The brain is one of the most important, and probably the most complicated, structures found within the human body. It is located within a hard, bony structure called the cranium. The strong bones of this structure protect the brain from damage. The average human brain is approximately 1,500 g with an approximate volume of 1,250 cubic centimeters (cm3) for men and 1,125 cm3 for women. The anatomy of the brain is very complex, being divided into four main areas: the cerebrum, diencephalon, brain stem, and cerebellum. While much of its anatomy has been mapped, there are still many functions that remain unknown. For example, scientists are researching the conversion of experiences into memories and how those memories are stored and recalled.
In addition to the cranium, the brain tissue is protected by three layers of connective tissue called meninges. These layers of tissue cover and protect the brain and protect blood vessels in the brain tissue. The dura mater is the outermost layer, surrounding the brain with two layers of tissue. These two layers are fused together, and in places, help to divide the cranial cavity, and prevent the brain from moving. The arachnoid layer is found under the dura mater and forms a loose brain covering. The innermost meninx is called the pia mater. This layer is composed of connective tissue and contains many blood vessels. It directly covers and attaches to the brain.
The cerebrum is the part of the brain that is the site of higher-order thinking, memory, centralization of the information received by the senses, and control of voluntary movements, including those that require specific muscle actions, such as using a pencil or running a race.
The diencephalon is the part of the brain that is composed of the thalamus, hypothalamus, and epithalamus. The thalamus is the large gray mass at the top of the diencephalon responsible for sending sensory and motor signals to the cerebral cortex. Each of these organs plays a role in the relay of sensory information and coordinates the autonomic nervous system, the part of the nervous system that controls involuntary movements such as breathing. The hypothalamus is a gland that regulates the release of other hormones by the anterior lobe of the pituitary gland. The hypothalamus also helps to control body temperature, hunger, water balance, and other physiological processes.
The brain stem includes the midbrain, pons, and medulla oblongata. The midbrain controls vision, hearing, and motor control. It also controls sleep cycles, alertness, and regulation of body temperature. The pons is part of the brain stem located between the midbrain and the medulla oblongata that serves as a relay center, transmitting signals from different parts of the nervous system to the brain and back to the effector organs. The pons works with the medulla oblongata, the structure that is continuous with the spinal cord and controls several involuntary functions such as breathing, swallowing, and heart rate.
The cerebellum is located beneath the cerebrum and is the part of the brain that processes information coming from motor neurons—those that create a response to a stimulus. The cerebellum also maintains a person's balance so they are able to stand upright.All parts of the brain sit in cerebrospinal fluid, the fluid that surrounds the central nervous system and fills ventricles of the brain and central canal of the spinal cord, protecting the central nervous system. This liquid provides buoyancy to the brain and prevents it from being crushed under its own weight. Cerebrospinal fluid also acts as a shock absorber, protecting the brain from impacts to the skull. The brain has a large blood supply, but this blood does not come into contact with the cerebrospinal fluid. Instead, the cerebrospinal fluid helps with the transport of nutrients from one part of the brain to another and also transports hormones. Cerebrospinal fluid is produced by ependymal cells in the choroid plexuses of the ventricles of the brain. Adults maintain a volume of cerebrospinal fluid of about 150 mL and replace it every eight hours.