 descending tracts are the tracts which descend from the cortex or from the brain stem and supply the muscles of the body for voluntary and involuntary control of the movements. So, what are these descending tracts? Basically, we have the lateral systems of the descending tracts with the tracts which descend literally in the spinal cord and then there are the medial group of descending tracts and these lateral and medial descending tracts differ in terms of whether they are for voluntary control or for involuntary control. So, lateral descending tracts are for voluntary control of movement and medial descending tracts are for involuntary control of movements. Now, there are other way of classifying descending tracts that is pyramidal and extra pyramidal tracts that we will also see. So, first of all in lateral descending tracts, we have the corticospinal tract or also known as the pyramidal tract. So, the name is corticospinal tract. Then in lateral spinal tract we also have the rubro spinal tract. Then in medial descending tracts we have vestibulospinal tract, then reticulospinal tract and tectospinal tract, tectospinal tract and in vestibulospinal tract again we have medial and lateral vestibulospinal tract and reticulospinal tract again we have quantine and medullary reticulospinal tract. Now, if you notice the names of these different descending tracts, we can tell from where are they originating. So, corticospinal tract, so it is originating from the cortex and going towards the spinal cord. So, it is terminating in the spinal cord. Then rubro spinal tract, it is originating in the red nucleus of the midbrain and going into the spinal cord. Then vestibulospinal tract, it is originating in the vestibular nuclei and going to the spinal cord. Reticulospinal tract, the names quantine and medullary. So, quantine one is originating in the quantine reticular formation and medullary one is originating in the medullary reticular formation and then we have the ectospinal tract. Basically, this arises from the collicular nuclei of the midbrain. So, this lateral descending tracts are for voluntary control of the movement and medial ones are for the involuntary control of the movements and as I told you before that there is another classification that is pyramidal and extra pyramidal tract simple. This corticospinal tract is also known as the pyramidal tract and it is the only tract which arises from the cortex and all these arise from the brain stem. So, they are the ones which are known as the extra pyramidal tracts. Now, let us go one by one into the details of these and see their course. How are they originating? What is their course? Where are they terminating and what is their functions? So, first let us discuss the pyramidal tract or the corticospinal tract. So, what is the course of pyramidal tract? They arise from the cortex and where in cortex there is motor cortex, there is not only motor cortex there is premotor area and supplementary motor area and also they arise from somatosensory cortex remember generally we forget that. See, ultimately the sensations also guide our movement. So, it is logical only that some fibers should arise from the somatosensory cortex as well. So, 30 percent of the fibers arise from premotor area 30 percent from motor cortex and 40 percent of the fibers arise from the somatosensory cortex. Then from different areas these fibers come together and this coming together of the fibers that is known as corona radiata and then it passes via the posterior limb of the internal capsule. So, internal capsule has anterior limb, genu posterior limb is there. So, via the posterior limb of the internal capsule all these fibers pass and why it is important to remember see if there is an injury into the posterior limb of the internal capsule then what will happen? All of the fibers of the pyramidal tract will be affected. On the contrary if there is an injury in the cortex maybe a particular area is affected then only some fibers will be affected. Isn't it? After passing through the posterior limb of internal capsule all these fibers descend and they descend up to the lower part of the medulla where 80 percent of these fibers cross okay and 20 percent of the fibers keep descending and when they are descending remember that these are the nerve fibers they descend in the white matter of the spinal cord. So, when they are descending they will descend in the white matter of the spinal cord but ultimately in the spinal cord they terminate in the ventral horn of the spinal cord okay and these 20 percent of the fibers also which have not crossed in the medulla in the level of the spinal cord ultimately they cross and again in the ventral horn of the spinal cord they synapse with other neurons and what are these neurons where they synapse? Either it is the alpha motor neuron, alpha motor neuron which is supplying the muscles okay or it may synapse to interneurons also so this is a interneuron which in turn makes contact with many alpha motor neurons supplying the muscles. So, this is basic pyramidal tract it's the origin from three areas in the cortex then coronary area then passing through the posterior limb of internal capsule descends in the brain stem and in medulla 80 percent of the fibers cross and 20 percent of the fibers descend and then these 20 percent of the fibers also cross in the spinal cord and make contact with the alpha motor neurons or interneurons in the spinal cord. Now why this tract is named as pyramidal tract? Well in medulla there is formation of pyramids okay there is a structure known as pyramids and that is formed by this corticospinal tract and hence the name pyramidal tract. Now there is some confusion here actually the cells in the motor cortex from which they arise that are also known as the pyramidal cells of beds so it is not because they are arising from these cells that their name is pyramidal tract their name is because of the medullary pyramids okay so pyramidal cells of beds so these are very large giant cells are there so the cells of these neurons are large bed cells and then they are descending and these bed cells are located in layer five of the cerebral cortex so that was about the course of the pyramidal tract coming to what is its function function as