Brain

Definition

The brain is the part of the central nervous system (CNS) inside the skull (the part outside the skull is the spinal cord). It gives rise to cognitive thought processes and controls various body functions including muscular activity, speech, sight, hearing, breathing, and digestion.

Description

The brain is the organ that is located inside the skull and is connected to the spinal cord. The brain has four major parts: the brainstem, the cerebellum, the diencephalon, and the cerebral hemispheres.

The brainstem is located at the base of the brain and connects the brain to the spinal cord. The brainstem has three parts: the medulla oblongata, the pons, and the mid-brain. Nine of the twelve cranial nerves originate in the brainstem. The brainstem is responsible for controlling basic functions that require no cognitive thought, such as blood pressure, digestion, breathing, and heart rate. The brainstem is also the information freeway between the cerebral hemispheres and the spinal cord.

The cerebellum is located immediately below the back part of the cerebral hemispheres and is attached to the brainstem through bands made of nerve fibers called peduncles. The cerebellum plays a major role in balance, coordination, and the learning of motor skills. Damage to the cerebellum can result in incoordination, otherwise known as ataxia.

The diencephalon lies above the midbrain and houses the thalamus and the hypothalamus. The thalamus serves as a major relay system between the brainstem and the cerebral hemispheres. The hypothalamus is involved in controlling a variety of functions that don't require conscious thought (autonomic functions) such as appetite, blood pressure, thirst, temperature, and sexual arousal.

The largest part of the human brain is the cerebrum. It is divided into left and right cerebral hemispheres by a deep groove called the longitudinal fissure. A band of fibers called the corpus callosum connects the hemispheres with each other. Each cerebral hemisphere has four major parts called lobes. In addition, there is a fifth small lobe called the island (insula) hidden deep inside each hemisphere. The four major lobes are named for the bones of the skull closest to which they lie: the frontal, parietal, temporal, and occipital lobes.

The frontal lobe is located in the front part of the brain and is responsible for higher cognitive functions such as speech, problem-solving, planning, organizing, awareness, motor activity, memory storage, and intelligence. The frontal lobe is also involved in emotions and other aspects of personality.

The parietal lobe is located behind the frontal lobe and is the highest part of the brain. The parietal lobe is involved in perceptions such as touch, pain, and pressure. It also discriminates fine sensations such as the weight of an object. In addition to sensory processing, the parietal lobe is also involved in understanding language and in writing.

The temporal lobes form the side parts of the cerebral hemispheres at the level of the ears. The temporal lobes control hearing, speech, smell, and memory.

The fourth lobe, located in the back of the head, is the occipital lobe. It is involved in the processing of visual information, such as the recognition of shapes and colors.

The outer surface of the cerebral hemispheres is arranged in convolutions known as gyri (singular "gyrus") which are separated by grooves called sulci (singular "sulcus"). Two sulci are especially important as borders between lobes: the central sulcus lies between the frontal and parietal lobes; the lateral sulcus separates the temporal lobe from the frontal and parietal lobes.

The brain is composed not only of solid matter but also of four ventricles. These ventricles are the two lateral ventricles inside the hemispheres, the third ventricle inside the diencephalon, and the fourth ventricle located among the cerebellum, the medulla oblongata, and the pons. The third ventricle is connected to the fourth ventricle by a narrow channel called the cerebral aqueduct. Cerebrospinal fluid (CSF) flows in the ventricles and in the cerebral aqueduct. The CSF serves to protect the brain by cushioning it from dangerous shocks that would otherwise injure it. It also carries nutrients to the brain cells and transports waste products away from them.

Other protectors of the brain include the blood-brain barrier and the meninges. The blood-brain barrier prevents foreign substances in the blood from entering the brain. The meninges are membranes consisting of connective tissue which cover the brain in three layers. The outermost of these layers, the dura mater, is the thickest and toughest of the three. The middle layer, the arachnoidea, is loosely attached to the third layer by fibers resembling a spider's web. The innermost layer, the pia mater, is made of a delicate connective tissue which has many blood vessels.

The names of the meninges have a fascinating story behind them. Medieval European scientists borrowed heavily from Arab anatomists who were in turn building upon ancient Greek science. The Arabs called the meninges "the tough covering, the spider-web covering, and the delicate covering." The Arabic word for "covering" could also mean "mother" or "matrix." The Arabic terms for the outer and inner layers were mistranslated into Latin for "hard" (dura) mother, and "tender, devoted" (pia) mother, but the Arabic made no such personification and was merely contrasting a rough, tough covering with a fine, delicate one. However, the Europeans did

get the Arabic for "like a spider's web" (Latin arachnoidea) correct.

