BRAIN FACTS BOOK
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This sheath target cell, such as the generation of an action potential, the is made by specialized cells called glia. In the brain, the glia contraction of a muscle, the stimulation of enzyme activity, that make the sheath are called oligodendrocytes, and in the or the inhibition of neurotransmitter release. An increased understanding of neurotransmitters in The brain contains at least ten times more glia than the brain and knowledge of the effects of drugs on these neurons.
Glia perform many jobs. Researchers have known chemicals — gained largely through animal research — for a while that glia transport nutrients to neurons, clean comprise one of the largest research efforts in neuroscience.
Current research is uncovering important become more knowledgeable about the circuits responsible new roles for glia in brain function. There are many different kinds of neurotransmitters, and they all play an essential role in the human body.
The next section provides a summary of key neurotransmitters and neuromodulators, chemicals that help shape overall activity in the brain. Neurotransmitters and Neuromodulators Acetylcholine The first neurotransmitter to be identified — about 80 years ago — was acetylcholine ACh. This chemical is released by neurons connected to voluntary muscles, causing them to contract, and by neurons that control the heartbeat. ACh is also a transmitter in many regions of the brain.
ACh is synthesized in axon terminals.
When an action potential arrives at the nerve terminal, electrically charged calcium ions rush in, and ACh is released into the synapse, where it attaches to ACh receptors on the target cells.
On voluntary muscles, this action opens sodium channels and causes muscles to contract. ACh is then broken down by the enzyme Neurons are cells within the nervous system that transmit information to other nerve cells, muscle, or gland cells.
Most neurons have a cell body, an axon, and dendrites. The cell body contains acetylcholinesterase and resynthesized in the the nucleus and cytoplasm. The axon extends from the cell body and often gives rise to many nerve terminal. Antibodies that block one smaller branches before ending at nerve terminals.
Dendrites extend from the neuron cell body type of ACh receptor cause myasthenia gravis, and receive messages from other neurons. Synapses are the contact points where one neuron a disease characterized by fatigue and muscle communicates with another. Much less is known about ACh in the blocks of proteins. Certain amino acids can also serve as brain. Recent discoveries suggest that it may be neurotransmitters in the brain. The neurotransmitters critical for normal attention, memory, and sleep.
The activity of GABA is increased by ways to restore this neurotransmitter is a goal of current benzodiazepines e.
Glutamate and aspartate act as throughout the body and the brain, serve as the building excitatory signals, activating, among others, N-methyl-dSociety for NeuroScieNce introduction to the brain BraiN factS 9 The stimulation of NMDA receptors may promote beneficial changes in the brain, whereas overstimulation can cause nerve cell damage or cell death. This is what happens as a result of trauma and during a stroke.
Developing drugs that block or stimulate activity at NMDA receptors holds promise for improving brain function and treating neurological and psychiatric disorders. Catecholamines The term catecholamines includes the neurotransmitters dopamine and norepinephrine.
Dopamine and norepinephrine are widely present in the brain and peripheral nervous system. Dopamine is present in three principal circuits in the brain. The dopamine circuit that regulates movement has been directly linked to disease. Another dopamine circuit is thought to be important for cognition and emotion; abnormalities in this system have been implicated in schizophrenia. Because drugs that block certain dopamine receptors in the brain are helpful in diminishing psychotic symptoms, learning more about dopamine is important to understanding mental illness.
In a third circuit, dopamine regulates the endocrine system. Dopamine directs the hypothalamus to manufacture hormones and hold them in the pituitary gland for release into the bloodstream or to trigger the release of hormones held within cells in the pituitary.
These conditions all lead to memory loss and a decline in cognitive functioning. Thus, researchers believe that norepinephrine may play a role in both learning and memory.
Norepinephrine is also secreted by the sympathetic nervous system throughout the body to regulate heart rate and blood pressure.
Acute stress increases release of norepinephrine from sympathetic nerves and the adrenal medulla, the innermost part of the adrenal gland. Serotonin This neurotransmitter is present in the brain and other tissues, particularly blood platelets and the 10 BraiN factS introduction to the brain lining of the digestive tract.
