The Human Brain

This article relates to Do No Harm

British neurosurgeon Henry Marsh, the author of Do No Harm, operates on the brain.

The Mayfield Clinic provides this succinct description of the organ: "Nothing in the world can compare with the human brain. This mysterious three-pound organ controls all necessary functions of the body, receives and interprets information from the outside world, and embodies the essence of the mind and soul. Intelligence, creativity, emotion, and memories are a few of the many things governed by the brain."

The brain, as we all know, is housed in a person's skull, specifically in the cranium - a bony enclosure formed by eight separate bones that fuse together during childhood. The inside of the cranium is divided into three separate areas: The anterior cranial fossa located at the front of the head, the middle cranial fossa, and the posterior cranial fossa located at the back. The posterior fossa has a large hole in it called the foramen magnum, where the spinal cord passes through to connect to the brain.

Below the skull one first comes to the meninges, three layers of membrane designed to protect the brain underneath. The dura mater is the thick layer closest to the skull. Below that is the arachnoid mater, a thin, elastic sheath covering the entire brain. Finally, closest to the brain is the pia mater, which hugs the organ's creases. As these layers are pulled back one next encounters the ventricles – cavities filled with cerebrospinal fluid (CSF) which is continuously manufactured by a ribbon-like area of the brain called the choroid plexus. CSF provides a cushion between the brain and skull to protect against injury.

The brain itself is divided into three major areas: The cerebrum, cerebellum and brain stem. The cerebrum is the section responsible for higher functions such as reasoning, emotions and speech. Its surface is called the cortex, and this heavily creased area has a grey-brown appearance which gives the brain one of its nicknames: "grey matter." It's believed that the folds in the cortex, called gyri (singular = gyrus), allow more neurons to fit inside the skull, providing for higher functions as humanity evolves. The cortex contains approximately 70% of the 100 billion nerve cells in the brain. The cerebellum, located under the cerebrum, controls motor functions such as muscle movement, while the brain stem is a sort of "relay center" that connects the other parts of the brain and is responsible for automatic actions like breathing and body temperature.

The cerebrum is divided into two halves, each of which control the opposite side of one's body – so, for example, a stroke in the left side of the brain would impact the right side of the body. The cerebrum is also segmented into lobes which further divvy up control of various bodily functions (for example, the occipital lobe located near the back of the cranium interprets what we see).

Buried within the brain are various "deep structure" glands and other cellular formations that help the brain and body function together. This includes the pituitary gland (which secretes hormones that control sexual development, help us fight disease, and governs our response to stressful situations) and the pineal gland (which helps regulate the body's internal clock). Other areas in this category are the limbic system - a set of separate brain structures that together support functions as various as emotion, behavior, long-term memory and smell; the thalmus which relays signals to the cerebral cortex and regulates consciousness, and the hypothalamus, which links the nervous system to the endocrine system (that produces hormones) and thus regulates functions such as hunger, thirst and blood pressure.

Twelve pairs of nerves are also housed in the center of the brain called the cranial nerves. With the exception of the nerves for smell and vision, which originate in the cerebellum, these fibers are located in the brain stem and govern movements such as facial expression, neck and shoulder movement, and the tongue muscles.

The brain is comprised mostly of two types of cells – nerve cells (aka neurons) and glia cells ("glia" is Greek for glue). The neurons are responsible for passing information between the various segments of the brain using electrical and chemical signals transmitted through gaps known as synapses. The glia cells (which make up about 90% of the brain) provide structural support, protection and nourishment to the neurons. Brain tumors most often involve an abnormality in the glial cells.

Picture of the brain from TheHumanBrainProject.org

Filed under Medicine, Science and Tech

This "beyond the book article" relates to Do No Harm. It originally ran in June 2015 and has been updated for the June 2016 paperback edition. Go to magazine.

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