Spinal cord modularity: evolution, development, and optimization and the possible relevance to low back pain in man

The words of Hughlings Jackson in 1884 were inspired by a consideration of what was then the recent work of Darwin on evolution. In the quoted article he applied this perspective to CNS evolution, development and subsequent neurological damage. His overall framework likely remains relevant today. Our paper’s goal is to review current perspectives on motor modularity and its mechanisms, especially at the spinal level (for Jackson ‘well-organized’, ‘simpler’ and ‘more automatic’), but also beyond. Modularity can be defined as the use of designs employing standardized components or units, allowing easy assembly, repair and flexible arrangements of the components. The simplest modules in a system, from which other larger modules might be made, can be termed primitives. This term derives from a combination of the biological definition of primitive as ‘occurring in or characteristic of an early stage of development or evolution’, the definition of primitive as an element assumed as a basis, and the computer science definition of ‘a basic or fundamental unit of machine instruction’. Like Jackson, we will argue that these modules and primitives are in significant part already organized at birth. At the end of this review, we discuss how these issues in spinal modularity and protective reflex structure may relate to trunk control and low back pain mechanisms in humans.