Topographical projections of segmental nerves to the frog glutaeus muscle during loss of polyneuronal innervation.

Abstract

The development of synaptic connexions to the frog (Limnodynastes tasmaniansis) glutaeus muscle from segmental nerves 8 and 9 was determined using glycogen depletion, contraction and electrophysiological methods between stages 54 and 60 (Nieuwkoop & Faber, 1975). There was no change in the number of muscle cells in the glutaeus from stage 55 onwards; the maturation of muscle cells was most advanced at the point of nerve entry on the ventral surface of the glutaeus and least advanced at the dorsal surface. Electrophysiological determination of the segmental innervation of the dorsal surface of the muscle in low calcium (0.5 mM) indicated that the muscle was almost uniquely innervated by nerve 8 at stage 54. Weak nerve 9 terminals were detected at stage 54 only if the calcium concentration was raised to 6.0 mM. Innervation by nerve 9 was first detected in low calcium at stage 55. There was then a progressive decrease in the innervation of the glutaeus muscle by nerve 8 in regions apposed to the iliofibularis muscle, until stage 59 when this part of the glutaeus was almost completely innervated by nerve 9. Glycogen depletion of muscle fibres following tetanic stimulation of either segmental nerve confirmed that the region of the glutaeus muscle apposed by the iliofibularis becomes progressively innervated by nerve 9 during development. Contraction studies of the segmental innervation of the glutaeus muscle in normal calcium (2 mM) indicated that the muscle was predominantly innervated by nerve 8 at stage 54. There was then a progressive increase in the number of nerve 9 motor units up to stage 56. The size of individual motor units declined for both nerves 8 and 9 until stage 59, when the average motor unit size was about 9% of the muscle. It is suggested that the decline in the innervation by nerve 8 of the side of the glutaeus muscle apposing the iliofibularis muscle is due to this region of the muscle selectively favouring synapse formation by nerve 9 over nerve 8.

Competition between segmental nerves at end‐plates in rat gastrocnemius muscle during loss of polyneuronal innervation.

“…During postnatal development, motor units to the rat gluteus muscle lose branches in such a way as to sharpen the topographical projection (Brown & Booth, 1983a), at the same time segmental nerves to the rat biceps brachii and pectoral muscles also sharpen their projections (Bennett & Lavidis, 1984a). Similar observations have been made in developing amphibian muscle Bennett & Lavidis, 1986).…”

“…Suppression of transmission from the branches of motor units in such a way as to sharpen the segmento-topic relationship has been observed for the rat glutaeus muscle (Brown & Booth, 1983a, b). Finally suppression of inappropriate segmental innervation has been observed in the early developmental period for both the frog gluteus muscle (Bennett & Lavidis, 1982, 1986 and the rat biceps brachi and pectoral muscles (Bennett & Lavidis, 1984a). This suppression suggests that the formation of synapses by motor axons is favoured in some parts of a muscle over others, depending on the position of the motoneurones in the spinal cord.…”

Rat muscle during post‐natal development: evidence in favour of no interconversion between fast‐ and slow‐twitch fibres.

“…In some rat and frog muscles individual motor units are selectively distributed across the width of the muscle, the rough position of the fibres in the motor unit being related to the segmental levels of the spinal nerve (Bennett & Lavidis, 1984: rat;Bennett & Lavidis, 1986: frog) and even to the rostrocaudal position in the ventral root (Brown & Booth, 1983: rat) at which the motor axon leaves the spinal cord. This segregation is much clearer after the period of synapse elimination than before, indicating that it is at least partly brought about by selective synapse elimination.…”