In the brain, serotonin has been identified as an important factor in sleep quality, mood, depression, and anxiety. Because serotonin controls different switches affecting various emotional states, scientists believe these switches can be manipulated by analogs, chemicals with molecular structures similar to that of serotonin.
Peptides Short chains of amino acids that are linked together, peptides are synthesized in the cell body and greatly outnumber the classical transmitters discussed earlier. In , scientists discovered receptors for opiates on neurons in several regions of the brain, suggesting that the brain must make substances very similar to opium.
Shortly thereafter, scientists made their first discovery of an opiate peptide produced by the brain. This chemical resembles morphine, an opium derivative used medically to kill pain. A simple hypothesis is that they are released by brain neurons in times of stress to minimize pain and enhance adaptive behavior.
Some sensory nerves — tiny unmyelinated C fibers — contain a peptide called substance P, which causes the sensation of burning pain. The active component of chili peppers, capsaicin, causes the release of substance P, something people should be aware of before eating them.
Trophic Factors Researchers have discovered several small proteins in the brain that act as trophic factors, substances that are necessary for the development, function, and survival of specific groups of neurons. These small proteins are made in brain cells, released locally in the brain, and bind to receptors expressed by specific neurons.
Researchers also have identified genes that code for receptors and are involved in the signaling mechanisms of trophic factors. These findings are expected to result in a greater understanding of how trophic factors work in the brain. While the nervous system uses neurotransmitters as Society for NeuroScieNce The pancreas, kidneys, heart, adrenal glands, gonads, thyroid, parathyroid, thymus, and even fat are all sources of hormones.
The endocrine system works in large part by acting on neurons in the brain, which controls the pituitary gland. The pituitary gland secretes factors into the blood that act on the endocrine glands to either increase or decrease hormone production. This is referred to as a feedback loop, and it involves communication from the brain to the pituitary to an endocrine gland and back to the brain.
This system is very important for the activation and control of basic behavioral activities, such as sex; emotion; responses to stress; and eating, drinking, and the regulation of body functions, including growth, reproduction, energy use, and metabolism. The way the brain responds to hormones indicates that the brain is very malleable and capable of responding to environmental signals.
The brain contains receptors for thyroid hormones those produced by the thyroid and the six classes of steroid hormones, which are synthesized from cholesterol — androgens, estrogens, progestins, glucocorticoids, mineralocorticoids, and vitamin D. The receptors are found in selected populations of neurons in the brain and relevant organs in the body.
Thyroid and steroid hormones bind to receptor proteins that in turn bind to DNA and regulate the action of genes. This can result in long-lasting changes in cellular structure and function. The brain has receptors for many hormones; for example, the metabolic hormones insulin, insulin-like growth factor, ghrelin, and leptin. These hormones are taken up from the blood and act to affect neuronal activity and certain aspects of neuronal structure.
In response to stress and changes in our biological clocks, such as day and night cycles and jet lag, hormones enter the blood and travel to the brain and other organs. In the brain, hormones alter the production of gene products that participate in synaptic neurotransmission as well as affect the structure of brain cells. As a result, the circuitry of the brain and its capacity for neurotransmission are changed over a course of hours to days. In this way, the brain adjusts its performance and control of behavior in response to a changing environment.
Severe and prolonged stress can impair the ability of the brain to function Society for NeuroScieNce normally for a period of time, but the brain is also capable of remarkable recovery. Reproduction in females is a good example of a regular, cyclic process driven by circulating hormones and involving a feedback loop: The neurons in the hypothalamus produce gonadotropin-releasing hormone GnRH , a peptide that acts on cells in the pituitary.
In both males and females, this causes two hormones — the follicle-stimulating hormone FSH and the luteinizing hormone LH — to be released into the bloodstream. In females, these hormones act on the ovary to stimulate ovulation and promote release of the ovarian hormones estradiol and progesterone.
In males, these hormones are carried to receptors on cells in the testes, where they promote spermatogenesis and release the male hormone testosterone, an androgen, into the bloodstream. Testosterone, estrogen, and progesterone are often referred to as sex hormones.
