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.