|
|
Active vocabulary
processing ['prqVsesIN] обробка даних
coding кодування, шифрування
complex комплексний, змішаний
hindbrain задній мозок
midbrain середній мозок
forebrain передній мозок
cerebellum мозочок
medulla oblongata довгастий мозок
vertebrates ['vE:tIbr(e)It] хребетні
relative size відносний розмір
as one ascends the phyloge-netic scale спускаючись по філогенетичним сходам
carnivore ['kQ:nIvO: ]м'ясоїдна тварина
primates примати
rind кора
relegated зд. передано, віддане
whereas незважаючи на те, що
ablation [q'bleIS(q)n] ампутація, видалення
stimulation стимуляція
recording запис (сигналів)
conjunction [kqn'dZANkf(q)n] з'єднання, сполучення, зчіпка
removing видалення
auditory слуховой
visual зоровий
chemicals речовини
elicit витягати
markedly different помітно відрізняється
stages of sleep стадії сну
Text 1.
Brain
The brain is the organ of the central nervous system responsible for the processing and coding of sensory and motor information, for the control of regulatory processes in the body, and for the mediation of complex processes, such as motivation, emotion, learning, and memory.
For anatomical convenience, the brain may be subdivided into three major divisions: the hindbrain, midbrain, and forebrain. The hindbrain contains the cerebellum, the pons, and the medulla oblongata. The midbrain stands as a separate portion of the brain responsible for the integration of eye movements and vestibular functions. The forebrain consists of the telencephalon, which is further subdivided into the neocortex, the basal ganglia, and the limbic system. The forebrain also contains the diencephalon, which is further subdivided into the thalamus and hypothalamus.
Various hindbrain structures are quite similar in all vertebrates. Considerable changes occur in the relative size and development of forebrain structures as one ascends the phylogenetic scale. In higher animals, such as carnivores and primates, there is tremendous development of the neocortex relative to other brain systems.
One of the most important aspects of brain development is that as one ascends the phylogenetic scale one rinds that less of the cortex is concerned with purely sensory and motor functions and more is concerned with complex integrative functions that seem to be the basis for complex learning. In the rat, most of the cortex is relegated to sensory and motor aspects of behavior whereas in the human, most of the cortex is relegated to associative and integrative functions, with only a small percentage involved directly in sensory and motor processes.
Research Techniques. The brain is studied primarily through three basic methods: ablation, stimulation, and recording. All three methods are used in conjunction with one another by brain researchers in order to obtain a picture of how various systems in the brain function dynamically in the mediation of specific behaviors.
Ablation, an experimental technique used for animals, involves selectively removing portions of the brain and trying to determine what the sum total of remaining parts can or cannot do. In this way it is possible, for example, to map out regions of the cortex responsible for auditory or visual functions or to determine which portions of the thalamus are involved in sensory or motor functions. The stimulation technique essentially involves placing small electrodes in specific portions of the brain and electrically activating them to observe the effect on some behavior or performed task. An alternative method of stimulation is through the implantation of small tubes that can carry chemicals to specific regions of the brain. Stimulation in certain portions of the hypothalamus with cholingeric drugs will elicit drinking, whereas stimulation of the identical regions with an adrenergic substance (norepinephrine) will elicit eating. As is the case in ablation, stimulation also produces an alteration in the normal functioning of the brain.
A third method, recording, is passive and produces the least change in ongoing cerebral activity. Recording involves placing recording electrodes in specific brain structures and observing the electrical activity of these regions as a function of different activities. It has been found, for example, that the electrical activity of the brain is markedly different during the various stages of sleep and certain aspects of learning and that it is related to attentional state.
Recent techniques for recording brain activity include computer axial tomography (CAT) and positron emission tomography (PET). These, coupled with EEC, are beginning to allow scientists to study the brain as it functions in normal activities.
Answer the questions
1. What do we call the brain?
2. What major parts can the brain be subdivided into?
3. What do each of these parts contain?
4. What is one of the most important aspects of brain development?
5. What are three basic methods to study the brain?
6. How are these methods and techniques applied?
7. Why brain researches use them?
Text 2.
