Fig. 1

Structure of muscle spindles and distribution of the DGC. Panel a shows a schematic representation of the sensory and fusimotor innervation of intrafusal fibers. The connective tissue capsule is indicated in orange. Muscle spindles contain three types of intrafusal fibers: nuclear bag1, nuclear bag2, and nuclear chain fibers. Different parts of intrafusal fibers are innervated by different neurons: The central (equatorial) part is in intimate contact with afferent proprioceptive sensory neurons, termed primary “group Ia afferents” (forming the annulospiral endings) and (if present) secondary or “group II afferents”, marked in green and red, respectively. In addition to the sensory neurons, intrafusal muscle fibers are innervated by efferent γ-motoneurons (marked in black) in both polar regions, were they form a cholinergic synapse. The polar regions of intrafusal fibers contain most of the contractile elements (sarcomeres are indicated in blue in panel a). This schematic representation is based on the well-characterized muscle spindles from the cat’s tenuissimus muscle [19]. However, interspecies differences exist. For example, mouse muscle spindles might not have a group II innervation [26], and in humans, the sensory nerve terminal does not form annulospiral endings and the secondary ending innervates nuclear bag as well as nuclear chain muscle fibers [27]. Panel b shows a confocal section of the central part of a mouse muscle spindle stained with anti-neurofilament antibodies. Note the annulospiral endings of the Ia afferents in the central region. The γ-motoneuron endplates are located outside the picture