The microanatomy of the brain is highly specialized and organized. The brain is composed of two major types of cells called neurons and glial cells.

There are several types of neurons including motor neurons, sensory neurons, and interneurons. Neurons are the basic operating cells of the brain. They are specially designed to communicate rapidly with other neurons and with organs. They do this by sending electrical signals known as action potentials down the length of their axons.

Axons are fiber processes of neurons (they are unique to neurons) that generally conduct electrical impulses away from the cell body of the neuron. The word "axon" comes from the Greek word for an axis and refers to the fact that the axon is the central part of the nerve fiber. Specialized glial cells called oligodendrocytes generally wrap the axons of the neurons in myelin.

A dendrite is a protoplasmic process of a neuron that conducts electrical impulses toward the cell body of the neuron. Usually it spreads out into many branches (its name comes from the Greek dendron meaning "tree" because of its resemblance to the branches of a tree).

The other type of glial cell in the brain is the astrocyte, which aids the neuron in its function.

Cell bodies of neurons, part of their processes, and glia form the gray matter of the brain. The gray matter which forms the outer layer of the cerebrum is called the cerebral cortex. This cerebral cortex contains from about nine to fourteen billion nerve cells and weighs on average1.3 lb (581 g). Of this 1.3 lb, only 0.044 lb (20 g) is made up of cell bodies (that is, approximately one part in thirty). Another part of the cerebral gray matter forms a few islands called basal ganglia. Basal ganglia are masses of gray matter located deep within each cerebral hemisphere. These groups of neurons help regulate body movement and facial expressions.

Inside, the cerebral hemispheres are made largely of white matter. Axons form this white matter, and it is the high lipid content of their myelin coating that gives the white matter its characteristically white color.

Neurons communicate by releasing chemical compounds called neurotransmitters. The major neurotransmitters in the brain are serotonin, dopamine, acetylcholine, gamma-aminobutyric acid (GABA), and glycine. Neurotransmitters bind to protein receptors on the surface of the neuron and cause changes to occur inside the neuron. It is believed that many psychiatric diseases are due to imbalances in these chemical neuro-transmitter systems.

Function

The brain is the ultimate controller of the human body and performs functions with and without conscious thought. The brain enables the mind to conduct conscious thoughts and feelings. The brain allows human beings to respond to the environment. It also regulates functions without conscious thought such as digestion, blood pressure, balance, and sleep.

The brain allows one to interpret and respond to the stimuli given to the five senses: taste, touch, hearing, smell, and vision. The brain helps us learn to recognize a certain smell or remember how to ride a bike. The brain enables human communication. The brain contains regions devoted to speech production and speech comprehension. Certain regions of the brain are employed in reading, and writing. The brain is also involved in reproductive behavior and regulates the release of sex hormones. The hypothalamus in the brain tells the body when it is time to drink and when it is time to eat. The brain also regulates sleep and biological rhythms. The brain is likewise involved in generating emotions and largely determines personality.

It is within the cerebral cortex that impulses are received, analyzed, and answered. The body contains nerve cells that are specialized for detecting environmental events and other cells that are specialized for producing movement. Information in the form of light, sound waves, odors, tastes as well as contact with objects is gathered from the environment by specialized cells which are called sensory neurons. In response to this information, movements are performed by the contraction of muscles, which are controlled by motor neurons. Communication between sensory neurons and motor neurons is carried on by interneurons.

Specialized sensory cells of the ear called cells of Corti perceive sound waves and send corresponding impulses to the brain, where they reach the projector auditory center which is located in the superior temporal gyrus (field 41 of Brodmann). The cells of this center receive and analyze the separate impulses. Near this field 41 is located field 37, the associative auditory area, cells of which form one integral or complete auditory image of the object from the separate impulses sent by the neurons of field 41. If we destroy field 37, the person can hear sounds but cannot make sense out of them. For example, he or she can hear the speech sounds "p" and "e" and "n" but does not imagine a pen as a result of hearing these sounds. The parietal lobe contains a sensory area which is located in the post-central gyrus (fields 1, 2, 3 of Brodmann), and in the nearby part of the superior parietal lobulus (fields 5, 7 of Brodmann). In this area, impulses of general sensation such as touch, pain, pressure, and temperature are interpreted. The occipital lobe contains the visual region which is located in the area of the calcarine sulcus (field 17 of Brodmann) and an adjacent area (fields 18, 19, of Brodmann). In this visual area, the impulses arising from the retina of the eye are interpreted.

