Mechanisms involved in the neural construction of a body-centered reference axis for extrapersonal directed movements. A hypothesis.

To localize objects in space, it is necessary to refer them to a set of coordinates that serve as a frame of reference. Advances in molecular aspects of evolutionary developmental biology reveal how axial coordinates are established in embryos. But we do not yet know how axes of reference are constructed by adult animals. The characteristics of epaxial musculature, spinal connectivity, and organization at the cortical level are reviewed. Although endowed with muscle spindles, epaxial muscles lack the monosynaptic but possess the tonic component of the stretch reflex. Motoneurons of epaxial muscles are devoid of recurrent inhibition and do not show crossed disynaptic inhibition. At motorsensory cortex (MSC), regions corresponding to the body axis receive somatosensory signals that always extend across the midline. Visual and vestibular input also converge in the zone corresponding to the body axis. This region is also endowed with a large number of callosal fibers that, by connecting the two halves of the body axis, may allow them to function and behave as a unity. In contrast, somatic signals from distal extremities are discrete, confined only to the contralateral MSC, and show short latency of responses. They do not receive either telereceptive or vestibular input. We propose that limb movements directed to extrapersonal space take place within a reference frame in which one of the axes is the result of integration at the MSC of telereceptive, proprio and somatosensory signals from the body. Vestibular input signals the effect of the force of gravity, providing directionality to the axis.