Me and a student for viva I asked what is limbic system can you describe limbic system the student says after some thought sir the limbic system is in animals they walk on their forelimbs and humans walk on the hind limbs or two limbs that's the limbic system Wow fantastic amazing hilarious thank you very much goodbye I do get such answers once a while, hilarious answers or amazing answers. There was one such answer on the limbic system. Probably the student wanted to connect it with limbs. So I thought of making a video for first year students for what you should write if it's a five mark short note.
So here is the limbic system. First of all, limbic system, limbus means a ring. And therefore, this term, the limbic system is applied to the parts of the cortical and subcortical structures. So, there will be cortical structures and some subcortical nuclei which form a ring around the brain stem.
So, limbic actually does not mean the limbs as in forelimbs, hindlimbs. Limbic or limbus means a ring. Alright, and here is a system which consists of some cortical structures and some subcortical nuclei which form a ring around the brain stem.
That's how the word originates limbic system. And the first thing that should come to your mind when you talk of limbic system is emotions. Emotions and behavioral responses.
That's the core of the limbic system. With that in mind, now let us... proceed further whenever there is a short note on this limbic system five marks or seven marks note first you should write an introduction then the structures involved in the limbic system the part of the limbic system that is and then functions of the limbic system and finally maybe the applied physiology these four headings uh under which you will answer this particular note all right so the limbic system uh this is the introduction of course the first part that you should write Limbic system consists of the limbic lobe or the limbic cortex and the related subcortical nuclei as I have already mentioned. Now, basically, limbic cortex would include or even before that, let me just explain to you about the cortex. Our cortex has got two parts.
Phylogenetically, the newer one is called as neocortex. And this neocortex forms 90% of our cortex. It includes all those frontal lobe, motor cortex, parietal lobe, sensory cortex, temporal lobe, auditory cortex and occipital lobe, visual cortex and all these cortices, primary, secondary, etc.
So that's 90% part which is called as neocortex. And the 10% remaining part is called as allocortex, phylogenetically older. So, allocortex and that comprises of the limbic system.
So, you may remember like this 10% cortex is made up of the limbic system and it's called as or referred to as allocortex. All right. Now, the structures, structures, that's the second part of your answer. Limbic cortex includes cingulate gyrus. Yes, cingulate gyrus, not cingulated.
Cingulate gyrus. isthmus hippocampal gyrus and uncas uh let me add one more point here there is no unanimity among the authors as to which structures should be included in the limbic system so what i have done is taken the most common structures which are included by everybody okay so uh what are the cortical structures here limbic cortex includes cingulate gyrus, isthmus, hippocampal gyrus and uncus. Four structures.
And the related subcortical nuclei amygdala. Remember most one of the most important parts. Amygdala which is a group of nuclei on the tip of the temporal lobe.
Okay limbic lobe structures are associated with the temporal lobe. limbic system structures closely associated with the temporal lobe septal nuclei and of course easy to remember hypothalamus hypothalamic nuclei and the anterior thalamic nuclei. So hypothalamus thalamus. Another important point to be highlighted hypothalamus forms the very important part of the limbic system.
Crucial critical part almost we can say centrally located or central role being played is that of the hypothalamus as we will see later. But is that clear now? The cortical structures are cingulate gyrus, isthmus, hippocampal gyrus, and uncus. And related subcortical nuclei, collection of nerve cells, that will include amygdala, hypothalamus, thalamus. All right.
Now, just an additional point here. Limbic system was formerly... called as rhinencephalon.
Rhinose, you know that rhinose means nose or olfactory sense. So, because it's relation to the olfaction, smell. Yes, it is closely related to the olfactory sense, the limbic system and olfactory sense are closely related, but that's only a small part of the limbic system function.
So, previously called as rhinencephalon at additional point the limbic cortex phylogenetically the oldest part of the cerebral cortex and referred to as the allocortex which i have already mentioned let's look look at the structures in detail and their connections so uh okay you can see here it's making a limbus or a ring around the brain stem structures. Somewhat goes like this, like a ring. So it will have cingulate gyrus over here, cingulate gyrus, then thalamus here, then hypothalamus, amygdala, a small portion, and hippocampus. So these are the important main structures of the limbic system. or let's have a better diagrammatic view better diagrammatic presentation of these structures cingulate gyrus then nucleus accumbens a very small area we will see its function later anterior thalamic nuclei septum and then of course beneath the thalamus below the thalamus there is a hypothalamus alongside it is the hippocampus so remember nucleus accumbens, anti-arthalamic nuclei, septum, hypothalamus, hippocampus, then beneath that the dentate gyrus and amygdala.
These four names will be crucial for your answer. Amygdala, hypothalamus, anti-arthalamic nuclei, cingulate gyrus and then the other structures like parahippocampal gyrus and close by is the orbitofrontal cortex. This diagram will be useful for your exam answer in the exam.
Now let's see the important connections and functions. So limbic system represents the primary area for the control of autonomic functions like heart rate, like blood pressure, like GI movements etc. How it happens is that hypothalamus is the highest seat of control for the ANS.
ANS function has got its control in the hypothalamus and hypothalamus is extensively connected with the limbic system structures. Hypothalamus itself is a part and it has got connections with the other structures of the limbic system. So emotions controlled by the limbic system connected with the hypothalamus and hypothalamus controls the ANS. in turn will control all these heart rate, GI movements, blood pressure, etc.
That is how emotions have the influence on our heart function and other functions. Alright, now amygdala. Amygdala is the emotional window. Remember this particular phrase.
Amygdala is the emotional window. through which a person sees the external world. Yes, you will find two people reacting very differently emotion wise to a same scenario or same situation. That's because of their functioning of the amygdala differs from each other.
