Presynaptic inhibition · Lobeworks/17

Presynaptic inhibition is a form of inhibition in which an interneuron synapses onto the terminal of another axon and reduces how much transmitter that terminal releases, and in the spinal cord it lets the nervous system turn down chosen afferent signals before they climb to the brain.

Presynaptic inhibition. Presynaptic inhibition is a form of inhibition in which an interneuron synapses onto the terminal of another axon and reduces how much transmitter that terminal releases, and in the spinal cord it lets the nervous system turn down chosen afferent signals before they climb to the brain.

The interneurons are GABAergic and make axo-axonic contacts on the terminals of sensory fibres. Opening GABA-A receptors there lets chloride out of the terminal, which depolarises it slightly (primary afferent depolarisation, PAD) and adds a conductance; both shrink the spike that reaches the release sites, so less calcium enters and fewer vesicles are released. GABA-B receptors on the same terminals reduce release without the depolarisation. The receiving neuron is left untouched, which is the point: one input can be silenced while the others onto the same cell keep their strength.

The brain decides what comes up. Pathways descending from the cortex and brainstem drive these interneurons, so the flow of touch, pain and muscle sense is regulated at the first relay, depending on the task.

It is selective where postsynaptic inhibition is not. Inhibiting the receiving cell quiets everything it hears; inhibiting a terminal quiets one source.

The classic view is being refined. Recent work on Ia fibres finds that the same GABA can help spikes pass the branch points of the axon, so its net effect depends on where along the fibre it acts.

The spinal cord edits sensation before the brain reads it.

By turning down chosen terminals it decides which signals pass, and how loudly.

Questions: How can the spinal cord turn down one sensory input without silencing the neuron that receives it? By acting on the terminal instead of the receiving cell. GABAergic interneurons synapse onto the endings of a sensory fibre; opening GABA-A receptors there depolarises the terminal slightly and adds a conductance, so the spike that reaches the release sites is smaller and fewer vesicles are released. The receiving neuron keeps its other inputs at full strength, and descending pathways from the brain decide which terminals are turned down.