VPB 121: Veterinary Physiology - II (2+1)

POSTURAL REFLEXES

• This 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.
• It is divided into
  • Supporting reflexes
  • Attitudinal reflexes

Supporting reflexes

• These reflexes involve activity that initiates co-ordinated 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.