I told you before is voluntary movement right so by supplying the alpha neuron interneurons in the spinal cord it is causing the voluntary movement now these 80% of the fibers the lateral corticospinal tract that is responsible for skilled movements fine skilled movement that is the main function however this 20% that is supplying the proximal muscles because that are also under voluntary control isn't it so 80% of the fibers for distal muscles causing fine skilled movements like writing and 20% of the fibers for proximal muscles for course movements so that is the main function but before I move on to other tracks when we are talking about corticospinal tract or pyramidal tract we cannot miss the terms upper motor neuron and lower motor neuron so the neuron from the cortex up till the level of the spinal cord where its synapses is known as upper motor neuron and alpha motor neuron in the spinal cord where the synapse is occurring this alpha motor neuron coming out from the ventral on of the spinal cord that is the lower motor neuron please remember detailed video on upper motor neuron lower motor neuron lesion and how to identify the site of the lesion I have made another video two videos are there I will give the link in the description section below you can have a look on that also one more thing we have spoken about the corticospinal tract but there is another aspect that is the corticobulbar tracts what is that when the term bulbar is there then we are talking about cranial nerves okay so there are fibers from the motor cortex which are also ending in the brain stem okay and where are they are ending they are ending in the cranial nerve nuclei so there are certain cranial nerves that is the cranial nerve fifth right seventh and twelfth okay and these cranial nerves are for voluntary movement so from the motor cortex information is also going in the brain stem terminating in the brain stem on these cranial nerve nuclei and those are known as corticobulbar fibers and you see that they are not crossing they are terminating on the same side it's literally crossing is happening in the lower end of the medulla so for movement of the facial muscles for movement of the tongue muscles we have the corticobulbar fibers with this let us move on to the extra pyramidal tracts and what are the extra pyramidal tracts we said they are rubro spinal tract vestibular spinal tract reticulus spinal tract and tectospinal tract so first what we'll discuss in extra pyramidal tract is the rubro spinal tract because it forms the lateral system that is it is responsible for voluntary control so it arises in the midbrain from the red nucleus of the midbrain okay and then it crosses in the midline just like the lateral corticospinal tract and synapses with the neurons of the lateral muscles however it synapses only with the interneurons and not directly with the alpha motor neurons okay so that fineness of movement which is provided by the lateral corticospinal tract that is missing here so for example if there is injury to the lateral corticospinal tract but rubro spinal tract is intact then the fine writing movements may disappear however the coarse wrist movements will only be there another thing it supplies only the flexor muscles of upper limb flexor muscles of upper limb not to the lower limb so it terminates in the upper spinal cord only so that is rubro spinal tract and this rubro spinal tract gets some facilitatory input from the cortex as well moving on to the next tract that is the tectospinal tract the tectospinal tract also arises from the midbrain what nuclei colloquial nuclei is there or the tectum okay and then this also crosses over to the other side and it supplies the cervical spinal cord it supplies the muscles in the cervical spinal cord remember cervical spinal cord and why cervical spinal cord because it supplies the muscles for head and neck movements so what will be the function of this tectospinal tract see suppose if there is some sudden sound you are focusing somewhere and somewhere you hear a sound then what happens suddenly you turn your head and eyes towards that sound right so how that is brought about that is a reflex movement and that is being brought about by this tectospinal tract you remember that colloquialia receive information in from the auditory pathway and the visual pathway and this information is being used by this tectospinal tract to bring about the head and eye movements so that was simple rubro spinal tract and tectospinal tract moving on to the other tract that is the reticulo spinal tract pontine reticulo spinal tract and medullary reticulo spinal tract so pontine reticulo spinal tract arises from pontine reticulo formation and medullary reticulo spinal tract arises from medullary reticular formation. So from pontine reticular formation pontine reticular spinal tract will descend in the spinal cord right and you see it is descending medially however the medullary reticular spinal tract descends little bit laterally so somewhere here it will descend right but you see there is no crossing over it doesn't cross and which muscles it supplies the reticular spinal tract supply the extensor muscles extensor muscles okay so these are the proximal muscles and they are important for posture maintenance so after descending into the spinal cord they synapse with the neurons which are supplying the extensor muscles and where they actually synapse they synapse on the gamma motor neurons remember reticulo spinal tract spinal tract they supply the gamma motor neurons and pontine reticular spinal tract is excited to gamma motor neuron while medullary reticular spinal tract is inhibitory to gamma motor neuron so what is the significance of this see first of all extensor muscles are very important for posture maintenance whenever there is increase in tone in the extensor muscles the person can stand up straight and when there is decrease in tone so that helps in changing the position to some other position so both are there one is excited to gamma motor neuron other is inhibitory to gamma motor neuron now suppose there is some voluntary movement which cortical spinal tract have to bring about say suppose you are about to run or say suppose you are going to sit in a particular position and carry