Afferent inhibition on various types of cats cuneate neurons induced by dynamic and steady tactile stimuli

Receptive field properties of somatosensory neurons in the cat superior colliculus

In general, knowledge of the internal organization of receptive fields has played an important role in shaping current understanding of sensory physiology. Such knowledge is particularly important for understanding the function of the superior colliculus, since this structure is at once implicated in spatial localization and has relatively large receptive fields. While this issue has been addressed in the visual and auditory modalities represented in the superior colliculus, there are no previous studies of its somatosensory receptive field organization. Here, the properties of somatosensory receptive fields in the cat superior colliculus were studied quantitatively to determine whether they contain internal non-homogeneities that might aid in the determination of stimulus detail. Of special interest was the possibility that these comparatively large receptive fields would contain areas of differential excitability that could aid in spatial resolution, that within-field spatial summation and/or inhibition would be exhibited, and that the borders of the excitatory receptive field would be flanked by inhibitory regions. The data demonstrate that while inhibition beyond the receptive field borders is a rarity, these somatosensory receptive fields nearly always contain a well-defined area of maximal sensitivity within which the size of the stimulus is a critical feature in determining the magnitude of the response. These best areas are systematically distributed across receptive fields as a function of their location in the structure, and indicate that the resolution of stimulus location and size may be greater than expected on the basis of receptive field size alone.

Spatial features of vibrotactile masking effects on airpuff-elicited sensations in the human hand

It was recently shown that the cutaneous sensitivity to airpuffs is decreased by a low-frequency vibrotactile masker in the hairy skin, and by a low-frequency but especially by a high-frequency masker in the glabrous skin. In the current study, the spatial features of this masking effect were determined in four healthy human subjects, using a reaction time paradigm. The masking effect decreased monotonically with increasing interstimulus distance, and identically in longitudinal and transverse (i.e., lateral) directions in the palm or dorsal surface of the hand. The masking effect was stronger in the glabrous than in the hairy skin, especially in the fingers. In the glabrous skin, the spread of masking effect produced by a high-frequency masker was more extensive than that produced by a low-frequency masker. The mechanical spread of high-frequency vibration was less extensive than that of low-frequency vibration in the skin. In the glabrous skin, a masker applied to the tip of the finger produced a stronger masking effect on sensations in the base of the finger than when the masker was located at the base and the test stimulus was located at the tip. It is concluded that mechanical spread of vibration in the skin is of minor importance in explaining the masking effects. Different peripheral neural mechanisms underlie the airpuff-elicited sensations in the hairy and glabrous skin. The afferent inhibitory mechanisms are stronger for signals coming from the glabrous skin of the fingers than for signals coming from the hairy skin. Furthermore, the peripheral innervation density and size of the cortical representational areas may be of importance in determining the magnitude of the masking effect.

Central projections of the rat radial nerve investigated with transganglionic degeneration and transganglionic transport of horseradish peroxidase

Transganglionic degeneration and transganglionic transport of HRP were used for investigation of the spinal cord and brainstem projections from the superficial, cutaneous (SR) and deep, muscular (DR) branches of the radial nerve. The HRP study included a numerical and size analysis of labelled dorsal root ganglion (DRG) cells. In degeneration experiments the SR nerve was found to project somatotopically to laminae III-IV, but degeneration was also found in lamina I and inconsistently in lamina II. Transection of the DR nerve was found to give rise to a small amount of degeneration, which in "sham" operations was established to result from the skin injury during dissection of the DR nerve. With the HRP method, the SR nerve was found to project somatotopically to laminae I-IV, whereas the DR nerve projected more diffusely to the medial part of laminae V-VII. HRP application to the SR and DR nerves resulted in labelling of a mean of 1,024 and 310 DRG cells, respectively. These labelled neurons had a median cell area of 381 and 562 micron 2 for the SR and DR nerves, respectively, and both small and large cells were labelled in both types of experiments. In the lower brainstem, projections from the SR nerve were found only in the ipsilateral dorsal part of the main cuneate nucleus (MCN) with both methods. Brainstem projections from the DR nerve that were found only with the HRP method were found in the ipsilateral ventral part of the MCN together with a projection to the ipsilateral external cuneate nucleus. No projections were found to the central cervical nucleus. The present results indicate that cutaneous compared to muscular primary sensory neurons are much more prone to react with transganglionic degeneration after peripheral nerve transection. Furthermore, in the rat the SR nerve projects somatotopically, whereas the DR nerve does not. Both nerve branches are connected to small and large spinal ganglion cells, although the median cell area is larger in muscular neurons.

