motor cortex is mostly consists of two regions here area four and areas six in broadened areas the primary motor cortex is right here in this strip called the central sulcus so again these two regions here are what we consider to be motor cortex area four is really the primary motor cortex so so this region here area four also called m1 or again primary motor cortex consists of this strip of the right or this gyrus right in front of the central sulcus which is also called the precentral gyrus so if you remember the this region we call the precentral gyrus and then this gyrus right behind it we call the post central gyrus and that is the primary somatosensory cortex so the primary motor cortex and primary somatosensory cortex are right next to each other so area four is again where most of the neurons that are going to form the descending spinal tracts have their cell bodies and the cell bodies are arranged in a somatic topic map again so that means if you look at the positions of neurons within this region so again this is we're talking about we're looking at a cross-section of the left side of the brain through primary motor cortex so this is a coronal section so you're seeing the the dorsal side of the brain up here and the the medial lateral side over here and so what this diagram says is that there are neurons in the these each of these regions that control different parts of the body and they're arranged in a way that more or less lines up with those parts of the body so neurons that are next to each other in the motor cortex control parts of the body that are next to each other so that means for example up here the parts of motor cortex that control the shoulder are right next to the neurons that control the elbow which are right next to the neurons that control the wrist and the hand and so on and of course we're crossing over so this is the left side of the brain we're looking at but these muscles control the right side of the bot so actually this picture is kind of backwards because this shows a left hand but really it controls the riking so the same goes for the face now again because the the nerves that control the facial muscles come out from the cranial nerves while the nerves that control this body come out through the spinal nerves the the part of the cortex that maps to the face is in a slightly different position but still within that region you have a somatic topic map so you have the muscles that control the eyelids and the eyeballs next to the ones that control the eyebrows and the facial muscles and then the neurons that control the lips and the jaw and the tongue are all next to each other - and again these are all parts of the body controlled by skeletal muscle and the only kind of internal part of the body that's under control of motor cortex are those of the tongue the throat so those involved in swallowing and speaking to are are found here in the motor cortex whereas the there are parts of the brain that can have influence over the muscles of say the digestive system and so on but that's that's not controlled by this region and you may notice this may look familiar because the primary somatosensory cortex is also a somatic topic and it's right next to the primary motor cortex so this again is the is the map these are the regions we're talking about so primary motor cortex is right here in red primary somatosensory cortex is right here in green they both have a somatic topic map that is pretty well lined up so the for example the parts of somatosensory cortex they receive touch input from the face are right across the central sulcus from the parts of primary motor cortex that control the face and so that's probably no no accident fact that may be why they are arranged that way in fact for whatever reason the in this diagram at least the the region of the somatosensory cortex they correctly drew as a as a right hand you because this is the left side of the brain whereas the motor protein I think that was just an error on the on the printing here this should be a left hand in this diagram because these muscles or this part of the brain controls the muscles on the left side of the hand but the somatic map again looks very similar to the somatosensory somatic topic map not just in position but also in the relative sizes so again with the somatosensory cortex you see a large over representation of the lips and the hand because again that's where you have a high degree of sensitivity of touch receptors in those regions and the same is true in the motor cortex so the hand the fingers the lips the jaw the tongue all have a much higher degree of representation in motor cortex then say the the arms for example even though there's much more muscle so in other words the number of muscles or the amount of muscle in the say the arms the shoulders of the trunk is much much greater than in the hand but the the hand requires so much more control the muscles of the hand require a lot more control than muscles of those other parts of the body because of the way we use that body part we use our hands to manipulate objects to use tools you know things like writing for example wires a lot more fine control of the individual finger muscles than say walking so that's also why like the hands have a lot more representation in the motor cortex than the feet you just don't need a lot of really fine control of the muscles in your feet to do to walk or to stand but you need a lot of fine control to independently control the fingers to to use tools and so on same goes for the lips jaw tongue so we use those body parts those muscles for you know for example speaking so speaking speech requires a lot of fine control of the muscles of the lips and the tongue and jaw and so that's why those muscles or those body parts are so over-represented in motor cortex because of the way we use those body parts not because of their size or because of the amount of muscle or anything like that in those regions and then adjacent to area for you have area six so area six is just anterior to the to area for what's called the primary motor