6. Specializations in the evolving CNS, part 2

Key Takeaways

the following content is provided under a Creative Commons license your support will help MIT open courseware continue to offer highquality educational resources for free to make a donation or view additional materials from hundreds of MIT courses visit MIT open courseware at ocw.mit.edu two things I want to do today just briefly go through chapter 6 which is about brain specialization it was meant to be sort of an interlude there was a little bit of a break before we uh get into the some of the forbrain connections but uh these all in involve specializations because of the evolution of head receptors and the resulting expansions of the brain that have occurred and uh we start with Electro reception in some fish that's led to enormous expansion of the cerebellum and the marids and my first question here was why what cranial nerves carry information from electro receptors and fish that have Electro reception and also why is it needed why don't they use Vision yeah they're cranial nerves that's right but what cranial what cranial nerves are they sorry it is actually up to six of them and what are they called the lateral line nerves because fish have these receptors along a a line along the side of their body they also have uh mechanical receptors there there's a mechanical lateral line row of receptors also but we're concerned here with the electro receptors okay so what are they doing why is it so important why aren't they just using Vision uh yeah go ahead yeah because they can't see they're living in Muddy Waters uh they thrive in those Waters but they've got to be able to find things has got to be able to find prey if they're feeding on them so what are they detecting there're there's two kinds of electric reception let's just deal with one of them the fish that generate an electric field around their body it's not a big electric field there are a few of them that can generate such a big jolt of electricity that they can stun prey but we're not talking about those we're just talking about them that use it as a sense they generate an electric field but they also detect the electric field all around the side of their body so any disturbance in that field they can detect so all that computation involved in these this specialized brain here the cerebellum this is the area in one of the morid uh are involved in com Computing the where the animal is in order to get a particular disturbance in the electric field it's not an easy task and it's a huge computational problem and there are computational neuroscientists trying still trying to figure that out all right uh no placental mammals have Electro receptor abilities but one nonplacental mammal does have such an ability do you know which one it is in the chapter huh sorry the duck build platypus yes okay how are this animals Electro receptor inputs different from electrosensory fish they have they use a different cranial nerve and somebody said it so anybody else know I think you said it before it's trinal nerve that's right but not in those fish just in the build platypus it's in their bill so how do they feed they stick that bill down in the muck and in the water and they're detecting electric Fields uh many animals do some kind of electric Fields I don't know the detailed for the duck field platypus I know a little more about the fish all right uh another sense that is not highly developed in mammals is infrared detect action in what animals is this particularly important and what cranial nerve is involved anybody else somebody that's not talked today yet the pit vipers yes snakes but only particular snakes snakes like the rattlesnake so here in this little picture I think this is the same one I used in the book I've added to the picture I found in a drawing in the hotos and carton book but this is the pit it literally is a pit it's recessed what would be the value of that why put the infrared detectors at the bottom of a little pit in their face yeah because you just imagine you know I I have a cup here and The receptors are back here now it's got directionality now it's not responding to things over there it's only responding to things in this dire so the head they have to move their head of course it doesn't give them directional selectivity if they keep their head still except they know it's right in front of them all right so and you can see it comes in through a branch of the trienal nerve to a particular group of secondary sensory cells and then through a circuit involving the trigal neurons it goes from there to the optic tectum remember we call it optic tectum but it actually gets many different sensory modalities distance receptors it gets auditory sensory and visual and now it we see it gets it gets information in the these snakes from their infared detection so they can use it to orient uh towards their pray no only the pit vipers have that and it would be interesting to look to see what other kinds of infrared detection you have in animals I'm sure other animals have evolved that it's it's too useful not to evolve only in one small group like that but okay bats are not the only mammal with Echo location ability another special sense so you'd expect specializations in the brain of the bat but what other mammals have Echo reception ability and the dolphin okay and what cranial nerve do you think has expanded I've actually studied that with Eric Monty who's a marine mammologist uh we did uh using MRIs we were