Neuro biologia

REVIEWS

PROPRIOCEPTION AND LOCOMOTOR DISORDERS
Volker Dietz
Advances in our understanding of movement control allow us to define more precisely the requirements for the rehabilitation of patients with movement disorders. Most purposeful, complex movements are programmed in the central nervous system (CNS) and adapted by proprioceptive feedback. The selection of and interaction betweendifferent sources of afferent input is task dependent. Simple stretch reflexes are thought to be involved primarily in the control of focal movement. For more complex motor behaviours such as locomotion, afferent input related to load and hip-joint position probably has an important role in the proprioceptive contribution to the activation pattern of the leg muscles. There is increasing evidence thatmovement disorders such as spasticity and Parkinson’s disease involve the defective use of afferent input in combination with secondary compensatory processes. This has implications for therapy, which should be directed to take advantage of the plasticity of the CNS.
The study of movement control has relevance to our general understanding of brain function. But it also has implications for specificfields, such as neurology, cognitive neuroscience, rehabilitation medicine and robotics. Our understanding of movement disorders and their appropriate treatment depends on knowledge of the neuronal mechanisms that underlie functional movements. Movement disorders are the focus of one of the most rapidly expanding fields in medicine, leading to increasing costs of treatment and rehabilitation.This review focuses on the role of proprioception during human locomotion, which can serve as a paradigm for functional movements. In a more general sense, locomotion is representative of movement control. It is a subconsciously performed, everyday movement that is highly reproducible. It is adapted automatically to existing conditions, such as ground irregularities, within a large safety margin.Knowledge about the neuronal control of human locomotion is also of broad interest for clinical reasons. Characteristic disorders of locomotion are often the first sign of a central or peripheral lesion of the motor system. However, impaired movement is not only the direct consequence of a central lesion, reflected in defective programming or reflex functioning. Rather, a movement disorder alsoreflects secondary compensatory processes that are induced by the primary lesion. In many cases, the altered motor behaviour can be considered as the optimal outcome for a given lesion of the motor system1. The complexity of primary and secondary effects of a lesion means that detailed analysis of a movement disorder is required to define the target of any treatment.
Basic aspects of locomotionCENTRAL PATTERN GENERATOR

A neural circuit that produces self-sustaining patterns of behaviour independently of sensory input.

ParaCare, Institute for Rehabilitation and Research, University Hospital Balgrist, Forchstrasse 340, CH 8008, Zurich, Switzerland. e-mail: dietz@balgrist.unizh.ch
doi:10.1038/nrn939

It is generally accepted that locomotion in mammals depends on neuronal circuits(networks of interneurons) in the spinal cord (the CENTRAL PATTERN GENERATOR, or CPG) that can act in the absence of any afferent input2. Afferent information influences the central (spinal) pattern and, conversely, the CPG selects appropriate afferent information according to external requirements2. In addition, proprioceptive information provides the basis for a conscious representation of our bodyin space, which becomes severely disturbed in deafferented individuals3. Both the spinal locomotor centre (CPG) and the reflexes that mediate afferent input to the spinal cord are under the control of the
VOLUME 3 | OCTOBER 2002 | 7 8 1

NATURE REVIEWS | NEUROSCIENCE

REVIEWS
Table 1 | Proprioceptive reflexes suggested to be involved in locomotion
Appropriate stimulus Dynamic muscle...