Reciprocal axo-axonal synapses between the common inhibitor and excitor motoneurons in crustacean limb muscles |
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Authors: | J. Pearce and C. K. Govind |
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Affiliation: | (1) Life Sciences Division, Scarborough College, University of Toronto, 1265 Military Trail, M1C 1A4 Scarborough, Ontario, Canada |
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Abstract: | Summary Nerve terminals of the common inhibitor motoneuron in a crab (Eriphia spinifrons) 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. |
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