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when and how did h2o come to earth?

Answered below

Explanation:

On piercing his foot, the pain receptors called nociceptors, found in the skin, are activated. Nociceptors exist at the nerve endings of the primary afferent neurons. Signals travel along the first order neurons which have cell bodies within the dorsal root ganglia of the spinal cord. In the dorsal root ganglia, the signals are taken up at the synapse with the second order neurons which have cell bodies in the nuclei of the cranial nerves within the brainstem. These neurons ascend cranially in the spinothalamic tract to the thalamus. The third order neurons lie within the ventral posterolateral nucleus of the thalamus. They terminate at the somatosensory cortex where the signals are being processed and returned as a sensation of pain.