Scratch reflex

The scratch reflex is a response to activation of sensory neurons whose peripheral terminals are located on the surface of the body. Some sensory neurons can be activated by stimulation with an external object such as a parasite on the body surface. Alternatively, some sensory neurons can respond to a chemical stimulus that produces an itch sensation. During a scratch reflex, a nearby limb reaches toward and rubs against the site on the body surface that has been stimulated. The scratch reflex has been extensively studied to understand the functioning of neural networks in vertebrates. Despite decades of research, key aspects of the scratch reflex are still unknown, such as the neural mechanisms by which the reflex is terminated. This article will focus on the neurological aspects of the reflex.

A number of animal models have been used to study, understand and characterize the scratch reflex. These models include the turtle, cat, frog, dog, and a variety of other vertebrates. In these studies, researchers made use of spinal preparations, which involve a complete transection of the animal's spinal cord prior to experimentation. Such preparations are used because the scratch reflex can be elicited and produced without the involvement of supraspinal structures. Researchers focused predominantly on investigating spinal cord neural circuitry responsible for the generation of the scratch reflex, limiting the system of study.

In studies of spinal preparations, researchers have experimented using preparations both with and without movement-related sensory inputs. In preparations with movement-related sensory inputs, the muscles and the motor neuron outputs to muscles are left intact, allowing sensory feedback from the moving limb. In preparations without movement-related sensory input, either of three strategies are used: (1) the axons of sensory neurons are cut by dorsal root transection; or (2) neuromuscular blockers are used to prevent contractions of muscles in response to motor neuron activity; or (3) the spinal cord is isolated in a bath of physiological saline.

Electromyographic (EMG) and electroneurographic (ENG) techniques are used to monitor and record from animals during experiments. EMG recordings are used to record electrical activity directly from muscles. ENG recordings are used to record electrical activity from motor neurons and spinal cord neurons. These techniques have enabled researchers to understand the neural circuitry of the scratch reflex on a single-cell level.

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