LOCOMOTOR CONTROL: FROM SPRINGLIKE REACTIONS OF MUSCLES TO NEURAL PREDICTION In: AThe Somatosensory System: Deciphering the Brain's Own Body Image@ edited by R. NELSON (original) (raw)

The sensory control of mammalian locomotion has been studied for around 150 years. Many systems are involved: skeletomuscular actuators, spinal reflex and pattern generating networks, propriospinal and brainstem networks, the vestibular apparatus, cerebellum, deep brain nuclei and the cerebral cortex. All of these systems are directly or indirectly affected by mechanical or sensory input related to the locomotor movements they help control. In this chapter we will argue that since locomotor movements vary tremendously according to task, terrain and context, sensory input is crucial in controlling them. Experimental results as well as control systems simulations will be used to show that sensory input is crucial for determining the timing of phase transitions and thus the cadence (cyclic frequency) of locomotion, as well as the relative scaling of muscle activation and thus the amplitude of locomotor movements. We will argue that stretch reflexes play a relatively minor role in this scheme, basically augmenting the load compensation that occurs by virtue of the intrinsic compliance characteristics of skeletal muscle. With the help of biomechanical models, we will show that in quadrupeds, a carefully constructed pattern of muscle activations can produce sustained locomotion over flat ground in the absence of sensory input. However, small variations in terrain, initial conditions, or biomechanical parameters can disrupt locomotor stability. The situation is more critical in bipedal locomotion, where sensory input is vital for stable step cycles. The main function of the huge flow of multimodal sensory information from limb mechanoreceptors to the CNS is therefore continuously to adapt the locomotor pattern to variations in terrain and posture and to mediate higher level prediction of requirements in upcoming step cycles. Sensory systems must therefore be considered as integral parts of the semi-autonomous locomotor pattern generator. Taking the broad view, at least five levels of locomotor control can therefore be identified: load compensation due to skelotomuscular properties, load compensation due to stretch reflexes, bodily motion resulting from cyclical pattern generation and adaptive and predictive control in relation to terrain and to behavioural goals and contexts.