Bernice Essenfeld. The Gale Encyclopedia of Science. Editor: K Lee Lerner & Brenda Wilmoth Lerner. Volume 1. Detroit: Gale, 2008.
The brain is a mass of nerve tissue located in an animal’s head that controls the body’s functions. In simple animals, the brain functions like a switchboard picking up signals from sense organs and passing information to muscles. The brain is also responsible for a variety of involuntary behavior, including keeping the heart beating, maintaining blood pressure, and temperature. In more advanced forms, particularly vertebrates, a more analytical brain coordinates complex behaviors. In higher vetebrates, the brain coordinates thinking, memory, learning, and emotions. The brain is part of an animal’s central nervous system, which receives and transmits impulses. It works with the peripheral nervous system, which carries impulses to and from the brain and spinal cord via nerves running throughout the body.
Nematodes (roundworms) have a simple brain and nervous system consisting of approximately 300 nerve cells, or neurons. Sensory neurons located in the head end of the animal detect stimuli from the environment and pass messages to the brain. The brain then sends out impulses through a ventral nerve cord to muscles, which respond to the stimulus. The way that the interneurons of the brain process the data determines the response.
The earthworm and other annelids, as well as insects and other arthropods, have more complex nervous systems. In these animals, there are paired ventral nerve cords that run from head to tail on the animal’s underside. Cell bodies of neurons in the cords form pairs of ganglia in each body segment. Four of the most anterior ganglia fuse in the head to form a brain. As a result, the brain ganglia are larger than the segmental ganglia, and also contain a larger proportion of sensory motor neurons. The brain ganglia have some dominance over the segment ganglia. The ventral nerve cords, brain, and segmental ganglia comprise the central nervous system. Neuronal fibers in the cords, bundled into nerves that carry communications between ganglia, make up the peripheral nervous system.
The earthworm’s brain consists of paired ganglia in the head end. An impulse, such as touch, light, or moisture, is detected by receptor cells in the skin. A pair of nerves in each of the earthworm’s segments carries the signal to the brain and smaller ganglia in each segment, where the signals are analyzed. The central nervous system then transmits impulses on nerves that coordinate muscle action, causing the earthworm to move.
In insects, specialized sense organs detect information from the environment and transmit it to the central nervous system. Such sense organs include simple and compound eyes, sound receptors on the thorax or in the legs, and taste receptors. The brain of an insect consists of a ganglion in the head. Some of the segmental ganglia are fused, allowing better communication between the segments. The information that insects use for behaviors such as walking, flying, mating, and stinging is stored in the segmental ganglia. In experiments in which heads are cut off of cockroaches and flies, these insects continue to learn.
The central nervous system of vertebrates consists of a single spinal cord, which runs in a dorsal position along the back, and a highly developed brain. The brain is the dominant structure of the nervous system. It is the master controller of all body functions, and the analyzer and interpreter of complex information and behavior patterns. We can think of the brain as a powerful neural computer. The peripheral nervous system, composed of nerves that run to all parts of the body, transmits information to and from the central nervous system.
The vertebrate brain is divided into three main divisions: the forebrain, the midbrain, and the hind-brain. The hindbrain connects the brain to the spinal cord, and a portion of it, called the medulla oblongata, controls important body functions such as breathing rate and the heart rate. The cerebellum, also in the hindbrain, controls balance. The forebrain consists of the cerebrum, thalamus, and hypothalamus. The fore-brain controls, among other things, the sense of smell in vertebrates.
During the first few weeks of development, the brain of a vertebrate looks like a series of bulges in the neural tube. It is hard to see a difference when we examine the early embryonic brains of fish, amphibians, reptiles, birds, and mammals. As the brain develops, the bulges enlarge, and each type of vertebrate acquires its own specific adult brain that helps it survive in its environment. In the forebrain of fish, the olfactory (smell) sense is well developed, whereas the cerebrum serves merely as a relay station for impulses. In mammals, on the other hand, the olfactory division is included in the limbic system, which also controls emotions, and the cerebrum is highly developed, operating as a complex processing center for information. Optic lobes are well developed in the midbrain of non-mammalian vertebrates, whereas in mammals the vision centers are mainly in the forebrain. In addition, a bird’s cerebellum is large compared to the rest of its brain, since it controls coordination and balance in flying.
