Development of muscle afferents in the spinal cord of the tammar wallaby.
The development of muscle afferents in the tammar wallaby was examined to address whether proprioceptive input contributes to the marked asymmetry of the fore and hindlimb movement. Anatomical tracing with biocytin showed that the muscle afferents had reached the brachial motor horn by postnatal day (P1), but were less advanced in the lumbar region. Labelled cells lying outside the motor horn, presumably filled via gap junctions, were evident in the neonatal lumbar cord. By the 4th postnatal week, the afferent innervation of both brachial and lumbar cords became similar. Afferent discharges from stretching the biceps muscle could be recorded at birth, but not until P4 from the hindlimb gastrocnemius muscle. The discharges were predominantly phasic until P35 when tonic activity could also be recorded. Short latency spinal reflex responses superimposed upon a longer lasting potential were present in the brachial cord at birth, appearing in the lumbar cord at P4. By the 3rd postnatal week, spinal reflex became comparable in both segmental levels. The time course of muscle afferent development was compared to the progression of natural cell death in the lumbar cord. Sensorimotor connections were established towards the end of the rapid phase cell death as observed in other vertebrates.
Ho
S M
SM
Developmental Neurobiology, Research School of Biological Sciences, Australian National University, Canberra, Australia. sho@rsbs.anu.edu.au
Stirling
R V
RV
D000200:Action Potentials / Q000502:physiology; D000344:Afferent Pathways / Q000502:physiology; D000818:Animals; D007614:Macropodidae / Q000254:growth & development*; D024510:Muscle Development; D018482:Muscle, Skeletal / Q000254:growth & development / Q000294:innervation*; D011930:Reaction Time / Q000502:physiology; D012018:Reflex / Q000502:physiology; D013116:Spinal Cord / Q000254:growth & development / Q000502:physiology*