Impulse, sudden wishes or urges that prompt an unpremeditated act or feeling.
For information on:
Source of impulsive feelings, according to Freudian theory, sees Id
Parts of personality controlling socially unacceptable impulses, according to Freudian theory, see Ego; Superego
Inabilities to resist the impulse to wager money or other items of value see Pathological Gambling
Repetitive counterproductive expressions of repressed impulses see Habit
Impulse to steal sees Kleptomania
Attention-deficit hyperactivity disorder as cause of impulsiveness, Attention-Deficit Hyperactivity Disorder
Lack of control over the impulse to set fires
For more information click her
Electrical Impulse, surge of electrical power in one direction.
For information on:
Measurement of electrical impulses sees Oscilloscope
Role of electrical impulses in physiology, see Acetylcholine; Physiology: Recent Advances
Electrical impulses in communication devices, see Facsimile Transmission; Phonograph; Sound Recording and Reproduction: Introduction; Telegraph: Telegraph Carrier Media; Telephone; Television: Television Receiver; Voiceprint Identification
Electrical impulses in nuclear weapon detonation see Nuclear Weapons: Detonation of Atomic Bombs
Role of electrical impulses in the nervous system, see Anatomy: Nervous System; Nervous System: Anatomy and Function; Neurophysiology: Electrical Transmission
Impulse in biology:
Brain (anatomy), portion of the central nervous system contained within the skull. The brain is the control center for movement, sleep, hunger, thirst, and virtually every other vital activity necessary to survival. The brain controls all human emotions—including love, hate, fear, anger, elation, and sadness—. It also receives and interprets the countless signals that are sent to it from other parts of the body and from the external environment. The brain makes us conscious, emotional, and intelligent.
Between the brain and the cranium—the part of the skull that directly covers the brain—are three protective membranes, or meninges. The outermost membrane, the dura mater, is the toughest and thickest. Below the dura mater is a middle membrane, called the arachnoid layer. The innermost membrane, the pia mater, consists mainly of small blood vessels and follows the contours of the surface of the brain.
A clear liquid, the cerebrospinal fluid, bathes the entire brain and fills a series of four cavities, called ventricles, near the center of the brain. The cerebrospinal fluid protects the internal portion of the brain from varying pressures and transports chemical substances within the nervous system.
From the outside, the brain appears as three distinct but connected parts: the cerebrum (the Latin word for brain)—two large, almost symmetrical hemispheres; the cerebellum (“little brain”)—two smaller hemispheres located at the back of the cerebrum; and the brain stem—a central core that gradually becomes the spinal cord, exiting the skull through an opening at its base called the foramen magnum. Two other major parts of the brain, the thalamus and the hypothalamus, lie in the midline above the brain stem underneath the cerebellum.
The brain and the spinal cord together make up the central nervous system, which communicates with the rest of the body through the peripheral nervous system. The peripheral nervous system consists of 12 pairs of cranial nerves extending from the cerebrum and brain stem; a system of other nerves branching throughout the body from the spinal cord; and the autonomic nervous system, which regulates vital functions not under conscious control, such as the activity of the heart muscle, smooth muscle (involuntary muscle found in the skin, blood vessels, and internal organs), and glands.
The two cerebral hemispheres are partially separated from each other by a deep fold known as the longitudinal fissure. Communication between the two hemispheres is through several concentrated bundles of axons, called commissures, the largest of which is the corpus callosum.
Several major sulci divide the cortex into distinguishable regions. The central sulcus, or Rolandic fissure, runs from the middle of the top of each hemisphere downward, forward, and toward another major sulcus, the lateral (“side”), or Sylvian, sulcus. These and other sulci and gyri divide the cerebrum into five lobes: the frontal, parietal, temporal, and occipital lobes and the insula.
The frontal lobe is the largest of the five and consists of all the cortex in front of the central sulcus. Broca's area, a part of the cortex related to speech, is located in the frontal lobe. The parietal lobe consists of the cortex behind the central sulcus to a sulcus near the back of the cerebrum known as the parieto-occipital sulcus. The parieto-occipital sulcus, in turn, forms the front border of the occipital lobe, which is the rearmost part of the cerebrum. The temporal lobe is to the side of and below the lateral sulcus. Wernicke's area, a part of the cortex related to the understanding of language, is located in the temporal lobe. The insula lies deep within the folds of the lateral sulcus.
