The Human Brain- An Introduction
Making sense of the brain's mind-boggling complexity isn't easy. It makes us human, gives people the capacity for art, language, moral judgment and rational thought. It's also responsible for each individual's personality, memories, movements, and how we sense the world.
The brain performs an incredible number of tasks:
1. It controls the body temperature, blood pressure, heart rate and breathing.
2. It accepts a flood of information about the world from various senses (seeing, hearing, smelling, tasting and touching).
3. It handles your physical movements when you walk, talk, stand or sit.
4. It lets you think, dream, reason and experience emotions.
All these tasks are coordinated, controlled and regulated by the brain --- an organ the size of a small head of cauliflower. Your brain, spinal cord and peripheral nerves make up a complex, integrated information-processing and control system known as the central nervous system. Together, they regulate all the conscious and unconscious facets of your life. The scientific study of the brain and nervous system is called neuroscience or neurobiology. Because the field is so vast -- and the brain and nervous system are so complex -- we will start with the basics and give you an overview of this complicated organ.
The Structure of Human Brain
We'll examine the brain structure and how each part controls our daily functions, including motor control, visual processing, auditory processing, sensation, learning, memory and emotions.
All this comes from a jellylike mass of fat and protein weighing about 3 pounds (1.4 kilograms). It is one of the body's biggest organs, having some 100 billion nerve cells that not only put together thoughts and highly coordinated physical actions but regulate our unconscious body processes, such as digestion and breathing.
The brain's nerve cells are called neurons, which make up the organ's so-called "gray matter." The neurons transmit and gather electrochemical signals that are communicated via a network of millions of nerve fibers called dendrites and axons. These are the brain's "white matter."
The cerebrum is the largest part of the brain, accounting for 85 percent of the organ's weight. The distinctive, deeply wrinkled outer surface is the cerebral cortex, which consists of gray matter. Beneath this lies the white matter. It's the cerebrum that makes the human brain—and therefore humans—so formidable. The capacity to think, reason, argue, remember, visualize --- all these faculties come from this part. So when you are reading this article, the most active part of your brain is the cerebrum. Whereas animals such as elephants, dolphins, and whales have larger brains, humans have the most developed cerebrum. It's packed to capacity inside our skulls, enveloping the rest of the brain, with the deep folds cleverly maximizing the cortex area.
Cerebral Hemispheres and Functional Localization
The cerebrum has two halves, or hemispheres. It is further divided into four regions, or lobes, in each hemisphere. The frontal lobes, located behind the forehead, are involved with speech, thought, learning, emotion, and movement. Behind them are the parietal lobes, which process sensory information such as touch, temperature, and pain. At the rear of the brain are the occipital lobes, dealing with vision. Lastly, there are the temporal lobes, near the temples, which are involved with hearing and memory.
Right Brain-Left Brain
In most respects, the left and right sides are symmetrical in function. For example, the counterpart of the left-hemisphere motor area controlling the right hand is the right-hemisphere area controlling the left hand. There are, however, several important exceptions, involving language and spatial cognition. In most people, the left hemisphere is "dominant" for language: a stroke that damages a key language area in the left hemisphere can leave the victim unable to speak or understand, whereas equivalent damage to the right hemisphere would cause only minor impairment to language skills.
The second largest part of the brain is the cerebellum, which sits beneath the back of the cerebrum. It is responsible for coordinating muscle movements and controlling our balance. Consisting of both grey and white matter, the cerebellum transmits information to the spinal cord and other parts of the brain.
Diencephalon, Thalamus, Hypothalamus
The diencephalon is located in the core of the brain. A complex of structures roughly the size of an apricot, the two major sections are the thalamus and hypothalamus. The thalamus acts as a relay station for incoming nerve impulses from around the body that are then forwarded to the appropriate brain region for processing. The hypothalamus controls hormone secretions from the nearby pituitary gland. These hormones govern growth and instinctual behavior such as eating, drinking, sex, anger, and reproduction. The hypothalamus, for instance, controls when a new mother starts to lactate.
The brain stem, at the organ's base, controls reflexes and crucial, basic life functions such as heart rate, breathing, and blood pressure. It also regulates when you feel sleepy or awake. The brain is extremely sensitive and delicate, and so requires maximum protection. This is provided by the surrounding skull and three tough membranes called meninges. The spaces between these membranes are filled with fluid that cushions the brain and keeps it from being damaged by contact with the inside of the skull.
Brain Injuries/ Disorders
Despite being protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood–brain barrier, the human brain is susceptible to damage and disease.
The most common forms of physical damage are closed head injuries such as a blow to the head, a stroke, or poisoning by a variety of chemicals which can act as neurotoxins, such as ethanol alcohol. Infection of the brain, though serious, is rare because of the biological barriers which protect it. The human brain is also susceptible to degenerative disorders like Parkinson's disease, and Alzheimer's disease (mostly due to aging) and multiple sclerosis. Many psychiatric conditions like schizophrenia and clinical depression are associated with brain dysfunctions, although they are not well understood.
Mental disorders like clinical depression, schizophrenia, bipolar disorder and post-traumatic stress disorder may involve particular patterns of neuropsychological functioning related to various aspects of mental and somatic functions. These disorders may be treated by psychotherapy, psychiatric medication, social intervention and personal recovery work or cognitive behavioural therapy; the underlying issues and associated prognoses vary significantly between individuals.
A dominant feature of the human brain is corticalization. The cerebral cortex in humans is so large that it overshadows every other part of the brain. Corticalization is reflected in function as well as structure. In a rat, surgical removal of the cerebral cortex leaves an animal still capable of walking around and interacting with the environment. In a human, comparable cerebral cortex damage produces a permanent coma. The cerebral cortex is a neural tissue, folded in a way that allows a large surface area to fit within the confines of the skull.
The cerebral cortex is nearly symmetrical with left and right hemispheres that are approximate mirror images of each other. Each hemisphere is conventionally divided into four "lobes", the frontal lobe, parietal lobe, occipital lobe, and temporal lobe.
Neural signals from eyes
Each hemisphere of the brain interacts primarily with one half of the body, but the connections are crossed: the left side of the brain interacts with the right side of the body, and vice versa. Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, both cross the midline at the level of the brainstem. The result is that connections from the left half of the retina in both eyes go to the left side of the brain, whereas connections from the right half of the retina go to the right side of the brain. Because each half receives light coming from the opposite half of the visual field, the functional consequence is that visual input from the left side of the world goes to the right side of the brain, and vice versa. Thus, the right side of the brain receives somatosensory input from the left side of the body, and visual input from the left side of the visual field—an arrangement helpful for visuomotor coordination.