able to reconstruct the brain of the Atlantic white-sided dolphin and it was confusing at the beginning because I had never seen I mean I was in helping him identify everything and i' never I thought it must be the trigeminal nerve you know any mammal I've ever seen the trial nerve is the biggest but not in that animal it was the eighth nerve okay the auditory nerve and similarly if you look at the surface of the midbrain in the dolphin look at this enormous inferior calculus the superior calculus is relatively smaller and it's the same in the back releas very similar the dolphin of course has an enormous end brain too and so I've taken a picture there where the embran has been removed in the dolphin but not the bat yeah sorry as humans that use some echolocation it does become specialized and there are indications that blind people that are more likely to be able to use such an ability do have changes in their cortex and we're going to talk a little bit about that to the end of the class very good question just notice here the other two animals in this picture it shows animals where the superior culus the more anterior uh surface of the midbrain is enlarged and they're very visual animals here's a goat a wild goat in eex and tarer a a prian primate that can literally locate using that structure it's got this ability to orient and reach out with his hand and grab insects literally right out of the air all right what is a brain manifestation of the specialization of primates provision that includes us most of the primates have very special they're very specialized for vision that's why monkeys are used so much in studies of vision because of similarity to humans you know rats are used to and cats used to be the most popular animal but no longer it's because of the specialization so you see what has expanded so much it doesn't isn't that tectum it's not like in the evex in the tarsia here but it's a part of the brain more involved in visual learning all right and this shows an owl monkey there's the stri cortex what we call primary Vil cortex but all every area that you see outlin with the black line and many of them are also in different colors uh are separate representations of the visual field or separate in some way in their in the like metrop physiological recording studies they all have at least a partial representation separate from the others of the visual field so you can see how the visual areas have expanded into the posterior parietal region and the temporal region from the middle temporal area here all the way down into the in inerior Temple lobe okay what's the specialization of small rodents they're using their whiskers what's happened in the brain of these animals they have a specialized sense too but it's not like any of the others we've just looked at and we don't have it the Barrel Fields each Barrel named because of the way it looks in the hystology represents one whisker and uh these are sections of the cortex that have been they try to flatten the cortex and cut it um or at least they cut it tangential to the surface and here they've taken different levels and they go down through this area of the parietal lobe and they see these barrels we'd have to blow that up to see how these really look like barrels they're crowded neurons packed neurons around the edges and very few in the middle and if you look in the middle of the barrels these would all the cells would be in the dark areas here this is stained for ACS you see the axons from the phalus come up representing each whisker coming up in the middle of those barrels and terminating so it's a very specialized organization it's been very useful for electric physiological studies other CTIC sensory specializations exist too like the hand and apes and raccoons and certainly humans because of we have such high Acuity and motor control uh it also expanded cerebellum for coordinating those movements and I mention here the pensol tale and spider monkey and you get expanded representation of that taale in sensory and motor neocortex and uh this just shows the raccoon this is a picture from the book and it's comparing the quadi Mundi and the raccoon they differ tremendously in their use of their Cora the raccoon literally has a sensitive hand just like we do and if you look in the cortex this whole area outlined in red there here's a an enlargement part of it there's a separate little gyrus for each digit they have very fine sensory ability and corresponding motor control of that hand whereas if you look at the quad Mundi who also uses his for plaws as many animals do in feeding the corresponding area is much smaller and he doesn't have a separate gyus for each uh for each digit um does the have sensory abili he does have enhanced sensory abilities yeah and uh what may be a brain specialization of the human ability for complex social interactions and this would apply to many other primates that live in groups with complex social behavior what do we think there's increasing amount of work on this now especially the people studying the social brain here in the building yes in the neocortex especially prefrontal areas which is frontal lobe association areas we'll be defining all that when we uh later these are the areas that they keep localizing higher functions to and they'll talk about their evolution and I just want to point out I want I want to ask you if you remember what the specialization that I illustrate in the book The in the Akida which is the spiny an Eater of