Dog Leg Spasm & Scratching the Tummy

Dog owners love to show their friends how they can make their dog's leg go into a spasm when they scratch their dog's tummy. Showing is one thing but explaining why it happens is another. The most plausible explanation is the scratch reflex, which is best described in a 1906 study published in the Journal of Physiology, as the rhythmic scratching movement of the hind leg in response to stimuli.

  1. Scratch Reflex

    • When a dog owner scratches a dog's tummy, a nerve impulse is activated, traveling to the dog's spinal cord. The spinal cord sends a message to activate the dog's hind leg, resulting in what appears to be a leg spasm.

    Sherrington's 1906 Study

    • The dog's leg spasm in response to a tummy scratch is actually an involuntary reflexive response. Charles Sherrington, in a 1906 study of the scratch reflex in dogs, noted that in former studies, evidence suggested this reflexive response was closely connected with the dog's hair roots. Sherrington noted however, that the scratch reflex also could be triggered on hairless areas, such as the dog's nipples.

    The Science of the Leg Spasm

    • The scratch reflex results from the wiring of nervous connections between the sensory receptors in the dog's tummy, motor neurons and interneurons located in the dog's brain stem and spinal cord. The scratch reflex is such a reliable tool that veterinarians use it to assess nerve and spinal cord function.

    Other Potential Explanations

    • Dog owners should be wary of always attributing a leg spasm to the scratch reflex. Leg spasms also can indicate underlying medical conditions that may require veterinary attention.