In turn, the increased levels of testosterone in males and estrogen in females act on the hypothalamus and pituitary to decrease the release of FSH and LH. The increased levels of sex hormones also induce changes in cell structure and chemistry, leading to an increased capacity to engage in sexual behavior. Sex hormones also exert widespread effects on many other functions of the brain, such as attention, motor control, pain, mood, and memory.
Sexual differentiation of the brain is caused by sex hormones acting in fetal and early postnatal life, although recent evidence suggests genes on either the X or Y chromosome may also contribute to this process.
Scientists have found statistically and biologically significant differences between the brains of men and women that are similar to sex differences found in experimental animals.
These include differences in the size and shape of brain structures in the hypothalamus and the arrangement of neurons in the cortex and hippocampus. Sex differences go well beyond sexual behavior and reproduction and affect many brain regions and functions, ranging from mechanisms for perceiving pain and dealing with stress to strategies for solving cognitive problems. That said, however, the brains of men and women are more similar than they are different.
Anatomical differences have also been reported between the brains of heterosexual and homosexual men. Research suggests that hormones and genes act early in life to shape the brain in terms of sex-related differences in structure and function, but scientists are still putting together all the pieces of this puzzle. Gases and Other unusual Neurotransmitters Scientists have identified a new class of neurotransmitters that are gases. These molecules — nitric oxide and carbon monoxide — do not act like other neurotransmitters.
Being gases, they are not stored in any structure, certainly not in storage structures for classical and peptide transmitters. Instead, they are made by enzymes as they are needed and released from neurons by diffusion. Rather than acting at receptor sites, these gases simply diffuse into adjacent neurons and act upon chemical targets, which may be enzymes. Although exact functions for carbon monoxide have not been determined, nitric oxide has already been shown to play several important roles.
For example, nitric oxide neurotransmission governs erection in the penis. In nerves of the intestine, it governs the relaxation that contributes to the normal movements of digestion. In the brain, nitric oxide is the major regulator of the intracellular messenger molecule cyclic GMP.
In conditions of excess glutamate release, as occurs in stroke, neuronal damage following the stroke may be attributable in part to nitric oxide. Lipid Messengers In addition to gases, which act rapidly, the brain also derives signals from lipids. Prostaglandins are a class of compounds made from lipids by an enzyme called cyclooxygenase.
These very small and short-lived molecules have powerful effects, including the induction of a fever and the generation of pain in response to inflammation. Aspirin reduces a fever and lowers pain by inhibiting the cyclooxygenase enzyme. These messengers control the release of neurotransmitters, usually by inhibiting them, and can also affect the immune system and other cellular parameters still being discovered.
Endocannabinoids play an important role in the control of behaviors. They increase in the brain under stressful conditions. Second Messengers After the action of neurotransmitters at their receptors, biochemical communication within cells is still possible.
Substances that trigger such communication are called second messengers. Second messenger effects may endure for a few milliseconds to as long as many minutes. They also may be responsible for long-term changes in the nervous system. An example of the initial step in the activation of a second messenger system involves adenosine triphosphate ATP , the chemical source of energy in cells.
ATP is present throughout the cytoplasm of all cells. For example, when norepinephrine binds to its receptors on the surface of the neuron, the activated receptor binds a G protein on the inside of the membrane. The activated G protein causes the enzyme adenylyl cyclase to convert ATP to cyclic adenosine monophosphate cAMP , the second messenger.
Rather than acting as a messenger between one neuron and another, cAMP exerts a variety of influences within the cell, ranging from changes in the function of ion channels in the membrane to changes in the expression of genes in the nucleus. Second messengers also are thought to play a role in the manufacture and release of neurotransmitters and in intracellular movements and carbohydrate metabolism in the cerebrum — the largest part of the brain, consisting of two hemispheres.
Second messengers also are involved in growth and development processes. In addition, the direct effects of second messengers on the genetic material of cells may lead to long-term alterations in cellular functioning and, ultimately, to changes in behavior.
The intricate communication systems in the brain and the nervous system begin to develop about three weeks after gestation. How this process unfolds and how it is relevant to an understanding of brain-based conditions and illnesses are discussed in Chapter 2. Society for NeuroScieNce Understanding the processes underlying how brain cells are formed, become specialized, travel to their appropriate location, and connect to each other in increasingly elaborate adaptive networks is the central challenge of developmental neurobiology.