Australia he's got an enormous prefrontal cortex why would he have that you think it has to do with this is the pink area here it's relatively larger even than in humans just in relative to the in in the colors here are the motor area it's not sensory area visual area in green and uh the darker green is the auditory area and these are we would call them association areas but look at that prefrontal area so it has I assume it must have to do with his specialized feeding he's an ant eater and ants are very small how does he maximize his intake you know I think there's tremendous Demand on his working memory in a single glance he has to remember where dozens of ants are where they're disappearing and he's got to be able to go after them but that has never been analyzed okay one way to get it to be analyzed is to put it in a book and raise the issue people will look at it this is a question for you what other special izations might have noticeable brain manifestations I want you to think of something that I didn't mention in the chapter what other specializations in animals that you know about must have some special brain representation sorry anybody anybody else okay that's never been investigated do they have a specialized vestibular system what would you expect in that case anybody else specializations I ask yeah go ahead Rachel um I know that taxi dri has a parial neocortex parietal Association the area where we retain long-term Memories the map of the environment that's right there is some evidence for that it's a correlation and it's difficult to prove cause and effect but it's a very interesting calization these are London taxi drivers we know that the cortex is affected by learning uh in pretty interesting ways some of which are still being discovered I asked my wife this question this morning and she came up with specializations in the nose of some animals what was the one you thought of star Mo the star NOS mole John C uh actually turns about out to be one of the guys who's wrote an endorsement of my book so I like John cus and he he has studied this star not stos mole and and published representation of that it looks like a almost like a hand multiple fingers it's on their nose and they use it when they go through the ground okay and they use it for feeding yes that's another thing that we'll be dealing with a little bit but you're right it there are changes that occur in blind people and that's been studied in animals in very interesting ways and I would just add the one other animal with a specialized nose the elephant you know he's got incredible motor and sensory ability in that nose in that trunk all right this is the bigger class we won't quite get through it today but we'll do our best I want to give you an overview of forbrain structures and introduce the neocortex I want to start with this question what can an animal do without a forbrain and this has actually been done it's a pretty gross lesion you know they they either disconnect the forbrain from the midbrain with a cut in some cases they leave some of the connections so we it's been done the initial studies were were done in the cat uh but they then they were done on rats and pigeons it's all in early literature uh and let me just go through some of that you call that a des serration basically remove the cereal hemispheres or disconn completely disconnect them from other parts of the brain so this is our earlier diagram and what I'm showing here is uh the division spinal cord hindbrain midbrain and everything from here on is the for brain but the cereal hemispheres are everything in front of this line here this is Thalamus and hypothalamus they're tween bra so they're disconnecting all of these structures and of course in the mammals that they're doing it this area has expanded tremendously the neocortex has grown okay so they they get rid of all of them why do you think they got huge differences between the results in the cat and in the rat they both are mammals they both have a netics they both have a corpus stoom you know and they in both animals they disconnected those areas from the lower brain stem or from the brain stem period from the entire they got big differences what is the main thing I claim in the book is the difference because the early interpretations were not this and there's been all kinds of arguments about it yes number of yeah the quantity the number of connections severed very very good why would you be disconnecting so many more connections if you're dealing with a calf than in a rat they all they have the same kinds of Connections in qualitative terms but think of the size of the cat hemispheres he's got the highly folded cortex he got a relatively large KN of Cortex whereas the rat's a smooth brain mammal much smaller in relative terms a much smaller cortex okay well that's what we're going to consider why why is it I claim it's quantitative let's go through this but let's there's another question here actually both rats and cats the only way you can keep them alive after this kind of leion is to force feed them why was that the case they couldn't keep themselves alive does that mean they weren't hungry well in a sense that's true they never showed signs of hunger but if you put food in their mouth they ate so it's not true that they wouldn't eat they just didn't show the signs of hunger motivation okay now that the extensive study of the cerebration in the cat was done by these guys Mar and mo published in 58 they called it the animal