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The Brain

What is the brain?

The brain is the most important organ in our body. It forms the central nervous system along with the spinal cord. It is a large organ weighing approximately 1.4 kg and containing more than 100 billion neurons. It is the chief information-processing unit of the body. It analyzes the stimuli it receives and provides an appropriate response. It is responsible for our thinking process, language, emotions, speech and physical movements, among other functions.

Where is the brain located?

The brain is located in the bony cage of the skull. Its cavity is called as the cranium. The brain is surrounded in this cavity by a fluid called as the cerebro-spinal fluid. This fluid extends into the vertebral column or the spine. This fluid acts as a shock absorber for the central nervous system. Three protective membranes called as the meninges also surround the brain and protect it from injury.

What is the brain made of?

The brain is made of:

  • Gray matter : This consists mainly of neuron cell bodies from which nerve impulses originate. Gray matter is seen in the outer layer of the brain called as the cerebral cortex.
  • White matter : This consists largely of nerve fibers and its main role is to transmit nerve impulses. It is seen beneath the gray matter of the cortex.
  • Basal ganglia : These are islands of gray matter within the white matter. They help coordinate movement.
  • The brain stem : This consists of the mid-brain, pons and the medulla. These areas are responsible for relay of nerve impulses between the spinal cord and the brain. The brain stem controls vital functions such as heart rate and breathing.
  • Ventricles of the brain : These are empty spaces in the brain and are filled with cerebrospinal fluid. These are 4 in number, two in both cerebral hemispheres and two elsewhere.

How can we describe the contents of the cranium and their functions?

1. Minor structures in the cranium:

  • The meninges : The brain is protected from outside by three coverings. These membranes are called as the meninges. These are the:
          » Dura mater, which is firmly adherent to the brain.
          » Pia mater, which lies over the dura mater and,
          » Arachnoid mater, which lies adherent to the bone of the skull, the cranium.
    The subarachnoid space is the region in the cranium surrounding the brain and lying below the arachnoid mater. It contains cerebrospinal fluid that protects and nourishes the brain.
  • The choroid plexus : It is the area in production of cerebrospinal fluid.
  • Arachnoid granulations : These absorb cerebrospinal fluid from the cranium into the blood and thus regulate the quantity of CSF in the brain.
  • Venous sinuses : These are several blood vessels that drain blood away from the brain.
  • Pituitary gland : This is a small gland regulating the other glands in the body.
  • Thalamus : This is the area through which sensory nerve impulses pass before going to the cerebral cortex.
  • The hypothalamus : This area controls the endocrine system of the body. It regulates sleep, sexual function, body temperature and water content.
  • Blood vessels in the brain : These are extensively spread throughout the area of the brain and supply the brain with nutrients in the form of glucose. Blockage of any vessel can cause death of that area of the brain. Rupture of blood vessels causes stroke.

2. Major structures in the cranium : The two major structures in the cranium are the cerebrum and the cerebellum.

The Cerebrum

It is a large organ that looks much like the inside of a walnut. It is brown in color and is made up of many ridges that appear like depressions and elevations on its surface. The elevations are called as the gyri and the depressions are called as the sulci. The cerebrum is the largest part of the brain. It is divided into two halves called as the hemispheres. These halves are joined together by the corpus callosum. This is a large bundle of about 300 million nerve fibers connecting the two hemispheres. The cerebrum is made of outer gray matter and inner white matter.

The cerebrum is also divided into several lobes. These are the:

  • Frontal Lobe : This lobe is present at the front of the brain. The frontal cortex is responsible for complex movements, intelligence, behavior, personality, speech formation, and voluntary movements.
  • Temporal Lobe : The lobe that appears as a projection from the cerebrum and lies on the side of the brain. The temporal cortex detects discrete qualities of sound, such as pitch and volume. Thus it recognises words and melodies. There is one more area called as the Wernicke’s area that interprets written and spoken language.
  • Parietal Lobe : This lobe is present towards the middle and the rear of the cerebrum. The primary sensory cortex present on this lobe is responsible for analyzing information about sensations.
  • Occipital Lobe : This lobe lies at the back of the cerebrum. This contains the visual cortex that receives nerve impulses from the eye.

Microscopic Structure
The outer layer of the brain is formed by the cerebral cortex. This layer controls the higher brain functions such as thinking. It also initiates movement and receives and processes sensations. The cerebral cortex is also called as the gray matter. The gray matter consists of neuron cell bodies or dendrites, from which nerve impulses originate. The white matter mainly contains nerve fibers or axons. Its main role is to transmit nerve impulses. In between the white matter, there are certain areas or islands of gray matter that are called as the basal ganglia. These areas are responsible for co-ordination of movement. Parkinson’s disease is a disease of the basal ganglia that causes disturbances in movements. It produces a characteristic tremor of the limbs.

Functions of the Cerebrum?

Special functions of the cerebrum
The brain is made of two hemispheres, one of which is the dominant hemisphere. The dominant hemisphere has certain specialized functions to perform such as interpretation of language. Handedness also depends on the dominant hemisphere. In 9 of 10 people the left hemisphere is the dominant hemisphere.

Higher functions of the cerebrum
The parts of the cerebral cortex, which perform the higher functions of the brain, can now be identified easily. These are as follows:

  1. Pre-motor cortex : This part of the brain coordinates complex movement sequences such as playing the piano.
  2. Pre-frontal cortex : This deals with the various aspects of behavior and personality.
  3. The Broca’s area : This area is important for the formation of speech.
  4. The primary auditory cortex : This area detects the fine qualities of sound like its pitch and volume.
  5. The auditory association cortex : This area is responsible for analysis of data pertaining to sound. This area recognizes words and tunes.
  6. Motor cortex : This area sends signals to muscles to cause voluntary movements.
  7. Primary sensory cortex : This area receives data about the sensations in the skin, joints, muscles and organs.
  8. Sensory association cortex : This area analyses the data received by the primary sensory cortex.
  9. Visual association cortex : This area of the brain forms images after the analyses of visual data.
  10. Primary visual cortex : This area of the brain is responsible for receiving nerve impulses from the eye.
  11. Wernicke’s area : This area interprets spoken and written language.

Movement and touch
Each side of the brain has its own motor and sensory cortices, which control movement and sense touch in the opposite side of the body. Movement signals are processed by an area at the top of the cerebrum in the motor cortex. An adjoining area called as the sensory cortex processes the information regarding touch. The parts, which are sensitive to touch or have complex movements, are assigned a larger area in the sensory cortex of the brain. For example, areas for the hands, fingers have a large area in the brain.

The Cerebellum

The cerebellum is that part of the brain that lies beneath the rear end of the cerebrum. This part of the brain is responsible for the balance of the body and control of muscle movement. It is divided into two parts called as the cerebellar hemispheres separated by a structure called as the vermis.

Microscopic Structure
The cerebellar cortex is thrown into folds called folia. There are certain masses of gray matter found within the cerebellum. The largest of these masses is called as the dentate nucleus.

Functions of the Cerebellum
The cerebellum plays an important role in the control of muscle tone and the coordination of muscle movements on the same sides of the body.