THE INFLUENCE OF THE CEREBRAL CORTEX 

 ON THE DORSAL COLUMN NUCLEI* 



S. J. Jabbur and a. L. Towe 



Department of Physiology and Biophysics, University of Washington 

 School of Medicine, Seattle, Washington 



The idea of centrifugal modulation of sensory input has developed rapidly 

 in the past decade. Hagbarth and Kerr (1954) described an inhibition of the 

 dorsal root reflex and the dorsal column relay following conditioning stimula- 

 tion of various supraspinal structures. Since that time, similar depressive 

 effects of central structures on nuclei of the somesthetic, auditory, visual, and 

 olfactory systems have been demonstrated. More recently, fibers have been 

 shown to course directly from the cerebral cortex, via the pyramidal tract, to 

 the dorsal column nuclei and the spinal trigeminal nucleus in both the cat and 

 several primate species (Walberg, 1957; Chambers and Liu, 1957; Kuypers, 

 1958 a, b. c). The following discussion describes the function of some of these 

 fibers in the cat. 



Neurons of the cuneate nucleus are excited not only by axons whose cell 

 bodies are located in dorsal root ganglia, but also by dorsal column relay 

 fibers (Hursh, 1940) and by axon collaterals originating within the cuneate 

 nucleus (Amassian and DeVito, 1957). The monosynaptic input can readily 

 be distinguished from the indirect inputs by its short and relatively invariant 

 response latency and its ability to respond to each shock in a train at fre- 

 quencies in excess of 100/sec. The indirect inputs produce a discharge after 

 a longer latency (greater than 8 msec) which is more variable; the spikes so 

 produced fail to follow repetitive shocks as high as 50/sec. The same neuron 

 may first be excited monosynaptically and then be excited by either or both of 

 the indirect routes following a single ipsilateral forelimb stimulus (Fig. 1a). 

 This usually results in a short silent interval during the discharge; the spikes 

 appear in two groups. 



Bipolar shocks applied to the surface of the precruciate or postcruciate 

 cortex of either hemisphere modify the response of cuneate neurons to a 

 subsequent ipsilateral forepaw shock (Jabbur and Towe, 1960). One cuneate 

 neuron in three is discharged from 5 to 30 msec after single shocks or trains 



* This work was supported by a grant (B396) from the National Institute of Neuro- 

 logical Diseases and Blindness, Department of Health, Education and Welfare. 



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