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Decoding the organization of spinal circuits that control locomotion
Ole Kiehn
Abstract | Unravelling the functional operation of neuronal networks and linking cellular activity to specific behavioural outcomes are among the biggest challenges in neuroscience. In this broadfield of research, substantial progress has been made in studies of the spinal networks thatcontrol locomotion. Through united efforts using electrophysiological and molecular genetic network approaches and behavioural studies in phylogenetically diverse experimental models, the organization of locomotor networks has begun to be decoded. The emergent themes from this research are that the locomotor networks have a modular organization with distinct transmitter and molecular codes and that their organization is reconfigured with changes to the speed of locomotion or changes in gait.
Locomotion is the motor function that allows humans and other animals to interact with their surroundings. It takes the form of swimming in fish, flying in insects and birds, and over-ground locomotion in limbed animals, and is the output of numerous integrated brain activities that allow the animal to find its way, escape predators or search forfood.
Although locomotion might seem effortless, it is a complex motor behaviour that involves the concerted activation of a large number of limb and body muscles. The planning and initiation of locomotion take place in supraspinal areas, including the cortex1, the basal ganglia24, the midbrain5,6 and the hindbrain79, but the precise timings and patterns of locomotor movements in vertebrates are generated by activity in neuron assemblies that are located in the spinal cord itself10,11 (FIG.1). These
neurons receive activating inputs from the brain and are able to produce the rhythms and patterns of loco- motion that are conveyed to motor neurons and then to the axial and limb muscles, as first shown by Thomas Graham Brown more than 100years ago in the cat12 and later confirmed in all vertebrates13. Additional layers of regulation come from the cerebellum, modulatory signals9,1416 and sensory feedback17,18.
Much of the early work on spinal locomotor networks was carried out in cats, in which it was shown that monoamine precursors could evoke locomotor-like neural activity in spinal cords that were isolated from the brain and sensory organs9,18,19. Vertebrate locomotion is now studied in several vertebrate models. Owing to their...