The living human brain is a soft, shiny, grayish white, mushroom-shaped structure. Encased within the skull, it is a 3 lb (1.4 kg) mass of nerve tissue that keeps us alive and functioning. On average, the brain weighs 13.7 ounces (390 g) at birth, and by age 15 grows to approximately 46 ounces (1,315 g). The human brain is composed of up to one trillion nerve cells—100 billion of them are neurons, and the remainder are supporting (glial) cells. Neurons receive, process, and transmit impulses, while glial cells (neuroglia) protect, support, and assist neurons. The brain is protected by the skull and by three membranes called the meninges—the outermost the dura mater, the middle the arachnoid, and the innermost the pia mater. Also protecting the brain is cerebrospinal fluid, a liquid that circulates between the arachnoid and pia mater in the subarachnoid space. Many bright red arteries and bluish veins on the surface of the brain penetrate inward. Glucose, oxygen, and certain ions pass easily from the blood into the brain, whereas other substances, such as antibiotics, do not. The capillary walls are believed to create a blood-brain barrier that protects the brain from a number of biochemicals circulating in the blood.
The parts of the brain can be studied in terms of structure and function. Four principal sections of the human brain are the brain stem (the hindbrain and midbrain), the diencephalon, the cerebrum, and the cerebellum.
The Brain Stem
The brain stem is the stalk of the brain, and is continuous with the spinal cord. It consists of the medulla oblongata, pons, and midbrain. A part of the brain stem, the medulla oblongata is a continuation of the spinal cord. All the messages that are transmitted between the brain and spinal cord pass through the medulla via fibers in the white matter. The fibers on the right side of the medulla cross to the left and those on the left cross to the right. The result is that each side of the brain controls the opposite side of the body. There are three vital centers in the medulla, which control the heartbeat, the rate of breathing, and the diameter of the blood vessels. Centers that help coordinate swallowing, vomiting, hiccoughing, coughing, and sneezing are also located in the medulla. The reticular formation occurs partially in the medulla and in other parts of the central nervous system. The reticular formation operates in maintaining our conscious state. The pons (meaning “bridge”) conducts messages between the spinal cord and the rest of the brain, and between the different parts of the brain. The midbrain conveys impulses from the cerebral cortex to the pons and spinal cord. It also contains visual and audio reflex centers involving the movement of eyeballs and head.
Twelve pairs of cranial nerves originate in the underside of the brain, mostly from the brain stem. They leave the skull through openings and extend as peripheral nerves to their destinations. Cranial nerves include the olfactory nerve that brings messages about smell from the nose and the optic nerve that conducts visual information from the eyes.
The diencephalon lies above the brain stem, and embodies the thalamus and hypothalamus. In the diencephalon, the thalamus is an important relay station for sensory information for the cerebral cortex from other parts of the brain. The thalamus also interprets sensations of pain, pressure, temperature, and touch, and is concerned with some of our emotions and memory. It receives information from the environment in the form of sound, smell, and taste. The hypothalamus performs numerous important functions. These include the control of the autonomic nervous system (a branch of the nervous system involved with control of a number of body functions, such as heartbeat rate and digestion). The hypothalamus helps regulate the endocrine system and controls normal body temperature. It tells us when we are hungry, full, and thirsty. It helps regulate sleep and wakefulness, and is involved when we feel angry and aggressive.
