The role of skin areas adjacent to extensor muscles in motor neurone excitability: evidence bearing on the physiology of Babinski's response.

Cutaneous afferent nerve fibres fall into two categories of fibre sizes, group II (6 to 16 ,t) and group III (2 to 3 ,i) (Ranson, Droegemueller, Davenport, and Fisher, 1935). Afferent fibres from the skin, together with high threshold joint afferents and groups II and III muscle afferents, are classed as flexion reflex afferents, part of the nociceptive reflex system of Sherrington (1906) subserving the basic function of protective withdrawal from potentially noxious stimuli. Yet flexion is not an invariable response to stimulation of cutaneous nerves and exceptional instances of ipsilateral extension were already compiled by Brown and Sherrington in 1912. Creed, Denny-Brown, Eccles, Liddell, and Sherrington (1932) later described the occasional reversal of reflex pattern with change of the reflex stimulus parameters, concluding that the reversal of response indicates that one of the reflexes is ordinarily concealed. Clearly this inversion is apposite to the problem of the pathophysiology of Babinski's reflex, in which the normal plantar flexion of the toes on stimulation of the skin of the lateral planta alters to extension of the large toe with fanning of the other toes. A peripheral extension from the concept of mutually antagonistic reflexes evocable from the same cutaneous nerve is that of preferential facilitation of either flexor or extensor motor neurones from stimulation of certain specific skin areas. This concept would provide the substrate for the occasional reversal of reflex on stimulation of a cutaneous nerve, composed of an intermingling of nerve fibres from diverse skin areas. Hagbarth (1952) first reported that extensor muscles were inhibited by stimulation of most skin areas of the hind limb in spinal and decerebrate cats, but were excited by stimulation of cutaneous areas over the extensor muscle; whereas flexor muscles were excited by