The whole body receptive field of dorsal horn multireceptive neurones.

Multireceptive neurones are found in the spinal dorsal horn and may be projection neurones and/or interneurones for polysynaptic reflexes. The cutaneous receptive field of a multireceptive neurone exhibits a gradient of sensitivity with the centre responding to any mechanical stimulus, including hair movements and light touch, while the periphery responds only to noxious stimuli. These neurones also receive signals from viscera, muscles and joints. This convergence of inputs means that multireceptive neurones are continuously capturing all the information from both the interface with the external environment (the skin) and the internal milieu (the viscera, muscles, etc.). This information constitutes a 'basic somaesthetic activity' that could help the somatosensory system build a 'global representation of the body'. In addition to be seen as a global entity, the output of multireceptive neurones should be understood in dynamic terms since the size of the peripheral fields of the individual neurones may change, as a result of the plasticity of both excitatory and inhibitory segmental processes. Furthermore, the activity of these neurones can be inhibited from most of the remaining parts of the body via supraspinal mechanisms. These diffuse noxious inhibitory controls (DNIC) are triggered by peripheral A delta- and C-fibres, involve brain structures confined to the caudal-most part of the medulla including the subnucleus reticularis dorsalis (SRD) and are mediated by descending pathways in the dorsolateral funiculi. A painful focus that both activates a segmental subset of neurones and inhibits the remaining population can seriously disrupt this basic activity, resulting in the distortion of the body representation in favour of the painful focus, which becomes pre-eminent and (relatively) oversized.