DURING PURPOSEFUL MOVEMENTS the central nervous system uses information from a vast array of sensory receptors to ensure that the pattern of muscle activity suits the purpose. Without this sensory information, movements tend to be imprecise, and tasks requiring fine coordination in the hands, such as buttoning one’s shirt, are difficult. The sensory-motor integration that makes the ongoing regulation of movement possible takes place at many levels of the nervous system, but the spinal cord has a special role because of the close coupling in the cord between sensory input and the motor output to the muscles.
Charles Sherrington was among the first to recognize the importance of sensory information in regulating movements. In 1906, he proposed that simple reflexes—stereotyped movements elicited by activation of receptors in skin or muscle—are the basic units for movement. He also emphasized that all parts of the nervous system are connected and that no part is ever capable of activation without affecting or being affected by other parts. In his words, the simple reflex is a convenient if not a probable fiction.
Laboratory studies of reflexes in animals from the 1950s and onward demonstrated that descending motor pathways and afferent sensory pathways converge on common interneurons in the spinal cord. Later research in intact animals and in humans engaged in normal behavior confirmed that the neural circuitries in the spinal cord take part in conveying and shaping the motor command to the muscles by integrating descending motor commands and sensory feedback signals. Nevertheless, the idea of simple reflexes is convenient for understanding the principles of organization of sensory-motor integration in the spinal cord and of how sensory input to different spinal circuits contributes to movement control.
In this chapter, we explain the principles underlying sensory-motor integration in the spinal cord and describe how this integration regulates movement. For this purpose, we must first have a thorough knowledge of how reflex pathways in the spinal cord are organized.
Reflex Pathways in the Spinal Cord Produce Coordinated Patterns of Muscle Contraction
The sensory stimuli that activate spinal reflex pathways act outside the spinal cord, on receptors in muscles, joints, and skin. By contrast, the neural circuitry responsible for the motor response is entirely contained within the spinal cord. The interneurons in the reflex pathways and the resulting reflexes have traditionally been classified based on the sensory modality and type of sensory fiber that activates the interneurons. As we shall see, this classification is inconsistent with the significant convergence of multiple modalities on common interneurons, but as a starting point, it is still useful to distinguish reflex pathways based on whether the principal sensory input originates from muscle or skin.
The Stretch Reflex Acts to Resist the Lengthening of a Muscle
The simplest and certainly the most studied spinal reflex is the ...