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Antidromically identified neurons projecting to the putamen (CPNs) and pyramidal tract neurons (PTNs) were recorded from motor and premotor cortex of a monkey which performed a load-bearing task with the wrist. CPNs appeared as a uniform population with very slowly conducting axons and low spontaneous activity. In contrast to PTNs, they exhibited weak, mostly insignificant correlation with graded steady-state forces, responded to torque perturbations with remarkably long latency, and seemed to discharge much later with active movement. Collateral branching of PTNs to the putamen was found to be infrequent (1%). We suggest that the putamen receives a cortical message that is strikingly different from that sent down the pyramidal tract.The role of the putamen as part of a 'motor' circuit linking the basal ganglia with the neocortex has been evidenced by the demonstration of somatotopically well-organized projections from the sensory and motor cortices s'L°'12 and by recent findings on the relationship of single cells to specific aspects of motor behavior ~-3'1~ in the primate putamen. However, the functional significance of the corticostriatal part of this 'motor loop' remains obscure, mainly because the actual projection neurons to the putamen have until now not been neurophysiologically identified and studied in any animal preparation. Recent anatomical work stresses the independence of the cells of origin of the various corticofugal systems 9, thus possibly indicating their functional specialization. Although still a matter of controversy 4"5, collateral branching to the striatum of other descending fibers, namely of the pyramidal tract, seems to be negligible in the monkey s."> . Other, more general questions posed by our study relate (1) to the issue of columnar organization of the motor cortex, i.e. do neighboring clusters of different types of efferent cell in cortical layer V constitute a common module9"16?, and (2) do similarities within a particular corticofugal system transcend areaspecific differencesT? We have therefore compared the properties of antidromically identified corticoputamenal neurons (CPNs) and pyramidal tract neurons (PTNs) recorded from the primary motor cortex (MI) and postarcuate premotor area (PMA) in the behaving monkey.A fully trained, male rhesus monkey (8.5 kg) was used for the experiments. Under Nembutal anesthesia and after craniotomy over the left MI-hand-arm region, a recording cylinder (i.d. 18 mm) was installed and tilted in the vertical plane, so as to allow transdural microelectrode penetrations perpendicular to the cortical surface. Stereotaxic procedures were aided by the use of X-rays and NMR-tomography. A pair of teflon-insulated stainless steel wires of 150 /xm diameter (tips exposed over 200/~m and vertically separated by 2.5 mm) attached to a guide shaft was implanted within the left pyramidal tract (PT) at the most caudal level of the inferior olive. Another, similar bipolar stimulating electrode aimed at the left putamen was inserted

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SUMMARY1. High frequency vibration was applied to the tendon of the noncontracting triceps surae muscle while recording the background discharges of single y efferents from the nerves to the gastrocnemius muscles in the decerebrate cat. For isolation of y fibres only small nerve bundles were cut, leaving most of the nerve supply to the triceps intact.2. 22 % out of a total of sixty-three y efferents were tonically inhibited by vibration. The inhibition appeared between 25 and 50 #sm peak-to-peak amplitude of vibration and increased to a plateau for amplitudes of about 100 gm. The dependence of the tonic vibration reflex of a efferents on the amplitude of vibration was found to be similar. Increasing the frequency of vibration from 150 to 300 Hz increased the degree of inhibition.3. 33 % of the fusimotor neurones investigated responded to muscle vibration with an increase in discharge rate. The threshold amplitudes of this reflex ranged from 20 to 50 /m. Some features of the reflex, in particular the parallel post-vibratory facilitation found in a and y efferents, pointed to a polysynaptic pathway organized in an c-y linkage.4. All y efferents inhibited by vibration showed inhibitory responses to antidromic stimulation of the parent ventral root, and most of them were inhibited by ramp stretch of the triceps. The y motoneurones facilitated by vibration, however, were excited by muscle stretch and were less susceptible to antidromic inhibition, some lacking it completely.5. Cutting the nerves to triceps abolished the inhibitory as well as the excitatory responses of y efferents to muscle vibration. Both fusimotor reflexes were preserved after spinal section and subsequent administration of L-DOPA. 6. It is concluded that both of the fusimotor reflex effects of vibration are caused by excitation of primary spindle endings within the triceps.