Human brain viewed through a mid-line incision. An anatomical illustration from Sobotta's Human Anatomy 1908

This section covers the medical basics of the human brain in relation to brain tumors and cancers occuring in the brain, ranging from the anatomical basics of the brain, covering the symptoms occuring in persons with brain tumors and the diagnostic methods used to detect potential brain tumors. It also covers the different brain tumor types and the grading system used to grade tumors (not necessarily in the brain).

Function of the Brain and the Nervous System

Human brain is the central organ of the human nervous system. The brain is the organ that controls almost all activities of the body. The brain receives information about the surrounding world from the sensory organs. It then analyzes and processes this information to make decisions about the instructions given to the muscles in our bodies. It also serves as memory storage for humans, which helps it make decisions about actions based on previous experiences. [1]

The brain together with the spinal cord make up the human central nervous system.

It is currently impossible to say precisely which part of the brain is responsible for processing or creating which impulses, so you may see several parts of the brain being labeled as facilitating the same function. This is because the "secret of the brain" is not yet fully understood and most of our understanding about which part of the brain is responsible for what function comes from observation of patient behaviour after damage to the brain. This is currently improving with the more wide-spread use of functional MRI and other sophisticated methods which can detect neuron activation.

Brain Anatomy

The brain consist of Cerebrum, Cerebellum, Brain stem and Diencephalon. These structures of the brain are formed by microscopic structures called neurons.[2]

Cerebrum

Cerebrum marked in red. Polygon data are from BodyParts3D maintained by Database Center for Life Science(DBCLS). Licensed under CC-BY-SA-2.1-jp

The largest part is the Cerebrum. It is divided vertically into two symmetrical halves (called the left and right Cerebral Hemispheres). The hemispheres, albeit being symmetric in appearance both have different responsibilities. The hemispheres are connected by nerve tracts known as commisures. [1]

The outer layer of the Cerebrum is called the Cerebral Cortex, this is the area with the highest concentration of neurons in the brain.

Both hemispheres are divided into 6 lobes, each of them is located in a specific part of the hemisphere and has a different role in the functioning of the brain.

Lobes 

Original image is by John A Beal, PhD. Dep't. of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport. Parcellation is by was_a_bee.Licensed under Creative Commons Attribution 2.5 Generic

The major lobes of each hemisphere are: [3] 

• Frontal - located in the top-front of the brain - contains the dopamine system associated with the reward system, motivation, attention, and short-term memory and planning.
• Parietal - located in the top-center of the brain - integrates sensory information from different modalities into the orientation sense and navigation. It is also the main receptive area for the sense of touch, temperature and pain receptors of the skin. It is also important in language processing.
• Occipital - located in the rear part of the brain - is the main visual processing center
• Temporal - located in the bottom-center of the brain - is the main auditory processing center and it processes sensory data to induce visual object recognition and language recognition. It is believed that new long-term memories are created in the temporal lobe and also the association of emotions to sensory input

There are also two areas of the cerebral cortex which hold special significance, so they are considered as specific lobes. These are:

• Limbic Lobe - located on the inside of the major lobes - main emotional center, combines higher mental functions and primitive emotion, new memory formation, fight-or-flight system, unconsious systems (breathing etc.), movement planning
• Insular Complex - located in a fold of Cerebral cortex deep in the cerebrum, between temporal, parietal and frontal lobes - the center of consciousness, self-awareness, cognitive functions, Also important for motor control.

Cerebellum 

Cerebellum marked in red. Polygon data are from BodyParts3D maintained by Database Center for Life Science(DBCLS). Licensed under CC BY-SA 2.1 JP

The Cerebellum is located below the rear part of the brain, its main function is movement coordination, precision and accurate timing of actions. It receives sensory inputs from the spinal cord and also from other parts of the brain and integrates these inputs to fine-tune motor activity. It also facilitates motor learning for actions requiring precision. Cerebellum is latin for little brain. 

Unlike the Cerebrum, the Cerebellum functions almost completely feed-forwardly (from input to output) and is unable to sustain self-generating patterns of neural activity. It is believed that the Cerebellum is used to process inputs to outputs based on a learned set of patterns.

Damage to the Cerebellum is most obvious in decreased motor function precision on the same side of the body as the damage to Cerebellum occured, but other severe symptoms are common, such as inability to plan actions properly, impaired abstract thinking and impaired spatial orientation. Damage to the cerebellum is also often associated with inactivity, distractibility and impaired verbal fluency. [4]

Brainstem

Brain stem marked in red. Polygon data are from BodyParts3D maintained by Database Center for Life Science(DBCLS). Licensed under CC BY-SA 2.1 JP

The brainstem consists of the midbrain, pons and medulia. It lies in the rear part of the skull, beneath the cerebrum. The midbrain is the top part of the brain stem and it is connected to the forebrain in the cerebrum.

 All sensory and motor information relayed between the cerebrum, cerebellum and the rest of the body goes through the brainstem. The brainstem also provides the motor and sensory innervation of the face and neck.The brainstem is also important in regulating the "automatic" functions of the body, such as breathing and heart-beat regulation. It also regulates the Central Nervous System and consciousness and regulates the sleep cycle of the body.

