Renshaw cell

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Renshaw cells are inhibitory interneurons found in the gray matter of the spinal cord, and are associated in two ways with an alpha motor neuron.

Firstly, they receive an excitatory collateral from the alpha neuron's axon as they emerge from the motor root, so that they are "kept informed" of how vigorously that neuron is firing.
Secondly, they send their own inhibitory axon to synapse with that alpha neuron.

The rate of discharge of the Renshaw cell is thus broadly proportional to the rate of discharge of the associated motor neuron, and the rate of discharge of the motor neuron is broadly inversely proportional to the rate of discharge of the Renshaw cell. Renshaw cells thus act as "limiters," or "governors," on the alpha motor neuron system, thus helping to prevent muscular damage from tetanus.

Renshaw cells utilize the neurotransmitter glycine as an inhibitory substance that synapses on the alpha motor fibers. Strychnine specifically acts on these cell's ability to control alpha motor neuron firing by binding to the glycine receptors on the motor neuron. This antagonistic poison will thus predispose someone to tetanic contractions, and can prove fatal if the diaphragm becomes involved.