 Broadly there are two types of rigidity posters, de-quarticate rigidity poster and de-celebrate rigidity poster. Let's try to understand the mechanism of how they occur. For that we need to understand about the muscle tone and what are the various tracks which influence the muscle tone. So we have to see it in both the flexor muscle and extensor muscle. So suppose this is a flexor muscle and this is an extensor muscle and we know that skeletal muscle has intrafusal fibres and extrafusal fibres where extrafusal fibres are supplied by alpha motor neuron and they contract when there is a stimulation of alpha motor neuron. On the other hand intrafusal muscle fibre is a sensory, right? So there are sensory nerve endings from intrafusal muscle fibres which reach to the spinal cord and this has connections with the alpha motor neuron. Then the ends of the intrafusal muscle fibre are supplied by gamma motor neuron and these ends are actually contractiles. So when there is excitation of the gamma motor neuron there is contraction at the ends of the intrafusal fibres and this intrafusal fibres stretches and because of this stretching there is increased impulses from these ascending fibres that is 1, and 2 which go and excite the alpha motor neuron. So this is the basic stretch reflex actually the connection between the sensory fibres and with that of the alpha motor neuron. Now these motor neurons that is the alpha motor neuron and gamma motor neurons are influenced from neurons from top and there are various influences there is influence from the cortex then from the midbrain where red nucleus is there from there then pons and medulla actually there is a reticular formation in pons and medulla and from there reticulo spinal tract arises which influences these neurons. So let us see some details about these. The flexor muscles and the extensor muscles receive different influences. Flexor muscles alpha motor neuron receives excitatory influence from cortex. So that means from the cortex the fibres which are coming they are exciting the alpha motor neuron of the flexor muscle also via red nucleus. The cortex influences these flexor muscles actually from red nucleus alpha motor neuron receives excitatory influence and this red nucleus receives inhibitory influence from the cortex. So this fibres which are coming from the red nucleus of the midbrain to the flexor muscle alpha motor neuron this is the rubro spinal tract and one thing you remember that this tract only descends up to thoracic spinal level. So it is not going to influence the flexor muscles which are coming below this level. Okay now let's see the influences on the extensor muscle. Extensor alpha motor neuron receives inhibitory influence from red nucleus. So are you getting it that the influences from the cortex and from the red nucleus basically are exciting the flexor alpha motor neuron that is causing its contraction. However they are inhibiting the extensor alpha motor neuron. Anyways now the extensive gamma motor neuron also have lots of influences. The gamma motor neuron supplying the extensor muscles receives excitatory influence from the pons reticular formation but inhibitory influence from medullary reticular formation. Okay now both these tracts that is the medullary reticular spinal tract and pontine reticular spinal tract both receive excitatory influences from cortex. So in summary corticospinal tract and rubro spinal tract from red nucleus they excite the flexor alpha motor neuron and then there is reticular spinal tract from pons and medulla. From pons reticular spinal tract is exciting the gamma motor neuron and from medulla there is inhibitory influence on the gamma motor neuron. So reticular spinal tract works by acting on gamma motor neuron. Fine now let's see the two types of rigidity posters which we spoke about in the beginning. So first one is the de-cerebrate rigidity. In de-cerebrate rigidity what is happening that these tracts that is the cortical influence on the alpha motor neuron and the red nucleus influence on the alpha motor neuron both are done away with this. So basically the lesion is somewhere here below the red nucleus and that is basically a mid-collicular lesion. So in this lesion all influences from cortex and midbrain are lost. So you can predict what is going to happen, where it will be affected. Simple you see excitatory influence to alpha motor neuron of flexor muscles is lost. And what will be more active it is the influence on the gamma motor neuron coming from pons and medulla. But we said that the influence from pons and medulla one is excitatory and the one is inhibitory. Then that means overall there is no influence no that doesn't happen like that. Because actually this excitatory pathway from pons reticular formation, pontine reticulo spinal tract dominates because the sensory signals from the intrafusal fibers which are coming facilitate this pathway. So in simple terms due to increased activity of pontine reticulo spinal tract which is basically having excitatory influence on gamma motor neuron the stretch reflex becomes more sensitive. As I told earlier that this is causing the gamma motor neuron is stretching the intrafusal fiber and this will further increase the afference activity coming from the intrafusal fiber and hence it is going to activate the alpha motor neuron. So if alpha motor neuron is activated extensor muscle is going to contract. So what is it? We are getting a posture in which the extensor muscles will be in contracted state but we have seen how flexor muscles will not be in contracted state because the influence from the red nucleus and from the cortex is missing. So in de-celebrate rigidity what we see is that both lower extremities and upper extremities are extended. Lower extremities are extended with toes pointed inward and upper extremities are also extended and neck and head are also extended. Now let's see the other rigidity that is the de-corticate rigidity. Now as the term indicates the de-corticate rigidity basically the influence from the cortex only is lost so the lesion is above the red nucleus. So only the cortex influence is lost and red nucleus influence is still active. So what will happen? See obviously the extensor muscles will be similar like that of de-celebrate rigidity because the reason is same the afferent impulses coming from the extensor muscles are activating the quantine reticular formation and because of that there is extension of the limbs. However this red nucleus excitatory influence to flexor muscles is intact and remember as I told you earlier that this influence is up till only to the thoracic spinal level. So if we consider the muscles right then this influence is only on upper limb muscles. So in de-corticate rigidity we get extensor hyperactivity in lower limbs but flexion of upper extremities at the level of the elbow. If you like the video do press the like button and do share the video with others and don't forget to subscribe to the channel Physiology Open. Thank you.