the remaining spikes from 

 interneurons are distributed in duration of action 

 potentials as though they were made up of many 

 fibers and fewer somata. 



Slow PdtnUiali 



As has been shown above it has not been possible 

 yet to distinguish clearly between cell somata and 

 dendrites with intracellular electrodes. But grouping 

 these two structures together a fairly clear-cut distinc- 

 tion is possible between soma-dendrites and fibers on 

 the basis of the slow potentials recorded from within 

 them. As seen in figure 13, a unit classified as a moto- 

 neuron soma shows a long lasting graded response to 

 subthreshold excitatory afiTerent stimulation which is 

 commonly called a synaptic potential. If this potential 

 reaches a critical level of depolarization, as shown in 

 figure 14, a spike is initiated and this spike is followed 



-> B 



f^ 



5 nnec 



lOmV 



-V- 



FIG. 13. Spikes and slow potentials recorded from a cat's 

 motoneuron following antidromic (o) and orthodromic (0) 

 stimulation. .All records made from same motoneuron at difler- 

 ent sweep speeds and amplifications. Note respective time and 

 potential scales, msec, marks shown in A, B and D. Note dorsal 

 root spike records in C and D which are recorded with nega- 

 tivity downward. [From Brock et al. (i i).] 



FIG. 14. Diagram illustrating terminology used to describe 

 antidromic (a) and orthodromic (0) spikes from cat's moto- 

 neurons and the level of polarization, Vth, which must be 

 achieved if a propagated spike is initiated. All potentials are 

 measured from the outside potential taken as o. Vm, resting 

 membrane potential, inside negative; Vs, total spike height; 

 Vov, spike overshoot, amount inside goes positive at peak of 

 spike. [From Frank & Fuortes (27).] 



FIG. 15. Spike and slow potential recorded from cat's 

 motoneuron following antidromic stimulation. Calibration : 



20 mv. Time: i msec. 



by an even longer lasting period of hyperpolariza- 

 tion, such as those in figures 1 5 and 1 3C. These slow 

 potentials are apparently attentuated in the axon so 

 that, at the gains employed with intracellular record- 

 ing, no slow potentials are recorded from fibers unless 

 they are penetrated close to the soma (18, 26). This 

 finding, which is based upon identifications made 

 using the criteria just discussed, itself becomes a 

 method of identifying long duration spikes as from 

 somata or dendrites and short spikes as from fibers. 



Steps in Development of Cell Spikes 



Once the identification of the various units pene- 

 trated in the central nervous system has been accepted. 