a purely reflex animal all he had was the reflexes they said he couldn't do any act that requires a series of reflexes that's the way they put it they were still very affected by the SR model of behavior and they thought they just couldn't put the reflexes together we would say it more broadly now in terms of sensor motor responses so let's look at what the animal can actually do and can it learn anything first of all the cats were an osmic and blind because those are two forbrain senses the cranial nerves for ofaction and vision come into the forbrain they're disconnecting the forbrain so of course vision and old action can affect their behavior they didn't eat spontaneously as we said but they also didn't groom themselves they showed no spontaneous sexual or other social behavior they could though elicit sexual reflexes by stimulating the genitals they could get lower dosis response from females they could get penile erection but it required stimulation they didn't show it in Norm in normal behavior okay let's look at what they could do they could stand could sit they could write themselves they tipped over they could walk they did show some abnormalities uh they even showed rage but only if you pinch them especially their tail they didn't bite or strike out normally they showed some autonomic regulation they showed pyo erection they would fluff up their hair in response to cold they had some thermal regulation it wasn't as good as normal their temperature would fluctuate more with changing temperatures of the environment but they still had some therm regulation so commonly if they wanted to have an easier time maintaining these animals and not have to give them injections of electrolytes and things they left the an is the the hypothalamus attached to the pituitary even though it was disconnected from the midbrain they left that hypo hypothalamic Island attached to the pituitary and then they had much better regulation of their internal environment they couldn't learn much they had condition eye blank condition respiratory changes but they didn't maintain it very long so what was it I just said this describe the behavior that shows how a cat without a forebrain shows no hunger and yet will eat when you put food in its mouth so what's he doing he's responding to the stimulation he will open his mouth touch his lip he will close his mouth on the food what does that if he's not hungry sorry procedural no I wouldn't say it's procedural memory I would say it's innate reflexes okay exactly like the baby suckling okay except babies the suckling will vary with how hungry they are here it didn't seem to but that's the question uh maybe it did vary a little bit with how hungry they were but the point is they didn't show any motivation yeah that's a big issue and how this how the uh the dopamine Pathways the role they play in feeding it's I the representation that we call representation of the motivational State the drive state is no longer it doesn't mean they don't have it if they left the forbrain there but just disconnected it but it means it can't influence the midbrain and hindbrain and spinal cord anymore all right so then this just summarizes if you do the same thing to a rat he can do more he's faster to recover his writing reflexes and Locomotion I said the C had writing reflexes and Locomotion but in fact he lost it for a while it took him a long time to recover the rats recover faster uh they did show some eating and drinking responses but again and more than the cat did it was a little more than the normal just simple reflexes but they didn't seek food so it was still fatal if they didn't force feed them and now the rats would groom themselves the cats never did so against that b and mock conclusion they did show a series of you could call it a series of reflexes if that's the language you want to use you could we would describe it as a fixed action pattern a inherited pattern of behavior they also showed some defensive behavior that went beyond what the cat did they would vocalized they would try to escape they would claw and bite they even showed some auditory localization in space remember the audition is the eighth cranial nerve and it comes into the hind brain uh and it can reach the midbrain and cause orany movements why did the rat do that not the cat so some people said the cat's forbrain has taken over more of the function than the rat Forin but as the upper parts of the brain involve especially the cortex it takes over lower functions that was the way people thought about it and I'm asking is that the best way to describe the results and they say no so some people said well you take the the cerebal hemisphere is away they lose all their learned behavior let's look at that by looking now at another animal where Des celbration has occurred and that was in the pigeon these were earlier than the other studies earlier than the cat and rat studies okay they were done by Dutch people and uh it was all written in German viser and radok and uh 1935 and 37 they published this uh they showed after the the cerebration they showed their basic repertoire of unlearned reactions they would fly if you threw them into the air but they wouldn't start flying spontaneously you had to throw them into the air and while they were flying they would avoid vertical sticks of course this was all done indoors they would land on horizontal sticks but then they found out they would land even on the back of a dog or a cat so what's happened they