out a certain task now for every movement there is a particular posture which needs to be there isn't it so how this posture is brought about that posture is brought about by the information from the cortex going to these reticulo spinal tracts okay and depending on what is the need of the posture this pontine and reticular spinal tracts are either activated or inhibited so the muscle tone of the extensor muscles is brought about by influencing the activity of reticulo spinal tract for carrying out any voluntary movement so please remember whenever we are going to carry out a voluntary movement along with the pyramidal tract this reticulo spinal tract is also activated or inhibited so this is known as kind of feed forward influence of the cortical influence on the reticulo spinal tract feed forward so for one activity it is changing the activity in the reticulo spinal tract to maintain a particular posture then what about vestibular spinal tract vestibular spinal tract again are to medial and lateral medial rises from medial and inferior vestibular nuclei okay so just remember the term medial right medial and instead of E remember I so medial vestibular spinal tract arises from medial and inferior vestibular nuclei and lateral vestibular spinal tract arises from lateral vestibular nuclei again these tracts do not cross over vestibular spinal tract from the medulla these vestibular nuclei are present in the medulla from the medulla actually they descend medially right and again they make synapse with the anti-gravity muscles anti-gravity muscles neuron supplying the anti-gravity muscles and what are these anti-gravity muscles these are flexile muscles of the upper limb and extensor muscles of the lower limb and they are excited tree to the alpha motor neurons of these anti-gravity muscles and what is their function again their function is posture and balance so how is it different from that of reticulo spinal tract this vestibular spinal tract mostly functions in a feedback mechanism reticulo spinal tract we said it is functioning in a feed-forward mechanism whenever voluntary movement is required reticulo spinal tract is getting active or inactive at different levels of the spinal cord vestibular spinal tract is is a feedback mechanism suppose when we are walking right and suddenly we tend to fall what will happen the information from the vestibular apparatus from years it will reach these vestibular nuclei and hence it will bring about the changes in the tone of these anti-gravity muscles and hence preventing us from falling so it is receiving sensory information and based on that it is correcting the tone so even if when we are performing some voluntary movement and a particular posture via because of the reticulo spinal tract if there is some disruption because of some unexpected activity the information from vestibular nuclei will change the tone of the anti-gravity muscles so that is the main function of the vestibular spinal tract before I forget I was talking about medial and lateral vestibular spinal tract well this medial vestibular spinal tract supplies the cervical spinal cord that is the movement of the neck is brought about by this medial vestibular spinal tract okay and it supplies bilaterally so there is crossing over also and supplies bilateral muscles and lateral vestibular spinal tract supplies the anti-gravity muscles of the limbs so that was fundamental about the various descending tracts and their function which I don't think you will find anywhere that what is the difference between the various tracts and their functions before we end let us summarize little bit so what we said that in descending tracts we have the lateral systems okay lateral systems and there are the medial systems okay in lateral systems we have the pyramidal tract then we have the rubro spinal tract in medial system we have the reticulo spinal tract where we talked about the pontine and medullary tracts then we have the which one vestibular spinal tract right so there is vestibular spinal tract where we have again the medial and lateral tracts and then we have the last one that is the tectospinal tract so summary entire course we discussed in pyramidal tract and where it is terminating on the alpha motor neurons and also on the interneurons to bring about the contraction of the group of the muscles and also the fine skilled movements I brought about this lateral pyramidal tract and then there is anterior pyramidal tract also 20 percent of the fibers which you don't cross they are supplying the proximal muscles as well then there is a rubro spinal tract again which ends on the interneurons and supplies the alpha motor neurons basically these interneurons then end on the alpha motor neurons and there is excitatory effect on the alpha motor neurons especially of the flexor muscles of the upper limb flexor muscles of upper limb so what we said that even if pyramidal tract is injured course movement of the flexor muscles of upper limb will remain intact due to the rubro spinal tract then coming to the medial descending tracts this reticular spinal tract they supply the gamma motor neuron where pontein is excitatory to the gamma motor neuron and medullary is inhibitory to the gamma motor neuron of its muscles of the extensor muscles and they work in the feed forward mechanism for the posterior maintenance for any voluntary control right then vestibular spinal tract where we have medial and lateral vestibular spinal tract medial supplies the neck muscles bilaterally and lateral supplies the anti-gravity muscles and where it supplies which neuron it supplies it is excitatory to the alpha motor neurons and it works mostly in feedback mechanism where the information sensory information is made use for correcting the posture and balance as required and pectospinal tract which arises from the collicular nuclei in the mid brain that is responsible for attention that is movement of the head and eyes towards an unexpected stimulus when it comes so that reflex movement is going on so that was all about descending tracts if you have any other questions post in the comment section below if you like the video press the like button share the video with others and don't forget to subscribe to the channel physiology open thank you