The cerebrum, constituting about 87.5% of the brain weight, spreads over the diencephalon. The cerebral cortex is the outer layer of the brain and is composed of gray matter made up of nerve cell bodies. It is about 0.08 inch (2 mm) thick and its surface area is about 5 square feet (1.5 sq m)—around half the size of an office desk. White matter, composed of nerve fibers covered with myelin sheaths, lies beneath the gray matter. With the rapid growth of the brain during embryonic development, the gray matter grows faster than the white matter and folds on itself. The folds are called convolutions or gyri, and the grooves between them are known as sulci. A deep longitudinal fissure separates the cerebrum into a left and right hemisphere. Each cerebral hemisphere is divided into frontal, temporal, parietal, and occipital lobes. The corpus callosum, a large bundle of fibers, connects the two cerebral hemispheres. The thalamus and subcortical nuclei, or basal ganglia, are areas of gray matter that exist below the white matter.
Sensory areas of the cerebrum interpret sensory impulses. Spoken and written language are transmitted to a part of the cerebrum called Wernicke’s area where meaning is extracted, and sent to Broca’s area, one of the motor areas of the cerebrum. Motor areas of the cerebrum control muscle movements. Within Broca’s area, thoughts are translated into speech, and muscles are coordinated for speaking. Impulses from other motor areas direct our hand muscles when we write, and our eye muscles when we scan the page for information.
Association areas of the cerebrum are concerned with emotions and intellectual processes, by connecting sensory and motor functions. In our association areas, innumerable impulses are processed that result in memory, emotions, judgment, personality, and intelligence.
Certain structures in the cerebrum and diencephalon make up the limbic system. These regions function in memory and emotions, and are associated with pain and pleasure.
By studying patients whose corpus callosa were destroyed, scientists realized that differences existed between the left and right sides of the cerebral cortex. The left side of the brain functions mainly in speech, logic, writing, and arithmetic. The right side of the brain, on the other hand, is more concerned with imagination, art, symbols, and spatial relations.
The cerebellum is located below the cerebrum and behind the brain stem, and is shaped like a butterfly. The “wings” are the cerebellar hemispheres, and each consists of lobes that have distinct grooves or fissures. The cerebellum controls the movements of our muscular system needed for balance, posture, and maintaining posture.
Studying the Brain
At the end of the nineteenth century, Santiago Ramón y Cajal (1852-1934), a Spanish scientist, studied neurons using stain developed by Camillo Golgi. Cajal realized that the brain was made up of individual units and not a continuous net as was believed at the time. His studies uncovered a large variety of neurons that differed in size and shape. He explained that neurons received signals on dendrites and transmitted impulses on axons. Since his work, researchers have learned that neurons carry information in the form of brief electrical impulses called action potentials that result when positively charged sodium ions travel across the axon membrane from the fluid outside to the cytoplasm inside. When a nerve impulse reaches the end of an axon, neurotransmitters are released at junctions called synapses. The neurotransmitters are chemicals that bind to receptors on the receiving neurons, triggering the continuation of the impulse. Fifty different neurotransmitters have been discovered since the first one was identified in 1920. By studying the chemical effects of neurotransmitters in the brain, scientists have made advances in finding medicines for the treatment of mental disorders, and determining the actions of drugs on the brain.
Researchers today are able to trace various molecules that are transported along axons during action potentials. Microelectrodes are used to detect the currents that cross synapses. Using this information, wiring diagrams are created that model the patterns of information flow within the brain.
Considerable knowledge about the human brain has been obtained during brain surgery by stimulating specific areas with a mild electric current, and from the observation of patients with brain damage. In the 1920s, a Canadian neurosurgeon named Wilder Penfield electrically stimulated different parts of the brains of some of his patients. He found this caused them to remember specific events from the past. For example, one patient heard someone from the past singing a particular song. From this and other studies, scientists realized that specific functions are localized in specific parts of the brain. Recently, scientists observed the behavior of a woman whose amygdala (an almond-shaped group of cells in the cerebrum) was destroyed. The amygdala plays a role in emotions and social relationships. The researcher realized that without an amygdala, the patient could not read facial expressions. As a result, she couldn’t judge the intentions of others, and often made poor social decisions.