During embryonic development, the brain first forms as a tube, the anterior end of which enlarges into three hollow swellings that form the brain, and the posterior of which develops into the spinal cord. Some parts of the brain have changed little during vertebrate evolutionary history.
Vertebrate evolutionary trends include
1. Increase in brain size relative to body size.
2. Subdivision and increasing specialization of the forebrain, midbrain, and hindbrain.
3. Growth in relative size of the forebrain, especially the cerebrum, which is associated with increasingly complex behavior in mammals.
The Brain Stem and Midbrain
The brain stem is the smallest and from an evolutionary viewpoint, the oldest and most primitive part of the brain. The brain stem is continuous with the spinal cord, and is composed of the parts of the hindbrain and midbrain. The medulla oblongata and pons control heart rate, constriction of blood vessels, digestion and respiration.
The midbrain consists of connections between the hindbrain and forebrain. Mammals use this part of the brain only for eye reflexes.
The Cerebellum
The cerebellum is the third part of the hindbrain, but it is not considered part of the brain stem. Functions of the cerebellum include fine motor coordination and body movement, posture, and balance. This region of the brain is enlarged in birds and controls muscle action needed for flight.
The Forebrain
The forebrain consists of the diencephalon and cerebrum. The thalamus and hypothalamus are the parts of the diencephalon. The thalamus acts as a switching center for nerve messages. The hypothalamus is a major homeostatic center having both nervous and endocrine functions.
The cerebrum, the largest part of the human brain, is divided into left and right hemispheres connected to each other by the corpus callosum. The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex, the most recently evolved region of the vertebrate brain. Fish have no cerebral cortex, amphibians and reptiles have only rudiments of this area.
The cortex in each hemisphere of the cerebrum is between 1 and 4 mm thick. Folds divide the cortex into four lobes: occipital, temporal, parietal, and frontal. No region of the brain functions alone, although major functions of various parts of the lobes have been determined.
The occipital lobe (back of the head) receives and processes visual information. The temporal lobe receives auditory signals, processing language and the meaning of words. The parietal lobe is associated with the sensory cortex and processes information about touch, taste, pressure, pain, and heat and cold. The frontal lobe conducts three functions:
1. motor activity and integration of muscle activity
2. speech
3. thought processes
Most people who have been studied have their language and speech areas on the left hemisphere of their brain. Language comprehension is found in Wernicke's area. Speaking ability is in Broca's area. Damage to Broca's area causes speech impairment but not impairment of language comprehension. Lesions in Wernicke's area impairs ability to comprehend written and spoken words but not speech. The remaining parts of the cortex are associated with higher thought processes, planning, memory, personality and other human activities.
The Spinal Cord
The spinal cord runs along the dorsal side of the body and links the brain to the rest of the body. Vertebrates have their spinal cords encased in a series of (usually) bony vertebrae that comprise the vertebral column.
The gray matter of the spinal cord consists mostly of cell bodies and dendrites. The surrounding white matter is made up of bundles of interneuronal axons (tracts). Some tracts are ascending (carrying messages to the brain), others are descending (carrying messages from the brain). The spinal cord is also involved in reflexes that do not immediately involve the brain.
The Brain and Drugs
Some neurotransmitters are excitory, such as acetylcholine, norepinephrine, serotonin, and dopamine. Some are associated with relaxation, such as dopamine and serotonin. Dopamine release seems related to sensations of pleasure. Endorphins are natural opioids that produce elation and reduction of pain, as do artificial chemicals such as opium and heroin. Neurological diseases, for example Parkinson's disease and Huntington's disease, are due to imbalances of neurotransmitters. Parkinson's is due to a dopamine deficiency. Huntington's disease is thought to be cause by malfunctioning of an inhibitory neurotransmitter. Alzheimer's disease is associated with protein plaques in the brain.
Drugs are stimulants or depressants that block or enhance certain neurotransmitters. Dopamine is thought involved with all forms of pleasure. Cocaine interferes with uptake of dopamine from the synaptic cleft. Alcohol causes a euphoric "high" followed by a depression.
