Tip a virtual platform under a real 3D ankle. Discover why one muscle gets more drive while its partner gets less, follow the spinal circuit, and change one connection to see the difference.
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What supports the explanation?
Human ankle pathways support reciprocal Ia inhibition
Crone et al. studied 60 humans using conditioned soleus and tibialis-anterior H reflexes. The extra central delay was inferred under a symmetry assumption; our drive gains are not taken from their data.
Human muscle spindle responses depend on mechanical conditions
Original recordings from human muscle spindle afferents during vibration, length and movement conditions. The lesson does not reproduce their firing rates.
Walking research separates measurements from estimated neural activity
Gervasio et al. 2017, PLOS ONE 12(1):e0168557. Complete Figure 3 documents the setup and is reused under CC BY 4.0. This paper is separate from Suzuki’s standing/walking comparison.
Edwards et al. original human study of task-related arousal, reflex threshold and pain ratings. The lesson uses the distinction, not a personal calibration.
Anatomy comes from selected BodyParts3D 4.0 right-sided muscles, tendon and bones. The platform, spindle locator, articulation and functional circuit are authored illustrations, not an observed reflex in motion.
The ankle uses an approximate single pivot and a smooth deformation of selected soft tissues. The platform prescribes the foot pose. Outputs do not solve joint torque, balance, muscle force, tendon strain or realistic spindle dynamics.
Model drive is a normalized teaching quantity. The gains and playback stages are authored; there are no physiological milliseconds, Hz, EMG or force units attached to them. Removing inhibition is a counterfactual circuit comparison, not a diagnosis.
The circuit selects one Ia stretch-response branch. It omits many neurons, inputs, presynaptic regulation, descending influences and task-dependent dynamics. Inhibition is shown at the opposing motor pool.
Withdrawal and crossed-response evidence concerns different inputs and experiments. The standing/walking comparison is qualitative evidence about an H reflex, not a gait simulator.
The scientific figure is a static diagram of a research setup. It is not a photograph, a result graph or a home experiment. Source geometry and figure attribution are downloadable. Independent subject and learner review of the finished lesson remains pending.
A sensory route and a motor route: Sensory fibers enter through dorsal, or posterior, roots. The sensory cell body is in a dorsal-root ganglion outside the cord; it is not an extra compulsory relay between two neurons. Motor axons leave through ventral, or anterior, roots. Our detached diagram has dorsal at the top and ventral at the bottom. It does not align spinal segments with vertebral levels.
Count central synapses carefully: The selected direct Ia afferent-to-motor-neuron branch has one central synapse. The reciprocal branch includes an inhibitory interneuron, giving two central synapses. Both include additional events elsewhere, including the neuromuscular junction. Neither count is a complete description of every stretch-related response.
The small model we actually calculate: Let u be authored stretch input from 0 to 1, b background drive, and g be 1 with the inhibitory connection or 0 with it removed. At the completed teaching stage: calf drive = clamp(b + 0.5u); opposing drive = clamp(b − 0.3gu). Clamp keeps outputs in [0,1]. The gains 0.5 and 0.3 are chosen for a readable comparison. They are not fitted nerve, EMG, force or movement measurements. During playback we reveal the result in stages, on an illustrative clock.
An extra relay is not the whole response time: Crone and colleagues studied reciprocal ankle pathways in 60 humans in 1987 using conditioned H reflexes. Assuming symmetric central connections, they inferred about 1 ms of additional central delay relative to monosynaptic Ia excitation. That comparison does not measure the whole time from an everyday stimulus to movement, and it says nothing about when someone becomes aware of pain.
Fast feedback can include the cortex: Pruszynski and colleagues combined human and rhesus-monkey experiments in 2011 to investigate rapid multijoint feedback and motor-cortex contributions. This evidence expands the simple spinal picture. It does not make our ankle illustration a model of every arm response or assign one delay to all people.
The other side changes with the task: Suzuki and colleagues compared crossed cutaneous effects on a soleus H reflex in humans. They found facilitation while standing at selected conditioning-to-test intervals, but suppression during early stance in walking. The measured endpoint was electrically probed reflex excitability, not whole-leg force. Gervasio and colleagues separately studied crossed responses during walking and distinguished measured data from modeled afferent activity.
Reflex measures are not a pain meter: Nociception concerns neural processing of potentially tissue-damaging input. Pain is a personal sensory and emotional experience. Experimental reflex measures and reported pain can dissociate. This lesson neither assigns a pain score nor tests a learner’s neurological function.
What has been checked
Analytical reference cases, conservation or transition invariants, finite drawing commands, bounded setup parsing, discovery and route integrity are checked automatically. These checks do not establish anatomical fidelity, learner outcomes or browser/device compatibility. Independent subject review, learner trials, comprehensive accessibility review and browser video encoding checks remain pending.
Each source supports the associated claim. Sources do not certify this implementation or its visuals.