Brain – Its Classifications & Functions

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Cerebrum (In Red) Source- Wikimedia Commons / Database Center for Life Science (DBCLS)- Japan
Cerebrum (In Red)
Source- Wikimedia Commons / Database Center for Life Science (DBCLS)- Japan

Human brain is a part of Central Nervous system as our nervous system is divided in to two parts, the central nervous system and the peripheral nervous system. The central nervous system consist of brain and spinal cord while the peripheral system connects the rest of the body to the central system.

Human Brain is surrounded by the primary sensory organs for visionhearingbalancetaste, and smell. The function of the brain is to exert centralized control over the other organs of the body. The brain controls the rest of the body by generating patterns of muscle activity and by driving the secretion of chemicals called hormones.

Very interestingly, a brain can be compared with the computer in the sense that it receives input from the sensory organs or the surrounding world, then stores it, and processes it in a variety of ways, and  eventually sends output to the muscles same as the central processing unit (CPU) in a computer.

Ancient Facts
In 2011, Scientists at the Anthropological Survey of India claim to have found the oldest evidence of an ancient brain surgical practice on a Bronze Age Harappan skull. It is believed to be around 4,300 years old, having a hole in skull that indicates a case of surgical practice known as trepanation. Trepanation is a way of surgery practised in prehistoric societies, involved drilling or cutting through the skull vault, often to treat head injury or to remove bone splinters or blood clots caused by a blow to the head.

There is another oldest known surgical treatise “The Edwin Smith Papyrus” is an Ancient Egyptian medical text named after Edwin Smith, an American Egyptologist, who was born in Connecticut (the southernmost state in the north-eastern region of the United States known as New England) in 1822. It describes number of cases relating to injuries, fractures, wounds, dislocations and tumours (Papyrus means- Paper made from the papyrus plant by cutting it in strips and pressing it flat; used by ancient Egyptians, Greeks and Romans). It dates to Dynasties 16–17 of the Second Intermediate Period in Ancient Egypt, ca.1500 BCE. He bought this text in Luxor, Egypt in 1862 from an Egyptian dealer named Mustafa Agha. The Edwin Smith papyrus is unique among the four principal medical papyri in existence that survive today.
Quick Brain Facts picture1
1. % brain of total body weight (150 pound human) = 2% 
2. Average brain width = 140 mm 
3. Average brain length = 167 mm 
4. Average brain height = 93 mm 
5. Intracranial contents by volume (1700 ml i.e.100%): Brain = 1400 ml (80%); Blood = 150 ml (10%); Cerebrospinal fluid = 150 ml (10%). 
6. Average number of neurons in the brain = 100 billion 
7. Total surface area of the cerebral cortex = 2,500 cm2  
8. Total number of neurons in cerebral cortex = 10 billion 
9. Thickness of cerebral cortex = 1.5-4.5 mm

Classification of Brain

It is classified in to several divisions; Telencephalon, Diencephalon, Mesencephalon, Pons, Medulla oblongata, Cerebellum and the seventh significant part is Limbic system.  picture8

1. Telencephalon

It consists of a cerebral hemisphere which is divided into approximately symmetric left and right cerebral hemispheres comprising of cerebrum, cerebral cortex (Upper layer i.e. grey matter), white matter, basal ganglia and lateral ventricles. The telencephalon refers to the embryonic structure from which the mature cerebrum develops. The cerebrum is a major part of the brain containing the cerebral cortex which plays major part in controlling emotions, hearing, vision, personality and much more. It controls all voluntary actions.

A. Cerebral hemispheres or Cerebrum

Cerebral hemispheres (100%) is divided in following four lobes, which are Frontal lobe (41%), parietal lobe (19%), temporal lobe (22%) and occipital lobe (18%).picture8

1. Frontal Lobe 

The frontal lobe located at the front of the brain, is one of the four major lobes of the cerebrum. It contains most of the dopamine (sensitive neurons in the cerebral cortex). Dopamine limits and selects the sensory information arriving from the thalamus to the forebrain. The dopamine system is associated with executive functions, planning, organizing, problem solving, behaviour control, reward, attention, short-term memory tasks, and choice between good and bad actions, similarities and differences between things or events, emotions and motivation.

