Role of peripheral afferents and spinal reflexes in normal and impaired human locomotion

For many years, electrophysiological investigations of locomotion were restricted to animals, largely the cat. They concentrated on and emphasized the role of spinal interneuronal networks responsible for the generation of the locomotor pattern. Following the introduction of perturbation impulses and electrical nerve stimulation during stance and gait, information became increasingly available concerning the role of the reflex systems involved in the regulation of gait, their afferent pathways and their control by supraspinal motor centres. During gait monosynaptic stretch reflexes are inhibited. From a knowledge of the behaviour of the cerebral potentials evoked during stance and gait, it can be deduced that during gait the signals of group I afferents are blocked at both segmental and supraspinal levels. Polysynaptic reflex responses are mainly responsible for the compensation of perturbations introduced during gait. They are most probably mediated by group II afferents via a spinal pathway closely connected with the spinal locomotor centres. The functioning of these responses depends on an intact supraspinal control. They are suggested to be incorporated in a more complex e.m.g. pattern mainly determined by central mechanisms. In contrast to the gait condition, segmental stretch reflex activity does contribute to activation of extensor muscles of the leg during fast movements, such as running and hopping. In children at an early stage in the development of gait (around 1 to 2 years of age), as well as in patients with spastic paresis, the polysynaptic reflex responses are reduced or absent, and isolated monosynaptic reflex potentials are present. This suggests a reciprocal modulation of mono- and polysynaptic reflex mechanisms, both being dependant on supraspinal control. When this control is either not yet matured (small children) or impaired (spastic paresis), inhibition of monosynaptic stretch reflexes is absent and associated with a reduced facilitation of polysynaptic spinal reflexes. In spastic muscle hypertonia, the tension developed at the Achilles tendon during gait cannot be explained by gastrocnemius activation alone. In patients with spastic hemiparesis gastrocnemius e.m.g. activity is reduced in the spastic leg as compared to the unaffected one. It can be concluded that the paretic muscle undergoes changes in its mechanical properties, secondary to the supraspinal lesion, which results in the development of spastic muscle hypertonia.(ABSTRACT TRUNCATED AT 400 WORDS)