The Brain-Part3
Evolution of the human brain.
Homeostasis.
Role played by the inner brain.
Endocrine system.
Homeostatic regulated functions.
Evolution of the human brain.
The human brain, is like a city, with a long history.
It has its old sections which are ancient.
These ancient sections performs basic survival activities.
We still retain those parts of the brain.
It has also newer sections, which developed around the older ones.
Finally, it has a modern section, which developed more recently,
and which are adapted to current functionality, of the modern human being.
It is fascinating to trace the development of the brain,
in conjunction with the evolution of human beings, from ancient forms of life.
Primitive forms of life, also had some type of primitive brains.
As more complex forms of life developed, a more complex brain also developed.
The more complex layers developed, on the foundation of the primitive layers.
Just like our DNA, carries our evolutionary history,
our brain also carries our evolutionary history.
We share the design and functionality, of our brain, with many other forms of life.
We will discuss three important phases, in the development of the brain.
The reptilian brain.
The limbic brain.
The neocortex.
The reptilian brain first appeared in fish.
This happened nearly 500 million years ago.
The amphibians developed, a more advanced version,
of this brain, about 250 million years ago.
The limbic system first appeared, in small mammals,
about 150 million years ago.
Lastly, the neocortex began its spectacular evolution, in primates.
This happened 2 to 3 million years ago.
We homo sapiens evolved from the primates.
Our brains carry a 500 million years history of design, inside it.
It is amazing, how nature never reinvents the wheel.
For the brain, once a design was satisfactory, it carried it forward,
and built more advanced versions, over it.
The reptilian brain.
The oldest part of the brain can be compared to the reptilian brain.
It controls the body’s vital functions, such as heart rate, breathing,
body temperature, etc.
Our reptilian brain, includes the main structures, found in a reptile’s brain.
They are:
The brain stem.
The cerebellum.
The limbic brain.
The limbic brain can be found in early mammals.
This part of the brain, records memories of behaviours,
that result in agreeable and disagreeable experiences.
Evolution favoured this kind of memory.
The mammals learnt to avoid disagreeable experiences,
and seek agreeable experiences.
What we call as emotions, in human beings, had its origin in this part of the brain.
The main structures of the limbic brain are :
The hippocampus.
The amygdala.
The hypothalamus.
The limbic brain is the seat of value judgements, we make.
These judgements are often unconsciously made,
and exert a strong influence on our behaviour.
Some of the primitive emotions, like fear are triggered,
in human beings, by the limbic brain.
Many homeostatic functions of human beings,
are carried out in the reptilian and the limbic brain,
which we inherited from our ancestral species.
Neocortex.
The cerebral cortex of the brain, is built over the limbic brain.
The neocortex is an evolutionary advance that took place, in the cerebral cortex.
The neocortex first assumed importance in primates.
This evolution culminated in the development of the human neocortex.
The neocortex has evolved significantly, from the first human beings,
or homo sapiens, to the modern human being that we are today.
It is still evolving.
The neocortex is present in the two cerebral hemispheres, and is responsible,
for our higher level of thinking.
It is involved in human language, abstract thought, imagination and consciousness.
The neocortex has almost infinite learning abilities.
The neocortex has played a significant role, in the development of human culture.
Our highly sophisticated neocortex, differentiates us, - the modern human being,
from all other species.
The neocortex endows us with advanced thinking capabilities.
We often associate the brain mainly with thinking.
However, we need to remember, that the primary evolutionary purpose,
of the brain, is to keep us alive.
We still retain the basic design, of the earlier structures of the brain,
which evolved from our ancestral species.
Our older brain is physically located, in the inner parts of our current brain.
We will refer to these parts collectively, as the inner brain.
The more recent developments were in the cerebral cortex, which is the outer layer,
which developed over the inner brain.
We will refer to this as the outer brain.
The terms inner brain, and outer brain is not in common use.
For convenience, we will use these terms,
to refer to the brain, which developed earlier and later.
We retain the basic structure and design of the inner brain,
which we share with other species of life.
This is understandable because, we share many of the basic life processes,
like breathing, maintaining body temperature, blood pressure, etc. with animals.
Sometimes we refer to the functions, of the inner brain,
as lower order functions.
This kind of statement, is purely relative.