forgot dogs and cats can be dangerous to pigeons uh there was a lot of indication more complicated Behavior patterns that depended some degree on learning during development were lost in these animals so my hypothesis is that the forbrain is very very important in linking together by learning the species typical action patterns fixed action patterns that are built in and therefore genetically genetically determined but we still learn to link those things together we inherit the ability to do this to do this but that doesn't mean we can you know we learn how to grasp a corner that doesn't mean we learn how to play the piano it's not not innate just the movements are but we link them in different ways by learning and that's mostly a uh function of the Corpus triom but first I want you to understand now's terms he uses these terms stability of the internal environment stability in space and stability in time and they involve different parts of the brain so without a forbrain NAA said they still had pretty good stability in space meaning they could balance they could write themselves they could move around they had quite a bit of stability the internal environment but that was much better if the hypothalamus was left intact but their stability in time was really messed up because their behavior is just their actions seemed to depend on the current inputs they had little or no motivation initiated Behavior little or no long-term memory and of course when you're dealing with mammals they lose a lot of sensory and motor Acuity they don't have fine movements uh they can't respond to detail the way they can with the cortex if you spare the Corpus Statum they can do a lot more uh I'm not going to spend a lot of time on that you can read this it's all in the chapter they've also varied instead of taking out the whole forebrain they just take out they just do decortications in various animals and the basic conclusion of those studies is that the more the cortex you take out the more deficient the animal is in a sensory motor control again a lot depends on quantity as much as people people tried to explain the behavior the effects of these lesions in terms of specific cortical areas removed and there was some correlation but it seemed that the biggest Factor was just the amount of Cortex they took up so that leads us to explaining this term invented by F mono uh long long time ago when we discussed cortical spinal diastasis some very interesting analyses what is meant by that term what does it mean diis you know what a Schism is cut something into two parts diis separation of two things so I want to know why is understanding of this phenomenon so important in interpreting these species differences in brain lesion effects and in fact in interpreting recovery of function in the human beings with brain damage and that's why I think it's so important to understand it in simple terms it means deaeration depression if you deafen a structure it REM me means you remove the inputs or at least a lot of them it's always a partial deaeration but with greater deaeration the greater the effect on those neurons so for example take the phenomenon of spinal shock do you know that term have you ever heard that has anybody ever heard of spinal shock before what is it but here you're thinking of dation as remover removal of sensory input and you're right that will cause deaeration of the dorsal horn neurons that are getting those inputs now let's talk about spinal shock it's a lesion of the spinal cord you if I get a spinal cord transsection say it bottom of the cervical region okay I don't just lose control of my legs and much of the control of my arms no I lose much more I lose even spinal reflexes I don't even show withdrawal reflexes or stretch reflexes the reflexes go but wait a minute in it they're spinal reflexes that's called spinal shock it takes a long time for spinal functions simple spinal reflexes that don't involve Pathways outside the spinal cords to to recover so here's my diagram of it here I've taken two species I'm showing something like a cortex here and an animal with a lot of it and an animal with less of it and here's a little diagram of the spinal cord and I'm showing a reflex pathway I just show neurons coming in and other neurons going out and they're connected in some way in the cord okay and then you do this you eliminate all those corticospinal connections this is really an a simple way simple way to depict what F Monaco described when he first described diastasis and he talked about cortical spinal diast thesis I just put into diagrams what he was saying I'm showing excitatory connections we'll ignore the inhibitory ones for now a lot more excitatory connections removed from the spinal cord in the animal with the larger cortex okay so what happens all these fibers are degenerated these neurons are less DEA and the smaller brained animal than these that means me the diis effect the degree of deaeration is greater in the larger brained animal so his spinal cord reflexes the spinal reflexes will take much longer to recover if I transect the spinal cord of a frog in not many days his spinal reflexes will recover if I do it in a dog it might take weeks and if I do it in humans might take months just to get the spinal reflexes back why because the neuron can fire Action potentials only if there is a depolarization of the membrane at the axon helic that reaches a critical level you know that from basic physiology okay if you remove a lot of it