This 3D printed plate is displayed below a rendering under the title "Spinal Cord" in the exhibit, although it is a 3D print showing a cross section of the brain.

Cajal observed that motor neuron axons originated at the end of the neuron facing away from the brain. Although he did not know the nature of the signal being sent, Cajal hypothesized that the command to move began in the motor cortex of the brain and traveled down the spinal cord to the muscles involved.

In the same way, he noticed that the axons of sensory neurons originated at the end of the neuron that was closer to the brain and inferred that sensory input travels up the spinal cord to be processed by the brain.

These observations were instrumental in the formulation of his ‘Law of Dynamic Polarization,’ which was extended further by Sir Charles Sherrington, winner of the 1932 Nobel Prize in Physiology or Medicine, who showed that spinal cord reflex circuits involved “reciprocal innervation” of opposing muscles.

This plate was created for an exhibit in the Porter Neuroscience Building at the NIH and is being provided here, courtesy of the Cajal Institute (Neuroscience Research Center of the Spanish Main Research Council) Producido con el permiso de El Instituto Cajal (IC), Consejo Superior de Investigaciones Científicas (CSIC).  As such, it's being shared under a creative commons license, so that reuse of the materials are attributed to the museum.