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Coordinated motor output in the hindlimb of the 7-day chick embryo
Insights
Early chick embryos exhibit organized motor output in leg muscles, suggesting innate neural circuits for movement control before sensory reflexes develop.
Area of Science:
- Neuroscience
- Developmental Biology
- Embryology
Background:
- Spontaneous motility in chick embryos begins around day 7 (stage 31).
- Skeletal muscle control in chick embryos is neurogenic, not myogenic.
Purpose of the Study:
- To investigate the organization of motor output in the chick embryo leg shortly after spontaneous motility begins.
- To understand the neural control mechanisms underlying early embryonic leg movements.
Main Methods:
- Electromyographic recordings were taken from individual ankle muscles (gastrocnemius, peroneus, tibialis) of 7-day chick embryos (stage 31).
- Analysis focused on motor unit activity patterns during spontaneous leg movements.
Main Results:
- Motor unit activity in ankle muscles occurred in alternating bursts and quiescent periods.
- Distinct timing of flexor and extensor muscle activation suggests differential central nervous system connections.
- Findings imply the presence of functional inhibitory synapses in the embryonic lumbosacral CNS.
Conclusions:
- The basic pattern of muscle activation is established early and is similar to older embryos.
- Neural circuits for selective muscle activation appear to be pre-programmed, independent of sensory reflexes.
- Motor output organization suggests a common drive for motoneurons innervating a single muscle.
Abstract:
Electromyographic recordings from individual identified ankle muscles of the 7-day chick embryo (stage 31) were used to determine the organization of motor output at a developmental stage shortly after the onset of spontaneous motility in the leg. During spontaneous motility of the embryo, the electromyographic recordings from the gastrocnemius, peroneus, and tibialis muscles displayed bursts of motor unit activity which alternated with periods of little or no activity. Since the control of skeletal muscle in the chick embryo is neurogenic rather than myogenic, these findings imply that the motoneurons to a given muscle are driven by a common source. Since flexor and extensor muscles are attivated at different times, different central connections to flexor and extensor motoneurons must be present in the central nerbous system of the 7-day embryo. Moreover, since inhibition is known to play an important role in the selective activation of agonist and antagonist muscles, the present results suggest that functional inhibitory synapses may be present in the lumbosacral central nervous system at this stage of development. The basic pattern of muscle activation observed in the 7-day embryo is similar to that seen in older embryos. Since these patterns appear prior to the time at which motor responses to sensory stimulation of the leg can be demonstrated, it is likely that the neural pattern generating circuits for selective activation of muscles are established in the central nervous system without reliance on functional reflexes.