The cerebrum receives information from all the sense organs and sends motor commands (signals that result in activity in the muscles or glands) to other parts of the brain and the rest of the body. Motor commands are transmitted by the motor cortex, a strip of cerebral cortex extending from side to side across the top of the cerebrum just in front of the central sulcus. The sensory cortex, a parallel strip of cerebral cortex just in back of the central sulcus, receives input from the sense organs.
Many other areas of the cerebral cortex have also been mapped according to their specific functions, such as vision, hearing, speech, emotions, language, and other aspects of perceiving, thinking, and remembering. Cortical regions known as associative cortex are responsible for integrating multiple inputs, processing the information, and carrying out complex responses.
The cerebellum coordinates voluntary movements by fine-tuning commands from the motor cortex in the cerebrum. The cerebellum also maintains posture and balance by controlling muscle tone and sensing the position of the limbs. All motor activity, from hitting a baseball to fingering a violin, depends on the cerebellum.
C Thalamusand Hypothalamus
The thalamus and the hypothalamus lie underneath the cerebrum and connect it the brain stem. The thalamus consists of two rounded masses of gray tissue lying within the middle of the brain, between the two cerebral hemispheres. The thalamus is the main relay station for incoming sensory signals to the cerebral cortex and for outgoing motor signals from it. All sensory input to the brain, except that of the sense of smell, connects to individual nuclei of the thalamus.
The hypothalamus lies beneath the thalamus on the midline at the base of the brain. It regulates or is involved directly in the control of many of the body's vital drives and activities, such as eating, drinking, temperature regulation, sleep, emotional behavior, and sexual activity. It also controls the function of internal body organs by means of the autonomic nervous system, interacts closely with the pituitary gland, and helps coordinate activities of the brain stem.
D Brain Stem
The topmost structure of the brain stem is the midbrain. It contains major relay stations for neurons transmitting signals to the cerebral cortex, as well as many reflex centers—pathways carrying sensory (input) information and motor (output) commands. Relay and reflex centers for visual and auditory (hearing) functions are located in the top portion of the midbrain. A pair of nuclei called the superior colliculus control reflex actions of the eye, such as blinking, opening and closing the pupil, and focusing the lens. A second pair of nuclei called the inferior colliculus, control auditory reflexes, such as adjusting the ear to the volume of sound. At the bottom of the midbrain are reflex and relay centers relating to pain, temperature, and touch, as well as several regions associated with the control of movement, such as the red nucleus and the substantia nigra.
Continuous with and below the midbrain and directly in front of the cerebellum is a prominent bulge in the brain stem called the pons. The pons consists of large bundles of nerve fibers that connect the two halves of the cerebellum and also connect each side of the cerebellum with the opposite-side cerebral hemisphere. The pons serves mainly as a relay station linking the cerebral cortex and the medulla oblongata.
D3 Medulla Oblongata
The long, stalklike lowermost portion of the brain stem is called the medulla oblongata. At the top, it is continuous with the pons and the midbrain; at the bottom, it makes a gradual transition into the spinal cord at the foramen magnum. Sensory and motor nerve fibers connecting the brain and the rest of the body cross over to the opposite side as they pass through the medulla. Thus, the left half of the brain communicates with the right half of the body and the right half of the brain with the left half of the body.
D4 Reticular Formation
Running up the brain stem from the medulla oblongata through the pons and the midbrain is a netlike formation of nuclei known as the reticular formation? The reticular formation controls respiration, cardiovascular function (see Heart), digestion, levels of alertness, and patterns of sleep. It also determines which parts of the constant flow of sensory information into the body the cerebrum receives.
