Posture. Prof. K. Sivapalan

Posture Regulating Mechanisms Posture regulation involves many nuclei in spinal cord, brain stem, and the cerebral cortex. They are concerned with static posture and initiation and control of movement. Segmental coordination developed in lower animals. These mechanisms are dominated and movements are integrated by higher structures as evolution progressed. Transaction of neural axis at different levels release the higher control. The local [or lower] mechanisms appear to be accentuated after transaction. 2/23/2019 Posture

Spinal Shock It is profound depression of all spinal reflex responses for a period following transaction of spinal cord. The duration of spinal shock is proportional to the degree of encephalization In frogs and rats it is only a few minutes. In dogs and cats it is about 1 to 2 hours It is few days in monkeys and 2 weeks in human. When reflex responses return, they are hyperactive. 2/23/2019 Posture

Responses in Chronic Spinal Animals and Humans Minor noxious stimuli may cause prolonged withdrawal of stimulated extremity and marked flexion – extension patterns in the other three limbs. Stretch reflex is hyper reactive. Positive supporting reaction [magnetic reaction]: when the sole of a spinal animal touched by a finger and withdrawn, the leg extends following the finger. This involves tactile and proprioceptive afferents to support against gravity. Extension of a limb results in negative supporting reaction. 2/23/2019 Posture

Locomotion Generator There are two pattern generators, one in cervical and the other in lumbar regions. They are influenced by discharge from midbrain- mesencephalic locomotor region. Spinal animal shows spontaneous diagonal stepping movements. This shows release of higher control and communication between the two centers through spinal cord. 2/23/2019 Posture

Autonomic Reflexes Bladder and rectum contract reflexly when distended without voluntary control. Bladder may not empty completely. Bladder will be shrunken by the hyperactive reflexes and fibrosis may occur. Afferent stimuli can radiate from one to another reflex and result in mass reflex. Blood pressure is normal at rest but baroreceptor reflex cannot operate and it will result in wide swings in pressure. Genetal manipulation can cause erection and ejaculation in males and copulatory position in females. 2/23/2019 Posture

Merdullary Components of Posture Decerebration- transaction of brain stem at the superior border of the pons. Decerebrate rigidity is seen soon after transaction. It is due to facilitation of stretch reflex by increased gamma motor discharge. It is more marked in extensors. 2/23/2019 Posture

Tonic Labyrinthine Reflexes in Decerebrate Animal. Rigidity in the limbs varies with position. When placed on the back, extension of all four limbs is maximal. As the animal is turned to either side, rigidity decreases and it is minimal when the animal is prone. The change is due to influence of vestibule. 2/23/2019 Posture

Tonic Neck Reflexes in Decereberate Animal. Turning the head to a side- jaw limbs become more rigid. Flexion of the head causes flexion in fore limbs and extension in hind limbs. Extension of head causes extension in fore limbs and flexion in hind limbs. Initiated by proprioceptors in neck. 2/23/2019 Posture

Midbrain Components of Posture. Mid brain animal- section at the superior border of midbrain. Decerebrate rigidity is seen the animal lies quietly on its back. They can raise to standing position, walk and right themselves. 2/23/2019 Posture

Righting Reflex. To stand in the normal position and keep head upright. When the animal is tilted by the body, the head is kept in position. This is by labyrinthine receptors and reflex coordinated in midbrain. When animal is placed on a side, the head is held upright because pressure on the side generates righting reflex. Labyrinth is not needed. It is body on head righting reflex. When head is righted, neck muscles are stretched. This rights thorax and a wave of correction of the entire body by neck righting reflex. Pressure on the side of the body also corrects the body by body on body righting reflex. [Optical righting reflexes need cerebral cortex.] 2/23/2019 Posture

Grasp Reflex In primates with intact thalamus, limb next to supporting surface are extended. Upper limbs are flexed. The hand on the upper side grasps firmly any object brought into contact with it. 2/23/2019 Posture

Effects of Decortication. Little motor effect in many species. More severe in primates but movement is still possible. Decorticate rigidity is seen in patients with lesions in internal capsule. Lack of inhibition to gamma motor efferent. It is obscured by phasic postural reflexes and seen only at rest. 2/23/2019 Posture

Decorticate Rigidity 2/23/2019 Posture