Advances in the study of brain development have become increasingly relevant for medical treatments. For example, several diseases that most scientists once thought were purely disorders of adult function, such as schizophrenia, are now being considered in developmental terms; that is, such disorders may occur because pathways and connections to the brain did not form correctly early in life.
Other research suggests that genes important for brain development may also play a role in susceptibility to autism spectrum disorders. And by applying knowledge about how connections form during development, regeneration following injury to the brain is now viewed as a future possibility.
Knowing how the brain is constructed is essential for understanding its ability to reorganize in response to external influences or injury. As the brain evolves from the embryo to the adult stage, unique attributes evolve during infancy and childhood that contribute to differences in learning ability as well as vulnerability to specific brain disorders.
Neuroscientists are beginning to discover some general Society for NeuroScieNce principles that underlie developmental processes, many of which overlap in time.
The Journey of Nerve Cells The development of neurons occurs through a delicate process. Even more astonishing is that this process takes place as the embryo is developing. Induction and proliferation are followed by migration, during which the newly formed neurons travel to their final destination.
Throughout life, the nervous system is active, making new connections and fine-tuning the way messages are sent and received. The activities of the everchanging nervous system are explained in more detail in the following sections.
Induction During the early stages of embryonic development, three layers emerge — the endoderm, the ectoderm, and the mesoderm.
These layers undergo many interactions to grow into organ, bone, muscle, skin, or nerve tissue. How does this process of differentiation occur, especially since each cell contains 25, genes, the entire sequence of DNA instructions for development?
The answer lies in signaling molecules released by the mesoderm. These molecules turn on certain genes and turn off others, triggering some ectoderm cells to become nerve tissue in a process called neural induction.
Subsequent signaling interactions further refine the nerve tissue into the basic categories of neurons or glia support cells , then into subclasses of each cell type. The remaining cells of the ectoderm, which have not received the signaling molecules diffusing from the mesoderm, become skin.
The proximity of cells to the signaling molecules largely determines their fate. For example, a particular signaling molecule, called sonic hedgehog, is secreted from mesodermal tissue lying beneath the developing spinal cord. As a result, the adjacent nerve cells are converted into a specialized class of glia.
Cells that are farther away, however, are exposed to lower concentrations of sonic hedgehog, so they become the motor neurons that control muscles. An even lower concentration promotes the formation of interneurons, which relay messages to other neurons, not muscles. Interestingly, the mechanism of introduction to the brain BraiN factS 13 By four weeks, major regions of the human brain can be recognized in primitive form, including the forebrain, midbrain, hindbrain, and optic vesicle, from which the eye develops.
Ridges, or convolutions, can be seen by six months. Migration Once neural induction has occurred, the next step for new neurons is a journey to the proper position in the brain. This process is called migration, and it begins three to four weeks after a human baby is conceived.
At this time, the ectoderm starts to thicken and build up along the middle. As the cells continue to divide, a flat neural plate grows, followed by the formation of parallel ridges, similar to the creases in a paper airplane, that rise across its surface. Within a few days, the ridges fold in toward each other and fuse to form a hollow neural tube. The top of the tube thickens into three bulges that form the hindbrain, the midbrain, and the forebrain.
After neurons stop dividing, they form an intermediate zone, where they gradually accumulate as the brain develops. The neurons then migrate to their final destination— with the help of a variety of guidance mechanisms. The most common guidance mechanism, accounting for about 90 percent of migration in humans, are glia, which project radially from the intermediate zone to the cortex.
In this way, glia provide a temporary scaffolding for ushering neurons to their destination. Through another mechanism, inhibitory interneurons, small neurons with short pathways usually found in the central nervous system, migrate tangentially across the brain.
Migration is a delicate process and can be affected by different factors. External forces, such as alcohol, cocaine, or radiation, can prevent proper migration, resulting in misplacement of cells, which may lead to mental retardation or epilepsy. Furthermore, mutations in genes that regulate migration have been shown to cause some rare genetic forms of retardation and epilepsy in humans.
