Of all the human body systems, the nervous system is the most complicated system in the body. The brain is the central part of the nervous system. It is an intriguing organ, that has been studied right from the time it develops in the fetus. The human brain weighs about 1.5 kg in adults. The cerebrum, which forms the bulk of this organ, is divided into two hemispheres, the right hemisphere and the left hemisphere.
Each hemisphere of the brain interacts with one half of the body, but for unknown reasons, it is the right side that controls the left half of the body and the left half controls the right half of the body. In most respects, the left and the right sides of the brain are symmetrical. However, there is one major difference, in most people, the left hemisphere is more involved in language and creativeness, while the right side is more involved in understanding and judgment. Although in brain functions, left and right hemispheres have the same essential role, the left is often solely attributed with personality and the right solely with intelligence. However, there is no concrete scientific proof to back this claim.
There are many ways of dividing the brain for studying its various aspects. However, it is conventionally divided into three parts: the forebrain, the midbrain and the hindbrain. The brain also contains four interconnected cavities called ventricles, which contain cerebrospinal fluid. We will study the diagram of the brain and its functions in this article, along with a detailed study of the brain anatomy.
Outer Morphology of the Brain
The forebrain is further divided into telencephalon and diencephalon.
This consists of the two cerebral hemispheres of the cerebrum and their inter connections.
The cerebrum is the largest part of the brain. Any kind of trauma or lesion in the cerebrum can lead to various diseases and disorders and mental illnesses. The cerebrum is divided into two cerebral hemispheres by the medial longitudinal fissure. These two hemispheres are connected to each other by a fibrous band of nerves called the corpus callosum. The cortex of each hemisphere is divided into four lobes - the frontal lobe, temporal lobe, parietal lobe and the occipital lobe.
The cerebral cortex is highly convoluted in its structure, which increases the area available for the neurons. The ridges present on the surface of the cortex are called gyri, whereas the grooves or fissures separating them are called sulci. Important sulci of the cerebrum include the central or Rolandic sulcus, the lateral or Sylvian sulcus, the parieto-occipital sulcus and the Calcarine sulcus. These sulci help to divide the cerebrum into its four lobes. Given below is a diagram of brain, its functions, detailing the four lobes and their associated structures.
Four Lobes of the Brain
This is the part of the cerebrum that lies directly below the frontal bone. It is the part that is present directly behind the forehead. It is separated from the parietal lobe by the central sulcus, and from the temporal lobe by the lateral sulcus. The frontal lobe is divided into a lateral, polar, orbital and medial part. The frontal lobe is seen in the above diagram; and its functions are described below.
The frontal lobe is involved with the main executive functions of the brain, which include:
- Judgment, that is, the ability to recognize future consequences resulting from ongoing actions. This activity mostly occurs in the pre-frontal area.
- Analytical and critical reasoning.
- Cognition and memory is mostly concentrated in the frontal lobe. Thus, any damage to this region leads to memory loss and dementia. In some people during old age, there is severe memory loss due to malfunctioning of the frontal lobe, which leads to Alzheimer's disease.
- Emotional traits of a person are stored in the frontal lobe.
- The motor cortex located in the frontal lobe is responsible for voluntary motor activity. Thus, this is one of the regions often implicated in Parkinson's disease.
- The premotor cortex is responsible for the storage of motor patterns and various voluntary activities.
- The frontal lobe is also responsible for storing the language skills of a person.
The parietal lobe is that lobe that is present superior to the occipital lobe and posterior to the frontal lobe. It is enclosed by the parietal bone of the skull. The parietal lobe is separated from the frontal lobe by the central sulcus, while the lateral sulcus separates the parietal lobe from the temporal lobe.
- The parietal lobe is concerned with appreciation of basic sensations, such as touch, pain, pressure, temperature (heat and cold) and various joint movements.
- It is also responsible for the discrimination of the intensity of various stimuli, like distinguishing warm from hot, ice-cold from cold, etc.
- It is responsible for storing data, which helps in later fine tuning tactile sensation, i.e., it helps in recognizing common familiar objects placed in our hand without looking at them.
- Parietal lobe helps in recognition of spatial relationships, that is, realization of the exact point of tactile sensation, the ability of discrimination between two points of tactile stimulation etc.
The temporal lobe is a region of the cerebral cortex that is present beneath the Sylvian fissure. It is present on both sides of the brain. This lobe is home to the primary auditory complex. This lobe contains the hippocampus.
- The temporal lobe is involved in auditory (sound) sensation and is where the Wernicke's Area (language recognition center) is located. The left temporal lobe is especially seen to be involved in speech.
- The sensation of smell is also registered in the temporal lobe.
- This lobe is also partly responsible for emotion, memory and speech.
The occipital lobe is the smallest lobe and is present at the back of the skull. It is present just overlying the occipital bone, hence the name. The occipital lobes rest on the tentorium cerebelli, which is a process of the dura mater, that separates the cerebrum from the cerebellum. At the edges of the occipital lobe, there are several lateral occipital gyri, separated by the lateral occipital sulcus.
The occipital lobe is primarily involved with interpreting visual impulses. This is done via the visual pathway.
Visual inputs follow a slightly complex rule - the optic nerves from the two eyes come together at a point called the optic chiasm, and here, half the fibers of each nerve split off to join the other. Thus, impulses received from the left half of the retina in both eyes go to the left side of the brain, while the impulses from the right half of the retina go to the right brain.
The retinal sensors that are present in the eye are stimulated when rays enter the eye and reach them. This stimuli then is conveyed to the cuneus (Brodmann's area 17). From here it goes on to the optic tracts and the lateral geniculate bodies of the thalamus, where optic radiations then continue onto the visual cortex. Each visual cortex receives raw sensory information from the outside half of the eye present on the same side and from the inside half of the eye present on the other side of the head. Finally, the image is then projected in the cortex. Given below is a diagram of the visual cortex of the brain.
