Reciprocal axo-axonal synapses between the common inhibitor and excitor motoneurons in crustacean limb muscles.
Nerve terminals of the common inhibitor motoneuron in a crab (Eriphia spiniforns) limb closer muscle and in a crayfish (Procambarus clarkii) limb accessory flexor muscle make neuromuscular synapses with the muscle membrane (postsynaptic inhibition) as well as axo-axonal synapses with the terminals of the excitatory axon (presynaptic inhibition). That transmission is from the inhibitor to the excitor terminals at these axo-axonal synapses is indicated by the occurrence on the inhibitor membrane of presynaptic dense bars denoting sites of transmitter release. Axo-axonal synapses with the opposite polarity, in which transmission is from an excitatory onto an inhibitory terminal, were occasionally seen either adjacent to or separate from the inhibitory axo-axonal synapse. Nerve terminals of the specific inhibitor in the crayfish opener muscle were seen to make numerous axo-axonal output synapses upon excitatory nerve terminals but excitor nerve terminals were not seen to make output synapses onto inhibitor terminals. Thus reciprocal axo-axonal synapses appear to be a feature of the common inhibitor but not of the specific inhibitor. The excitor-to-inhibitor component of these reciprocal synapses may serve to limit transmitter output in the common inhibitor axon by activating glutamateB receptors which facilitate efflux of K+ and hyperpolarization of the membrane.
Pearce
J
J
Life Sciences Division, Scarborough College, University of Toronto, Ontario, Canada.
Govind
C K
CK
D000818:Animals; D003400:Astacoidea; D001369:Axons / Q000502:physiology* / Q000648:ultrastructure; D003386:Brachyura; D005121:Extremities; D008854:Microscopy, Electron; D009046:Motor Neurons / Q000502:physiology* / Q000648:ultrastructure; D009132:Muscles / Q000502:physiology / Q000648:ultrastructure; D009411:Nerve Endings / Q000502:physiology; D009469:Neuromuscular Junction / Q000502:physiology* / Q000648:ultrastructure; D013569:Synapses / Q000502:physiology* / Q000648:ultrastructure