Structural Reflex

 STRETCH REFLEX

This reflex is normally present and serves particularly to maintain the body in an upright position. Such reflexes are, therefore more pronounced in extensor muscles.

POSTURAL REFLEXES

Postural reflexes are the reflexes which are responsible for maintenance of posture. Afferent impulses for the maintenance of posture arise from proprioceptors, vestibular apparatus and retina of eye and reach the centers in central nervous system (CNS). The centers, which maintain the posture, are located at different levels of CNS particularly cerebral cortex, cerebellum, brainstem and spinal cord. These centers send motor impulses to the different groups of skeletal muscles so that appropriate movements occur to maintain the posture.

CLASSIFICATION OF POSTURAL REFLEXES

Postural reflexes are generally classified into two groups:

A. Static reflexes

B. Statokinetic reflexes.

STATIC REFLEXES

Static reflexes are the postural reflexes that maintain posture at rest. Static reflexes are of four types:

I. General static reflexes or righting reflexes

II. Local static reflexes or supporting reflexes

III. Segmental static reflexes

IV. Statotonic or attitudinal reflexes.

I. General Static Reflexes or Righting Reflexes

General static reflexes are otherwise called righting reflexes because these reflexes help to maintain an

upright position of the body. Righting reflexes help to govern the orientation of the head in space, position

of the head in relation to the body and appropriate adjustment of the limbs and eyes in relation to the

position of the head, so that upright position of the body is maintained. When a cat, held with its back downwards, is allowed to fall through the air, it lands upon its paws, with the head and body assuming the normal attitude in a flash. A fish resists any attempt to turn it from its normal position and if it is placed in water upon its back, it flips almost instantly into the normal swimming position. All these actions occur because of the righting reflexes. Righting reflexes consist of a chain of reactions, which occur one after another in an orderly sequence. Each reflex causes the development of the succeeding one.

Righting reflexes are divided into five types:

1. Labyrinthine righting reflexes acting on the neck muscles

2. Neck righting reflexes acting on the body

3. Body righting reflexes acting on the head

4. Body righting reflexes acting on the body

5. Optical righting reflexes.

First four reflexes are easily demonstrated on a thalamic animal or a normal animal, which is

blindfolded.

1. Labyrinthine righting reflexes acting on the neck muscles

When a thalamic animal (rabbit) is suspended by holding at the pelvic region, its head turns up, until it

assumes its normal position. It is because of reflexes arising from labyrinth, the sensory organ concerned with equilibrium of head, in regard to the position of the body. Turning the body of animal through air into different positions is followed by compensatory movements of the head. After extirpation of labyrinths, the head shows no compensatory movements when the rabbit is suspended. It hangs simply like that of a dead rabbit

2. Neck righting reflexes acting on the body

It is noticed that during labyrinthine righting reflexes, the head raises up to normal position. It is because of the contraction of neck muscles. Now, the contraction of neck muscles produces proprioceptive impulses, which act on the body and rotate the body in relation to position of head. This reflex is well noticed, if the animal is laid down in resting position upon its side on a table.

3. Body righting reflexes acting on the head

Labyrinthine righting reflexes are not the only reflexes acting on neck muscles to cause rotation of head. If the animal is laid down upon its side on a table, the unequal distribution of pressure on that particular side of the body stimulates exteroceptors on the skin. Impulses thus generated by exteroceptors, act on neck muscles and rotate the head.

4. Body righting reflexes acting on the body

When the same animal is laid down on the table on its side, with head held down to table, to eliminate

labyrinthine and neck righting reflexes, the body attempts to right itself by raising the lower parts. It is

because of the impulses from exteroceptors on that side of body acting on the body itself.

5. Optical righting reflexes

Optical righting reflexes are initiated through the retinal impulses. Center for optical righting reflexes is in the occipital lobe of cerebral cortex. So, these reflexes are absent in thalamic animal. Optical righting reflexes help to correct the position of the body or head with the help of sight. It is proved in a labyrinthectomized animal. When such an animal is suspended, it rotates its head to normal position with the help of sight. But, the movements of head do not occur if eyes of the animal are closed.

Summary of righting reflexes

Following are the sequential events of righting reflexes:

i. When the animal is placed upon its back, labyrinthine reflexes acting upon neck muscles turn the head

into its normal position in space, in relation to body

ii. Proprioceptive reflexes of neck muscles then bring the body into its normal position in relation to position of head

iii. When resting upon a rigid support, these reflexes are reinforced by the body righting reflexes on the

head as well as on the body

iv. If the animal happens to be a labyrinthectomized one, then it makes an attempt to recover its upright

position as a result of operation of the optical righting reaction. If the optical righting reflexes are abolished by covering the eyes, the righting ability is lost. Optical righting reflexes are also demonstrated in 3 or 4 weeks old baby. When laid down on belly, i.e. prone position, the baby tries to raise the head to a vertical position.

Centers for righting reflexes

Centers for the first four righting reflexes are in red nucleus situated in midbrain. Center for optical righting reflexes is in the occipital lobe of cerebral cortex.

 

Post a Comment

0 Comments