Contents

6.1 Introduction

6.2 Cells vs. Systems

6.2.1 Historical Development

6.3 The Effect on Locomotion of the Loss of Sensation

6.4 Receptors Involved in the Control of Locomotion

6.4.1 Mechanoreceptors

6.4.2 Vision, Vestibular Input, and Hearing

6.5 Structure and Response Properties of Proprioceptors

6.5.1 Muscle Spindles: Structure

6.5.2 Passive Response Properties of Spindle Afferents

6.5.3 Fusimotor Action

6.5.4 Spindle Models

6.5.5 Tendon Organs: Structure and Response Properties

6.5.6 Tendon Organ Models

6.5.7 Mechanoreceptors in Joints, Ligaments, and Skin

6.5.8 Overview of Proprioceptive Firing during Locomotion

6.6 Simple Locomotor Reflexes

6.6.1 The Stretch Reflex

6.6.2 Flexion and Extension Responses, "Reflex Stepping," and "Reflex Walking"

6.7 Central Pattern Generators and Sensory Feedback

6.7.1 Cyclical Motor Patterns Generated without Sensory Input

6.7.2 Interaction between CPGs and Sensory Feedback

6.7.3 Human Locomotion

6.7.4 Robots

6.7.5 Virtual Animals

6.7.6 If-Then Rules Governing Phase Switching and the Selection of "Hazard" Responses

6.8 Prediction and Adaptation

6.9 Summary Points

6.10 Appendix

6.10.1 Stretch Reflex Model Acknowledgment References

6.1 INTRODUCTION

Dr. P.R. Burgess organized a Society for Neuroscience symposium in 1992 to discuss his contention that "You can only control what you sense." The question is, what is being controlled in locomotion and which of the many sensory inputs to the CNS are the main players? There are numerous ways of answering this, each implying a different level of control and different neural systems. For example, at one level, that which is controlled is support and movement of the body with respect to uneven terrain. The control problem at this level is to cope with support surfaces of variable orientation, consistency, stability, friction, and compliance. At another level, that which is controlled is movement of the body with respect to a moving target (e.g., as in the hunting of prey). At this level, the problem is to anticipate future positions of the target and to control and adapt one's own trajectory accordingly, taking into account obstacles and hazards in the way.

6.2 CELLS VS. SYSTEMS

Stuart et al. (2001) recently pointed out that sensorimotor control has been studied either "inside-out" from cellular and molecular mechanisms within small neuronal networks (the cellular level) or "outside-in" from complex behaviors to reflexes (the systems level). Although these two approaches often remain far apart, more and more laboratories are tackling specific problems from each end (Rossignol 1996; Jordan 1998; Kiehn and Kjaerulff 1998; O'Donovan et al. 1998; Grillner et al. 2001).

6.2.1 Historical Development

The outside-in approach started centuries ago, when it was suggested that complex behaviors including locomotion comprised chains or assemblies of simple behaviors or reflexes (Descartes 1664; Mettrie 1745; Spencer 1855; Sechenov 1863). These ideas gained credibility with early experimental work that showed that after removal of the cerebrum in birds, frogs, and quadruped mammals, the brainstem and spinal cord could still generate complex movements such as righting reflexes and locomotion (Flourens 1823; Goltz 1869; Freusberg 1874; Goltz 1892; Brown 1911).

The inside-out approach gained momentum with the neuroanatomical studies of Ramon y Cajal (Cajal 1894) and the technical breakthrough of electronic recordings of single-neuron activity (Adrian and Zotterman 1926). Within years the glass microelectrode had allowed intracellular potentials to be measured and the scene was set for the detailed study of the ionic mechanisms of the action potential, the synaptic actions of sensory axons on motoneurons and interneurons (Eccles et al.

1957a; Eccles et al. 1957b), the role of neurotransmitters in simple reflex behavior (Eccles et al. 1954; Jankowska et al. 1967; Jankowska et al. 2000) and the neuronal analysis of reflexes elicited in anesthetized or mid-collicular decerebrated animals (Chen and Poppele 1978; Terzuolo et al. 1982). In the 1970s, patch-clamping and molecular techniques allowed the functioning of membrane channels and their associated intracellular mechanisms to be studied in detail (Neher and Sakmann 1976).

In the 1960s, it was shown by the Moscow group of Shik, Orlovsky, and their colleagues that locomotion in decerebrate and spinal animals provided an excellent basis for electrophysiological studies at both cellular and systems levels (Shik et al. 1969). This in-between approach has provided much useful insight and, combined with pharmacological and molecular techniques, has begun to allow the first comprehensive analyses of locomotor control ranging from ions to behavior (Grillner et al. 2000).

To return to the main theme of this chapter, deafferentation studies, modelling, and lessons that have been learned from designing walking robots all show that it is important to have sensory information throughout the step cycle about the terrain and obstacles ahead; ground reaction forces, and displacements; internal forces and displacements; and relative velocities of the body segments. Numerous reviews have been written in the last few years on one or more of these topics (Pearson 1995; Horak and MacPherson 1996; Prochazka 1996b; Rossignol 1996; Buschges and Manira 1998; Marder and Pearson 1998; Pearson et al. 1998; Duysens et al. 2000). We will focus mainly on control mediated by mechanoreceptors, but key aspects of the visual control of locomotion will be included toward the end of the chapter.

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