Polysensory interactions in the cuneate nucleus

ERG alterations induced by sound

The existence of efferent neurons from the CNS to the retina mediating centrifugal effects is an old and much debated hypothesis. In order to test whether the human ERG can be influenced by another sensory input, we have recorded the retina response to light when a sound was presented simultaneously. It was found that the b-wave of the ERG increased when a flash was paired with a sound. The sound-induced increment of the b-wave showed the following characteristics: a) Short time course, b) No relation to the frequency and intensity of the sound, c) Habituation specific to the frequency of the sound, d) Adaptation when the sound lasted more than 2 sec, e) Dependence on localization cues of the sound and f) Enhancement by low doses and suppression by high doses of diazepam. Further experiments on rabbits showed that it is not suppressed by ether anesthesia, by local application of atropine or by cutting the external ocular muscles. The results suggest that this effect is brought about by a reticulo-retinal pathway, the function of which is most probably to control the input of the visual route.

Nucleus gracilis: an integrator for visceral and somatic information

The nucleus gracilis (NG) receives an abundance of visceral input from various abdominal organs and is proposed to play an important role in visceral pain processing. The purpose of this study was to investigate the necessity of the NG for colorectal input into the ventral posterolateral (VPL) nucleus of the thalamus. Single-cell recordings were made from nine VPL cells isolated in nine different male Sprague Dawley rats anesthetized with pentobarbital sodium. Responses of the VPL cells to colorectal distension (CRD) and to cutaneous stimuli were obtained before and after lesioning of the NG. Electrolytic (n = 5) and chemical (n = 4) lesions of the NG were made in different preparations. The chemical lesions were made by injecting a solution of kainic acid into the NG. Kainic acid presumably kills neuronal cell bodies and spares axons of passage. The results indicate that a lesion of the NG, regardless of its type, reduces dramatically the responses of VPL neurons to innocuous cutaneous stimuli, and, to a lesser extent, the responses to CRD. Attenuation of VPL neuronal responses to CRD as well as to innocuous cutaneous stimuli by the NG lesions emphasizes the role of the dorsal column in visceral nociception and suggests that the NG is an integration center for visceral and cutaneous information flowing into the VPL nucleus.

Auditory influences via cochlear nucleus on cuneate neurons in decerebrate-decerebellate cats

In decerebrate-decerebellate cats, clicks and tones evoked negative and positive potentials on the surface of the cuneate nucleus and modulated the activity of cuneate neurons. These effects appear to be mediated through direct projections from the cochlear to the dorsal column nuclei.

Inhibition of dorsal column nuclei by stimulation of trigeminal afferents in decerebrate-decerebellate cats

In decerebrate-decerebellate cats, stimulation of trigeminal afferents inhibited neurons in dorsal column (DC) nuclei driven by activation of DC input and produced primary afferent depolarization in DC primary afferent terminals. This inhibition was most likely mediated by a trigeminal-brainstem-DC nuclear loop.

Electrophysiological and sensory properties of the thalamic reticular neurones related to somatic sensation in rats

1. In urethane-anaesthetized rats, studies were made on unit discharges of neurones of the thalamic reticular nucleus (tr) receiving the somatosensory input. Such units were found distributed in a part of the tr (s-tr) surrounding the anterior and anterolateral borders of the ventrobasal nuclear complex (vb). 2. A series of five to ten bursts of grouped discharges were elicited in the s-tr neurones as orthodromic responses to single shock stimulation of the medial lemniscus (ml) or the somatosensory cortex (smc). In contrast to this, the vb relay cells were fired by ml impulses only once at short latencies, though later there occurred a series of two to three bursts of grouped discharges. The same response pattern was seen by stimulating the smc, except that the initial response to smc shocks was of antidromic origin. 3. The mean response latency of the s-tr neurones to ml shocks was about 0.6 msec longer than that of the vb relay cells. Ml fibres connected to the s-tr neurones had the same distribution of conduction velocity as those connected to the vb relay cells. It is suggested that most, if not all, of the s-tr neurones are innervated by axon collaterals of the vb relay cells. 4. About half the s-tr units were activated from vibrissae. For most of the remaining half either movements of hairs or touch to the skin were effective. In the vb, the units discharged by movements of hairs were most numerous and those activated from vibrissae were less frequent. While responses of the s-tr neurones to adequate stimuli were almost exclusively of phasic type, 30% of the responses of the vb relay cells were tonic and the remainder were phasic. 5. By exploring the tr extensively, the s-tr was found to be contiguous caudally to the perigeniculate reticular nucleus which is immediately adjacent to the dorsal lateral geniculate nucleus and contains reticular neurones activated by visual stimuli. Though few in number, units were found in the tr which received a convergence of somatosensory and visual inputs or of somatosensory and auditory inputs.

Electrophysiological identification of pontomedullary reticular neurons directly projecting into dorsal column nuclei

In cats 'encephale isole', cerebellectomized and curarized, neurons projecting from the bulbopontine reticular formation to the cuneate nucleus were identified by stimulating their axons in this nucleus by means of Eide micropipettes and by recording both field potential and antidromic responses from single somata in the nucleus gigantocellularis. Of 130 neurons responsive to cuneate stimulation, 17 (12%) were found that were antidromically activated. The small proportion of antidromic responses might reflect the small size of the projection pathway. However, there are reasons to believe that a certain number of neurons might have been missed because of biasing factors introduced by the microstimulation technique. Neurons that were activated orthodromically were also observed, responding after latencies spread over rather wide ranges. The possible pathways for this activation are discussed.