Proprioceptive eye position signals are still missing a sensory receptor.

Editor's Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more information on the format and purpose of the Journal Club, please see Review of Zimmermann et al. To properly control body movements, the brain needs to keep track of body position. Visual and cognitive studies have paid particular attention to eye position signals, which are considered crucial to many aspects of visuomotor behavior and may help to maintain perceptual stability. A long-standing controversy surrounds the nature and origin of eye position signals, which could potentially derive, on the sensory side, from extraocular eye muscle (EOM) pro-prioceptors, and on the motor side, from a corollary discharge of velocity-to-position neuronal integrators, without precluding the possibility that both signals coexist (Donaldson, 2000). Eye position signals from brainstem neuronal integrators play an undisputed role in eye movement control , and pathways that carry those signals to cortical regions with eye position-modulated activity have also been demonstrated (Prevosto et al., 2009). In contrast, whereas eye position signals have been found in primary somatosensory cortex (Wang et al., 2007), the paucity of knowledge on sensory organs and pathways for proprioceptive eye position signals has long put into question the relevance of EOM sensory inputs for visuomotor control. It has been found that in most species, EOMs essentially lack classical proprio-ceptors. In species where muscle spindles and Golgi tendon organs are found in EOMs, those sensory receptors are generally poorly developed (Maier et al., 1974; Donaldson, 2000). However, mammalian EOMs possess a unique structure, the palisade endings, which have long been pos-ited to serve proprioceptive functions (Ruskell, 1978). EOMs differ from skeletal muscles in several respects. The nearly continuous activity of EOMs is carried out by specialized fatigue-resistant muscle fibers dominantly found in the muscle layer that connects to the orbit (orbital layer; Fig. 1). EOMs are also characterized by the presence of slow-contracting muscle fibers which do not propagate action potentials. Those fibers are innervated by multiple, " en grappe, " terminals and consequently are called multiply innervated muscle fibers (MIFs). Palisade endings are found at the myotendinous (muscle–ten-don) junction of MIFs that connect to the sclera, in the global layer of the EOMs (Fig. 1; Ruskell, 1978). Axons of palisade endings extend into the tendon, before turning …