E Brain Cells
There are two main types of brain cells: neurons and neuroglia. Neurons are responsible for the transmission and analysis of all electrochemical communication within the brain and other parts of the nervous system. Each neuron is composed of a cell body called a soma, a major fiber called an axon, and a system of branches called dendrites. Axons, also called nerve fibers, convey electrical signals away from the soma and can be up to 1 m (3.3 ft) in length. Most axons are covered with a protective sheath of myelin; a substance made of fats and protein, which insulates the axon. Myelinated axons conduct neuronal signals faster than do unmyelinated axons. Dendrites convey electrical signals toward the soma, are shorter than axons, and are usually multiple and branching.
Neuroglial cells are twice as numerous as neurons and account for half of the brain's weight. Neuroglia (from glia, Greek for “glue”) provide structural support to the neurons. Neuroglial cells also form myelin, guide developing neurons, take up chemicals involved in cell-to-cell communication, and contribute to the maintenance of the environment around neurons.
F Cranial Nerves
Twelve pairs of cranial nerves arise symmetrically from the base of the brain and are numbered, from front to back, in the order in which they arise. They connect mainly with structures of the head and neck, such as the eyes, ears, nose, mouth, tongue, and throat. Some are motor nerves, controlling muscle movement; some are sensory nerves, conveying information from the sense organs; and others contain fibers for both sensory and motor impulses. The first and second pairs of cranial nerves—the olfactory (smell) nerve and the optic (vision) nerve—carry sensory information from the nose and eyes, respectively, to the undersurface of the cerebral hemispheres. The other ten pairs of cranial nerves originate in or end in the brain stem.
III HOW THE BRAIN WORKS
Dendrites of one neuron receive signals from the axons of other neurons through chemicals known as neurotransmitters. The neurotransmitters set off electrical charges in the dendrites, which then carry the signal electrochemically to the soma. The soma integrates the information, which is then transmitted electrochemically down the axon to its tip.
At the tip of the axon, small, bubblelike structures called vesicles release neurotransmitters that carry the signal across the synapse, or gap, between two neurons. There are many types of neurotransmitters, including norepinephrine, dopamine, and serotonin. Neurotransmitters can be excitatory (that is, they excite an electrochemical response in the dendrite receptors) or inhibitory (they block the response of the dendrite receptors).
One neuron may communicate with thousands of other neurons, and many thousands of neurons are involved with even the simplest behavior. It is believed that these connections and their efficiency can be modified, or altered, by experience.
Scientists have used two primary approaches to studying how the brain works. One approach is to study brain function after parts of the brain have been damaged. Functions that disappear or that are no longer normal after injury to specific regions of the brain can often be associated with the damaged areas. The second approach is to study the response of the brain to direct stimulation or to stimulation of various sense organs.
Neurons are grouped by function into collections of cells called nuclei. These nuclei are connected to form sensory, motor, and other systems. Scientists can study the function of somatosensory (pain and touch), motor, olfactory, visual, auditory, language, and other systems by measuring the physiological (physical and chemical) changes that occur in the brain when these senses are activated. For example, electroencephalography (EEG) measures the electrical activity of specific groups of neurons through electrodes attached to the surface of the skull. Electrodes inserted directly into the brain can give readings of individual neurons. Changes in blood flow, glucose (sugar), or oxygen consumption in-groups of active cells can also be mapped.
Although the brain appears symmetrical, how it functions is not. Each hemisphere is specialized and dominates the other in certain functions. Research has shown that hemispheridominance is related to whether a person is predominantly right-handed or left-handed(see Handedness). In most right-handed people, the left hemisphere processes arithmetic, language, and speech. The right hemisphere interprets music, complex imagery, and spatial relationships and recognizes and expresses emotion. In left-handed people, the pattern of brain organization is more variable.
Hemispheric specialization has traditionally been studied in people who have sustained damage to the connections between the two hemispheres, as may occur with stroke, an interruption of blood flow to an area of the brain that causes the death of nerve cells in that area. The division of functions between the two hemispheres has also been studied in people who have had to have the connection between the two hemispheres surgically cut in order to control severe epilepsy, a neurological disease characterized by convulsions and loss of consciousness.
