THE CEREBELLUM 



cells may also be accompanied or produced by a) 

 excessive depolarization following intense activation 

 and by b) a transient depolarization lasting 15 to 30 

 msec, and having the character of an organized con- 

 trol mechanism. Direct stimulation of cerebellar 

 neurons (139) was found to be possible with very 

 low voltage positive pulses applied near superficial 

 units (see also 49, 50). With single and repetitive 

 stimulation, the Purkinje cells could be excited, 

 inhibited, triggered, driven and thrown into higher 

 levels of spontaneous activity. 



Single neurons of the fastigial nuclei have also 

 been observed in spontaneous activity (271). It has 

 been possible to alter the activity of these neurons by 

 vermal polarization, labyrinthine polarization and 

 sensorv nerve stimulation. 



sis' and has been attributed to inhibitory reverberat- 

 ing circuits between the mesencephalon and cere- 

 bellar cortex (142, 144-147, 204). However, it has 

 been pointed out that with stimulation parameters 

 of these values, the brain stem is directly activated 

 by spread of stimulating current and that the stimuli 

 exceed the thresholds for physiological responses by 

 5 to 50 times (7, 239, 292). It has also been demon- 

 strated that when stimuli which are threshold for 

 the production of convulsive after-discharge are pro- 

 longed for 20 to 30 sec, electrical silence super- 

 venes — an effect which is independent of the level of 

 mesencephalic transection (214). It thus seems un- 

 necessary to postulate inhibitory mesencephalo- 

 cerebellar relations when evidences of simple ex- 

 haustion are adequate to account for the observations. 



Cerebellar Activity as Altered bv Surface Stimulation 



SINGLE SHOCKS. The effects of surface polarization of 

 the cerebellum on unit activity have already been 

 alluded to (49, 50J. This technique has been utilized 

 to study cerebellar influences on units of the reticular 

 formation (130, 213, 293, 350). 



Dow (logj has recorded surface negative responses 

 to single shocks with evidence that these are slowly 

 conducted (0.5 m per sec.) through fibers of the molec- 

 ular layer. Dow also presents evidence that the con- 

 ducted activity may cause Purkinje cell discharge. 

 This observation has recently been repeated (139). 



REPETITIVE STiMUL.'\TioN. Repetitive stimulation of 

 the surface of the cerebellum is followed by a variable 

 sequence of events depending, apparently, upon the 

 intensity and duration of the stimulus train. At the 

 site of stiiTiulation, there follows a period of electrical 

 silence if the period of stimulation has been short, or 

 a period of convulsive after-discharge, represented 

 by an increase in amplitude and frequencv of back- 

 ground activity if the stimuli are more intense or the 

 train is slightly prolonged (i, 106, 214). Following 

 the con\ulsive pattern, there ensues a period of re- 

 duced activity or of electrical silence, after which 

 the normal electrogram once more appears. This 

 sequence of electrical changes has been shown to be 

 accompanied by alterations of spinal cord function, 

 and therefore must be related to alterations in cere- 

 bellar neuronal discharge (106). 



An electrical silence following intense (up to 200 

 v., 40 ma peak, i msec, pulses at 280 per sec.) pro- 

 longed (30 sec.) stimulation of the cerebellum or 

 brain stem has been termed 'cerebellar electronarco- 



Cerehellar Activity as Altered by Cerebellopelal Impulses 



FORM OF EVOKED PDTENTi.AL. Since the first observa- 

 tions (131) and the subsequent studies by Snider & 

 Stowell (311) and by Adrian (2) of potentials at the 

 surface of the cerebellum produced by sensory stimu- 

 lation, it has been noted that the form of the evoked 

 potential is essentially the same whatever the origin 

 of the causative impulses. From this it may be in- 

 ferred either that a) all of the incoming activity thus 

 far tested arrives at the cortex over mossy fibers (see 

 169) or that b) mossy and climbing fibers both pro- 

 duce a similar sequence of sources and sinks by post- 

 synaptic activation. The latter inference seems uzi- 

 warranted on purely anatomical grounds. 



The arrival of a synchronized volley of impulses 

 in corticipetal fibers produces an initially positive 

 wave form of o.i to 0.3 mv and a duration of 20 to 

 30 msec. This is succeeded by a more leisurely, 

 usually smaller, negative wa\e which may occupy 30 

 to 40 msec. Such responses are highly localized in 

 anesthetized or depressed animals when the initiat- 

 ing stimuli are confined to a small population of 

 afferents. This localized characteristic has led to the 

 widespread use of the evoked response in electro- 

 anatomical studies which will be discussed later. In 

 unanesthetized preparations, the evoked response is 

 frequently succeeded by a period during which the 

 spontaneous rhsthm of the cortex is increased in 

 frequency and amplitude (2, 38, 39). This altered 

 rhythm may resemble convulsive activity, but it is 

 promptly blocked by the elicitation of a second 

 evoked response. 



The genesis of the surface sign of evoked response 

 has been studied using simultaneous leads from the 