cortex and this is then subdivided into two regions the the supplementary motor area or the SMA oops and the pre motor area or the PMA and there's also some a topic map within this region the this SMA controls mostly the distal motor unit so that means the the muscles of the arms and legs and then the promoter area is more involved with the muscles of the are the proximal motor unit so that means muscles that are closer to the midline of the body like the the shoulders and hips and and so on and the the the neurons in area 6 both area for an area six neurons and control motor neurons in the spinal cord so they they control the muscles of the body that they map onto and area for again it seems to send direct commands to will actually talk later about how area for controls muscles but area sixes job seems to be at least in part involved in planning of motor actions so in other words before a movement is actually initiated there seems to be some activity in area six that is necessary for sort of planning out movement so again we're talking about sort of the tactics strategy tactics level of movement here and so this is an example this is an experiment that kind of demonstrates the role of area six so this is a experiment where they trained a monkey to push these buttons on this board in front of it so the buttons are these little circles and the monkey is trained to push one of the buttons when it lights up but it's trained to wait or it's trained to to look at the board because before the button it's supposed to push lights up another light the little square boxes will light up to tell the monkey which button it's going to have to push but it knows and again you can train monkeys to do this after a few trials but it's trained that to wait for the red lights to come on one of these four boxes and that's going to be the instruction stimulus so that's the instruction to the monkey that's gonna say all right this little blue button down here is about to light up and when it lights up you're going to push it and and so it does it waits until the the so called trigger stimulus comes on so we call the big square lights the instruction stimulus and then the blue light the trigger statement so there's a delay between when the instruction stimulus comes on and the trigger stimulus comes on so the assumption is that during that time I mean it's just a few seconds during that time some part of the monkey's brain is is preparing to move in other words sending out or preparing the the motor cortex to carry out that that action and so over here they're recording from in this case a neuron in the in the PMA the pre motor area presumably on the right side so of course if the monkeys moving its left arm then then the right motor cortex would be involved in this movement and so this shows that you know when there's no stimulus at all even though even though the monkey knows it's gonna have to move at some point this particular neuron that they're recording from is inactive or it has sort of a baseline level of activities all little vertical lines again are action potentials and then it starts to fire as soon as the instruction stimulus comes on so again even though the monkey hasn't moved yet that neuron responds to the instruction stimulus and it keeps firing again until the trigger stimulus comes on but then about a split second after the trigger stimulus it comes on and after the movement initiates so as soon as the trigger stimulus comes on the monkey starts moving its arm but at that moment or a few seconds later that neuron shut off so the the implication or the assumption here is that this neuron that they're recording from is active during this sort of waiting phase and the assumption is that that neuron is somehow involved in the circuit that sort of planning out the movement so it's sort of storing perhaps the sequence of commands that are going to be sent to the primary motor cortex into the spinal cord to actually carry out the movement so again how exactly that movement stored or how that command is sword no one really knows for sure but that that just sort of illustrates perhaps what this this part of the brain does and then a outside of motor cortex there are a number of other cortical regions that send input to motor cortex so the motor cortex itself is what commands or sends commands to the spinal cord into the brainstem motor neurons to initiate movement so the question is who tells the motor or Dex what to do so there are a couple different regions that that send their inputs to motor cortex including the posterior parietal areas so these are the posterior parietal areas back here areas 5 & 7 for the most part oops so areas 5 & 7 are just posterior to the primary somatosensory cortex and we have heard of these before because we talked about the posterior parietal cortex in the context of the somatosensory system because of course primary somatosensory cortex is right here just anterior to those 2 and posterior parietal cortex if you'll recall is seemingly necessary with necessary for sort of integrating sensory input and creating ones sort of sense of the world and their place in it because and we know this from people who have had damage to this region they have what's called hemispatial neglect this is where again someone has trouble sort of processing or dealing with things on the side of their body or the side of the world essentially that's contralateral to the injury so this would be probably someone with a right posterior parietal injury and they are given a task where they have to copy a picture so they could give they give them a clock to copy or a picture of a flower and they can sort of get some of the image but they always have trouble with the the opposite side so with the clock they they draw the circle but they don't draw all the numbers they only draw the numbers on the right side or the flower they draw the stem and and the the middle circle but they leave off the petals and leaves on the the opposite side and again people with this condition you know men only shave one side of their face women