So it's the emotional window through which a person sees the external world and it is known to regulate emotional behavior of an individual. The emotions of rage and fear are elicited on stimulation of the limbic system, particularly the amygdala. So we have seen something about hypothalamus, we have talked about the amygdala emotional window, an important phrase.
Then limbic system plays an important role in the motivational drive of an individual. Some people are highly motivated, some people lack motivation. So, this all depends on the limbic system uh the drive that emotional drive depends on the functioning of the limbic system and it is also concerned with olfaction now uh just a quick additional information we uh suppose there is common cold we are suffering from common cold and there is loss of smell we can't smell the things You might have observed that We also can't feel the taste.
Have you experienced this? In common cold associated with fever or something. There is no smell.
That's fine. The nose is choked or blocked. Congestion of the nose. So there is no olfactory sense.
But then you also can't feel the taste. There's nothing wrong with the taste pathway. Why can't you appreciate the taste?
Remember this. That sense of olfaction. First of all all the sensory signals, they have to pass through the thalamus to reach the cortex, right? Olfactory sense does not go through the thalamus.
So it's quite separate from the other sensations, including taste. Is there any place where smell and taste meet in the CNS? Yes, there are two places, two locations where smell gives a collateral.
Olfactory sense, olfactory pathway gives a collateral. to excite the sense of taste or potentiate the sense of taste one is in the amygdala and the other one is the insular cortex so uh olfactory sense will give the collateral and potentiate the sense of taste uh especially at these two places which i mentioned and therefore if there is loss of smell loss of olfaction then this potentiation also will not be present and the person will not be able to appreciate the taste sensation all right then hippocampus yes we all know that the hippocampus is the seat for long-term memory all right seat for the long-term memory hippocampus converts working memory into the long-term memory so we have seen the important structures and the functions performed by them starting with hypothalamus hypothalamus uh controls the ans and hypothalamus connected with the limbic system parts other parts other than the hypothalamus so emotions having some bearing on the ans function is via the hypothalamus and thereby controlling the heart rate blood pressure gi movements amygdala emotional window through which a person sees the external world then hippocampus long-term memory you the seat of long-term memory. All right, now James Papes, the neuroanatomist, he gave the emotional basis of the human behavior.
Why do humans behave in a certain way in certain situations? What's the emotional basis? That emotional basis was given by James Papes, the neuroanatomist, and he gave a circuit which is called as Papes circuit. So that's also to be included in the discussion on limbic system. I hope it is visible.
Okay. Now prefrontal cortex has connections with the limbic system. Frontal lobe, motor cortex and in front of it, most anterior in the cerebrum is the prefrontal cortex.
So it's connected with the cingulate gyrus. a part of the limbic system then hippocampus hippocampus where the memory long-term memory will be stored then the mammillary bodies hypothalamus hypothalamus to thalamus anterior thalamic nuclei which is the connection is called as mmt mammulothalamic tract So mammillary bodies to anterior thalamic nuclei and thalamus back to the cingulate gyrus. So this circuit establishes the emotional basis of our behavioral responses. How do we behave? What are the external manifestations of our emotions?
All that is given by this circuit. Just remember it once. Once again, have a look at it. Prefrontal cortex to cingulate gyrus to and fro connection. So, uh...
whatever information is there in the prefrontal cortex upon a daily basis of course that information given to the cingulate gyrus and it goes through the circuit of the limbic system so cingulate gyrus hippocampus mammillary bodies that's hypothalamus and from there to the anterior thalamic nuclei back to the cingulate gyrus all right that's the papage circuit all right finally let's see the role of or before that uh there is just one part that we have missed out so let's let me just describe that also nucleus accumbens nucleus accumbens we saw it's a part of the limbic system it has got dopaminergic neurons very densely populated created dopaminergic neurons and you know the dopamine is a neurotransmitter associated with reward reward and pleasure okay addictive behavior so nucleus accumbens is associated with the reward pleasure and addictive behavior in humans so just that one more function to be added in your note finally the role of hypothalamus see how central role is played by the hypothalamus the non-hypothalamic parts of the limbic system receive information from the cortical association areas prefrontal cortex as we saw so cortical association areas will send the signals to the non-hypothalamic parts of the limbic system cingulate gyrus etc from there the signals will be sent to the hypothalamus and from the hypothalamus control of ans will be executed. So the emotional meaning of the external stimuli, information gathered from the memory and understanding is passed on to the hypothalamus. The hypothalamus then integrates look the emotional stimuli, various situations that a human will face.
All those things will be first going to the cortex from cortex to the limbic system structures. and there to the hypothalamus. Now hypothalamus will integrate all this to elicit endocrine autonomic and some other activities so that there will be an external manifestation of the emotions. So you know already there is a known fact that hypothalamus controls the master endocrine gland and thereby It controls the endocrine function of the body and it also is the seat of ANS control.
So heart rate increasing or if there is fight and flight, there is a release of catecholamines. All this will be initiated by the hypothalamus upon receiving the external emotional stimuli. So that's the function of the hypothalamus.
These all things you may write in the in the short note finally applied aspect you know clinical application of all of this degeneration of neurons in the limbic system will kind of lower your emotional responses so there may be dementia Alzheimer's disease if there is degeneration of neurons in the limbic system or anterograde amnesia with the legion of hippocampus loss of smell and taste in the case of lesions of the lesions of the other structures in the limbic system so that is the clinical application or things that are happening with the lesions of the limbic system so this in a nutshell is the limbic system as the answer you can write in the exam certainly you will get full marks after writing this much