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.

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Author Info: , The Gale Group Inc., Gale, Detroit, Gale Encyclopedia of Nursing and Allied Health, 2002

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