Topographic studies relating distribution of Ia- and gamma-fibres in spinal cord and position of muscle spindles in cat tibialis anterior muscle.

The segmental distribution of 115 Ia- and 115 paired gamma-fibres of the tibialis anterior muscle was studied in anaesthetized cats. All Ia-fibres recorded were found in the lumbar segments L6 and L7, from caudal L6 to middle L7. The paired gamma-axons were also mainly found in these parts of the spinal cord, only 7 gamma-fibres were localized in caudal L7. A total of 70% of all fibres was found in L7. Of the fibres constituting 'muscle spindle units' of the tibialis anterior 92.2% enter the same segment (a 'muscle spindle unit' is here defined as a muscle spindle with its Ia-fibres and one gamma-fibre innervating it). More than that, 88% of the afferent and efferent fibres of muscle spindle units were found in the same part of the segment. For the first time, the position of the muscle spindles was related to the location of their Ia- and gamma-fibres in the spinal cord. In general, the muscle spindles located in the proximal muscle region project to the more cranial part of the spinal cord and the muscle spindles localized distally in the muscle project to the more caudal part of the spinal cord. The topographic pattern of the muscle spindle units is discussed with respect to the topographically arranged monosynaptic reflex loop.