Marijuana, material from the Indian hemp plant (Cannabis sativa), has a potent chemical THC (tetrahydracannibinol) that in low, concentrations causes a euphoric high (if inhaled, the most common form of action is smoke inhalation). High dosages may cause severe effects such as hallucinations, anxiety, depression, and psychotic symptoms.
Cocaine is derives from the plant Erthoxylon coca. Inhaled, smoked or injected. Cocaine users report a "rush" of euphoria following use. Following the rush is a short (5-30 minute) period of arousal followed by a depression. Repeated cycle of use terminate in a "crash" when the cocaine is gone. Prolonged used causes production of less dopamine, causing the user to need more of the drug.
Heroin is a derivative of morphine, which in turn is obtained from opium, the milky secretions obtained from the opium poppy, Papaver somniferum. Heroin is usually injected intravenously, although snorting and smoking serve as alternative delivery methods. Heroin binds to ophioid receptors in the brain, where the natural chemical endorphins are involved in the cessation pain. Heroin is physically addictive, and prolonged use causes less endorphin production. Once this happens, the euphoria is no longer felt, only dependence and delay of withdrawal symptoms.
Senses
Input to the nervous system is in the form of our five senses: pain, vision, taste, smell, and hearing. Vision, taste, smell, and hearing input are the special senses. Pain, temperature, and pressure are known as somatic senses. Sensory input begins with sensors that react to stimuli in the form of energy that is transmitted into an action potential and sent to the CNS.
Sensory Receptors
• Sensory receptors are classified according to the type of energy they can detect and respond to.
• Mechanoreceptors: hearing and balance, stretching.
• Photoreceptors: light.
• Chemoreceptors: smell and taste mainly, as well as internal sensors in the digestive and circulatory systems.
• Thermoreceptors: changes in temperature.
• Electroreceptors: detect electrical currents in the surrounding environment.
Mechanoreceptors vary greatly in the specific type of stimulus and duration of stimulus/action potentials. The most adaptable vertebrate mechanoreceptor is the hair cell. Hair cells are present in the lateral line of fish. In humans and mammals hair cells are involved with detection of sound and gravity and providing balance.
Hearing
Hearing involves the actions of the external ear, eardrum, ossicles, and cochlea. In hearing, sound waves in air are converted into vibrations of a liquid then into movement of hair cells in the cochlea. Finally they are converted into action potentials in a sensory dendrite connected to the auditory nerve. Very loud sounds can cause violent vibrations in the membrane under hair cells, causing a shearing or permanent distortion to the cells, resulting in permanent hearing loss.
Orientation and Gravity
Orientation and gravity are detected at the semicircular canals. Hair cells along three planes respond to shifts of liquid within the cochlea, providing a three-dimensional sense of equilibrium. Calcium carbonate crystals can shift in response to gravity, providing sensory information about gravity and acceleration.
Photoreceptors Detect Vision and Light Sensitivity
The human eye can detect light in the 400-700 nanometer (nm) range, a small portion of the electromagnetic spectrum, the visible light spectrum. Light with wavelengths shorter than 400 nm is termed ultraviolet (UV) light. Light with wavelengths longer than 700 nm is termed infrared (IR) light.
Eye
In the eye, two types of photoreceptor cells are clustered on the retina, or back portion of the eye. These receptors, rods and cones, apparently evolved from hair cells. Rods detect differences in light intensity; cones detect color. Rods are more common in a circular zone near the edge of the eye. Cones occur in the center (or fovea centralis) of the retina.
Light reaching a photoreceptor causes the breakdown of the chemical rhodopsin, which in turn causes a membrane potential that is transmitted to an action potential. The action potential transfers to synapsed neurons that connect to the optic nerve. The optic nerve connects to the occipital lobe of the brain.
Humans have three types of cones, each sensitive to a different color of light: red, blue and green. Opsins are chemicals that bind to cone cells and make those cells sensitive to light of a particular wavelength (or color). Humans have three different form of opsins coded for by three genes on the X chromosome. Defects in one or more of these opsin genes can cause color blindness, usually in males.
Answer the questions
1. What does hearing involve?
2. Where are orientation and gravity detected?
3. What detects vision and light sensitivity?
4. How does the eye function?
|