Central Nervous System: brain and its parts

The brain is one of the largest and most complex organs of the human body. It is the central organ of the nervous system and along with the spinal cord, it makes up the central nervous system. 

The adult human brain contains about 95% of the neural tissue and weighs about 1.4 kilograms. The brain is contained and protected by the skull bones of the head and is suspended in cerebrospinal fluid and isolated from the bloodstream via the blood-brain barrier.

It is responsible for controlling almost all of the major activities performed by the body such as processing and relaying information received by the sensory organs, making decisions based on the bodily needs and controlling various organs to suit the same, and maintaining the basic body functions which are involuntary in nature.

Structural Organisation of the Brain and functions

• On initial observation, we see that the brain is divided into two halves: Left and Right hemispheres.
• Each hemisphere is divided into four lobes- frontal, temporal, parietal and occipital lobes.
• Furthermore, there are six major divisions in the adult brain: medulla oblongata, the pons, mesencephalon, diencephalon, cerebellum, and the cerebrum (containing the two hemispheres).
• The medulla oblongata, the Pons, and the mesencephalon are collectively referred to as the brain stem.
• The brain stem contains important processing centers and also relays information to and from the cerebrum or cerebellum.
• The cerebrum connects to the spinal cord via the brainstem and, the cerebellum is connected to the brainstem by pairs of tracts.
• The cerebrum, brainstem, cerebellum, and spinal cord are covered by three membranes called meninges.
• These membranes are the outer, tougher Dura mater; the middle arachnoid mater and the more delicate inner pia mater.
• The space between arachnoid mater and pia mater is where we find cerebrospinal fluid.

The Cerebrum

• The cerebrum is divided into the left and right hemispheres by a deep groove called the longitudinal fissure.
• Each hemisphere is divided into four lobes as discussed above and each one has one or two specialized functions, although some of these functions might overlap.
• The outer part of the cerebrum is the cerebral cortex and it is made up of grey matter arranged in layers that are folded to give a convoluted appearance that we usually see.
• Beneath this cortex is the cerebral white matter.

The cerebral cortex

• It is divided into two main functional areas – sensory cortex and motor cortex.
• As the name suggests, the motor cortex sends axons down the motor neurons and the spinal cord to receive and transmit impulses pertaining to the motor functions.
• The sensory cortex is responsible for obtaining and processing the sensory information received by the sensory nerves and regions of the brain such as the occipital lobe.
• The cerebrum contains the ventricles where the cerebrospinal fluid is produced and circulated.

The cerebellum

• It is divided into three lobes – anterior lobe, posterior lobe, and the flocculonodular lobe.
• The cerebellum rests at the back of the cranial cavity underneath the occipital lobes.
• It is responsible for adjusting the motor activities on the basis of sensory information and memories of learned patterns of movement.

The brainstem

• It lies below the cerebrum at the back of the skull.
• The brainstem continues below in the body as the spinal cord protected by the vertebral column.
• There are twelve pairs of cranial nerves, out of which ten arise from the brainstem.
• The brainstem is involved in the regulation of many essential processes of the body such as breathing, controlling eye movement, balance, etc.
• Many nerves that pass information to and from the cerebral cortex pass the brainstem thus making it one of the most important parts of the brain.

Fluids associated with the Brain

• The neural tissue in the Central Nervous System has a vast blood supply, yet it is isolated from the general circulation by the blood–brain barrier (BBB).
• BBB maintains a constant environment, for controlling and proper functioning of CNS neurons.
• Cerebrospinal fluid (CSF) completely surrounds and bathes the exposed surfaces of the central nervous system.
• CSF has several important functions, including supporting the brain, preventing contact of the delicate neural structures and the surrounding bones, transporting nutrients, chemical messages, and waste products in and out of the neural tissue.

Brain Injuries and Diseases

1. The brain is one of the most delicate organs is highly susceptible to injuries and diseases. These can ensue in many ways such as a traumatic brain injury resulting in a sports accident or a fall that may induce problems such as bleeding in the brain, compress the blood tissue or damage the blood supply.
2. Neurodegenerative diseases such as Alzheimer’s or Parkinson’s syndrome or dementia bring about progressive degeneration of the brain tissue and function.
3. Mental disorders such as depression, OCD, schizophrenia, etc deteriorate the functioning of the brain. Epileptic seizures due to abnormal electrical activity in the brain or a stroke as a result of decreased blood supply also have the tendency of altering the brain’s overall functionality.
4. Finally, brain death which results in total and irreversible loss of brain function and is usually characterized by coma, loss of reflexes, or apnoea.