2. Parietal lobe

The parietal lobe is positioned above the occipital lobe and behind the frontal lobe. It incorporates sensory information majorly coming from skin such as sense of touch, Pain, temperature, light, sound, taste, pressure and smell. It helps in determining the object, creating perception and writing on skin through touch alone without any visual input. Commonly known sensory systems responsible for sensory information are vision, auditory (hearing), somatic sensation (touch), gustatory (taste), olfaction (smell) and vestibular (balance/movement). Parietal lobe also play important role in language processing and navigation.

3. Temporal Lobe

The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the brain. The temporal lobe plays a key role in the formation of long-term memory and emotional association. Adjacent areas of the temporal lobes are involved in process such as hearing, through primary auditory cortex which receives the information from ears and process it into meaningful speech and words for language comprehension. It is also involved in visual processing of intricate stimuli such as faces, different scenes, creating perceptions and recognition of objects for the appropriate retention of long term visual memories. It also plays a key role in understanding of meanings (Semantics) in vision and speech, naming and verbal memory. The hippocampus also lies in the centre of temporal lobes, which is very significant in retainment of long term memory.

4. Occipital Lobe

The occipital lobe is the visual processing centre of the brain containing the primary visual cortex i.e. Brodmann area 17, commonly called V1 (visual one). It is specialized for different visual tasks, such as creating visual perception among objects (visuospatial processing), colour differentiation, and motion perception. The occipital lobe is divided into several functional visual areas. Each visual area contains a full map of the visual world. Each visual cortex receives raw sensory information from the Retinal sensors which convey stimuli through the optic tracts to the lateral geniculate bodies or nucleus of the thalamus before projecting to the cortex.

B. Basal ganglia 

captureThe basal ganglia is a group of interconnected structures in the forebrain. The primary function of the basal ganglia appears to be action selection. It sends inhibitory signals to all parts of the brain that can generate motor behaviors, and in the right circumstances can release the inhibition, so that the action-generating systems are able to execute their actions.

C. Cerebral cortex

picture1The cerebral cortex is the brain’s outer layer made up of Nervous tissue or Grey matter which is 2 to 4 millimetres (0.079 to 0.157 in) thick. The cerebral cortex is the outer layer depicted in dark violet.  It is composed of gray matter, consisting mainly of cell bodies and capillaries. On the other hand, white matter mainly consist of the white myelinated sheaths of neuronal axonsMyelin is a  white coloured fatty substance that surrounds the axon of some nerve cells to form an electrically insulating layer to transfer signals. It is essential for the proper functioning of the nervous system.  The cerebral cortex is connected to the thalamus and the basal ganglia, which sends sensory information to them and receive information from them.

2. Diencephalon

picture7The diencephalon appears at the upper end of the brain stem i.e. situated between the cerebrum and the brain stem. It is made up of four distinct components: the thalamus, the subthalamus, the hypothalamus, and the epithalamus.

A. Epithalamus

The epithalamus is a posterior segment of the diencephalon. The epithalamus includes the habenula and their interconnecting fibers, the habenular commissure, the stria medullaris and the pineal body.

The function of the epithalamus is to connect the limbic system to other parts of the brain. Some functions of its components include the secretion of melatonin by the pineal gland (involved in circadian rhythms), and regulation of motor pathways and emotions.

B. Thalamus

The thalamus is a symmetrical structure of two halves, situated between the cerebrum and the midbrain. The thalamus is located in the forebrain superior to the midbrain, near the center of the brain, with nerve fibers projecting out to the cerebral cortex in all directions.  Some of its functions are the relaying of sensory and motor signals to the cerebral cortex, and the regulation of consciousness, sleep, and alertness. For example, the received inputs from the retina are sent to the lateral geniculate nucleus, which is a relay center in the thalamus for the visual pathway, which in turn projects to the visual cortex in the occipital lobe. Overall, thalamus serves like a switchboard operator for the body i.e. it receives information, interpreting the information, and send it to the brain.

C. Hypothalamus

The hypothalamus is located below the thalamus, and just above the brainstem. Its main functions are;

One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system (collection of glands of an organism that secrete hormones) via the pituitary gland. It releases certain neurohormones that in turn stimulate the pituitary gland to release hormones or pituitary hormones.

The hypothalamus is responsible for certain metabolic processes like heart rate and blood pressure, fluid and electrolyte balance and other activities of the autonomic nervous system.

The hypothalamus controls body temperaturehunger, important aspects of parenting and attachment behavioursthirst, fatiguesleep cycles, Appetite and body weight.

D. Subthalamus

The main function of the subthalamus is the regulation of movements produced by skeletal muscles.