inhibit excitatory inputs then there will be more inhibitory inputs inputs from spinal inter neurons that are left and you just can't bring the the the cell to threshold anymore okay so then what happens after such lesions these two things happen there's collateral sprouting in the cord coll lateral sprouting by remaining inputs there's also what we call denervation Super sensitivity that means the neuron adjusts by increasing the number of receptors that respond to the neurotransmitters of the axons that are now missing so it will respond more strongly to other input that use the excitatory neurotransmitter okay so if we this animal will show some sprouting here this animal because he has more decantation will show more sprouting and there will be more receptors on these neurons and it will be more on the animal with a larger cortex you'll get more Super sensitivity those are the two factors and just over the time course of collateral sprouting that we know about and that has been studied you get quite a bit of recovery okay from this diation so basically these quantitative effects have to be considered whenever I get a brain lesion hopefully I won't but if I did okay if I get a brain leash I can't just consider well what were the functions now removed I've got to consider the areas that that area was connected to and how much deaeration that's caused I will lose midbrain function too if it projects heavily to the midbrain that's been studied corticotectal diastasis all right what part of the forbrain is most involved in the changes that occur during habit formation we call it procedural learning or implicit learning anybody it's the Corpus triot and we'll be talking about that we already mentioned it remember I said early inputs the olda very early when old faction was the main sense of the before there were anything except old faction coming into the the forbrain or the end brain was all old factory they went to the Statum that was the link to the lower structures that control movement and those connections were plastic they could be changed depending on the results the movement all right so what about the tween brain if you look at the just think of the main inputs to the tween brain and I it's relevant to this question according to the suggestions in the chapter what is a reason why sensory Pathways ascending to the forebrain almost always have a connection in the phus even the old factory actually to get to the cor neocortex have to go through the tween bra but why would that be why don't they just go directly so let's look at the early inputs optic inputs were very early and this is the picture of it here I'm showing them coming into the pineal area the pineal eye and here they are here's a lateral eye the ear the most primitive connection there goes directly to high the pels what is it doing it's controlling the activity rhythm changes during with the day night cycle the cycle of the Sun and just about all animals that is a system that's modulating the entire central nervous system that's what sleep and a waking is it's a cycle of change that affects the whole nervous system all right there's many other systems wide systemwide modulations too the here another area where secretions affect the whole animal the whole nervous system okay and there's more than that hypothalamus is affecting what goes through the phalus it's a modulator there are connections from hypothalamus to the thalamus we'll go over that in little more detail later and I think it was that modulation of the whole system that made it was important enough that I think it made it prevented the evolution of more direct connections there are a few more direct connections that have evolved but very few they almost all the sensory Pathways that go to the new cortex stopped in the in the Fon okay another part in those desate animals that you're removing is what we call the limic and brain basically all the parts that uh are connected to one structure you know what I'm talking about the lyic systems characterized by close interconnections with one portion of the upper brain stem what is it you can Define the lyic system those those structures that The Fringe of our hemispheres limic me means Fringe okay they're the more primitive parts that were there before any neocortex uh evolved and expanded what was the structure you didn't get that sorry what was it not the neocortex because the limic system is more primitive than it was there before there was a new cortex okay it's this structure here okay and I'm showing here hypothalamus and how it is interconnected not only with the factory system but with all these structures of the older parts of the brain the whole ancient pum we call them the dorsal p ium the medial pum and dorsal pum up here medial pum is the campus and then the lateral pum which is olfactory cortex and the ventral pum which also gets olfactory input includes the amular area those areas are all interconnected with the hypothalamus and that pretty much defines the lmic system and there are structures also in the mid brain that now at a point it out should be part of that because they are also closely interconnected with the hypothalamus and with the lindic system uh further forward all right I want you to read the rest of these I'll review a little bit of it at the beginning of the next class but uh I will mark this this is where we stop and uh I will post these online and uh we'll be able to finish this quickly next time and go on uh to uh class 7even