Efferent cortical activity from the cerebellum is carried by the Purkinje cells (P-cells, see Fig. 1) which are the final common path (P.e.) of two major synaptic organizations. One is a complex polysynaptic network of dendrites and stellate cells in the superficial molecular or plexiform layer to which apparently both cerebellar afferent systems contribute though its main supply is from the mossy fibres (Mo.a.) over the granular cells (Gr.c.). The other afferent system is the monosynaptic one over the climbing fibres (Cl.a.) to P-cell dendrites along which they climb. These are held to contribute also to the polysynaptic network (Estable, 1923;Scheibel & Scheibel, 1954) and many other complexities are encountered, but to a first approximation the physiologist has to be content with a general subdivision of cerebellar afferents into mono-and polysynaptic ones along the lines suggested.His next concern will be with the two additional systems centered on the P-cells one of which consists of two types of recurrent collaterals (P.r.): Cajal's (1911, fig. 12) 'plexus sous-cellulaire' spreads among the cells below the P-cells and his 'plexus sus-cellulaire' traverses the molecular layer giving off branches on its way. Both types of recurrent collaterals do, however, end up on P-cells.Finally, it would seem highly desirable to be able to elucidate the role of the unique system of basket cells (B-cells) with dendritic receptors in the molecular layer (and possibly elsewhere) and efferent axons (B.e.) surrounding the P-cell itself as well as the non-medullated and part of the medullated portion of its efferent axon (for good illustrations, see Estable, 1923; also Retzlaff, 1954).

SUMMARY1. The precentral and postcentral banks of the Rolandic fissure of the arm area of the baboon's cortex have been probed to their depths with extracellular micro-electrodes under nitrous oxide and oxygen anaesthesia, supplemented by minimal intravenous pentobarbitone or chloralose.2. Afferent volleys were sent in from the deep (motor) radial nerve and the deep palmar (motor) branch of the ulnar nerve. Their entry into the central nervous system was timed at the dorsal root entry zone. The nerves were stimulated in continuity and the effects of stimuli below threshold for the motor axons were investigated.3. Area 3a, in the depths of the postcentral bank, which is cytoarchitectonically transitional between areas 3 and 4, is the receiving area for afferent impulses from muscle.4. Evoked potential waves and unitary discharges began 4 msec, and the majority of units discharged between 5 and 10 msec, after the afferent volley reached the dorsal root entry zone.5. Similar responses were elicited by a brief pull (70 #u in 1 msec) or brief vibration (50 t at 250-400 Hz) applied to the tendons of m. extensor digitorum communis.6. No potential waves were evoked in area 4, even in the depths adjacent to area 3a, by muscle afferent volleys.

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SUMMARY1. Intracellular records were obtained from motoneurones innervating muscles of the baboon's forearm and hand. Monosynaptic excitatory postsynaptic potentials (EPSPs) were elicited by stimulation of motor cortex (CM EPSPs) and peripheral nerves (Ia EPSPs).2. CM EPSPs were larger on average in motoneurones innervating intrinsic hand muscles and extensor digitorum communis (EDC) than in neurones of other forearm muscles.3. Among motoneurones of the median nerve, the CM EPSP tended to be larger for cells with more rapidly conducting axons than for those with more slowly conducting axons. Among motoneurones of EDC the opposite tendency was found.4. The afferent fibres responsible for the Ia EPSP nearly always had a lower stimulus threshold than that of motor axons in the same nerve. Some observations were made concerning the distribution of heteronymous Ia EPSPs.5. Among motoneurones of a given nerve, those with large Ia EPSPs tended to receive larger CM EPSPs than did cells in which the I a EPSP was small.6. The results are discussed in relation to problems concerning the pyramidal control of hand and finger movement.

Using microcapillary electrodes in the motor cortex of cats under hexobarbitone, extracellular records were made from seventy-one Betz cells. Nineteen of these cells also gave intracellular records. The cells were identified by antidromic pyramidal stimulation. The extracellular records were diphasic positive-negative spikes of the type previously recorded from cerebellar Purkinje cells and other central neurones. The initial phase of the positive-going component (the " prepotential ") represents the " initial segment " spike of Coombs, Curtis and Eccles [1957] and the "A" spike of Fuortes, Frank and Becker [1957]. Disintegration of the Betz-cell spike into its components by two closely-spaced stimuli has been the exception rather than, as in spinal motoneurones, the rule. Pyramidal stimulation with one to four fine silver wires, with 0-2 msec. current pulses of strength up to 300 MsA, repeated at up to 420 c.s., caused no movement of the limbs, though some axons were stimulated at strength 18 MA. Shocks too weak to excite the axons of the recorded Betz cells were used to test the reaction of these cells to activity in other pyramidal axons. The usual consequence was an early facilitating-depolarizing reaction, followed by depressed excitability associated with increased polarization of the membrane. The most probable, but not the only possible, cause of these reactions is impulse traffic in the recurrent axon collaterals of the activated Betz cells.