The visual system of humans is one of the most advanced sensory systems in the body (see Vision). More information is conveyed visually than by any other means. In addition to the structures of the eye itself, several cortical regions—collectively called primary visual and visual associative cortex—as well as the midbrain are involved in the visual system. Conscious processing of visual input occurs in the primary visual cortex, but reflexive—that is, immediate and unconscious—responses occur at the superior colliculus in the midbrain. Associative cortical regions—specialized regions that can associate, or integrate, multiple inputs—in the parietal and frontal lobes along with parts of the temporal lobe are also involved in the processing of visual information and the establishment of visual memories.
Language involves specialized cortical regions in a complex interaction that allows the brain to comprehend and communicate abstract ideas. The motor cortex initiates impulses that travel through the brain stem to produce audible sounds. Neighboring regions of motor cortex, called the supplemental motor cortex, are involved in sequencing and coordinating sounds. Broca's area of the frontal lobe is responsible for the sequencing of language elements for output. The comprehension of language is dependent upon Wernicke's area of the temporal lobe. Other cortical circuits connect these areas.
D The Autonomic Nervous System
The autonomic nervous system regulates the life support systems of the body reflexively—that is, without conscious direction. It automatically controls the muscles of the heart, digestive system, and lungs; certain glands; and homeostasis—that is, the equilibrium of the internal environment of the body (see Physiology). The autonomic nervous system itself is controlled by nerve centers in the spinal cord and brain stem and is fine-tuned by regions higher in the brain, such as the midbrain and cortex. Reactions such as blushing indicate that cognitive, or thinking, centers of the brain are also involved in autonomic responses.
IV BRAIN DISORDERS
Brain disorders have a wide range of causes, including head injury, stroke, bacterial diseases, complex chemical imbalances, and changes associated with aging.
A Head Injury
Head injury can initiate a cascade of damaging events. After a blow to the head, a person may be stunned or may become unconscious for a moment. This injury, called a concussion, usually leaves no permanent damage. If the blows are more severe and hemorrhage (excessive bleeding) and swelling occurs, however, severe headache, dizziness, paralysis, a convulsion, or temporary blindness may result, depending on the area of the brain affected. Damage to the cerebrum can also result in profound personality changes.
Damage to Broca's area in the frontal lobe causes difficulty in speaking and writing, a problem known as Broca's aphasia. Injury to Wernicke's area in the left temporal lobe results in an inability to comprehend spoken language called Wernicke's aphasia.
An injury or disturbance to a part of the hypothalamus may cause a variety of different symptoms, such as loss of appetite with an extreme drop in body weight; increase in appetite leading to obesity; extraordinary thirst with excessive urination (diabetes insipidus); failure in body-temperature control, resulting in either low temperature (hypothermia) or high temperature (fever); excessive emotionality; and uncontrolled anger or aggression. If the relationship between the hypothalamus and the pituitary gland is damaged (see Endocrine System), other vital bodily functions may be disturbed, such as sexual function, metabolism, and cardiovascular activity.
Injury to the brain stem is even more serious because it houses the nerve centers that control breathing and heart action. Damage to the medulla oblongata usually results in immediate death.
A stroke is damage to the brain due to an interruption in blood flow. The interruption may be caused by a blood clot (see Embolism; Thrombosis), constriction of a blood vessel, or rupture of a vessel accompanied by bleeding. A pouchlike expansion of the wall of a blood vessel, called an aneurysm (see Artery), may weaken and burst, for example, because of high blood pressure.
Sufficient quantities of glucose and oxygen, transported through the bloodstream, are needed to keep nerve cells alive. When the blood supply to a small part of the brain is interrupted, the cells in that area die and the function of the area are lost. A massive stroke can cause a one-sided paralysis (hemiplegia) and sensory loss on the side of the body opposite the hemisphere damaged by the stroke.
C Brain Diseases
Some brain diseases, such as multiple sclerosis and Parkinson disease are progressive, becoming worse over time. Multiple sclerosis damages the myelin sheath around axons in the brain and spinal cord. As a result, the affected axons cannot transmit nerve impulses properly. Parkinson disease destroys the cells of the substantia nigra in the midbrain, resulting in a deficiency in the neurotransmitter dopamine that affects motor functions.
Cerebral palsy is a broad term for brain damage sustained close to birth that permanently affects motor function. The damage may take place either in the developing fetus, during birth, or just after birth and is the result of the faulty development or breaking down of motor pathways. Cerebral palsy is nonprogressive—that is, it does not worsen with time.
