Postural Reflexes
Postural Reflexes is also known as postural reactions. It is defined as the position or arrangement of the body and its limbs.
It is controlled by the activity of the skeletal muscles of the neck and limbs. Control of posture involves mechanisms that alter the excitable activity of α-motor neurons of the spinal cord. Structures concerned for maintenance of posture are located at all levels of brainstem, medulla and pons.
Postural Reflexes is divided into- Supporting reflexes and Attitudinal reflexes.
1. Supporting reflexes
Supporting reflexes involve activity that initiates coordinated skeletal muscle activity to cause the limbs to be fixed into supporting columns against the pull of gravity. The supporting reflexes are further classified as local supporting reflexes and segmental supporting reflexes.
Local supporting reflexes
These reflexes of the limbs produce a fixed standing posture, which prevents collapse under the force of gravity.
They involve only a few segments of the spinal cord for reflex actions.
Local supporting reflexes include the myotatic reflex, and extensor thrust reflex. These reflexes are initiated by the stimulation of muscle spindle or cutaneous receptors of the limbs involved or by stimulation of the opposite limbs with noxious stimulus.
Extensor thrust reflex
When pressure is applied to the footpads of a dog, that limb is extended into a supporting column by the extensor muscles of the elbow, which are initiated by myotatic reflex.
Segmental supporting reflex
These reflexes use many segments of the spinal cord. Segmental supporting reflex arcs are not solely limited to the spinal cord, but also involve the brain stem.
The reflex is represented by crossed extensor reflex. These reflexes involve sensory input to a local area and produce reflex activity over a wide region of the spinal cord.
The same reflex occurs in the opposite limb. When a hind limb is extended, the opposite forelimb also is extended.
2. Attitudinal reflexes
Attitudinal reflexes involves modification of posture as a result of varying positions of the head. Displacement of one part of the body is followed by postural changes in other parts so that a new posture is assumed.
This is classified into two types- Cortical reflexes and Extra cortical reflexes.
Cortical reflexes
Cortical reflexes is mediated through cerebral cortex and can be divided into- Placing reflex, Hopping reflex and Vestibular placing reflex.
Placing reflex
Blind folded animal if held in space in a dropping position near a supporting surface to facilitate either of the parts of the body like feet, ventral surface of the body, neck or chin in contact with the surface, the animal quickly assumes a posture to place either of the limbs with the supporting surface.
This reflex requires involvement of both spinal and cerebral cortex by way of corticospinal motor system. Decorticate animals or animals wherein descending motor pathways are abolished are not in a position to exhibit this reflex.
Hopping reflex
If the animal is made to stand on one limb and if simultaneously is moved horizontally the supporting limb will hop to maintain support for the body. This is supported by spinal reflex, myotactic reflex and tactile placing reflex. This reflex is temporarily abolished by transecting the corticospinal motor pathways. But due to involvement of myotactic reflex of the limbs, there will be weak exhibition of this reflex.
Vestibular placing reflex
Blind folded animal if dropped head down position towards the ground; the animal assumes specific supporting postures. The forelimbs are greatly extended. Toes are spread apart. Tries to get support against an expected contact with the ground. Here, direction of acceleration within the field of gravity is not a deciding factor to initiate this reflex.
Extracortical reflexes
Extracortical reflexes is mediated through spinal cord, medulla and pons. It is of two types – the tonic neck reflexes and tonic labyrinthine reflexes.
Tonic neck reflex
Neck receptors include muscle stretch receptors and receptor organs that sense the position of joints between cortical vertebrae related to head position.
Extension of the head to dorsal position causes flexion of the hind limbs, but extension of the forelimbs occurs when the head is moved downwards.
Turning of head to right side causes the extension of both the right limbs and the flexion of both the left limbs to support the body against gravity; this helps to prevent falling. If the turning is on the left side, it results in the extension of the left limbs and the flexion of the right limbs.
Tonic labyrinthine reflexes
These reflexes are initiated by receptor organs of the semicircular canal. Rotational acceleration of the head stimulates vestibular semicircular canal. Maintenance of upright posture on rotation if kept on rotating platform and altering the position of head and body to the opposite of the rotation.
On stoppage of the rotation, the animal’s head will be turned and animal lean towards the opposite direction. Semicircular canals help to maintain the animal’s balance on sudden change in the head’s direction.