Local Static Reflex

Local Static Reflexes or Supporting Reflexes

Local static reflexes or supporting reactions support the body in different positions against gravity and also protect the limbs against hyperextension or hyperflexion.

Supporting reactions are classified into two types:

1. Positive supporting reflexes

2. Negative supporting reflexes.

1. Positive supporting reflexes

Positive supporting reflexes are the reactions, which help to fix the joints and make the limbs rigid like pillars, so that limbs can support the weight of the body against gravity. It is brought about by the simultaneous reflex contractions of both extensor and flexor muscles and other opposing muscles. The impulses for these reflexes arise from proprioceptors present in the muscles, joints and tendons and the exteroceptors, particularly pressure receptors present in deeper layers of the skin of sole. While standing, the positive supporting reflexes are developed in the following manner:

i. When an animal stands on its limbs, the pressure of the animal’s paw upon the ground produces

proprioceptive impulses from flexor and extensor muscles of the limbs, particularly in

terminal segments of the limbs like digits, ankle or wrist. The proprioceptive impulses cause reflex contraction of the muscles of limbs making the limbs rigid.

ii. Excessive extension at the joints is checked or guarded by the myotatic reflexes setting up in

the flexor muscles. When the flexor muscles are simultaneously contracting, extensor muscles

cannot be stretched beyond the physiological limits. Similarly, over activity of the flexor muscles is prevented by the stretch reflexes developed in the extensor muscles.

 

iii. Impulses arise even from exteroceptors while standing, when the sole remains in contact

with the ground. It causes stimulation of the pressure receptors, which are present in deeper layers of the skin. These impulses from pressure receptors reinforce the rigidity of the limbs caused by the proprioceptive impulses.

2. Negative supporting reflexes

Relaxation of the muscles and unfixing of the joints enable the limbs to flex and move to a new position.

It is called negative supporting reaction. It is brought about by raising the leg off the ground and plantar

flexion of toes and ankle. When the leg is lifted off the ground, the exteroceptive impulses are stopped.

When the toes and ankle joints are plantar flexed, the stretch stimulus for the plantar muscles is stopped. So, unlocking of the limbs occurs. Moreover, by the plantar flexion of the toes and ankle, the dorsiflexor muscles are stretched, causing relaxation of the extensors and contraction of the flexors of the knee.

The positive and negative supporting reactions are demonstrated well on a decorticate animal. The

centers for the supporting reflexes are located in the spinal cord.

III. Segmental Static Reflexes

Segmental static reflexes are very essential for walking. During walking, in one leg, the flexors are active

and the extensors are inhibited. On the opposite leg, the flexors are inhibited and extensors are active.

Thus, the flexors and extensors of the same limb are not active simultaneously. It is known as crossed

extensor reflex. It is due to the reciprocal inhibition and the neural mechanism responsible for this reflex

is called Sherrington reciprocal innervation.  Segmental static reflexes are demonstrated in

spi nal animal. And, the centers for these reflexes are situated in the spinal cord.

IV. Statotonic or Attitudinal Reflexes

Statotonic or attitudinal reflexes are developed according to the attitude of the body and are of two types:

1. Tonic labyrinthine and neck reflexes acting on the limbs

2. Labyrinthine and neck reflexes acting on the eyes.

1. Tonic labyrinthine and neck reflexes acting on the limbs

Tonic labyrinthine and neck reflexes decrease or increase the tone of the skeletal muscles of the limbs

in accordance to the attitude or position of the head. These reflexes are best studied in decerebrate animal. The proprioceptors concerned with these reflexes are in the labyrinthine apparatus. Whenever the position of the head is altered, the receptors present in the labyrinth are stimulated and generate impulses. The impulses are also generated from the neck muscles when the position of the head is altered. The impulses from labyrinth produce the same effect on all the four limbs. But the impulses from neck muscles cause opposite effects in the forelimbs and hind limbs.

The labyrinthine reflexes are particularly effective on extensor muscles. When the head is dorsiflexed, all

the four limbs are extended maximally and when the head is ventriflexed, all the four limbs are flexed. In a labyrinthectomized animal where only neck reflexes are operated, during dorsiflexion of the head,

there is extension of the forelimbs and flexion of the hind limbs. The ventriflexion of the head causes flexion of the forelimbs and extension of the hind limbs. The importance of these reflexes is understood

well, while observing the movements during change in the attitude of a normal animal. When an animal

turns to one side, the limbs of that side become rigid in order to support the weight of the body. A cat looking upwards, keeps the hind limbs flexed but forelimbs remain extended. It gives a suitable inclination to the back of the animal, which improves the positions of the head and eyes. When the cat looks down, forelimbs are flexed and hind limbs are extended, giving the proper supported inclination at the neck region.

2. Labyrinthine and neck reflexes acting on the eyes

According to the changes in the position of the head and neck, the eyes are also moved. These reflexes

arise from labyrinth and neck muscles. Turning the head downward causes upward movement of the eyes. The eyes remain in this position as long as the position of

the head is retained. When the head is moved down, the tone in the superior recti and inferior oblique are increased and tone of inferior recti and superior oblique are reduced, so that the eyeballs move upwards. When the head is turned to one side, a corresponding compensatory movement of the eyes occurs.

When the head is turned to one side, the eyes deviate outward or inward in relation to the head. The

eyes are moved in a direction opposite to that of the head movement. It is because of external and internal recti. Centers for statotonic reflexes are present in the medulla oblongata.

STATOKINETIC REFLEXES

Statokinetic reflexes are the postural reflexes that maintain posture during movement. These reflexes

are concerned with both angular (rotatory) and linear (progressive) movements. The vestibular apparatus

is responsible for these reflexes. So, it is essential to study the structure and functions of vestibular apparatus to understand the statokinetic reflexes.

 

 

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