Damage to the brainstem induces loss of motor and sensory function. Depending on the severity of the damage up to complete paralylis and loss of all sensory information. Severe damage to the brainstem may also cause permanent unconsciousness and loss of basic reflexes such as gag, cough and blink reflex. Less severe damage can cause visual distortion, hearing loss, changes in sensation and muscle weakness.[2]

Diencephalon 

Diencephalon marked in red. Image credit : Life Science Databases(LSDB). - from Anatomography, licensed under CC BY-SA 2.1 JP

This region of the brain, located above the Brainstem and between the left and right hemisphere forms the third brain ventricle and consists of the following structures [5]

• Thalamus - a structure facilitating the transfer of neural impulses from peripheral nerves into the cerebral cortex
• Metathalamus - a part of the visual and auditory pathways
• Hypothalamus - the center of many vegetative, endocrinal and emotional functions
• Epithalamus - regulates motor pathways and emotions, contains the pineal gland that facilitates the secretion of melatonin and other hormones,
• Subthalamus - regulates muscle tension, motor control, contains the substantia negra which creates the serotonin hormone
  
  

Ventricular system and cerebrospinal fluid (CSF) 

Lateral ventricles marked in red, third and fourth ventricles in blue. Image credit : Life Science Databases(LSDB). - from Anatomography, licensed under CC BY-SA 2.1 JP

The Ventricular system of the brain is composed from 4 ventricles (cavities). These cavities contain ependymal cells responsible for the production of the Cerebrospinal fluid. [6]

• Lateral ventricles - biggest, located in the left and right hemisphere of the Cerebrum (red in the animation)
  

• Third ventricle - smallest, enclosed by the Diencephalon, particularly between the left and right Thalamus (blue, top, small one)
• Fourth ventricle - located in the pons or the upper part of the medulla of the brain stem (blue, bottom one)
  
  

Cerebrospinal fluid (CSF)

The Cerebro-Spinal fluid is a clear, colorless body fluid found in the brain and spinal cord. It's produced in the Ventricular system of the brain.

The brain is protected from impacts by floating in the CSF. The fact that it's floating in a liquid also means that the brain doesn't need any muscles or tendons to be held in place - it stays buoyant in CSF.
Without CSF the brain would deform under it's own weight and thus damage or cut off the blood supply from its lower parts.

Aside from mechanical protection of the brain, the CSF provides basic immunological protection (clearing metabolical toxins and foreign bodies from the brain) and the vital function of regulating cerebral blood flow. It also provides homeostasis to the brain (keeping the environment of the brain stable).

The CSF circulates through the ventricular system, around the cerebral cortex into the spinal cord and back. This circulation allows for the intra-cranial pressure to be reduced when needed by letting more CSF into the spinal cord thus reducing the amount of liquid in the brain.

The CSF is constantly created by the brain at a rate of circa 25 ml/hour and "used" CSF is continously reabsorbed so that there is only about 150-200 ml present in the body at any given time.

Blood-brain barrier 

The blood-brain barrier is a highly selective, semi-permeable membrane that separates circulating blood from the brain's extracellular fluid. This barrier is formed around most blood vessels that supply the brain. 
It is formed by brain endothelial cells which are connected by tight junctions. [7][8]

The function of this barrier is to provide an additional level of protections from pathogens to the brain. This effect is achieved by severely limiting the permeability of the capilary pores. In the rest of the body nutrients and other materials may be extracted from blood using passive transport - this means no cells are required to get the nutrients from the blood, traditional diffusion and osmosis will cause them to go from an area with higher concentration to an area with lower concentration.
In the brain, all substances that are to be taken out of the blood and into the brain cells must be picked up by active transport by specialized transporter molecules or by diffusion into the cells of the BBB from where they can be extracted by other specialized molecules and delivered to the brain cells.

The blood-brain barrier allows passage of water, some gases and lipid-soluble molecules. It also has specialized transporter molecules for the extraction of glucose and amino-acids that are crucial for brain cell function.
It restricts the diffusion of microscopic objects (e.g.bacteria) and large or hydrophilic or lipophilic molecules.

Sometimes, trauma to the brain or the spinal cord can cause a breach in the BBB, which may cause serious complications, as the brain cells are no longer protected from potential neurotoxins and bacteria.

Meninges

The brain is the most important organ in our bodies and so it is essential for it to be well protected. The protection from pathogens and toxins is done at the cellular level by the Blood-brain barrier. 

Henry Gray (1918) Anatomy of the Human Body - Public Domain

The physical protection from impacts is composed of multiple layers. Aside from skin and bone protecting the brain, the brain and the spinal cord are protected by meninges.

Meninges line the skull and the spinal cord to protect the brain and spinal cord from impacts and other deformations. The meninges are composed of 3 layers.