Brain Facts: A Primer On The Brain And Nervous System
Making Connections Once the neurons reach their final location, they must make the proper connections so that a particular function, such as vision or hearing, can emerge. Unlike induction, proliferation, and migration, which occur internally during fetal development, the next phases of brain development are increasingly dependent on interactions with the environment.
After birth and beyond, such activities as listening to a voice, responding to a toy, and even the reaction evoked by the temperature in the room lead to more connections among neurons. Neurons become interconnected through 1 the growth of dendrites — extensions of the cell body that receive signals from other neurons and 2 the growth of axons — extensions from the neuron that can carry signals to other neurons.
Axons enable connections between neurons at considerable distances, sometimes at the opposite side of the brain, to develop. In the case of motor neurons, the axon may travel from the spinal cord all the way down to a foot muscle.
Receptors for netrins were then found in worms, a discovery that proved to be invaluable in finding the corresponding, and related, human receptors. Once axons reach their targets, they form connections with other cells at synapses. At the synapse, the electrical signal of the sending axon is transmitted by chemical neurotransmitters to the receiving dendrites of another neuron, where they can either provoke or prevent the generation of a new signal. The regulation of this transmission at synapses and the integration of inputs from the thousands of synapses each neuron receives are responsible for the astounding information-processing capacity of the brain.
For processing to occur properly, the connections must be highly specific. Some specificity arises from the mechanisms that guide each axon to its proper target area. Additional molecules mediate target recognition when the axon chooses the proper neuron. They often also mediate the proper part of the target once the axon arrives at its destination. Over the past few years, several of these recognition molecules have been identified.
Researchers have successfully identified ways in which the synapse differentiates once contact has been made. The tiny portion of the axon that contacts the dendrite becomes specialized for the release of neurotransmitters, and the tiny portion of the dendrite that receives the contact becomes specialized to receive and respond to the signal.
Special molecules pass between the sending and receiving cells to ensure that the contact is formed properly and that the sending and receiving specializations are matched precisely.
These processes ensure that the synapse can transmit signals quickly and effectively. Finally, still other molecules coordinate the maturation of the synapse after it has This is a cross-sectional view of the occipital lobe, which processes vision, of a three-month-old monkey formed so that it can accommodate the fetus brain.
The center shows immature neurons migrating along glial fibers. These neurons make changes that occur as our bodies mature transient connections with other neurons before reaching their destination. A single migrating neuron, and our behavior changes. Defects in some shown about 2, times its actual size right , uses a glial fiber as a guiding scaffold. Researchers have discovered many special molecules that help guide growth cones. Some molecules lie on the cells that growth cones contact, whereas others are released from sources found near the growth cone.
The growth cones, in turn, bear molecules that serve as receptors for the environmental cues. The binding of particular signals with receptors tells the growth cone whether to move forward, stop, recoil, or change direction. These signaling molecules include proteins with names such as netrin, semaphorin, and ephrin. In most cases, these are families of related molecules; for example, researchers have identified at least fifteen semaphorins and at least nine ephrins.
Perhaps the most remarkable finding is that most of these proteins are common to many organisms—worms, insects, and mammals, including humans. Each protein family is smaller in flies or worms than in mice or people, but its functions are quite similar. As a result, it has been possible to use the simpler animals as experimental models to gain knowledge that can be applied directly to humans.
The loss of other molecules may underlie the degradation of synapses that occurs during aging. A combination of signals also determines the type of neurotransmitters that a neuron will use to communicate with other cells. For some cells, such as motor neurons, the type of neurotransmitter is fixed, but for other neurons, it is not.
Scientists found that when certain immature neurons are maintained in a dish with no other cell types, they produce the neurotransmitter norepinephrine. In contrast, if the same neurons are maintained with specific cells, such as cardiac, or heart, tissue, they produce the neurotransmitter acetylcholine.
Just as genes turn on and off signals to regulate the development of specialized cells, a similar process leads to the production of specific neurotransmitters. Many researchers believe that the signal to engage the gene, and therefore the final determination of the chemical messengers that a neuron produces, is influenced by factors coming from the location of the synapse itself. Neurons communicate with electrical and chemical signals at special contact points called synapses. Meagan A.
