Spinal withdrawal reflexes in the human lower limbs.

In his original description of the nociceptive flexion reflex in vertibrate animals Sherrington (1910) declares that even though the reflex is most readily elicited from the foot, it has a receptive field which includes "the skin of the whole limb as far up as the groin in front, the perineum medially, and the ischial region behind". He states that noxious stimuli within this field cause not only a contraction of the flexors of hip and knee and the dorsiflexors of foot and digits, but also a relaxation of antagonistic extensors and plantar flexors. In addition, he points out that even though the ensuing movement is always a flexion of the limb "there comes to be some accentuation of movement at this joint or that according as the skin-point stimulated lies in this limb-region or that" (the phenomenon of "local sign"). It was recognized quite early, however, that ipsilateral extension reflexes could also be elicited from the skin or the cutaneous nerves. The "*extensor thrust" represents an example of such a reflex, and it was also noted that modification of the stimulus applied to sensory nerves sometimes changed the motor response obtained from a flexion to an extension reflex (Sherrington and Sowton, 1911; Brown, 1912; Brown and Sherrington, 1912). These authors' final conclusion was that in vertibrate animals there exist ipsilateral extension reflexes of cutaneous origin; they are often concealed by the dominant flexion reflex but they can be uncovered by varying the site and nature of the stimulus. In an attempt to make a further analysis of these spinal extension responses, Hagbarth (1952) mapped the receptive skin fields for various flexor and extensor muscles in the cat's hind leg and found that local extension reflexes could be elicited from certain skin areas which were mainly localized over the extensor muscles themselves. Formerly many clinicians believed that extension reflexes could not appear after a complete transection of the spinal cord in human beings, but observations of Kuhn (1951) and other workers have shown that this is not so. An extensive study of the cutaneous reflexes obtainable from the lower limbs of patients with spastic paraplegia was carried out by Marshall (1954) who demonstrated that ipsilateral extension reflexes could be elicited from various skin areas on the leg, thigh, and perineum and that the nature of the skin stimulus often determined whether flexion or extension ensued. He found no clear evidence that extension reflexes were more easily elicited from skin overlying extensor muscles than from flexor surfaces. The nociceptive cutaneous reflexes in the lower limbs of man have also been investigated with an electromyographic technique (Kugelberg, 1948; Hoffmann, Schenck, and Tonnies, 1948; Dodt and Koehler, 1950; Pedersen, 1954) but so far no systematic analysis of the receptive fields for individual muscles has been performed, and thus it is still unknown if the local extension reflexes seen in the cat exist also in man. It seemed that such an analysis of spinal skin reflexes in healthy subjects might help to clarify the mechanisms responsible for "abnormal" reflex patterns characteristic of upper motor neuron disease.