Responses of neurons in VPL and VPL-VL region of the cat to algesic stimulation of muscle and tendon

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Abstract
The responses evoked by electrical stimulation of cutaneous and muscle nerves, by noxious and innocuous mechanical stimulation of muscle, tendon and cutaneous tissues, and by intra-arterial (i.a.) injection of algesic substances (K, bradykinin) into arteries supplying the gastrocnemius-soleus muscle (GS) were studied in single neurons located in the ventroposterolateral nucleus (VPL) and in the transitional zone between VPL and the ventrolateral nucleus (VL) of cats lightly anesthetized with thiopenthal. Such chemical stimulation of the muscles activated muscular groups III and IV axons specifically and presumably was nociceptive in character. Neurons (108) were tested; 83 of the units responded only to various types of cutaneous stimulation of the hindlimb. The other 25 responded to algesic stimulation of muscle and/or tendon. Of these latter 25, 7 had no apparent cutaneous receptive field athough 4 of them responded to electrical stimulation of the common peroneal and/or sural nerve. Only 3 neurons responded exclusively to algesic chemical and noxious mechanical stimulation of the muscle. Of the other 18 neurons, 14 had cutaneous receptive fields restricted to the hindlimb and often responded to non-noxious repetitive light stroking and to noxious pinching with a high-frequency discharge. Four cells (2 of which had cutaneous input only from low-threshold mechanoreceptors) had complex and large receptive fields extending to more than one limb. K was a more potent muscle receptor stimulant than bradykinin, the latter only weakly exciting 3 neurons of 24 tested with both substances. The responses to K were rapid (.apprx. 4.0 s in latency) and tended to be greater (have higher response rates) for the units that responded to cutaneous as well as muscle/tendon stimulation. Most neurons that responded to noxious deep stimulation had a threshold for the GS nerve volley in the group III fiber range. The few neurons with thresholds slightly below the group III range did not respond to activation of group I or II muscle spindle afferents by intra-aterial application of succinylcholine or by stretching the muscle. Neurons with responses to any of the muscle, tendon or cutaneous nocicpetive stimuli were located at the ventral and dorsal periphery of VPL and in the VPL-VL transitional zone. Results suggests that there exist regions within the lateral diencephalon of cats that are capable of processing nociceptive information and that these regions are located at the periphery of VPL.