The frontal lobe contains the motor area (field 4 of Brodmann) with about 25,000 giant pyramidal neurons. It is located in the precentral gyrus, the superior frontal gyrus, and the paracentral lobulus. The uppermost part of the precentral gyrus together with the paracentral lobulus controls the movement of the legs. The hindmost part of the superior frontal gyrus controls the movement of the torso. The middle part of the precentral gyrus controls the movement of the arms, and the lowest part of the precentral gyrus controls the movement of the neck and head. The premotor area located in front of the motor area is responsible for coordination and integration of movements. If one suffers destruction of the back part of the middle frontal gyrus, one can still wiggle one's fingers, but one cannot write, although the movement of the corresponding muscles is normal, because writing requires a high degree of coordination and integration of hand and finger movements.

The brain controls the functions of the body that do not require conscious thought (the autonomic functions). Located in the medulla oblongata are the centers controlling digestion, breathing, the functioning of the heart, blood vessels, of the urinary system, and of the glands which produce saliva, tears, and sweat.

The sympathetic and parasympathetic nerves extending from these centers cause the arousal and inhibition of these systems. The sympathetic nerves speed up the heart, raise the blood pressure, dilate the pupils of the eyes, contract the sphincters of the hollow organs, and relax the longitudinal muscles. They prepare a person for crisis situations and they remain active during stress.

The parasympathetic nerves slow the heart, lower the blood pressure, constrict the pupils of the eyes, relax the sphincters of the hollow organs, and help promote the digestion and absorption of nutrients.

Common diseases and disorders

There are hundreds of diseases and disorders of the brain. There are conditions in which learning is impaired, for example, disorders of speech called aphasias and disorders in writing called dyslexia. There are disorders of thought, such as schizophrenia and Tourette syndrome. Mood disorders include depression, mania, and anxiety. There are also disorders of sleep such as insomnia and narcolepsy. The brain is subject to strokes and to cancer. The brain can also cause seizures in which neurons uncontrollably fire electrical signals. This is the hallmark of epilepsy. The basis of drug abuse and addiction is intertwined with reward pathways in the brain. There are several diseases which are involved in mental retardation, such as Fragile X syndrome and lissencephaly. There are also several diseases in which neurons degenerate or die. These diseases are collectively known as neurodegenerative diseases, and clinical symptoms depend on where in the brain the neurons are dying.

• Alzheimer's disease. Alzheimer's disease usually occurs later in life and is characterized by a decline in cognitive functions such as memory, judgment, and reasoning. The hallmark of Alzheimer's disease is deposits in the brain of a protein called amyloid beta. These deposits are found in abnormal structures called neurofibrillary tangles. Risk factors for Alzheimer's disease include mutation in genes that are responsible for production of the following proteins: amyloid precursor protein, presenilin-1, presenilin-2, and apolipoproteins. However, the role of these proteins in the development of the disease is not known.
• Huntington's disease. Huntington's disease is a genetic disease in which neurons in the brain that are involved in controlling movement die. This leads to uncontrollable, jerky, and spastic movements. There is also slowness of movement, difficulty in swallowing, and dementia. Huntington's disease occurs in the fourth or fifth decade of life and usually results in death 10 to 12 years after the first symptoms appear. The gene involved in the disease is responsible for producing a protein called huntingtin, but the role this protein plays in causing the disease is not clear.
• Parkinson's disease. Parkinson's disease is a disorder of bodily movement caused by the death of neurons that release the neurotransmitter dopamine. Symptoms include tremor, slowness of movement, rigidity, and a loss of reflexes. It is a progressive disease in which patients become unable to move. There are cases of inherited Parkinson's disease in which a gene linked to alpha-synuclein has been identified. However, the role alpha-synuclein may play in Parkinson's disease is not clear.

KEY TERMS

Axon—A fiber process of a neuron that generally conducts electrical impulses away from the cell body of the neuron.

Cell body—The part of a neuron that contains the nucleus and other cell organelles.

Dementia—Decline in mental ability.

Dendrite—A protoplasmic process of a neuron that conducts electrical impulses toward the cell body of the neuron. Usually it spreads out into many branches.

Neuron—The highly specialized cell that is the basic structural and functional unit of the nervous system.

BOOKS

Carlson, N. R. Physiology of Behavior. 6th ed. Needham Heights, MA: Allyn and Bacon, 1998.

Cohen, Barbara and Dena Wood. Structure and Function of the Human Body. 7th ed. Philadelphia: Lippincott Williams and Wilkins, 2000.

ORGANIZATIONS

Society for Neuroscience. 11 Dupont Circle, NW, Suite 500, Washington, DC 20036. (202) 462-6688. <htpp://www.sfn.org>.