It means that we human beings, have higher order thinking capabilities.
Other animals do not have these thinking capabilities.
It is quite true, that we have a complex and sophisticated brain system,
which is capable of very high order thinking.
The more sophisticated and complex brain design and structures,
that where developed, over the inner brain.
This outer brain is capable of higher order thinking.
This does not mean, that lower order functions of the brain are unimportant.
The so called lower order functions, are critical to our living.
It is just that, we are not conscious of it, and take it for granted.
It is worthwhile to know, how the brain regulates critical life processes,
to keep us alive.
To engage in higher order functions, first we need to be alive.
Homeostasis.
There are millions of biochemical processes, happening in the human body.
All these processes need to be co-ordinated.
Homeostasis in this context,
refers to the regulating of complex life processes in the body,
to maintain a dynamic equilibrium.
The inner brain is responsible for maintaining homeostasis, in the body.
We traditionally think of five sensory organs.
The senses perceived from the sense organs are:
Visual sense from the eye.
Auditory sense from the ears.
Sense of balance and orientation from the ears.
Sense of smell from the nose.
Sense of taste from the tongue.
Sense of touch, temperature, pressure, etc from the skin.
We also have a sense of proprioception,
which gives us a sense of where our body parts are.
Apart from these senses, we also have other senses.
For example, We unconsciously sense body temperature and blood pressure.
By convention, these other senses, may not be considered,
as part of the sensory system.
We are not conscious of these other senses, but they are critical,
to maintain homeostasis.
Some examples of homeostatic functions are:
Maintaining a constant internal body temperature.
Maintaining a given range of blood pressure.
Maintaining the level of oxygen in the blood stream.
Maintaining the level of glucose.
For maintaining all these, and other homeostatic functions,
the brain needs to continuously sense other body parameters.
For example, it senses
body temperature,
blood pressure,
oxygen levels,
blood glucose levels,
blood ph levels, etc.
We are not conscious of these senses.
But these senses, are also continuously monitored, by specialised receptors,
in the body.
The brain senses these parameters.
Using these, it controls the body functions, to achieve homeostasis.
Role played by the inner brain.
Many of the homeostatic functions, are carried out in the inner brain.
The inner brain, is also involved in some primal emotions, like fear.
We will discuss, a simplified model of the inner brain system.
The inner brain system is located in the core area of the brain.
It includes the brain stem.
Its location is close to the lateral ventricle, the thalamus and corpus callosum.
Some of the main constituents of the inner brain are:
The cingulate gyrus.
The amygdala.
The hippocampus.
The hypothalamus.
The corpus callosum connects the two lateral hemispheres of the brain.
The cingulate gyrus is located above the corpus callosum.
It looks like a belt around the corpus callosum.
The hippocampus is located below the corpus callosum.
The thalamus is located below the cerebrum, and above the brain stem.
The hypothalamus is located below the thalamus.
The amygdala is located near the hypothalamus.
Cingulate gyrus.
The cingulate gyrus gives us the functionality of cognitive flexibility.
The cingulate gyrus plays a role in, we being socially adaptable.
Under normal conditions, we tend to be social animals.
We get along fine with other human beings.
When this malfunctions, we tend to be excessively obstinate and argumentative.
Malfunctions in this area, is also associated with,
obsessive compulsive disorder, or OCD, and addictions.
That is in normal functioning it is adaptable,
and in abnormal functioning,
it kind of gets stuck into a habit, or view point.
To be socially adaptable, we pickup emotional cues, from fellow human beings.
This helps us to adapt to different social situations.
The cingulate gyrus helps us to avoid negative situations.
It also learns to seek pleasure from positive situations.
Amygdala.
The amygdala receives sensory and processed information,
from different parts of the brain.
It also has pathways which can stimulate motor responses.
The fear response is encoded in the amygdala.
This is the most important primal response, that we share with other animals.
This is the response to a threat.
This response becomes critically important, when a prey senses a predator.
It is a survival instinct.
Though we do not face the actual threats, that other animals,
and our stone age ancestors faced,
we continue to use this response, to real or perceived threats.
There is an interesting pathway in the brain to the amygdala.
Sensory information is almost always routed via the thalamus.
There is one pathway which routes sensory information, to the cortex,
and then routes the processed information, to the amygdala.
