What is Scratch Reflex?

Scratch reflex is a response to activation of sensory neurons whose peripheral terminals are located on the surface of the body. During a scratch reflex, a nearby limb reaches toward and rubs against the site on the body surface that has been stimulated.

Rubbing the skin in response to itching is a type of scratch reflex.

For scratch reflex, the sensory neurons can be activated by stimulation with an external object. Presence of a parasite on the skin is one such example.

Some sensory neurons can respond to a chemical stimulus that produces an itch sensation.

Key aspects of the scratch reflex are still unknown, such as the neural mechanisms by which the reflex is terminated.

The scratch reflex is generally a rhythmic response.

Experimental studies have revealed that  spinal neural networks known as central pattern generators (CPGs) are responsible for the generation and maintenance of the reflex.

It is interesting to note that  supraspinal structures are not necessary for the generation of the reflex. The scratch response is programmed into the spinal cord.

These spinal CPGs are capable of producing the reflex in the absence of movement-related sensory feedback.

Few Definitions related to Scratch Reflex

Form

A movement-related strategy used by the animal to perform the scratch. To scratch the upper back, humans are limited to one scratch form, involving the elbow raised above the shoulder to provide access to the upper back.

Pure form domain

A region on the surface of the body, that when stimulated, elicits only one form of the scratch reflex.

Transition zones

These  can be successfully targeted by more than one form of the reflex. It usually lies at the boundary of two pure form domains.

Pure movement

It is a movement in which only one form of the scratch response is utilized to respond to the stimulus.

Switch movement

It  occurs in a transition zone, and is characterized by the smooth switching between two different scratch forms in response to the stimulus.

Hybrid movement

It is seen in transition zones as well, and is characterized by two rubs during each scratch cycle. Findings on switch and hybrid movements suggest a modular organization of unit generator CPGs used in combination to achieve a task.

It is noteworthy about scratch reflex is that a scratch response continues even after afferent input from the stimulated zone ceases.

For a few seconds after the cessation of the scratch, the neural networks involved in the generation of the scratch reflex remain in a state of heightened sensitivity. During this, stimuli normally too weak to trigger a scratch response are capable of eliciting a scratch response in a site specific manner.

Though  the scratch reflex is programmed into the neural circuits of the spinal cord but the neurons in the motor cortex play a role in the modulation of the scratch reflex as well.