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The changes in the environment are detected by the receptors and transmitted by the sensory neurons to the brain and spinal cord. This information is transmitted in the form of electrochemical signals, which travel from one neuron to the next.  First there is generation of an electrical impulse that is passed down the length of one neuron. Then the electrical impulse is converted to a chemical signal to cross a synapse and reach the next neuron, where it starts a similar electrical impulse.

(Information is transmitted to and from the brain and spinal cord by the peripheral nervous system composed of the cranial and spinal nerves)