only put on makeup on one side and so on so the posterior parietal is is at least involved in sort of integrating somatosensory and Meishan with visual input and other modalities and so the assumption is that the parietal posterior parietal area communicates with the motor cortex to again sort of coordinate movement on that side of the body with the sensory input coming from that same side and then you have the prefrontal cortex so the prefrontal cortex is basically the rest of the frontal lobe that's not part of the motor cortex so again motor cortex strictly speaking is just area of areas four and six and prefrontal cortex is just everything else so and it's pretty much what it sounds like it's it's the front of the front so this is the part of your brain that's all the way at the anterior end it's right behind your forehead and prefrontal cortex has a large input to motor cortex it seems to be involved in a lot of things obviously with motor planning so planning out movements but especially at the higher levels of our more complex levels of cognition so motor planning both in short term and also long term so prefrontal cortex that seems to be important for decision-making kind of anticipating consequences of actions long-term thinking a lot of abstract thought seems to be involved the prefrontal cortex we we've seen the prefrontal cortex before way way way back at the beginning of the class we talked about Phineas Gage so he's an a famous example of someone with a prefrontal cortex injury and if you remember among other things he had problems with planning for the future and making decisions and so that seems to be with the prefrontal cortex he's always partly involved in and so it it sends input to the motor cortex not necessarily to to initiate movement but there's reason to think that it it actually inhibits movement in other words it sort of prevents the motor cortex from initiating movements that are are not sort of goal-oriented so we'll talk about about that more later now again how the the motor cortex actually encodes movement or sends specific movement commands to the spinal cord is not entirely understood and it's probably very complicated it's not the case that individual parts of motor cortex send commands to individual muscles so that that's one possible way the motor cortex could be organized you could have individual neurons or groups of neurons that send commands to individual muscles to contract and then you get movement in the corresponding direction but that doesn't seem to be the way it works it seems to be the case that different parts of the motor cortex command different parts of the body and more specifically command movements in a particular direction and so this can be demonstrated by showing that that neurons in primary motor cortex have what are called Direction vectors and the way show this is by recording activity from motor cortex neurons and then looking for how those neurons are active during movements in particular directions so an example of this kind of experiment as this one here you have a monkey again trained to sit in front of a board like this with a little joystick and the joystick sits in the center of this table and it's surrounded by these these eight lights and so the monkey is trained that whenever one of these lights comes on and they just sort of come on a random to move the joystick in the direction that the light came on so in other words if if the light to the left comes on the monkey knows to move the joystick straight to the left so this is just just a way to train a monkey to initiate a simple movement in a particular direction and if you record from neurons in the brain so again if this is the monkey using its right arm then it would be over here from the left motor cortex and presumably the part of the motor cortex that controls the left arm and what it shows is that the these neurons don't fire with just any kind of movement neurons in motor cortex seem to prefer seem to be most active when the motion is in a particular direction so this is showing for one particular neuron the firing rate for different motions so zero degrees in this case would mean moving the joystick rocket to the right 90 degrees would be moving the joystick directly forward 180 would be moving the joystick to the left and and so on and 270 would be moving the joystick toward the monkeys body and so for example this particular neuron seems to fire the most when the monkey moves joystick to the left straight to the left it also fires at other angles so it fires a little bit you know here between 180 and nine you know a little bit between 180 and 270 still fires a little bit at 90 degrees and at 270 but then once you get close to zero degrees meaning moving directly to the right this neuron does not fire much at all so that seems to be the direction vector for that particular neuron so the assumption is there would be then another neuron or another set of neurons for each direction so there'd be a neuron that preferred or had a direction vector for 90 degrees a direction vector for zero and so on and and all the degrees in between it's just that this particular experiment only allowed the monkey to move any directions and in one plane but then the the idea is that a movement is made if when all the neurons sort of collectively add in their their direction vectors or fire at different rates given their direction vector and that determines the actual direction of motion and so the the sum or the average of a bunch of different direction vectors from different neurons all firing together produces what's called the population vector so this is another example actually a pop population coding which we've seen before in the sensory systems so again if you have these two cells here one cell it's it's preferred direction is ninety degrees by the Rin directly up and this cell cell to its direction vector is straight to the right if those two are firing at about the same rate the same firing rate then there their net movement would be about forty five degrees up and to the right and so that's