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Last updated: Sun, Mar 9, 2025
A number of phenomena related to inhibition and facilitation of the spinal cord were described in Pain Phenomena of the Spine and Periphery. The mechanisms of inhibition and facilitation that are described in the current section work together with the mechanisms that are local to the spinal cord and the sensory neurons.
The tonic or steady-state effect of the brain and brain stem on the spinal cord is inhibitory. This effect comes in part from the brain stem and the midbrain, since it persists even if the forebrain (the cortex) is separated from the rest of the CNS. The forebrain also has an effect. When the forebrain is separated, for example, lamina VI neurons that normally respond to muscle stretch respond also to cutaneous stimulation.
The descending control system is not regulated just by nociceptive stimulation. Activation of muscles has an effect on nociception.
The withdrawal reflex, which was described earlier as a simple reflex occurring in the spinal cord, is actually regulated by the descending control system. A type of cell in the RVM normally sends a continual “off” signal down to the spinal cells that provide the reflex. These cells must be shut down before the withdrawal reflex can occur. This mechanism allows the withdrawal reflex to be suppressed when that is needed.