Scratch Reflex

An especially important cord reflex in some animals is

the scratch reflex, which is initiated by itch or tickle sensation.

It involves two functions: (1) a position sense that

allows the paw to find the exact point of irritation on

the surface of the body, and (2) a to-and-fro scratching

movement.

The position sense of the scratch reflex is a highly

developed function. If a flea is crawling as far forward

as the shoulder of a spinal animal, the hind paw can still

find its position, even though 19 muscles in the limb

must be contracted simultaneously in a precise pattern

to bring the paw to the position of the crawling flea. To

make the reflex even more complicated, when the flea

crosses the midline, the first paw stops scratching and

the opposite paw begins the to-and-fro motion and

eventually finds the flea.

The to-and-fro movement, like the stepping movements

of locomotion, involves reciprocal innervation

circuits that cause oscillation.

Galloping Reflex. Another type of reflex that occasionally

develops in a spinal animal is the galloping reflex,

in which both forelimbs move backward in unison

while both hindlimbs move forward.This often occurs

when almost equal stretch or pressure stimuli are

applied to the limbs on both sides of the body at the

same time; unequal stimulation elicits the diagonal

walking reflex. This is in keeping with the normal patterns

of walking and galloping, because in walking,

only one forelimb and one hindlimb at a time are stimulated,

which would predispose the animal to continue

walking. Conversely, when the animal strikes the

ground during galloping, both forelimbs and both

hindlimbs are stimulated about equally; this predisposes

the animal to keep galloping and, therefore, continues

this pattern of motion.