The brain:

An organ made of a nervous tissue, the function of the brain is very wide as the brain controls every aspect of our body. In fact, the brain is in charge of thinking, imagining, remembering, feeling, moving, sensing, talking, and communicating. Without doubt, for the brain to mature completely, it needs 20 to 25 years. Besides, the brain and the spinal cord forms the central nervous system (CNS). As well as that, the brain compose 2% of the total body weight. On the other hand, the brain at rest requires an energy of 20% and 25% when active. As a matter of fact, the brain utilizes different sources of energy depending on their levels. For instance, glucose is the main energy source for the brain; however, when glucose is low in the blood, the brain uses ketone bodies. Also, when we exercise, our brain uses lactate as an energy source. 

Function of the brain according to each part:

Cerebrum: 

In short, spreading over the right and the left hemispheres, the cerebrum has a cerebral cortex. To illustrate, the cerebral cortex is an outer surface made of grey matter. Moreover, below the grey matter there exist white matter. Obviously, the cerebral hemispheres have folds spread on the surface, and between the folds there's ridges called gyri. Indeed, between gyris there's valleys known as sulci, and deep ones are called fissures. In general, the cerebrum relays many functions and it's the largest part of the brain. 

The cerebrum is responsible for:

• Regulating body temperature
• Initiating movement
• Thinking
• Acquiring logic
• Being able to talk
• Having judgement
• Learning
• Solving problems
• Having emotions
• The sense including vision, hearing, and touching 

Cerebellum:

The cerebellum which is known as the "little brain" includes a cerebellar nuclei and a cerebellar cortex. In fact, the latter has three layers: the granular layer, Purkinje layer, and the molecular layer. Moreover, it spreads over both hemispheres. Additionally, the cerebellum location is at the lower back of the head and is attached to the brain stem via the cerebellar peduncle.

The cerebellun is responsible for:

• Balance
• Posture
• Muscle movements (motor coordination)

Brain stem:

Located between the spinal cord and the base of the cerebrum, the brain stem "middle-brain" has 3 parts (the midbrain, the pons and the medulla).

The midbrain: It's the uppermost part of the midbrain originating from the mesencephalon. It also has 3 parts: the cerebral peduncle, the tegmentum and the colliculi. Furthermore, the midbrain contains a layer called substancia nigra which synthesizes dopamine, which leads to Parkinson's disease upon damage. The midbrain is responsible for:

• Processing visual signals
• Processing auditory signals
• Coordination of movement
• Suppressing pain
• Staying alert

The pons: The pons mean a bridge in latin, since it's the connecting bridge between the midbrain and the medulla. Indeed, four cranial nerves originate in the pons and makes it responsible for:

• Body balance
• Hearing
• Vision focus 
• Producing tears
• Blinking
• Chewing
• Facial expressions

The medulla: Without doubt, the spinal cord meets the brain at the medulla. The medulla is a very vital part because it's responsible for regulating:

• Breathing
• The levels of oxygen and carbon dioxide 
• The flow of the blood
• Heart rhythm
• Reflex actions such as coughing, sneezing, swallowing and vomiting

Brain covering layer (meninges): 

In general, the meninges has 3 layers whose purpose is to protect the brain and the spinal cord. In brief, the layers are the dura mater, the arachnoid mater and the pia mater. 

Dura mater: Composed of the periosteal layer and the meningeal layer and present right below the skull, it's the toughest layer. In addition, the region between these 2 layers is a space for arteries and veins to exchange blood with the brain and the body. 

Arachnoid: Clearly, this layer is a connective tissue that is thin and looks like a web. Unlike the dura mater, tbe arachnoid doesn't have blood vessels or nerves passing. Nevertheless, below the arachnoid mater, there's a CNS cusion fluid, the cerebrospinal fluid (CSF). Also, it's role is to get rid of unneeded substances.

Pia mater: This is the deepest part of the meninges, it covers directly the brain. Additionally, it only makes sense for veins and arteries to run aggressively in this layer. 

Function of the brain based on lobes:

At each hemisphere, there's four lobes of the brain that plays different roles.

Frontal lobe: 

As the name indicates, it's located at the front end of the head. Besides, this is the largest lobe, and it controls the:

• Personality
• Movement
• Speech
• Smell recognition
• Making decisions

Temporal lobe:

Present at both sides of the head, this part is responsible for:

• Speech
• Hearing
• Recognizing smell
• Short-memory retention
• Musical rhythm
• Spacial and visual perception

Occipital lobe:

Certainly, the occipital lobe is present at the far back end of the head. Also, it's responsible for:

• Vision

Parietal lobe:

In fact, the middle part of the head is composed of the parietal lobe. Furthermore, it involves:

•  Knowing where we are in space
• Identifying objects
• Understanding talk 
• Touch and pain sensation

The function of the brain cranial nerves:

The brain contains 12 cranial nerves. The cranial nerve:

• I: The olfactory nerve which is responsible for the smell sense.
• II: The optic nerve responsible for vision.
• III: The oculomotor nerver meant for pupil and eye montion control.
• IV: Which is the trochlear nerve, it controls the muscles of the eyes.
• V: The trigeminal nerve not only allows chewing, but also transports sensation of facial parts, mouth parts, sinuses, scalp, jaw and teeth to the brain.
• VI: The abducens nerve provides nerves to the eyes.
• VII: Known as the facial nerve, and it's responsible for the taste, facial movements, and gland functions.
• VIII: The vestibulococlear nerve supporting balance and hearing.
• IX: The glossopharyngeal nerve has many functions including throat movement, ear movement and taste. 
• X: The vagus nerve controls involuntary body actions such as digestion, immune system and heart rate. 
• XI: The accessory nerve supplies nerves for the shoulders, neck and head.
• XII: known as the hypoglossal nerve and controls movement of the tongue.

References:

-Maldonado KA, Alsayouri K. Physiology, Brain. [Updated 2021 Dec 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. https://www.ncbi.nlm.nih.gov/books/NBK551718/

-Cleveland clinic "Gastroparesis, Vagus nerve stimulation and syncope"

-Johns Hopkin's medicine "Brain anatomy and How the brain works"

-Cleveland clinic "Cranial nerves"

-Image credits: https://www.freepik.com/vectors/objects