When a GABAergic neuron synapses onto the axon of a sensory neuron, what happens?
Turns out, in the proprioceptors of the locust leg (chordotonal somatosensory neurons), "All the available evidence indicates that the inputs that occur during walking are depolarizing, inhibitory, synaptic inputs."
Depolarizing, inhibitory. How's that possible? Depolarizing because of where the resting potential of the axon is (which is different than the dendrite's), and the particularities of the GABA receptor on the receiving axons. Inhibitory because the depolarization is subthreshold, reducing the membrane voltage potential and, therefore, reducing neurotransmitter release when a spike arrives.
What is this good for? Implementing an efference copy signal: "The rhythmic depolarization at different phases of the step cycle in different sensory neurons may represent a predictive action of the CNS when generating a motor pattern." That is, predicting when the proprioceptor will fire, and by how much, as a function of the self-induced motion. So this signal is subtracted in order to be able to measure (and react to appropriately) any unexpected signal (such as the leg skidding or hitting a rock).
Wolf and Burrows, 1995. "Proprioceptive sensory neurons of a locust leg receive rhythmic presynpatic inhibition during walking" https://www.jneurosci.org/content/15/8/5623.short
Burrows and Matheson, 1994. "A presynaptic gain control mechanism among sensory neurons of a locust leg proprioceptor" https://www.jneurosci.org/content/14/1/272.short
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