How does a nervous stimulus enter the spinal cord?

A nervous stimulus enters the spinal cord primarily via the posterior horn and the spinal ganglion. The spinal ganglia, which are clusters of nerve cell bodies located just outside the spinal cord, contain sensory neurons that transmit information from the periphery of the body to the central nervous system. When a stimulus occurs, sensory neurons are activated and their axons enter the spinal cord through the dorsal (posterior) roots. This pathway allows the sensation to be processed and either result in a reflex action or be sent to the brain for further processing.

The input through the posterior horn is crucial because it serves as the entry point for sensory information. The anatomy of the spinal cord features the anterior horns primarily associated with motor functions, where motor neurons reside. The options pertaining to direct connections from the brain do not accurately reflect the typical sensory pathway, as most sensory information reaches the brain only after passing through the spinal cord. Similarly, while the lumbar region contains spinal nerves, the principle mechanism for the entry of nervous stimuli involves the posterior horn and spinal ganglion.