There is also another parallel pathway that connects, the sensory information,
directly to the amygdala.
Neural circuits in the amygdala, are like programable circuits.
They can be programmed to directly respond to selected sensory stimuli.
This response is not mediated by higher regions in the cortex of the brain.
It is an automatic response.
This leads to a immediate and fast response.
For example, if we notice a plastic scorpion in the dress,
the amygdala might trigger a scream response.
Moments later, we might get a highly processed signal, from the brain,
that the threat is not a real threat, and it is only a harmless plastic toy.
This inhibits or cools down, the initial response.
We can imagine a mouse trying to avoid a cat.
This fast and immediate response, comes in very handy in these situations.
When its life is under threat, there is no time to think it over and respond.
Under some conditions the animal can decide to fight.
Let us imagine a situation, when we pretend to throw a stone, at a stray dog.
In some cases, it will start to run, and escape.
This is called the flight response.
In some cases it will make an aggressive growl with bared teeth.
This is called the fight response.
Both these responses originate in the amygdala.
This response is called a flight or fight response.
We can find similarities of this response in human beings also.
Feelings of aggression is associated with the amygdala.
This is involved in triggering the fight response.
The absence of aggression, is mellowness, which is related to amygdala.
When a very strong emotion is experienced,
the amygdala permanently encodes, the experience,
along with all the circumstances, in which the emotion was experienced.
Recall of these experiences is easy, fast and richly vivid.
This would have been very useful for animals,
to recall dangerous, life threatening experiences, and react swiftly to it.
Instantaneous and vivid recording of strong emotions,
are present in human beings also.
We might notice that we can easily and vividly recall, strong emotional experiences.
The amygdala also plays a central role in pleasure experiences.
The pleasure of eating food, when hungry, is one of the basic pleasure experience.
Avoiding fearful experiences, and seeking pleasurable experiences,
are the basic emotional instincts of animals.
Human beings also have these primitive instincts,
but they are well modulated, by higher regions in the brain,
specially the pre frontal cortex.
The amygdala has got direct neural connections, to the thalamus.
Sensory information converging in the thalamus, can directly reach the amygdala.
The amygdala also has direct neural pathways, which trigger physiological responses.
The amygdala has got direct connections to the hypothalamus.
On experiencing an emotion like fear, the amygdala can trigger the hypothalamus,
to release specific hormones.
For example, it might trigger the release of a stimulating hormone,
which causes adrenaline to be released from the adrenaline glands.
This results in increased heart rate, increased blood circulation,
increased oxygen intake, and increased energy production,
by carbohydrate metabolism.
All this helps in the flight or fight response of the muscles in the body.
Human beings have a very special input to the amygdala.
This comes from the pre frontal cortex, unique to human beings.
This is a highly processed, well thought out response to the stimuli.
In the example, related to the plastic scorpion, the pre frontal cortex,
processes the sensory information, and evaluates it, as harmless.
This input comes to the amygdala, which helps it to cool down.
Unlike animals, we human beings, are in a unique position,
to manage our emotions.
We do not have to let emotions directly control our behaviour,
and jump to conclusions.
We have the option, to think about it, and give a thoughtful response.
This skill of regulating emotion, via a higher level thinking process,
can be cultivated.
Hippocampus.
The hippocampus is just below the cingulate gyrus.
It is located deep in the temporal lobe.
The hippocampus is strongly associated with the function of memory.
It is believed to play a central role in translating short term memory,
to long term memory.
When we get a sensory input, it is first held in short term memory.
Hippocampus processes short term memory, and selectively stores some of them,
in long term memory.
This is the kind of memory, we can recall days or even years later.
Strong emotions helps to translate short term memory, to long term memory.
If we are involved deeply, with what we are doing,
it is more likely that we can recall it.
If we enjoy what we are doing, it is more likely that we can recall it.
Many senses can also be associated with memory.
A sight of something, can trigger a memory event, which happened long ago.
A sense of smell, can trigger a long forgotten memory event.
The brain has a unique way of inter relating and associating,
seemingly unrelated things.
A sight, a smell, a memory, and an emotion, can all be strongly interrelated.
Hypothalamus.
An emotion is frequently, typically involves a physiological response.