Inhibitory action of Purkinje cells in the posterior vermis on fastigio-prepositus circuit of the cat

Neuronal connections between the posterior vermis including the fastigial nucleus and the prepositus nucleus were studied with electrophysiological and neuropharmacological methods in the cat. Spontaneous discharges of neurons in the prepositus nucleus were depressed by microstimulation of the contralateral vermis, lobule VI. This depressive effect was blocked by the injection of bicuculline into the caudal part of the fastigial nucleus. These prepositus neurons also showed long-lasting poststimulus facilitation following microstimulation of the contralateral fastigial nucleus. These prepositus neurons were reciprocally connected with neurons in the contralateral fastigial nucleus. These findings suggest that Purkinje cells in lobule VI of the vermis depress propositus neuronal activity through the fastigial nucleus and that the reciprocal connectivity between the prepositus and fastigial nuclei generates a significant background facilitation in the two nuclei.

Cutaneous excitatory and inhibitory input to neurones of the postsynaptic dorsal column system in the cat

1. In chloralose-anaesthetized cats single-unit microelectrode recordings were made from axons in the dorsal columns, at the lumbar level, identified as belonging to the postsynaptic dorsal column (PSDC) system. 2. Excitatory and inhibitory receptive field arrangements of a sample of seventy-five PSDC neurones were examined in detail using natural cutaneous stimuli. 3. The sample was characterized by a high degree of convergent input: 80% of units were activated by both light tactile and noxious mechanical stimuli and more than half of those examined were excited by noxious radiant heat. In addition, three-quarters of the units had inhibitory receptive fields on the ipsilateral limb. 4. Twenty-three units (27%) were influenced by input from areas of both hairy and glabrous skin covering the foot and distal limb. Neurones in this group had complex receptive fields, many of which occupied several discontinuous areas of skin. Background and evoked activity of these units could frequently be inhibited by light tactile and/or noxious stimuli. Their inhibitory receptive fields occupied small areas of skin overlapping or adjacent to excitatory fields. 5. Fifty-two units (73%) had receptive fields restricted to areas of hairy skin on the thigh and upper hindlimb. Half the units in this group had coextensive low- and high-threshold excitatory areas but about one-third had a concentric receptive field organization; a high-threshold excitatory component extending beyond, or around, a central low-threshold area. The discharge of these units could be inhibited only by light tactile stimuli. Their inhibitory receptive fields covered extensive areas of skin, sometimes completely surrounding the excitatory field. 6. The complex receptive field arrangements observed for neurones of the postsynaptic dorsal column system are discussed in relation to previous observations on dorsal horn neurones of other ascending tracts.

Vibrotactile masking effects on airpuff-elicited sensations vary with skin region in the human hand

Inhibitory interactions between two tactile signals take place predominantly within mechanoreceptive submodality channels. This finding was utilized in the present study to determine the mechanoreceptive channels contributing to tactile sensations elicited by brief airpuff stimuli applied to the hairy and glabrous skin of the human hand. A reaction time paradigm was used to estimate the sensitivity of four subjects to airpuffs without and during continuous vibration (masker) of low (30 Hz) or high (240 Hz) frequency. The sensitivity to airpuffs (test stimuli) was decreased by a low-frequency masker in the hairy skin and by low- and especially by high-frequency maskers in the glabrous skin. The masking effect was enhanced in both skin areas by increasing the intensity of the masker and by decreasing the intensity of the test stimulus. The results suggest that the mechanisms underlying airpuff-elicited sensations consist of the low-frequency channel in the hairy skin, and of both the low- and high-frequency channels in the glabrous skin.

The termination of forelimb nerves in the feline cuneate nucleus demonstrated by the transganglionic transport method

The projection of forelimb nerves to the cuneate nucleus was studied in the cat by the transganglionic transport method. The cut ends of the median, ulnar, musculocutaneous, medial cutaneous, lateral brachial and antebrachial cutaneous nerves and of the superficial and deep branches of the radial nerve were exposed to horseradish peroxidase (HRP) or to HRP conjugated to wheat germ agglutinin. Nerves innervating the skin terminated in a somatotopical pattern on the cell clusters in the middle region of the cuneate nucleus. Afferents from the paw occupied the largest area and were situated dorsally, with the ulnar part represented medially and the radial part laterally. The palmar side of the digits seemed to be represented superficial to the dorsal side. The projection from the arm was split into a ventromedial and a ventrolateral area. Superimposed on this somatotopy, a reversed termination pattern was also present. Thus medially projecting nerves also had a small separate projection to the lateral part of the nucleus and vice versa. The rostral region of the nucleus was organized in a similar way except for the rostral pole where the somatotopy was lost. The caudal region differed from the middle one in that it appeared to lack representation of the upper and lower arm. The deep branch of the radial nerve terminated in the middle-ventral, 'reticular' region of the cuneate nucleus, with a sparse projection also to the ventral parts of the rostral and caudal regions, including the base of the dorsal horn. Also the musculocutaneous, median and ulnar nerves, but not the pure cutaneous nerves, had projections to these areas, indicating a modality segregation in the cuneate nucleus. The rostral pole of the nucleus, however, appeared to constitute an area of overlap between projections from deep and superficial receptors.