A bacterial infection in the cerebrum (see Encephalitis) or in the coverings of the brain (see Meningitis), swelling of the brain (see Edema), or an abnormal growth of healthy brain tissue (see Tumor) can all cause an increase in intracranial pressure and result in serious damage to the brain.
Scientists are finding that certain brain chemical imbalances are associated with mentdisorders such as schizophrenia and depression. Such findings have changed scientiunderstanding of mental health and have resulted in new treatments that chemically correct these imbalances.
During childhood development, the brain is particularly susceptible to damage because of the rapid growth and reorganization of nerve connections. Problems that originate in the immature brain can appear as epilepsy or other brain-function problems in adulthood.
Several neurological problems are common in aging. Alzheimer's disease damages many areas of the brain, including the frontal, temporal, and parietal lobes. The brain tissue of people with Alzheimer's disease shows characteristic patterns of damaged neurons, known as plaques and tangles. Alzheimer's disease produces a progressive dementia (see Senile Dementia), characterized by symptoms such as failing attention and memory, loss of mathematical ability, irritability, and poor orientation in space and time.
V BRAIN IMAGING
Magnetic resonance imaging (MRI), introduced in the early 1980s, beams high-frequency radio waves into the brain in a highly magnetized field that causes the protons that form the nuclei of hydrogen atoms in the brain to reemit the radio waves. Computer to create thin cross-sectional images of the brain analyzes the reemitted radio waves. MRI provides the most detailed images of the brain and is safer than imaging methods that use X-rays. However, MRI is a lengthy process and also cannot be used with people who have pacemakers or metal implants, both of which are adversely affected by the magnetic field.
Computed tomography (CT), also known as CT scans, developed in the early 1970s. This imaging method X-rays the brain from many different angles, feeding the information into a computer that produces a series of cross-sectional images. CT is particularly useful for diagnosing blood clots and brain tumors. It is a much quicker process than magnetic resonance imaging and is therefore advantageous in certain situations—for example, with people who are extremely ill.
Changes in brain function due to brain disorders can be visualized in several ways. Magnetic resonance spectroscopy measures the concentration of specific chemical compounds in the brain that may change during specific behaviors. Functional magnetic resonance imaging (fMRI) maps changes in oxygen concentration that correspond to nerve cell activity.
Positron emission tomography (PET), developed in the mid-1970s, uses computed tomography to visualize radioactive tracers (see Isotopic Tracer), radioactive substances introduced into the brain intravenously or by inhalation. PET can measure such brain functions as cerebral metabolism, blood flow and volume, oxygen use, and the formation of neurotransmitters. Single photon emission computed tomography (SPECT), developed in the 1950s and 1960s, uses radioactive tracers to visualize the circulation and volume of blood in the brain.
Brain-imaging studies have provided new insights into sensory, motor, language, and memory processes, as well as brain disorders such as epilepsy; cerebrovascular disease; Alzheimer's, Parkinson, and Huntington's diseases (see Chorea); and various mental disorders, such as schizophrenia.
VI EVOLUTION OF THE BRAIN
The more highly evolved the animal, the more complex is the brain structure. Human beings have the most complex brains of all animals. Evolutionary forces have also resulted in a progressive increase in the size of the brain. In vertebrates lower than mammals, the brain is small. In meat-eating animals, particularly primates, the brain increases dramatically in size.
The cerebrum and cerebellum of higher mammals are highly convoluted in order to fit the most gray matter surface within the confines of the cranium. Such highly convoluted brains are called gyrencephalic. Many lower mammals have a smooth, or lissencephalic (“smooth head”), cortical surface.
There is also evidence of evolutionary adaption of the brain. For example, many birds depend on an advanced visual system to identify food at great distances while in flight. Consequently, their optic lobes and cerebellum are well developed, giving them keen sight and outstanding motor coordination in flight. Rodents, on the other hand, as nocturnal animals, do not have a well-developed visual system. Instead, they rely more heavily on other sensory systems, such as a highly developed sense of smell and facial whiskers.
VII RECENT RESEARCH