The hypothalamus is responsible for the physiological responses,
to the stimuli which triggers an emotion.
The hypothalamus is involved in regulating the autonomic nervous system.
The hypothalamus and the pituitary gland, controls the endocrine system.
For example, the heart rate, blood pressure, breathing etc.,
are mainly regulated from here.
Fear is associated, with the physiological response of increased heart rate,
blood pressure, rapid breathing, sweating etc.
These physical responses are triggered in the hypothalamus.
We have discussed some important structures of the brain, involved in emotions.
What we have discussed is a simpler model of the emotional brain.
Today, we know that many other parts of the brain are involved in emotions.
This includes the pre frontal cortex.
The pre frontal cortex is the most complex, advanced part of the human brain,
involved in higher order thinking.
Emotions can influence the functionality, of higher order thinking.
A persistent or lingering feeling, can create so called “moods”,
influences overall functioning, including the pre frontal cortex.
Positive moods can stimulate it, and negative moods inhibit it.
Learning can be much more effective, if we are really interested in it,
and it is a joyful experience.
Another interesting fact about our brain,
is that, thinking can influence our emotions.
It is like a two way process,
emotions, influence thinking, and thinking influence emotions.
Human beings have the capability to cultivate the mind,
so as to regulate emotions.
Endocrine system.
The endocrine system, is a functional system, in the brain.
The endocrine system plays an important role in homeostatic processes.
The endocrine system comprises of,
the Hypothalamus, and the pituitary gland, in the brain,
and other glands like the thyroid gland, the adrenal gland, etc.
located in different parts of the body.
To regulate the life processes or metabolic functions, the endocrine system,
translates electrical impulses, from the brain to chemical signals.
The hypothalamus generates the electrical signals.
The pituitary gland, translates the electrical signals,
to generate bio chemicals, called hormones.
These hormones circulate in the blood stream,
and control the other glands like the thyroid gland, and the adrenal gland in the body.
The pituitary gland, is considered as the master gland, in the body.
The hypothalamus of the brain, can be considered as the master,
of the pituitary gland.
The hypothalamus in the brain, and the pituitary gland work closely together,
to achieve the homeostatic function of the endocrine system.
Hypothalamus.
The hypothalamus is located below the thalamus, and above the brain stem.
It is considered as part of the inner brain.
It is a size of an almond.
The thalamus is the master router of the brain.
The thalamus routes information from the body to different parts of the brain,
and routes information from the brain, to different parts of the body.
The hypothalamus along with the pituitary gland,
can be considered as the master of homeostatic regulation,
of critical metabolic processes.
The endocrine system in the body, comprises of many glands.
The pancreas, the thyroid, and the adrenal glands,
are some examples of endocrine glands.
These glands secrete hormones, which are released in the blood stream.
These hormones reach selective target cells, and help to regulate their behaviour.
The hypothalamus and the pituitary gland, regulate the functioning of these glands.
It sends out instructions to these glands, to increase or inhibit the production,
of hormones.
These instructions are sent as biochemical molecules, called hormones.
These hormones act as control hormones, for the endocrine glands.
To effectively regulate critical life processes, the brain needs to monitor,
several body parameters.
For example, it needs to sense the body temperature, the blood pressure,
the oxygen levels, the glucose levels, etc.
There are specialised sensors in the body to do this.
The brain receives the data of the body parameters, from the sensors.
After some complex processing, it decides, what should be regulated,
and how much should be regulated.
These decisions are translated in the hypothalamus and pituitary gland,
into biochemical signals or hormones.
These hormones are released into the blood stream.
They reach the target glands through blood circulation,
and regulate the target glands.
The hypothalamus comprises of many different nuclei.
A nuclei is a collection of nerve cells.
Typically the same type of nerve fibres, dock into a particular nuclei.
Each nuclei in the hypothalamus, specialises in a particular type of function.
They are like a specialised department within the hypothalamus.
Each nuclei receives and processes sensory information.
For example, one nuclei might process body temperature parameter.
It then decides whether to trigger signals, to increase or decrease body temperature.
The decision is translated to an electrical or chemical signal,
and communicated to the pituitary gland attached to it.
The pituitary gland then releases, the appropriate hormone.