Thus, damage to the occipital lobe results in vision loss due to field cuts in each eye. If there is damage to the primary visual cortex, then this could lead to loss of vision of the opposite field, or in severe cases, seeing stars and flashes of light (visual hallucinations). It could also lead to color agnosia, that is, a problem with identifying colors (not to be confused with color blindness).
The diencephalon sits beneath the middle of the cerebrum and on top of the brain stem. It contains two important structures called the thalamus and the hypothalamus, of which the hypothalamus connects with the pituitary complex.
The thalamus is a paired structure that is a part of the diencephalon. Each thalamus is a large, egg-shaped cluster of nuclei (gray matter). The two thalami lie close together, and are joined across the mid-line by a mass of gray matter called massa intermedia. This is the center that acts as a relay station for incoming sensory nerve impulses, thus sending these impulses to the required appropriate regions of the brain for further processing. Most sensory signals, like auditory signals, visual signals and somatosensory signals go through this structure before being further processed in the brain. It also plays a major role in motor control, and is responsible for control of muscular movements through its various connections with the basal ganglia, cerebellum and the motor cortex. Basically, it is responsible for letting the brain get information on what is happening outside the body.
This is a small structure present in the diencephalon that plays a vital role in maintaining homeostasis, that is, a state of equilibrium within the body. The thalamus keeps conditions in the body constant, preventing any sudden change from occurring. It regulates various sensations, such as hunger, thirst, temperature, libido among other things. It is also responsible for the circadian rhythm which is exerted in the body (the daily sleep and awake cycle). It also plays a vital role in emotions, autonomic functions and motor functions. It tries to maintain homeostasis by exerting control on the pituitary gland.
The pituitary gland, or the hypophysis, is a small pea-sized gland, that is present in the skull, resting in a cavity of the skull called the sella turcica. It is an endocrine gland that is involved with secreting various hormones, and thus, establishing hormonal balance in the body.
The various hormones secreted by the pituitary gland include the human growth hormone, adrenocorticotropic hormone, anti-diuretic hormone, follicle stimulating hormone, luteinizing hormone, oxytocin and thyroid stimulating hormone among other hormones. Thus, it is often called the master gland. Given below is a diagram outlining the main brain functions and parts.
Vertical Section of the Brain and its Functions
The midbrain is divided into two parts by the Aqueduct of Sylvius, which is the duct that connects the IIIrd ventricle in the midbrain with the IV ventricle in the pons and medulla oblongata. The ventral part of it is called the cerebral peduncle, which is chiefly made up of white matter and it unites the pons with the thalamic region of the cerebrum. The dorsal part is called the tectum, which consists of two elevations, the superior and inferior colliculi.
This section serves as a relay center for sensory information from the ears to the cerebrum. It also controls the reflex movements of the muscles of the head, neck and the eye. Thus, it provides a passage for different neurons going in and coming out of the cerebrum.
Hindbrain or Rhombencephalon
The midbrain, pons and medulla oblongata are often together termed as the brain stem (sometimes only the pons and medulla oblongata are referred to as the brain stem). All cranial nerves are situated in the brain stem. The hindbrain is made up of the brain stem and the cerebellum.
The word 'pons' literally means bridge. This is the structure that helps connect the two parts of the medulla oblongata and is often seen as a slight bulge present just above the medulla oblongata. The pons has a role in the level of arousal or consciousness and sleep. It also helps in relaying sensory impulses to and from the brain. Furthermore, it is involved in controlling autonomic body functions.
The medulla oblongata forms the lower half of the brain stem. It is an extremely important part, as it deals with vital and basic activities of the human body, as it contains the cardiac, respiratory and vasomotor centers. Thus, it is responsible for functions such as breathing, maintaining a steady heart rate and blood pressure, inciting regurgitation (vomiting), swallowing, urination, defecation and in coordinating lifesaving reflexes. Thus, it is the medulla oblongata that executes the most important function of the brain, that is, regulating our life processes. Normally, when a person is hanged to death, it is the medulla oblongata that is compressed to an extent that the person's breathing and heartbeat stops. Thus, any kind of damage to this structure, due to either a brain stem injury or a brain stem stroke, is potentially life-threatening. Given below is a labeled diagram showing the brain stem and its related structures.
Brain Stem and Structures
The word 'cerebellum' literally means little brain. It is the second largest part of the brain, and is located at the back, below the occipital lobe, beneath the cerebrum and behind the brain stem. It contains an outer gray cortex and an inner white medulla, and has horizontal furrows, which makes it look different from the rest of the brain.
- Coordination of voluntary muscular movement. Thus, damage to the cerebellum results in cerebral palsy. Even uncontrolled movements, due to malfunctioning of the cerebellum can lead to seizures, which manifests as epilepsy. Thus, severe trauma to this region can even lead to paralysis.
- Maintaining balance and equilibrium while walking, swimming, riding, etc.
- Storing memory for reflex motor acts. It is basically involved with learning new movements, and coordinating and executing them properly.
- It is also involved with coordinating simultaneous subconscious actions, like eating while talking or listening etc.
The extension of the central nervous system is through the medulla oblongata, which continues as the spinal cord, by exiting the skull through the foramen magnum and continuing in the spinal column. This is where the spinal cord gives out nerves which form the peripheral nervous system. Explaining complex brain functions is quite a task, since brain is the only organ that makes humans capable of learning language, art, rational thinking and judgment. These gray cells define who we are. So, I hope this comprehensive article on the human brain has helped you become a mini neurologist in your own right!