Jenkins, et al. The myelin sheath covering axons serves a similar purpose. Myelination, the wrapping of axons by extensions of glia, increases the speed at which signals may be sent from one neuron to another by a factor of up to x. This advantage is due to how the sheath is wrapped. In between the myelin are gaps, called nodes of Ranvier, that are not covered in myelin. The electrical signal moves faster over the insulated portion, jumping from one node to another.
The process of myelination occurs throughout the lifespan. Paring Back After growth, the neural network is pared back to create a more efficient system. Only about half the neurons generated during development survive to function in the adult. Entire populations of neurons are removed through apoptosis, programmed cell death initiated in the cells.
Apoptosis is activated if a neuron loses its battle with other neurons to receive life-sustaining chemical signals called trophic factors. These factors are produced in limited quantities by target tissues. Each type of trophic factor supports the survival of a distinct group of neurons. For example, nerve growth factor is important for sensory neuron survival.
Recently, it has become clear that apoptosis is maintained into adulthood and constantly held in check. This discovery — and its implication that death need not follow insult — have led to new avenues for therapy. Brain cells also form excess connections at first. For example, in primates, the projections from the two eyes to the brain initially overlap and then sort out to separate territories devoted to one eye or the other. Furthermore, in the young primate cerebral cortex, the connections between neurons are greater in number and twice as dense as those in an adult primate.
Communication between neurons with chemical and electrical signals is necessary to weed out the connections. The connections that are active and generating electrical currents survive, whereas those with little or no activity are lost.
Thus, the circuits of the adult brain are formed, at least in part, by sculpting away incorrect connections to leave only the correct ones. Critical Periods Genes and the environment converge powerfully during early sensitive windows of brain development to form the neural circuits underlying behavior.
Although most neuronal cell death occurs in the embryo, the paring down of connections occurs in large part during critical periods in early postnatal life.
During these moments in time, the developing nervous system must obtain certain critical experiences, such as sensory, movement, or emotional input, to mature properly.
Such periods are characterized by high learning rates as well as enduring consequences for neuronal connectivity. After a critical period, connections diminish in number and are less subject to change, but the ones that remain are stronger, more reliable, and more precise. It is important to note that there are multiple critical periods, organized sequentially, as individual brain functions are established.
The last step in the creation of an adult human brain, the frontal lobes, whose function includes judgment, insight, and impulse control, continues into the early 20s. Thus, even the brain of an adolescent is not completely mature. Injury or deprivation of environmental input occurring at specific stages of postnatal life can dramatically reshape the underlying circuit development, which becomes increasingly more difficult to correct later in life.
In one experiment, a monkey raised from birth to 6 months of age with one eyelid closed permanently lost useful vision in that eye because of diminished use.
Similarly, cochlear implants introduced in infancy are most effective in restoring hearing to the congenitally deaf. Cognitive recovery from social deprivation, brain damage, or stroke is also greatest early in life. Conversely, research suggests that enriched environments or stimulation may bolster brain development, as revealed by animals raised in toy-filled surroundings. They have more branches on their neurons and more connections than isolated animals.
Many people have observed that children can learn languages or develop musical ability absolute pitch with Society for NeuroScieNce greater proficiency than adults. Heightened activity in the critical period may, however, also contribute to an increased incidence of certain disorders in childhood, such as epilepsy.
Fortunately, as brain activity subsides, many types of epilepsy fade away by adulthood. Plasticity The ability of the brain to modify itself and adapt to challenges of the environment is referred to as plasticity.
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Plasticity itself is not unique to humans, but the degree to which our brains are able to adapt is the defining attribute of our species.
Plasticity can be categorized as experienceexpectant or experience-dependent. Experience-expectant plasticity refers to the integration of environmental stimuli into the normal patterns of development.
Certain environmental exposures during limited critical, or sensitive, periods of development are essential for healthy maturation. For example, finches need to hear adult songs before sexual maturation in order for them to learn to sing at a species-appropriate level of intricacy. Scientists hope that new insight into brain development will lead to treatments for those with learning disabilities, brain damage, and neurodegenerative disorders, as well as help us understand aging.