In a typical process, the hypothalamus will send out, a neurohormone,
called a releasing hormone to the pituitary gland.
The pituitary gland, will receive the releasing hormone,
and generate a stimulating hormone.
The stimulating hormone, will be released to the blood stream.
The blood circulation will carry the stimulating hormone, to the target gland.
The target gland will release a hormone, which again enters the blood stream.
These hormones will dock into target cells, and modulate and regulate,
its behaviour.
In some cases the nerve fibres of the hypothalamus,
extends into the pituitary gland, and triggers the hormone production.
The homeostatic regulatory process is a closed looped system.
Signals are sent out to target glands.
Action is taken by target glands.
Sensory receptors send feedback to the hypothalamus.
The hypothalamus processes the information, and sends revised signals.
This closed loop system is very essence of the homeostatic functions, of the brain.
Pituitary gland.
The pituitary gland is an endocrine gland, the size of a pea.
The pituitary gland is attached to the bottom of the hypothalamus.
The pituitary gland is the master gland, and the hormone factory.
The pituitary gland has two parts.
The anterior pituitary gland.
The posterior pituitary gland.
The anterior pituitary gland controls many metabolic processes,
like stress, growth, reproduction and lactation.
The hypothalamus sends releasing hormones, to this part of the gland.
It then produces stimulating hormones which are released into the blood stream.
Some examples, of stimulating hormones released by the anterior pituitary gland are:
Growth hormone.
Growth hormone is involved in cell regeneration, and in human development.
Thyroid stimulating hormone.
This hormone stimulates the thyroid gland.
The thyroid gland in turn, produces hormones which stimulate the metabolism,
of most tissues in the body.
The posterior pituitary gland is directly connected to nerve fibres,
emanating from the hypothalamus.
These nerve fibres stimulate the production of hormones.
Oxytocin and vasopressin are examples of hormones produced in this part of the gland.
Oxytocin is involved in parenting functions.
Vasopressin is involved in blood pressure regulation.
The pituitary gland is involved in producing several such hormones.
All of them are involved in homeostatic regulation of metabolic functions.
Using chemical messenger signals, in the form of hormones, the pituitary gland,
is able to regulate many of the metabolic processes.
It requires the hypothalamus to give it instructions.
The hypothalamus requires feedback from specific sensors,
to process and give instructions to the pituitary gland.
Homeostatic regulated functions.
Overview.
Homeostasis is a process by which a system maintains, a dynamic equilibrium .
Homeostatic functions are a closed looped system.
It has a control mechanism, which acts upon the target subsystems.
It has a feedback mechanism, from the sub systems.
Using the feedback mechanism, the process can generate revised control signals.
All critical life processes, or metabolic functions have homeostatic regulations.
The hypothalamus and pituitary gland are the most important,
homeostatic regulatory organs.
Most of these functions are achieved by hormone regulations.
The autonomic nervous system is involved in some processes.
We will discuss a few examples, of homeostatic regulated functions.
Body temperature.
Animals and human beings, maintain the body at a constant temperature.
Human beings maintain a body temperature of around 37 degree celsius.
Regardless of whether it is summer or winter, body temperature,
is always maintained at the same standard set point of 37 degrees.
What is even more amazing is, human beings living in a hot desert, like the Sahara,
or the eskimos living in the artic zone, maintain the same body temperature.
Maintaining this body temperature, is critical for our metabolic processes.
This concept is similar to the thermostat used by air-conditioners.
Thermostats regulate the temperature, so as to maintain it, at a given set point.
There are many temperature receptors in the body.
Most of them are located, near the skin.
They sense hot or cold sensations.
These signals are sent to the hypothalamus.
The hypothalamus is the homeostatic thermostat of the brain.
It plays a major role in maintaining constant body temperature,
regardless of environmental conditions.
There are several strategies that the brain uses to regulate body temperature.
We will discuss some of these strategies.
When the temperature gets too hot, the hypothalamus signals,
to activate the sweat glands.
Sweat glands are located near the skin of the body.
Sweat glands, when stimulated release a watery fluid or sweat, to the surface of the skin.
Due to the surrounding heat, the sweat tends to evaporate.
When the sweat evaporates, it carries away latent heat.
This results in cooling the body.
