7/25/2019 fisiologi sp.docx

http://slidepdf.com/reader/full/fisiologi-spdocx 1/2

REFLEXES OF POSTURE AND LOCOMOTION

I. Postural and Locomotive Reflexes of the Cord

a.  Positive Supportive Reaction

Pressure on the footpad of a decerebrate animal causes the limb to extend against the pressure applied to the foot. Indeed, this reflex is so strong that if an animal whose spinal

cord has been transected for several months—that is, after the reflexes have become

exaggerated—is placed on its feet, the reflex often stiffens the limbs sufficiently to

support the weight of the body. This reflex is called the positive supportive reaction.

The positive supportive reaction involves a complex circuit in the interneurons similar to

the circuits responsible for the flexor and cross extensor reflexes. The locus of the

 pressure on the pad of the foot determines the direction in which the limb will extend;

 pressure on one side causes extension in that direction, an effect called the magnet

reaction. This helps keep an animal from falling to that side.

b.  Cord "Righting" Reflexes

hen a spinal animal is laid on its side, it will make incoordinate movements trying to

raise itself to the standing position. This is called the cord righting reflex. !uch a reflex

demonstrates that some relatively complex reflexes associated with posture are integrated

in the spinal cord. Indeed, an animal with a well"healed transected thoracic cord between

the levels for forelimb and hindlimb innervation can right itself from the lying position

and even walk using its hindlimbs in addition to its forelimbs. In the case of an opossum

with a similar transection of the thoracic cord, the walking movements of the hindlimbs

are hardly different from those in a normal opossum—except that the hindlimb walking

movements are not synchroni#ed with those of the forelimbs.

II. Stepping and Walking ovements

a.  Stepping and Walking ovements Rh!thmical Stepping ovements of a Single

Limb.$hythmical stepping movements are fre%uently observed in the limbs of spinal animals.

Indeed, even when the lumbar portion of the spinal cord is separated from the remainder

of the cord and a longitudinal section is made down the center of the cord to block

neuronal connections between the two sides of the cord and between the two limbs, each

hindlimb can still perform individual stepping functions. &orward flexion of the limb is

7/25/2019 fisiologi sp.docx

http://slidepdf.com/reader/full/fisiologi-spdocx 2/2

followed a second or so later by backward extension. Then flexion occurs again, and the

cycle is repeated over and over.

This oscillation back and forth between flexor and extensor muscles can occur even after

the sensory nerves have been cut, and it seems to result mainly from mutually reciprocal

inhibition circuits within the matrix of the cord itself, oscillating between the neurons

controlling agonist and antagonist muscles.

The sensory signals from the footpads and from the position sensors around the 'oints

 play a strong role in controlling foot pressure and fre%uency of stepping when the foot is

allowed to walk along a surface. In fact, the cord mechanism for control of stepping can

 be even more complex. &or instance, if the top of the foot encounters an obstruction

during forward thrust, the forward thrust will stop temporarily; then, in rapid se%uence,

the foot will be lifted higher and proceed forward to be placed over the obstruction. This

is the stumble reflex. Thus, the cord is an intelligent walking controller.

b.  Reciprocal Stepping of pposite Limbs.

If the lumbar spinal cord is not split down its center, every time stepping occurs in the

forward direction in one limb, the opposite limb ordinarily moves backward. This effect

results from reciprocal innervation between the two limbs.

c.  #iagonal Stepping of $ll %our Limbs ark &ime Reflex

If a well"healed spinal animal (with spinal transection in the neck above the forelimb area

of the cord) is held up from the floor and its legs are allowed to dangle. In general,stepping occurs diagonally between the forelimbs and hindlimbs. This diagonal responseis another manifestation of reciprocal innervation, this time occurring the entire distance

up and down the cord between the forelimbs and hindlimbs. !uch a walking pattern is

called a mark time reflex.

d.  'alloping Reflex

*nother type of reflex that occasionally develops in a spinal animal is the gallopingreflex, in which both forelimbs move backward in unison while both hindlimbs move

forward. This often occurs when almost e%ual stretch or pressure stimuli are applied to

the limbs on both sides of the body at the same time; une%ual stimulation elicits the

diagonal walking reflex. This is in keeping with the normal patterns of walking andgalloping, because in walking, only one forelimb and one hindlimb at a time are

stimulated, which would predispose the animal to continue walking. +onversely, when

the animal strikes the ground during galloping, both forelimbs and both hindlimbs arestimulated about e%ually; this predisposes the animal to keep galloping and, therefore,

continues this pattern of motion.