"a(n) is a neuron that carries information from the external environment, or from the body, back to the central nervous system."

The given blank can be filled with sensory neuron.

A neuron is the cell, which carries the neural signals from one place another in the body of the organism. The neurons, which carry the message from the brain to body is called motor neuron.

The neuron, which carry the message from the body to brain is the sensory neuron. The external stimuli is detected by the receptors present in various location of the body. These receptors generates the action potentials in the sensory neurons as a response to the stimulus.