The subthalamus or Prethalamus is a part of the diencephalon. Its major part is the subthalamic nucleus. The subthalamus connects to the globus pallidus, part of the telencephalon. The globus pallidus is involved in the constant and smooth regulation of movement without any conscious effort to function that allows people to walk, talk, and engage in number of other activities with a minimal level of disruption.

3. Mesencephalon

picture5The mesencephalon or midbrain is the portion of the brainstem that connects the pons and cerebellum with cerebral hemispheres. It is associated with vision, hearing, motor control, sleep/wake, arousal (alertness), and temperature regulation. The other functions are controlling the respiratory muscles, controlling the vocal cords enabling a person to phonate, controlling pharyngeal (relating to the throat), oral as well as nasal passages that are known to cause resonance effect and controlling the palate (The upper surface of the mouth that separates the oral and nasal cavities), tongue, lips and mandible ( lower Jaw ) which are involved in articulation control. The coordinated activities of laryngeal and oral facial tissues are controlled by the midbrain due to which it has control over the way we laugh and cry. That is the reason why even if the remaining brain is dead, a person can still laugh and cryDopamine is more profusely produced inside the midbrain. This Dopamine plays an important role in motor & cognition functioning and motivation.

4. Pons

picture4The pons is part of the brainstem, which is above the medulla, below the midbrain, and anterior to the cerebellum. The pons contains nuclei that relay signals from the forebrain to the cerebellum, along with nuclei that deal primarily with sleep, respiration, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture, chewing, swallowing, and the secretion of saliva and tears. The pons in humans measures about 2.5 cm or 1 inch in length. Posteriorly, it consists mainly of two pairs of thick stalks called cerebellar peduncles. They connect the cerebellum to the pons and midbrain.

5. Medulla oblongata

medulla-oblongataThe medulla oblongata is the lower half of the brainstem, which connects the higher levels of the brain to the spinal cord . It is situated just underneath the pons. The medulla oblongata is responsible for several functions of the autonomous nervous system (control system that acts unconsciously and regulates) which involves functions related to;

Cardiac centre; Heart rate and blood pressure,

Respiratory Centre; The chemoreceptors detect changes in acidity of the blood due to carbon dioxide, thus send electrical signals to the muscle tissue in the lungs  to maintain the acidity or PH value by re-oxygenating the blood.

Vasomotor ; The actions upon a blood vessel which alter its diameter,

Reflex centers of vomiting, coughing, sneezing, and swallowing.

6. Cerebellum

picture2Anatomically, the cerebellum is a separate structure attached to the bottom of the brain, connected underneath the cerebral hemispheres. It can be called a “little brain”. The cerebellum is a region of the brain that plays an important role in motor control and is necessary for several types of motor learning (involves improving the smoothness and accuracy of complicated movements such as speaking, playing the piano, and climbing trees, riding a bicycle. However, it does not initiate movements, but it contributes to coordination, precision, and accurate timing. Cerebellum contains more neurons than the total from the rest of the brain and stands for 10% of the total brain volume.

7. Limbic System

brain_headborderThe Limbic System involves in functions related to emotions, motivation, olfaction (sense of smell) and formation of long term memories.

The limbic system is made up of the limbic lobe, which is an arc-shaped region of cerebral cortex on the medial surface of each cerebral hemisphere and other deep-lying complex structures located on both sides of the thalamus just inside the cerebral hemisphere. The limbic lobe consists of the parahippocampal, cingulate, and subcallosal gyrus and dentate gyrus. The deep-lying structures include, the hippocampus, amygdala, mammillary body, habenula, anterior thalamic nuclei, and olfactory bulb.

Hippocampus
It plays significant part in the formation of long-term memories and in maintenance of cognitive maps for navigation. It converts short term memories or incidences in to long term memories that one will remember for a longer period. A person cannot build new memories if Hippocampus is damaged and new memories just fades away while older memories before the time of damage remains unchanged. This kind of condition is shown in Hollywood movie "50 first dates".

Amygdala
It is Involved in signaling the cortex of motivationally significant stimuli such as those related to reward and fear in addition to social functions such as mating. Furthermore, the anatomy of amygdalae are two almond-shaped masses of neurons on either side of the thalamus at the lower end of the hippocampus. The amygdalae stimulate the hippocampus to remember many details surrounding the situation, as well.

References
1. http://www.thehindu.com/features/friday-review/history-and-culture/brain-surgery-during-harappan-civilisation/article2118388.ece