If we can figure out a way to lift the brakes that restrict adult plasticity — either pharmacologically or by circuit rewiring — it may be possible to correct damage done through mistimed critical periods or other means.
By understanding normal functions of the brain during each developmental stage, researchers hope to develop better age-specific therapies for brain disorders. This chapter discussed how cells differentiate so that they can perform specific functions, such as seeing and hearing. Those are just two of the senses we rely on to learn about the world. The senses of taste, smell, and touch also provide key information. Through intricate systems and networks, the brain and the nervous system work together to process these sensory inputs.
Part 2, called Sensing, Thinking, and Behaving, describes how these systems work and complement each other. It begins with a look at senses and perception. Vision is one of our most delicate and complicated senses. Many processes must occur simultaneously in order for us to see what is happening around us. Information about image size and shape, color, motion, and location in space all must be gathered, encoded, integrated, and processed.
Performing these activities involves about 30 percent of the human brain — more than for any other sense. Vision has been studied intensively. As a result, neuroscientists may know more about it than any other sensory system.
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Most information about initial stages of visual transduction, or how light is converted into electrical signals, comes from studies of Drosophila fruit flies and mice, whereas visual processing has been mostly studied in monkeys and cats. It all Starts with Light Vision begins with light passing through the cornea, which does about three-quarters of the focusing, and then the lens, which adjusts the focus.
Both combine to produce a clear image of the visual world on a sheet of photoreceptors called the retina, which is part of the central nervous system but located at the back of the eye. Photoreceptors gather visual information by absorbing light and sending electrical signals to other retinal neurons for initial processing and integration. The signals are then sent via the optic nerve to other parts of brain, which ultimately processes the image and allows us to see.
Objects to the right of center project images to the left part of the retina and vice versa; objects above the center project to the lower part and vice versa. The size of the pupil, which regulates how much light enters the eye, is controlled by the iris.
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The shape of the lens is altered by the muscles just behind the iris so that near or far objects can be brought into focus on the retina.
Primates, including humans, have well-developed vision using two eyes, called binocular vision. Visual signals pass from each eye along the million or so fibers of the optic nerve to the optic chiasm, where some nerve fibers cross over. This crossover allows both sides of the brain to receive signals from both eyes. When you look at a scene with both eyes, the objects to your left register on the right side of the retina.
This visual information then maps to the right side of the cortex. A similar arrangement applies to movement and touch: Each half of the cerebrum is responsible for processing information received from the opposite half of the body. Scientists know much about the way cells encode visual information in the retina, but relatively less about the lateral geniculate nucleus — an intermediate way station between the retina and visual cortex — and the visual cortex.
Studies about the inner workings of the retina give us the best knowledge we have to date about how the brain analyzes and processes sensory information.
Photoreceptors, about million in each human eye, are neurons specialized to turn light into electrical signals. Two major types of photoreceptors are rods and cones. Rods are extremely sensitive to light and allow us to see in dim light, but they do not convey color. Rods constitute 95 percent of all photoreceptors in humans.
Most of our vision, however, comes from cones that work under most light conditions and are responsible for acute detail and color vision. The human eye contains three types of cones red, green and blue , each sensitive to a different range of colors. Because their sensitivities overlap, cones work in combination to convey information about all visible colors. You might be surprised to know that we can see thousands of colors using only three types of cones, but computer monitors use a similar Society for NeuroScieNce Near the center of the gaze, where visual acuity is highest, each ganglion cell receives inputs — via the middle layer — from one cone or, at most, a few, allowing us to resolve very fine details.
Near the margins of the retina, each ganglion cell receives signals from many rods and cones, explaining why we cannot see fine details on either side. Whether large or small, the region of visual space providing input to a visual neuron is called its receptive field. If light covers the entire receptive field, the cell responds weakly.
Thus, the visual process begins by comparing the amount of light striking any small region of the retina with the amount of surrounding light. Visual information from the retina is relayed through the lateral geniculate Vision begins with light passing through the cornea and the lens, which combine to produce a clear nucleus of the thalamus to the primary image of the visual world on a sheet of photoreceptors called the retina. As in a camera, the image on the retina is reversed: Objects above the center project to the lower part and vice versa.