When we engage in activity, involving usage of muscles,
such as playing, dancing, exercising,
heat is generated by the muscles in use.
This causes the body temperature to go up.
The hypothalamic thermostat activates the same sweat glands,
to cool down the body.
When the temperature gets too cold, the hypothalamic thermostat,
signals the muscles to shiver.
Shivering is a involuntary way of exercising the muscles.
It generates heat, helping to heat up the body, to the set point.
The brain might use multiple strategies to control body temperature.
When it gets too cold, several responses are possible.
Blood vessels near the skin, can get constricted.
This reduces the blood supply near the skin, there by reducing heat loss.
The brain signals the cells in the body to burn more nutrients,
to generate more heat.
When there is a gradual reduction of temperature, like when the season,
changes from summer to winter,
the hypothalamus can release more Thyroid stimulating hormones,
which induces the Thyroid gland to release hormones ,
like the T3 and T4 hormones.
This increases the basic metabolic rate, which helps to increase body temperature.
Then hypothalamus can use several such responses, to regulate temperature,
at a constant set point of 37 degree centigrade.
Maintaining a constant body temperature, in a changing environment,
is a good example of a homeostatic process, controlled by the brain.
When the temperature goes up, brain initiates action, to bring it down.
When the temperature goes down, brain initiates action, to bring it up.
This dual control, is typical for homeostatic process.
The feedback, received via., the sensory system,
is also typical, for a homeostatic process.
Breathing.
The respiratory centre, is an area of the brain,
which regulates the breathing function.
The respiratory centre, is located in the brain stem.
Breathing is an essential function, of all animals, including humans.
From the evolutionary perspective, the brain stem is the oldest part of the brain.
It is no surprise that the respiratory centre, is located in the brain stem.
Evolution took care of the basics first.
Some chemoreceptors are located, close to the brain stem.
Chemoreceptors chemically recognise, certain critical parameters for life.
They generate action potentials, which is sent to the brain stem.
Central chemoreceptors are a group of receptors,
which sense, the level of carbon dioxide, and pH, in the blood.
There are other forms of receptors, located further away.
Some are located in the Aortic body.
The Aortic body located in the Aortic arch, above the heart.
Some are located in the Carotid body.
The Carotid body is located in the Carotid artery, in the throat.
These chemoreceptors are called peripheral chemoreceptors.
They mainly detect levels of oxygen in the blood.
They can also detect levels of carbon dioxide, and pH levels, of the blood.
There are also mechanoreceptors, which sends signals to the respiratory centre.
For example, mechanoreceptors in the nose,
can send signals if it detects an unwanted foreign particles.
This might trigger a sneeze reflex action.
Mechanoreceptors in the lung, convey information, on the amount of stretch,
or expansion of the lung.
This may trigger an exhalation in the lungs, after an inhalation.
The respiratory centre is also connected to motor neurons,
which control muscles.
One group of muscles that it controls is the diaphragm muscles.
Diaphragm muscles help us to breathe.
Another group of muscles, it is connected to,
is the intercostal muscles.
Intercostal muscles are located between the ribs.
These and other muscles work, in a coordinated way,
to help the lungs to inhale and exhale.
Sensing the levels of oxygen, carbon dioxide, and pH,
help to regulate the breathing rate.
When oxygen levels are low, or carbon dioxide levels are high,
the lungs are regulated to breathe at a faster rate.
When we run or exercise, the body needs more oxygen.
This triggers the lungs and the heart to beat faster.
Blood pressure.
There are special sensors in the body, to sense blood pressure.
They are called baroreceptors.
They are located in the carotid sinus, in the neck.
The impulses from here are carried to the vasomotor sensor, in the brain stem.
This helps to regulate and maintain, consistent blood pressure.
Baroreceptors are a type of mechanoreceptors.
They are excited by the stretch of the blood vessel.
This provides the signal for measuring blood pressure, which is sent to the brain.
The brain uses these signals to regulate blood pressure.
One mechanism for controlling the blood pressure, is altering the heart rate.
Heart rate can be controlled by the autonomous nervous system.
The sympathetic and parasympathetic divisions of the autonomic nervous system,
are involved in the control of heart rate.
Increase in the heart rate, leads to increased blood pressure.
Decrease in the heart rate, leads to decreased blood pressure.
