Renshaw cell

**Anatomy/Physiology:** - Location: ventral (anterior) horn of the spinal cord gray matter, near alpha motor neurons. - Input: recurrent axon collaterals of alpha motor neurons; acetylcholine is the excitatory transmitter, acting on both nicotinic and muscarinic receptors on the Renshaw cell. - Output: inhibitory synapses onto the same alpha motor neuron (and neighboring ones, as well as Ia inhibitory interneurons) using glycine (predominant) and GABA. - Function: recurrent (feedback) inhibition. It stabilizes and limits motor neuron output, prevents excessive or prolonged firing, sharpens motor signals through lateral inhibition (the active motor neuron pool is focused while neighbors are suppressed), and helps regulate the firing frequency of motor neurons.

**Clinical correlations (high-yield):** - **Tetanus toxin (Clostridium tetani):** a zinc-dependent protease (tetanospasmin) that is retrogradely transported to the spinal cord and cleaves SNARE proteins (synaptobrevin/VAMP). It blocks release of glycine and GABA from Renshaw cells and other inhibitory interneurons. The result is unchecked alpha motor neuron firing, causing spastic paralysis (rigidity, trismus/lockjaw, risus sardonicus, opisthotonos). - **Strychnine poisoning:** a competitive antagonist at postsynaptic glycine receptors (on the motor neuron). It causes the same loss of Renshaw-mediated inhibition, with violent muscle contractions and convulsions while consciousness is preserved. - **Contrast with botulinum toxin:** it also cleaves SNARE proteins but acts at the peripheral neuromuscular junction, blocking ACh release and causing flaccid paralysis. - **Pharmacology:** anticholinesterases and other drugs that raise ACh at Renshaw synapses increase recurrent inhibition. Strongly activating Renshaw cells can reduce motor neuron output. Cholinergic agents act on them through nicotinic receptors, and nicotinic antagonists such as dihydro-β-erythroidine block this excitation. - **Other associations:** altered Renshaw cell function has been implicated in spasticity and in ALS (reduced recurrent inhibition is reported), but this is mostly beyond Step 1 scope.

**Mnemonic:** Renshaw = "Return/Recurrent" inhibition. The motor neuron's own collateral excites the cell, which then shuts the motor neuron down. Remove the brake (tetanus toxin, strychnine) and you get spasm.

**Board pearls:** - Transmitter at the alpha motor neuron to Renshaw cell synapse: ACh. Transmitter from Renshaw cell to motor neuron: glycine. - Renshaw cells are a classic example of negative feedback in the nervous system. - Tetanus = inhibitory neurotransmitter release blocked (spastic paralysis). Botulism = ACh release blocked (flaccid paralysis).