Control of locomotion in the decerebrate cat.

Many of the general concepts regarding the control of walking were described years ago by: Sherrington (1906) Integrative Actions of the Nervous System. Yale University Press: New Haven, CT; Sherrington (1910a) Remarks on the reflex mechanism of the step, Brain 33, 1-25; Sherrington (1910b) Flexor-reflex of the limb, crossed extension reflex, and reflex stepping and standing (cat and dog), J. Physiol. (Lond.) 40, 28-121; Sherrington (1931) Quantitative management of contraction in lowest level coordination, Brain 54, 1-28; Graham-Brown (1912) The intrinsic factors in the act of progression in the mammal, Proc. R. Soc. Lond. 84, 308-319; Graham-Brown (1914) On the nature of the fundamental activity of the nervous centres; together with an analysis of the conditioning of rhythmic activity in progression, and a theory of the evolution of function in the nervous system, J. Physiol. 49, 18-46; Graham-Brown (1915) On the activities of the central nervous system of the unborn foetus of the cat, with a discussion of the question whether progression (walking, etc.) is a 'learnt' complex, J. Physiol. 49, 208-215; Graham-Brown (1922) The physiology of stepping, J. Neur. Psychopathol. 3, 112-116. Only in recent years, however, have the mechanisms been analyzed in detail. Quite a few of these mechanisms have been described using the decerebrate cat. Locomotion is initiated in decerebrate cats by activation of the mesencephalic locomotor region (MLR) that activates the medial medullary reticular formation (MRF) which in turn projects axons to the spinal cord which descend within the ventrolateral funiculus (VLF). The MRF region regulates as well as initiates the stepping pattern and is thought to be involved in interlimb coordination. Afferent feedback from proprioceptors and exteroceptors can modify the ongoing locomotor pattern. Recently, the types of afferents responsible for signaling the stance to swing transition have been identified. A general rule states that if the limb is unloaded and the leg is extended, then swing will occur. The afferents that detect unloading of the limb are the Golgi tendon organs and stimulation of these afferents (at group I strengths) prolongs the stance phase in walking cats. The afferents that detect the extension of the leg have been found to be the length- and velocity-sensitive muscle afferents located in flexor muscles. Plasticity of locomotor systems is discussed briefly in this article. Descerebrate animals can adapt locomotor behaviors to respond to new environmental conditions. Oligosynaptic reflex pathways that control locomotion can be recalibrated after injury in a manner that appears to be functionally related to the recovery of the animal.