Until recently, scientists believed that brain cells do not regenerate, thereby making brain injuries and brain diseases untreatable. Researchers are now trying to help such patients with neuron transplants, introducing nerve tissue into the brain. They are also studying substances, such as nerve growth factor (NGF), that someday may be used to help regrow nerve tissue.
Since the 1950s, scientists have begun to understand the process of sleep. They find that sleep occurs in different stages. One stage is called rapid eye movement (REM) sleep. We dream during REM sleep, a period when there is a lot of brain activity and eye movements and the body is inactive. The pons, an area of the brainstem, sets off REM sleep and dreaming. During REM sleep, the brain emits characteristic brain waves. Non-REM sleep usually comes first, takes up about 75% of our sleep, and is much quieter. Its stages get deeper and deeper. Non-REM sleep also has its own particular brain waves. The two types of sleep alternate during the night. Scientists are beginning to understand the factors that control sleep and wakefulness. These include a biological clock, a group of about 10,000 neurons in the hypothalamus that trigger off waking up; homeostasis, the body’s tendency to maintain equilibrium in physiological systems; and changes in the level of norepinephrine and serotonin, neurotransmitters in the brain.
Where and how does memory occur? This is another question that has puzzled scientists for decades. Recent information suggests that memory is not stored in a single brain center, but instead is part of numerous processing systems in the cerebral cortex. Scientists believe that memory involves chemical and structural changes in neurons, as well as changes in the strength of synapses.
Technology provides useful tools for researching the brain and helping patients with brain disorders. An electroencephalogram (EEG) is a record of brain waves, electrical activity generated in the brain. An EEG is obtained by positioning electrodes on the head and amplifying the waves with an electroencephalograph, and is valuable in diagnosing brain diseases such as epilepsy and tumors.
Scientists use three different techniques that involve scans to study and understand the brain and diagnose disorders:
(1) Magnetic resonance imaging (MRI) uses a magnetic field to display the living brain at various depths as if in slices. Not blocked by bone, MRI allows the viewer to zoom in on any region and obtain reliable pictures of brain tissue.
(2) Positron emission tomography (PET) produces color images of the brain on the screen of a monitor. During this test, a technician injects a small amount of a substance, such as glucose, that is marked with a radioactive tag. The marked substance shows where glucose is consumed in the brain. PET is used to study the chemistry and activity of the normal brain and to diagnose abnormalities such as tumors.
(3) Magnetoencephalography (MEG) measures the electromagnetic fields created between neurons as electrochemical information is passed along. When under the machine, if the subject is told, “wiggle your toes,” the readout is an instant picture of the brain at work. Concentric colored rings appear on the computer screen that pinpoint the brain signals even before the toes are actually wiggled.
Using an MRI along with MEG, physicians and scientists can look into the brain without using surgery. They hope to use these techniques for the early diagnosis of disorders such as Alzheimer and Parkinson diseases. They hope that these techniques could help paralysis victims move by supplying information on how to stimulate their muscles, or indicating the signals needed to control an artificial limb. Furthermore, by understanding what areas of the brain are active while performing particular tasks, and by mapping information regarding increased or decreased metabolism in particular regions of the brain, a variety of questions regarding various disease states, as well as a variety of questions regarding the phenomenonal potential of the human brain, may be answered.
Researchers are studying any number of issues regarding brain functioning. Fascinating research revealing the presence of tiny magnetic bits within the brain has suggested that humans (like some insects and birds) have the potential to navigate via interactions with Earth’s magnetic field. Technological research into computers and artificial intelligence are furthered by a good understanding of the amazing, computerlike intelligence of the human brain; conversely, studies of the human brain have been furthered by efforts to create artificial intelligence. Research into the human brain is actually thought to be in its infancy; myriad topics of investigation must be explored to grasp the complexities and intricacies of the human brain.