Spinal motor circuits: interneurons, posture and stepping
- Concept
- For completeness
The spinal cord is not just a cable: its own circuits can support weight, step, scratch and lock muscles in spasm.
Also called: Renshaw cells, Central pattern generators, Recurrent inhibition
See it in 3D: Spinal cord · Grey matter of the cord
What it is
The anterior horn holds alpha motor neurons, gamma motor neurons and far more interneurons, which shape every command before it reaches muscle. Inhibitory interneurons such as Renshaw cells sharpen and limit motor output. Isolated cord circuits can produce supporting reactions, rhythmic stepping and scratching, and reflex spasm around painful tissue.
Key facts
- Positive supportive reaction: pressure on the sole makes the limb stiffen into extension so it can bear weight.
- Cord righting: even a spinal animal makes coordinated attempts to move toward an upright position.
- Stepping in one limb is coupled to the opposite limb (reciprocal stepping) and diagonally between fore- and hindlimbs in quadrupeds.
- The scratch reflex finds the irritated spot (position sense) and produces a to-and-fro rhythmic movement.
- Reflex spasm: pain from a fracture or an inflamed peritoneum drives local muscles into sustained contraction (splinting, abdominal guarding).
- Cramps can arise when local irritation or ischemia starts a contraction that excites its own pain and stretch receptors, a positive feedback loop.
How it works, step by step
- Descending tracts and sensory fibers synapse mostly on interneurons, not directly on motor neurons.
- Interneuron networks combine inputs, set reciprocal inhibition of antagonists and coordinate several segments.
- When a motor neuron fires, an axon collateral excites Renshaw cells, which inhibit that neuron and its neighbors with glycine.
- This recurrent inhibition limits runaway firing and sharpens the boundary between active and inactive motor pools.
- Remove the glycine brake (tetanus toxin, strychnine) and minor inputs cause intense, sustained co-contraction of agonists and antagonists.
Clinical notes
- Board-like rigidity of the abdominal wall in peritonitis is a cord reflex spasm triggered by inflamed parietal peritoneum.
- Baclofen, a GABA-B agonist, reduces spasticity by dampening cord reflex circuits.
Test yourself
Think of the answer, then tap the question to see it.
What do Renshaw cells do, and which transmitter do they use?
Excited by motor-axon collaterals, they inhibit the same and nearby motor neurons with glycine (recurrent inhibition), limiting and focusing motor output.
Why does tetanus cause trismus and generalized spasms?
The toxin travels into the cord and blocks glycine and GABA release from inhibitory interneurons, so motor neurons fire without restraint.
What is a central pattern generator?
A spinal interneuron network that produces rhythmic alternating flexor and extensor activity, such as stepping, without needing rhythmic input from the brain.
Why does the abdominal wall become rigid in peritonitis?
Pain signals from inflamed parietal peritoneum drive a cord reflex that keeps the overlying muscles in sustained contraction (guarding).
Connections
See it in 3D2
The structures of Durgical's 3D anatomy where it happens.
In Biochemistry1
In Pharmacology1
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Neuroanatomy, Spinal Cord Morphology (StatPearls (NCBI Bookshelf), 2025); Neuroanatomy, Gray Matter (StatPearls (NCBI Bookshelf), 2025).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.