A study of the functional relationship between the dorsal column and spinocervical system of the cat

Electrophysiological experiments have shown that stimulation of the ipsilateral DLF, caudal to a transection of the dorsal columns (at C4), inhibits cells in the dorsal column nuclei. The presence of such inhibition was shown both by focal potential, and single unit, recording. It was found that the amplitude of a volley, conducted down the dorsal columns in response to a shock applied to the cuneate nucleus and recorded from the killed ends of dorsal column fibres at the rostral and of a transection of the dorsal columns, was increased by a preceding tetanus applied to the ipsilateral DLF. This evidence of depolarisation of dorsal column fibre terminals suggests that presynaptic inhibition contributes to the inhibition produced by DLF stimulation. A contribution from postsynaptic inhibitory mechanisms was suggested by the observation, made on several occasions, that antidromic activation of a unit from the contralateral mid-brain could be prevented by a preceding shock applied to the ipsilateral DLF. In some oases inhibition of units was produced by squeezing one or more of the cat's paws; some of these units projected into the contralateral mid-brain.
In addition to an inhibitory action from the ipsilateral DLF on the cells of the dorsal column nuclei, electrophysiological experiments have also demonstrated the presence of an excitatory action. The majority of the cells transynaptically activated stimulation of the DLF were found in the region andminus;3 to +2andnbsp;mm (the obex being aero); however this may have been because the majority of electrode penetrations were also made in this region. Many of the unite so excited were shown to project into the contralateral mid-brain, thereby demonstrating the existence of a genuine excitatory effect on the dorsal column nuclei. Approximately one third of the units excited by stimulation of the DLF were also excited by peripheral mechanical stimulation. In the majority of cases the effective stimulus was cutaneous stimulation. A number of units, deep in the cuneate nucleus, ware found to be excited by "noxious" mechanical stimulation; no such units wore found in the gracile nucleus. Only one of those tested was found to project into the contralateral mid-brain, suggesting that these units may be inhibitory interneurones.
The excitation and inhibition, produced by stimulation of the ipsilateral DLF, were abolished by transection of the DLF. They were net affected by transection of the brain stem at a level immediately rostral to the dorsal column nuclei. None of the cells transynaptically activated by stimulation of the DLF were also activated by stimulation of the anterior lobe of the cerebellum; this suggests that dorsal column nuclei cells were not excited by collaterals of direct spinocerebellar tract fibres.
The presence of direct connections between the dorsal column nuclei and the DLF was demonstrated electrophysiologically by the recording, in the dorsal column nuclei, of short constant latency responses with properties indicative of external recordings from fibres. Whether such fibres were afferent to, or efferent from, the cells of the dorsal column nuclei could not be established from such recordings. However, evidence for the existence of both such categories was found.
Some cells in the dorsal column nuclei were antidromically activated by stimulation of the ipsilateral DLF. Some of these cells were orthodromically activated by afferents coursing in the DLF. The presence of such orthodromic activation of those cells led to the speculation that the cells with descending projections may synapse, directly or indirectly, with the cells of origin of the afferent pathway, and thereby form a feed-back circuit within this system.
Anatomical experiments have demonstrated the existence of direct afferent fibre connections between the DLF and the dorsal column nuclei. Terminal degeneration was found, after lesions of the DLF, in the rostral parts of both cuneate and gracile nuclei. The terminal degeneration extended from 1andnbsp;mm caudal to the obex up to the rostral poles of both nuclei. Such anatomical evidence of a direct afferent pathway does not, of course, exclude the presence of indirect pathways; such pathways, if synapsing before reaching the dorsal column nuclei, would not be demonstrated by anatomical methods. It is not possible, from the results described, to determine whether the direct pathway is formed by collaterals from a tract already known, such as the spinocervical tract, or whether it exists as a separate entity.