Another mechanism to control blood pressure,
is to contract or expand the arteries.
Contracting the arteries increases the blood pressure.
Expanding the arteries decreases the blood pressure.
The brain uses a combination of these methods, to regulate blood pressure.
This is a homeostatic process.
External events could trigger a change in blood pressure.
Stressful situations, for example, leads to increased heart rate, and blood pressure.
The homeostatic blood pressure control mechanism, helps to bring it back to normal.
Hunger.
Hunger is a mechanism for the body to signal, to the brain that it needs food.
Fuel for the bodies functioning is required at the cellular level.
A complex bio chemical mechanism is made use of,
by the cells to signal need for more fuel.
The hormone ghrelin is a hormone which is used to signal hunger.
When blood glucose levels go down, ghrelin is generated.
This hormone is circulated in the blood, and is sensed by the hypothalamus, in the brain.
The sensation of hunger, is experienced, when the hypothalamus senses this hormone.
This induces us to eat food.
After eating, the food is digested, and blood glucose level increases.
The cells are able to take in the fuel they needed.
After sufficient fuel is available, another hormone is generated.
This hormone is called leptin.
It is a satiation hormone.
This hormone is sensed by the hypothalamus, in the brain.
This results in we experiencing a sense of fullness.
This induces us to stop eating.
Ghrelin, leptin and some other biochemical molecules, are used by the brain,
to achieve energy homeostasis.
Under normal conditions it helps to regulate food intake, digestion,
fuel generation, fuel usage, and fuel storage.
Thirst.
Thirst is a sensation for the body to signal the need for water.
Water is essential for most biochemical processes.
It is the most abundant molecule in the body.
About 70% of our body weight, comprises of water.
It performs a wide variety of important functions.
For example, it is involved in transporting minerals, vitamins, hormones etc.
It is also involved in lubricating the joints, the intestine, the eyes etc.
A good part of the fluid inside every cell has water as its base component.
It is very important to maintain the right balance of water, in the body.
Thirst centre of the brain, is located in the hypothalamus.
Special sensors in the hypothalamus, constantly monitor the concentration of sodium,
and other substances in the blood.
This is an important indicator of the amount of water in the blood.
The hypothalamus also receives sensory input, of blood volume and pressure.
It uses a combination of these sensory inputs, to judge the need for water.
If more water is needed, we feel a sensation of thirst.
The brain also uses other methods, to regulate water levels.
When water levels are low, the hypothalamus stimulates the production of,
anti diuretic hormone called vasopressin, which is secreted by the pituitary gland.
This hormone travels in the blood stream, to the kidneys.
It signals the kidneys to reabsorb, water from the urine, and conserve the water.
The body typically has multiple methods, to achieve functions like water balance,
and it uses a combination of these methods, to achieve homeostasis.
Sleep.
Sleep is an essential requirement for most animals and human beings.
The body apparently recharges itself, during sleep.
The functionality of sleep, is still being researched.
There is no doubt, however, the sleep is essential for life.
Currently it is generally accepted that adults, need about 7 to 8 hours of sleep.
If we are deprived of normal sleep, we tend to be less alert, and feel tired.
Severe long term sleep deprivation, can even be fatal.
The body has a natural circadian clock, which regulates sleep-wake homeostasis.
Biologically the most important circadian clock, currently known,
is a dense cluster of neurons, located above the optic chiasm.
The optic chiasm is the place at which, the nerves from the left and right eye,
cross each other, on the way to the visual cortex.
The stimulus for the clock, seems to come from the eyes sensing day light.
The clock however trains the organism, in the day night cycle.
This is called as entrainment.
Once entrained the body does not necessarily require daylight stimulus,
to regulate the circadian rhythm.
The circadian clock, has a direct neural connection with the pineal gland.
The pineal gland is located in the epithalamus, near the centre of the brain.
This gland releases the melatonin hormone.
Hormones are a way by which the brain communicates, with the rest of the body,
using biochemical messages.
In human beings, the hormone melatonin, induces sleep.
The inner brain is an evolutionary inheritance from our ancestral species.
We still retain the basic design and function, of this inner brain.
The inner brain regulates and manages most of the homeostatic functions,
essential for our living.
It is also involved in many basic sensations and emotions.