The brain is a complex organ that controls thought, memory, emotion, touch, motor skills, vision, breathing, temperature, hunger and every process that regulates our body. Together, the brain and spinal cord that extends from it make up the central nervous system, or CNS. Weighing about 3 pounds in the average adult, the brain is about 60% fat. The remaining 40% is a combination of water, protein, carbohydrates and salts. The brain itself is a not a muscle. It contains blood vessels and nerves, including neurons and glial cells.

The cerebrum is the largest part of the brain, located superiorly and anteriorly in relation to the brainstem. Its large size is the result of a progressive centralization (telencephalization) of the various higher sensory and motor centres of the brain during evolution.

The cerebrum is a heavily convoluted bilobed structure. The two lateral halves are called cerebral hemispheres. When the two cerebral hemispheres are viewed together from above, they assume the shape of an ovoid mass, which is broader behind than in front. The widest transverse diameter corresponds with a line connecting the two parietal tuberosities.

A deep median cleft, the longitudinal cerebral fissure, incompletely separates the two cerebral hemispheres. Both in front and behind, the cleft is complete, but in the central part the cleft extends downwards up to the corpus callosum which is a large mass of white fibres joining the two cerebral hemispheres across the median plane.

Externally, the cerebrum has a highly convoluted appearance, consisting of sulci (grooves or depressions) and gyri (ridges or elevations). It is divided into two anatomically symmetrical hemispheres by the longitudinal fissure, a major sulcus that runs in the median sagittal plane. The falx cerebri (a fold of dura mater) descends vertically to fill this fissure. The two cerebral hemispheres are connected by a white matter structure, called the corpus callosum.

Each cerebral hemisphere presents six borders, viz. superomedial, superciliary, inferolateral, medial orbital, medial occipital and inferomedial. The superomedial border separates the superolateral surface from the medial surface. The superciliary border is at the junction of superolateral and orbital surfaces. It lies just behind the superciliary arch hence its name strictly speaking, it is the orbital part of the inferolateral border. The inferolateral border separates the superolateral surface from the tentorial surface. Posteriorly this border exhibits a notch, the preoccipital notch about 3 cm in front of the occipital pole. This notch is used as a useful surface landmark. The medial orbital border separates the medial surface from the orbital surface. The inferomedial/hippocampal border surrounds the cerebral peduncle. It is formed by the medial aspect of the uncus and parahippocampal gyrus. The medial occipital border separates the medial surface from the tentorial surface.

In spite of an apparent congruity in size, shape and feas of the left and right cerebral hemispheres, they are nures not mirror images' as far as certain neuronal activities are concerned. There are certain established functional differences between the two hemispheres (certain brain functions are lateralized. 

The frontal lobe is situated anterior to the central sulcus and posterior to the lateral fissure. The frontal lobe consists of an inferior, middle and superior frontal gyri, orbital area and primary motor cortex. It controls our motor and executive functions. It assists in our problem-solving, personality, behaviour, planning, concentration, writing, speaking and intelligence.

The parietal lobe is situated anterior to the parieto-occipital sulcus and posterior to the central sulcus. It controls sensations and perceptions such as the sense of touch and temperature.

The temporal lobe is situated inferiorly to the lateral fissure. It is further divided into inferior, middle and superior temporal gyri. This lobe assists in memory, hearing and language comprehension.

The occipital lobe is located posteriorly to the parieto-occipital sulcus. It helps us in facial recognition, interpreting colour, depth, distance and vision.

The cerebrum as a whole functions to control our emotions, vision, personality and intelligence. The upper motor neurons in the motor cortex send information via axons to the spinal cord and brainstem where they synapse with lower motor neurons that innervate the muscles.

The cerebral cortex receives all the audioty, visual, somatosensory and gustatory information and processes it with the cortices to make us feel these perceptions.

The olfactory bulb is a small structure located under the frontal lobe. It processes the olfactory information and sends them directly to the olfactory cortex instead of thalamus. Damage to the olfactory bulb can lead to loss of smell. The cerebral cortex controls the speech and language traits.

Wernicke's area, situated at the temporal-parietal lobe junction, is attributed to speech comprehension. Broca's area, situated in the frontal lobe, controls the language functioning. The hippocampus and the medial temporal lobe are associated with memory.

Each hemisphere has its own contributions, none is more important than the other). The left hemisphere is more efficient as far as handedness, perception of language, speech, writing and calculation (numerical skills) are concerned. The right hemisphere is more efficient with spatial perception (geometrical and spatial relationships), recognition of faces, creative acts of arts and music, and non-verbal ideation. 

The term dominant hemisphere refers to the side concerned with the perception and production of language and speech. According to this concept, the left hemisphere is dominant in over 90% of people, in whom the right hemisphere is described as the minor or non-dominant hemisphere.

The left hemisphere controls the right side of the body, including the skilful right hand. Consequently over 90% of the adult population is right-handed. During childhood, one hemisphere slowly comes to dominate over the other, and it is only after the first decade that the dominance becomes fixed.

Published : Oct 18 2023