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From the beginning of vertebrate evolution, the appearance of neuromechanical loops was associated with locomotion. Locomotion is essential in searching for food, for escaping harm and predators, and for reproduction. Locomotion is the ability to move over different terrains, including swimming, slithering, walking, or flying. Although this is not the place to recapitulate the long paleontological history of the evolution of locomotion1, understanding of the neural mechanisms underlying locomotion is central to revealing many of the unique features of the vertebrate nervous system.
In vertebrates, the spinal cord is organised into segments corresponding to the vertebrae. Locomotion involves coordinated activation of segmental spinal sensory-motor circuits. As proposed above for the intestine, during locomotion, the continuous interaction between organisms and their environment means that the sensory-motor circuits act as neuromechanical loops.
From: https://en.wikipedia.org/wiki/Spinal_cord
From: https://en.wikipedia.org/wiki/Reflex_arc
What was not clear initially is whether the neural circuits for locomotion require sensory inputs to initiate the activity, or whether there are some intrinsic properties of the spinal neural circuits that generate locomotor patterns. The original assumption by Charles Sherrington in the early 1900s that sensory inputs are essential for any movement to be initiated, was tested by Thomas Graham-Brown, one of his pupils. He found that rhythmic spontaneous activity in the spinal cord underlying walking in experimental animals occurred even in absence of sensory inputs2. He wrote “The rhythmic sequence of the act of progression is consequently determined by phasic changes innate in the local centres, and these phases are not essentially caused by peripheral stimuli.”3 Sherrington was never convinced of the findings of his pupil who, because of this opposition, abandoned his academic career and became a mountaineer. Amongst other exploits, he found and named three new routes ascending Mont Blanc.
Graham-Brown’s results implied that there must be some features of the spinal neural circuits that sustain the locomotor patterns. Since then, the neuroscience of locomotion has been significantly advanced thanks to research on the swimming of a primitive vertebrate, the lamprey.
- For more, see Henry Gee (2022): A (Very) Short History Of Life On Earth: 4.6 Billion Years In 12 Chapters. Pan MacMIllan. ↩︎
- Thomas Graham-Brown (1911): The intrinsic factors in the act of progression in the mammal. Proceedings of the Royal Society B 84, 308-319. ↩︎
- Thomas 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. Journal of Physiology 49, 18-46. ↩︎