36: Locomotion

Introduction

• A Complex Sequence of Muscle Contractions Is Required for Stepping

• The Motor Pattern for Stepping Is Organized at the Spinal Level

  • Contraction in Flexor and Extensor Muscles of the Hind Legs Is Controlled by Mutually Inhibiting Networks

  • Central Pattern Generators Are Not Driven by Sensory Input

  • Spinal Networks Can Generate Complex Locomotor Patterns

• Sensory Input from Moving Limbs Regulates Stepping

  • Proprioception Regulates the Timing and Amplitude of Stepping

  • Sensory Input from the Skin Allows Stepping to Adjust to Unexpected Obstacles

• Descending Pathways Are Necessary for Initiation and Adaptive Control of Stepping

  • Pathways from the Brain Stem Initiate Walking and Control Its Speed

  • The Cerebellum Fine-Tunes Locomotor Patterns by Regulating the Timing and Intensity of Descending Signals

  • The Motor Cortex Uses Visual Information to Control Precise Stepping Movements

  • Planning and Coordination of Visually Guided Movements Involves the Posterior Parietal Cortex

• Human Walking May Involve Spinal Pattern Generators

• An Overall View

The ability to move is essential for the survival of animals. Although many forms of locomotion have evolved—swimming, flying, crawling, and walking—all use rhythmic and alternating movements of the body or appendages. This rhythmicity makes locomotion appear to be repetitive and stereotyped. Indeed, locomotion is controlled automatically at relatively low levels of the central nervous system without intervention by higher centers. Nevertheless, locomotion often takes place in environments that are either unfamiliar or present unpredictable conditions. Locomotor movements must therefore be continually modified, usually in a subtle fashion, to adapt otherwise stereotyped movement patterns to the immediate surroundings.

The study of the neural control of locomotion must address two fundamental questions. First, how do assemblies of nerve cells generate the rhythmic motor patterns associated with locomotor movements? Second, how does sensory information adjust locomotion to both anticipated and unexpected events in the environment? In this chapter we address both of these questions by examining the neural mechanisms controlling walking.

Although most information on neural control of walking has come from studying the cat's stepping movements, important insights have also come from studies of other animals as well as rhythmic behaviors other than locomotion. Therefore, we shall also consider the more general question of how rhythmic motor activity can be generated and sustained by networks of neurons.

Several critical insights into the neural mechanisms controlling quadrupedal stepping were obtained nearly a century ago when it was found that removing the cerebral hemispheres in dogs did not abolish walking—decerebrate animals are still able to walk spontaneously. One animal was observed to rear itself up in order to rest its forepaws on a gate at feeding time. It was soon discovered that stepping of the hind legs could be induced in cats and dogs after complete transection of the spinal cord. The stepping movements in these spinal preparations (Box 36–1) are similar to normal stepping. Nonrhythmic electrical stimulation of the cut cord elicits stepping at a rate related to the intensity ...