you know all the different neurons firing at different levels or with different frequencies together will produce the actual population vector which determines the direction of the movement for that body part so that's that's presumably how movements are encoded by by motor cortex one last thing to say about motor cortex is that like somatosensory cortex and pretty much like every other part in the brain in fact it exhibits plasticity that means that connections within the cortex can change due to development or due to experience so here's an example of that where they have mapped the motor cortex of a rat so the and of course the motor cortex rat is not as granular as that of a human are harder to identify individual body parts but you can still see regions where you have four limb control here in orange that means that if you were to stick an electrode into this part of the brain of a rat it would the the rat and then stimulate that that region with the electrode the the muscles of the opposite forearm which would be the right forearm would move and then down here in yellow you have the peri ocular muscles under control so peri ocular means the the muscles are round and attached to the eye so in this case you've got in the yellow region neurons that would control the muscles of the in this case the right eye in between are the neurons that control the muscles of the vibrissae vibrissae is just a fancy word for whiskers so rats and other animals that rely on their whiskers has important sensory organs also have muscles attached to those whiskers that allow them to move or twitch and so the vibrissae muscles have their own mener ons in motor cortex that control them obviously this is something that wouldn't exist in a human or a primate brain but that's okay and then what they did in this experiment is they went in and surgically either cut the vibrissae or cut the nerve that controls the vibrissae muscles and so that means that the neurons that control those muscles are still there and presumably initially would still try to send commands to the whisker muscles to move but when those muscles don't send feedback back to the brain remember we've talked about how how sensory structures in muscles can also tell the brain what the muscles are doing so if the brain isn't getting that sensory feedback from from the muscles that they're that they're moving then over time if you go back in and map the motor cortex of this rat later you see that there are no more neurons obviously that control the vibrissae anymore because the vibrissae can't move but the question is what happened to those neurons they're the neurons are still there so somewhere in this region there are neurons they used to control the vibrissae muscles but now when we stimulate those muscles are those neurons instead for example these neurons here that used to control the vibrissae now they activate peri ocular muscles or as here you had neurons that used to control the vibra side now when they are stimulated they cause movement of the forelimb so again that's an example of plasticity the neurons in here now are reconnected have rewired themselves essentially to the neurons that control adjacent body parts again same thing happens with people so this is an example where this is an experiment where they they trained people to read Braille with one hand so so Braille you probably know is a way of printing text as little bumps so that people that are vision impaired can read and of course just like any other form of language it requires a lot of training so here what they did is they recorded this is a functional MRI image from some brain this these are averages of multiple people and they looked at what was going on in their brain when they were they were reading Braille and then the little green areas represent the parts of the brain they were active while they were doing this task and you can see up here that when they first did it before they before they actually trained the people to read Braille the left and right sides give you the same level of activity when trying to do the task because they've never done it before and so you would expect both sides to kind of contribute the same amount but then they they trained these people to read Braille over a year so this is a long a lot of practice with reading Braille and here what you can see is that on the left side so presumably they used the they were using the right hand to read Braille now when they when they record or look at brain activity while people are reading Braille after a year of training the parts of the brain that are active on the on the left side are much much larger than they were before and much larger than they are on the opposite side of the brain which presumably has not been trained to to read Braille so the point is that now you have this plasticity where the parts of the brain that now control the left I'm sorry the right hand have expanded now they haven't physically expanded so in other words the brain didn't get bigger necessarily in that region but the the connections between those neurons and the neurons that control other muscles have expanded and presumably perhaps at the expense of other parts of the body so that is one thing about plasticity if one region is expanding another region is reducing its connectivity but in any case this is just an example of how this would work in MA and the same thing would be true for any other kind of motion that's used a lot or thing you're trained to do so if you look again at the motor cortex of people who have learned to play a musical instrument you might see a similar kind of expansion of that part of the that kind of circuit of the brain and again this happens in pretty much every part of the brain so it's not limited to motor cortex or sensory cortex or even cortex generally plasticity or changing connectivity within the brain due to experience is common to pretty much every part of the nervous system alright so next time we will talk about some of the non cortical or subcortical parts of the brain that are also involved in movement