Testimony Prepared for the Standing Committee on Mental Health of the Assembly of the State of New York.
October 5, 1978.
by Dr. Peter Sterling, Ph.D.
Associate Professor of Neurobiology
Department of Anatomy
School of Medicine
University of Pennsylvania

Scope and Complexity of the Brain

The brain is the controlling organ of the body. It receives information from the outside world through the 5 senses. It also receives information from the inside of the body regarding all the body's internal functions: heart rate, blood pressure, amount of glucose (sugar), oxygen, carbon dioxide, hormones, etc., in the blood. It also contains information, coded originally in the genes, regarding the needs that all humans share: the drives for hunger, thirst, sex, and so on. As particular kinds of information is taken into the brain by the various sensors, it is stored. Old information, when needed, is retrieved for comparison with new information so that decisions can be made.

These decisions include the obvious, conscious ones, such as whether we shall get out of bed in the morning, or what clothes we shall wear. They also include decisions of which we are not conscious that have consequences for every cell in the body, such as how high the blood pressure should be, how much of a particular hormone should be secreted, how much blood should be distributed to one organ or another.

The Demands of Complexity

It is a general rule that the more complex a structure is, the more closely regulated its operation must be. In a complex structure, foundations must be firmer and the tolerances closer. The safeguards against disruption must be numerous and of a "fail-safe" variety. A simple hut needs no foundation but there can be no mistakes in the planning or construction of a skyscraper. The simpler the structure, furthermore, the less vulnerable it is to disruption. A hut will most likely survive an earthquake and in any case can be repaired, but a skyscraper, even with all its safeguards, is subject to irreparable collapse. This rule applies to the body as well. Let us compare a relatively simple tissue, the skin, to the most complex organ, the brain.

The skin is exposed directly to the environment and is frequently damaged by mechanical trauma. Its cells have the capacity to divide; new cells easily replace worn or damaged ones. Skin cells must be supplied with nutrients and oxygen from the blood, but their requirements are quite flexible. They can metabolize a variety of substances: fatty acids, glucose, amino acids; they can operate for a while without oxygen and can tolerate wide variations in blood supply. The skin cells are not very sensitive to temperature -- that is why we can sit in the sun or plunge into ice water without damage.

The brain is entirely different. Its 10-100 billion neurons are all present at birth. Nerve cells do not divide to replace their losses. Therefore, any loss of cells is permanent. The death of a single neuron represents a loss of up to 100,000 inputs and 100,000 outputs for a total loss of 10 billion connections. Obviously, the brain must be protected from mechanical trauma.

The brain, unlike the skin, has virtually no metabolic flexibility. It can metabolize only glucose and not fatty acids or amino acids. This is one reason why the glucose levels in the blood must be maintained at all times. A sharp fall in blood glucose leads rapidly to failure of brain function and coma. Oxygen supply to the brain must also be maintained for there is hardly any reserve supply. If a pressure cuff is placed around the neck and inflated, a human subject goes blind and loses consciousness in 6 seconds. If he breathes pure nitrogen, consciousness is lost in 17-20 seconds. After 3-4 minutes without oxygen at normal body temperature, there is generalized brain damage; after 4-5 minutes, the damage is irreversible.

Brain temperature must also be closely regulated. Everyone is familiar with their own experience with the deterioration of brain function that occurs in fever where temperature rises only a degree or so above normal. Aspirin, by restoring normal temperature, brings relief. Temperature rises that are only slightly greater than a common fever may cause convulsions and can do permanent damage. Clearly, brain function cannot withstand the extreme changes in blood glucose, oxygen supply, or temperature that bother the skin not at all.

Protective Mechanisms of the Brain

Mechanical protection. The first level of mechanical protection for the brain is the thick bone of the skull. There is an active protection too: because the skull has sensitive nerve endings on the outside, we learn early not to bang it into hard objects. Inside the skull, the brain is protected by 3 separate layers of casings. There is a tough, fibrous outer casing called the "dura" ("hard"). Beneath the dura there is a second, more delicate membrane called the "arachnoid". This encases the brain in a special fluid called "cerebrospinal fluid" ("cSF"). Thus, the brain is suspended in fluid in the same way the delicate embryo is suspended within the womb. Within limits, mechanical shocks to the head are absorbed by this fluid and are not transmitted to the brain. The third membrane layer is called the "pia". It is applied directly to the brain's surface, forming the last major protective barrier. Blood vessels must penetrate the pia to reach neural tissue itself.

Protection of blood supply. Even though the brain is only about 2% of the body's weight, it uses 20% of the body's oxygen supply because of its high rate of metabolism. The brain controls its own blood flow and gives itself highest priority along with the heart. If there is not enough blood to go around, the blood supply is shut down to the gut, kidney, skin and muscle -- always to preserve flow to brain and heart. If blood pressure falls so low that the heart has difficulty pumping blood to the head, the brain shuts off messages to muscle, causing collapse (fainting). With the head at the same level as the heart as a result of fainting, the crucial blood supply can more easily be maintained. Thus, fainting is an important protective mechanism for the brain.