The human brain is a complex organ that allows us to think, move, feel, see, hear, taste, and smell. It controls our body, receives, analyzes, and stores information (our memories). The brain produces electrical signals, which, together with chemical reactions, let the parts of the body communicate. Nerves send these signals throughout the body. The average human brain weighs about 3 pounds (1300 – 1400 g). At birth, the human brain weighs less than a pound (0.7 – 0.88 pounds or 350 – 400 g). As a child grows, the number of cell remains relatively stable, but the cells grow in size and the number of connections increases. The human brain reaches its full size at about 6 years of age. Although the brain is only 2% of the body weight, it uses 20% of the oxygen supply and gets 20% of the blood flow. Blood vessels (arteries, capillaries, and veins) supply the brain with oxygen and nourishment, and take away wastes. If brain cells do not get oxygen for 3 to 5 minutes, they begin to die.
The brain consists of gray matter (40%) and white matter (60%) contained within the skull. Brain cells include neurons and glial cells. The main parts of the brain are: the cerebrum (the forebrain) made up of the right and left cerebral hemispheres; the cerebellum (the hindbrain); the brain stem.
The cerebrum is the largest area of the brain and is concerned with all higher mental functions, such as thinking and memory. It’s made up of two halves or hemispheres. The right cerebral hemisphere controls the left side of the body and the left cerebral hemisphere controls the right side of the body. Each cerebral hemisphere is divided into four areas, known as lobes: the frontal, parietal, temporal and occipital lobes. Each lobe controls a different range of activities. The frontal lobes control skilled motor behaviour, including speech, mood, thought, and planning for the future. The parietal lobes interpret sensory input from the rest of the body and control body movement. The occipital lobes interpret vision. The temporal lobes generate memory and emotions.
Collections of nerve cells lie at the base of the cerebrum in structures called basal ganglia, thalamus, and hypothalamus. The basal ganglia help to smooth out movements; the thalamus generally organizes sensory messages to and from the highest levels of the brain (cerebral cortex); and the hypothalamus coordinates some of the more automatic functions of the body, such as controlling sleep and wakefulness, maintaining body temperature, and regulating the balance of water within the body.
The cerebellum is at the back of the brain, below the cerebrum. It’s a lot smaller than the cerebrum at only 1/8 of its size. But it’s a very important part of the brain. Cerebellum is concerned with balance and coordination. These activities are carried out automatically (subconsciously) by this area of the brain and are not under a person’s control.
The brain stem sits beneath the cerebrum and in front of the cerebellum. It connects the rest of the brain to the spinal cord. It controls the basic functions essential to maintaining life, including blood pressure, breathing, heartbeat, eye movements and swallowing.
Within the cerebral hemispheres and brain stem are a series of cavities called ventricles. These spaces are contiguous with the central canal of the spinal cord and, like the spinal cord, they are filled with cerebrospinal fluid. The largest of the ventricles are the first and second (lateral) ventricles, which extend into the cerebral hemispheres and occupy portions of the frontal, temporal, and occipital lobes. The third ventricle is in a narrow space in the midline of the brain and connects with the lateral ventricles through openings in the front of it, which are called interventricular foramina. The fourth ventricle is located in the brain stem, just in front of the cerebellum. It is connected to the third ventricle by a narrow canal, the cerebral aqueduct, which passes lengthwise through the brain stem. This ventricle is contiguous with the central canal of the spinal cord and has openings in its roof that lead into the meninges (membranes that cover the brain and spinal cord).
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 many types of 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 wide variety of chemicals that can act as neurotoxins. Infection of the brain is rare because of the barriers that protect it, but is very serious when it occurs. The human brain is also susceptible to degenerative disorders, such as Parkinson’s disease, multiple sclerosis, and Alzheimer’s disease. A number of psychiatric conditions, such as schizophrenia and depression, are widely thought to be caused at least partially by brain dysfunctions, although the nature of such brain anomalies is not well understood.