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Formation of transient inappropriate sensorimotor synapses in developing rat spinal cords
B S Seebach1, L Ziskind-Conhaim
1Department of Physiology, University of Wisconsin Medical School, Madison 53706.
Summary
During development, rat spinal cords show incorrect connections between muscle nerves and motor neurons. These inappropriate connections, crucial for motor control, decrease after birth as neural activity and myelination increase.
Area of Science:
- Neuroscience
- Developmental Biology
- Motor Control
Background:
- Muscle spindle afferents typically connect to motoneurons of the same or synergistic muscles.
- Antagonistic muscles are usually innervated by inhibitory interneurons, not directly by afferents.
Purpose of the Study:
- To investigate the specificity of primary afferent projections to motoneurons in developing rats.
- To determine the developmental timeline of appropriate and inappropriate afferent-motoneuron connections.
Main Methods:
- Stimulating ankle flexor and extensor muscle nerves in embryonic and neonatal rat spinal cords.
- Recording synaptic potentials (EPSPs and IPSPs) in identified motoneurons.
- Identifying monosynaptic potentials based on latency and fatigue resistance.
Main Results:
- Around 30% of motoneurons received inappropriate innervation from antagonistic muscle afferents at late gestation (days 18-21).
- These inappropriate connections persisted at birth but significantly decreased within 3-5 days post-birth.
- The reduction in inappropriate synapses correlated with increased spontaneous neural activity and myelination onset.
Conclusions:
- Developing rat spinal cords exhibit a significant proportion of initially inappropriate primary afferent-motoneuron connections.
- The refinement of these connections is a critical developmental process for establishing precise motor control.
- This synaptic refinement is temporally linked to increased neural activity and myelination during early postnatal life.