When a single neuron fires, the active potential rapidly changes the concentration of sodium (Na+) ions inside the cell and potassium (K+) ions outside the cell. Within milliseconds, thousands of sodium-potassium ion channels in the thin lipid bilayer of the cell wall must move billions of those ions, two or three at a time between inside and outside the cell wall, to restore the reduced chemical potential.
Ion channels do it with emergent biological machinery that exerts downward causation on the ions, powered by ATP energy carriers (feeding on negative entropy). Random quantum indeterministic motions of the ions put some near the pump opening, and quantum collaborative forces capture them in a lock-and-key structure.
The sodium-potassium pump in our neurons is as close to a Maxwell's Demon as anything we are ever likely to see.
When many motor neurons fire, innnervating excitatory post-synaptic potentials (EPSPs) that travel down through the thalamus and the spinal cord where they cause muscles to contract, that is as literal as downward causation gets between the mind and the body.