274 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY' 1 



a further study can be made of the details of their 

 behavior. The most definitive of these studies has 

 been made on the large motoneurons of the cat by 

 Eccles (i8), Fuortes et al. (29) and Fatt (23), and in 

 the toad by Araki et al. (9), and Araki & Otani (8). 

 While these studies are too detailed to report in full, 

 it would seem pertinent to this discussion of the 

 identification of single unit activity to descrilje briefly 

 the model which has been developed to account for 

 the activity of certain nerve cells (fig. 16). All nerve 

 cells certainly do not behave according to this 

 model, but some features of the electrical activity of 

 cells are so general that they justify some generaliza- 

 tions from a model Ijuilt for a particular cell. 



A neuron in the central nervous system might be 



NORMAL THRESHOID 



FIG. 16. Diagram to illustrate initiation of impulses in a 

 motoneuron. A spherical soma is indicated and its axis passing 

 through the center of the axon is used as abscissa for the plot. 

 Ordinate: membrane depolarization mejisured from resting 

 membrane potential. The dashed line indicates relative po- 

 tential changes evoked in various positions of neuron by 

 synaptic activity in soma and dendrites (not differentiated) or 

 by currents applied through a microelectrode in the sphere. 

 The solid line represents the depolarization level required to 

 evoke firing of different parts of the membrane. B' designates 

 that part of the neuron having a high threshold, and 'A' the 

 transitional region between this area and the lovi' threshold 

 axon. In normal conditions threshold stimuli, either synaptic 

 or applied through microelectrode, will initiate an impulse in a 

 region where the dashed line first crosses above the solid line 

 (If depolcirization is sudden, attenuation along the axon will be 

 steeper than that for steady depolarization due to capacity of 

 the membrane.) The dotted line is intended to indicate the 

 depolarization required to elicit firing shortly after activity 

 which has involved only the cross-hatched areas of the neuron, 

 e.g. after a blocked antidromic impulse. [From Fuortes et al. 

 (29)J 



considered to consist of its soma and dendrites which 

 are connected to its axon by a thin unmyelinated seg- 

 ment arising from the axon hillock. Presynaptic fibers 

 make synaptic connections with this cell through 

 terminal knobs which end on its soma and dendrites. 

 Some of these specialized endings are excitatory and 

 others inhil)it activity of the cell. Probably through a 

 mechanism of secretion of excitor or inhibitor trans- 

 mitter substances by the presynaptic terminals, the 

 membrane of the postsynaptic cell is made selectively 

 permealjle to certain inorganic ions (16). Normally 

 these transient changes in ion permeability alter the 

 equilibriiuii potential of the membrane and either 

 depolarize it (excitation) or hyperpolarize it (inhibi- 

 tion). The sum of these synaptic potentials in the soma 

 and dendrites spreads electrotonically with decrement 

 to a sensitive target area, probably the thin initial seg- 

 ment of the axon and part of the axon hillock. The 

 threshold of this region, that is the magnitude of the 

 depolarization necessary to start an action potential in 

 it, is lower here than in the soma and dendrites 

 (normally perhaps one third l Thus, in spite of the 

 fact that the synaptic potential is larger in the soma 

 and dendrites than in the thin segment, the lower 

 threshold of this region permits it to be the site of 

 origin of the propagated action potential. The action 

 potential is then propagated out the axon and may or 

 may not spread Ijackwards over the soma and den- 

 drites. Following activity the soma-dendritic region 

 remains refractory longer than the target area so that 

 a second spike elicited during this period may be con- 

 ducted in the axon without spreading to the soma or 

 dendrites as seen in figure lo.-l and in the paper by 

 Fuortes et al. (29). It is proljable that the safety factor 

 for propagation from target area to soma-dendritic 

 region varies, not only with the condition of the cell, 

 but also from one type of cell to another so that 

 normal ijehavior in cells may differ in this respect. 

 But cells in so many different parts of the nervous 

 system show similar electrical properties that the 

 main features of the model just described may well be 

 of general application. 



Stimulation Through Microelectrodes 



Electric currents delivered through intracellular 

 microelectrodes produce excitation similar to that 

 caused by conducted impulses. Such direct stimula- 

 tion plays a role in the identification of penetrated 

 units, and is so generally useful in studying the proper- 

 ties of cells that some reference should be made to the 