• Dura matter - thick, durable membrane that is attached to the bone (skull/vertebrae), composed of dense, fibrous tissue
• Arachnoid matter - thin, transparent membrane - in the middle of the meninges - impenetrable to fluids and it's function is to act as a barrier between cerebrospinal fluid and subarachnoid space and the blood circulation in the dura matter
• Pia matter - very thin, delicate membrane, adheres to the surface of the brain and the capilaries from Pia matter supply the brain with blood.

In the middle, between the Arachnoid and Pia layers is filled with Cerebrospinal fluid.

Brain histology

    Main article : Brain Histology

The Central Nervous System is composed of roughly hundred billion of microscopic cells called neurons. [2] Neurons are the core components of the brain, spinal cord, and peripheral nerves,

Neurons are electrically excitable and they process and transmit information. They communicate with each other via chemical and electrical synapses, in a process known as synaptic transmission.

There are several types of neurons with, each found in different parts of the brain and with a different function.

Unlike most other cells in our bodies, neurons do not gradually decompose and aren't replaced by new ones. We are born with most of the neurons that we have through our life, although some neurogenesis (the birth of neurons) does happen through our lives. The creation of new neurons and neural pathways is a controversial topic and we do not yet posses a true understanding when and how new neurons are created and how they join the existing neural pathways.

Symptoms of Brain Tumors

    Main article : Symptoms of Brain Tumors

The most common symptoms of brain tumors are:

• Headaches - permanent, throbbing, paired with sickness
• Seizures - attack due to abnormal electrical activity in the brain
• Loss of memory - due to tumors or treatment
• Mood swings and cognitive changes - problems with mental abilities and changes in thinking
• Depression - persistent feelings of sadness as illness
• Loss of motive control, hearing or vision - problems with physical abilities
• Deep venous thrombosis - blood clots
• Fatigue - abnormal tiredness

To see more details, go to the main article Symptoms. [10][11]

Diagnosis

    Main article : Brain Tumor Diagnosis

The different options for diagnosing a brain tumor are:

• Neurological examinations
• Brain scans with different imaging modalities
• Laboratory tests

Neurological examinations include a series of tests to analyze a person’s mental state, nerves, senses, muscle strength and reflexes. Typical the motive control is tested, as well as abstract thinking and memory. If the clinician suspects a brain tumor after the neurological examination, a brain scan might follow. Brain scans generate images of the brain in order to see if the patient has a tumor. The most common imaging modalities used for diagnosis are MRI and CT. Additionally contrast agent can be used. Laboratory tests include invasive and noninvasive procedures.They are used to verify the presence of a brain tumor and also to test the treatment response.

The same tests can be used to monitor the development of the tumor (growth, spread, other changes). [10]

Brain Tumor Types

    Main article : Brain Tumor Types

 There are over 130 types of brain tumor due to the World Health Organisation. Classifying, grading and staging tumors is done to determine the appropriate treatment, to have a consistent terminology for communication between physicians and to propose a prognosis for the patient. [2] #11

Grading and staging 

Tumors (not necessarily of the brain) are classified into 4 groups, depending on the size, growth rate and cellular appearance of the tumor.[14]

Bibliography

1) Anatomy of the Brain, http://www.abta.org/brain-tumor-information/brain-anatomy/ (access 15/05/17)

2) Jacobson S. et al. (2008) Neuroanatomy for the Neuroscientist, Springer, ISBN: 978-0-387-70970-3, chapter 1

3) Lobes of the brain, https://en.wikipedia.org/wiki/Lobes_of_the_brain (access 15/05/17)

4) Chapter 5: Cerebellum, http://neuroscience.uth.tmc.edu/s3/chapter05.html (access 15/05/17)

5) Diencephalon, https://www.thoughtco.com/diencephalon-anatomy-373220 (access 15/05/17)

6) Ventricular system, https://en.wikipedia.org/wiki/Ventricular_system (access 15/05/17)

7) Blood-Brain Barrier & The Spinal Cord, https://www.christopherreeve.org/blog/research-news/blood-brain-barrier-the-spinal-cord (access 15/05/17)

8) Boonstra E. et al. (2015) Neurotransmitters as food supplements: the effects of GABA on brain and behavior, http://hplusmagazine.com/2015/11/24/neurotransmitters-as-food-supplements-the-effects-of-gaba-on-brain-and-behavior/ (access 15/05/17)

9) Cerebrum, https://en.wikipedia.org/wiki/Cerebrum (access 15/05/17)

10) Brain Tumor Symptoms (inter alia), http://www.abta.org/brain-tumor-information/symptoms/ (access 15/05/17)

11) Brain tumour symptoms in adults (inter alia), www.thebraintumourcharity.org (access 15/05/17)

12) Types of Brain Cancer, http://www.webmd.com/cancer/brain-cancer/brain-tumor-types#1 (access 15/05/17)

13) Brain Tumor: Statistics, http://www.cancer.net/cancer-types/brain-tumor/statistics (access 15/05/17)

14) Tumor Grade, http://www.abta.org/brain-tumor-information/tumor-grade/ (access 15/05/17)