Anatomy of an itch
In a common itch like a mosquito bite, cells in the skin release a
chemical called histamine. Certain sensory neurons respond to the
histamine and carry the "itch" message up to the spinal cord. There the
message is relayed to other neurons that run to a part of the brain
called the thalamus. These neurons are part of a bundle of spinal
neurons called the spinothalamic tract, or STT.
From the thalamus, the "itch" message is passed to the cerebral
cortex, which interprets the signals and produces the sensation of itch.
From the Discover Magazine: "How Does Scratching Relieve an Itch?" (2)
Neuroscientists now say they’ve found part of the answer in a new study of macaque monkeys. Previous research has suggested that a specific part of the spinal cord – the spinothalamic tract – plays a key role. Nerve cells in this area have been shown to be more active when itchy substances are applied to the skin. The latest work … found that scratching the skin blocks activity
of nerve cells in the spinothalamic tract during itchiness – preventing the spinal cord from transmitting signals from the scratched area of skin to the brain.