The visual cortex — a thin sheet of tissue less information from the retina — in the form of electrical signals — is sent via the optic nerve to other than one-tenth of an inch thick , a bit parts of the brain, which ultimately process the image and allow us to see.
The central part of the brain. The primary visual cortex is densely packed with human retina, where light is focused, is called the fovea, which cells in many layers, just as the retina is. In its middle layer, contains only red and green cones. The area around the fovea, which receives messages from the lateral geniculate nucleus, called the macula, is critical for reading and driving.
Death of scientists have found responses similar to those seen in photoreceptors in the macula, called macular degeneration, is the retina and in lateral geniculate cells. Cells above and a leading cause of blindness among the elderly population in below this layer respond differently. They prefer stimuli in developed countries, including the United States.
Further studies have shown that different The rod and cone photoreceptors in the first layer send signals cells prefer edges at different angles or edges moving in a to the middle layer interneurons , which then relays signals particular direction.
The axons of the ganglion cells form the optic nerve. One system appears to process information mainly about shape; a second, mainly about color; and a third, movement, location, and spatial organization. Human psychological studies support the findings obtained through animal research.
These studies show that the perception of movement, depth, perspective, the relative size of objects, the relative movement of objects, shading, and gradations in texture all depend primarily on contrasts in light intensity rather than on color. Perception requires various elements to be organized so that related ones are grouped together. How do all these systems combine to produce the vivid images of solid objects that we perceive?
The brain extracts biologically relevant information at each stage and associates firing patterns of neuronal populations with past experience. Research Leads to More effective Treatment Vision studies also have led to better treatment for visual disorders.
Information from research in cats and monkeys has improved the therapy for strabismus, a condition in which the eyes are not properly aligned with each other and point in different directions. It is also termed squint, cross-eye, or walleye. Children with strabismus initially have good vision in each eye. But because they cannot fuse the images in the two eyes, they tend to favor one eye and often lose useful vision in the other.
Vision can be restored in such cases, but only during infancy or early childhood. Beyond the age of 8 or so, the blindness in one eye becomes permanent. Until a few decades ago, ophthalmologists waited until children reached the age of 4 before operating to align the eyes, prescribing exercises, or using an eye patch. Now strabismus is corrected very early in life — before age 4 — when normal vision can still be restored. Extensive genetic studies and use of model organisms have allowed us to identify defects in inherited eye diseases, making it possible to design gene or stem cell-based therapy and discover new drugs for treatment.
Loss of function or death of photoreceptors appears to be a major cause of blindness in many diseases that are currently incurable. Recently, gene therapy for a small group of patients with severe blindness allowed them to see. Work also is in progress to bypass lost photoreceptors and send electrical signals directly to the brain via ganglion cells.
National eye Institute, National Institutes of health] 20 BraiN factS sensing, thinking, and behaving Often considered the most important sense for humans, hearing allows us to communicate with each other by receiving sounds and interpreting speech. Our hearing system does not blend the frequencies of different sounds, as the visual system does when different wavelengths of light are mixed to produce color. Instead, it separates complex sounds into their component tones or frequencies so that we can follow different voices or instruments as we listen to conversations or to music.
Whether from the chirping of crickets or the roar of a rocket engine, sound waves are collected by the external ear — the pinna and the external auditory canal — and funneled to the tympanic membrane eardrum to make it vibrate. Attached to the tympanic membrane, the malleus hammer transmits the vibration to the incus anvil , which passes the vibration on to the stapes stirrup.
The stapes pushes on the oval window, which separates the air-filled middle ear from the Society for NeuroScieNce Produced in partnership with The Kavli Foundation and the Gatsby Foundation, Brain Facts gives an overview of the brain and nervous system, covering a variety of important topics in understandable language. Recently, SfN launched the eighth edition of the book , which was scientifically reviewed by nine members of the Dana Alliance, among others, to make sure the information is as credible and up-to-date as possible.
Besides a new look and hot-from-the-lab science, the book includes some new features. In addition, there are 30 new photos, 80 new glossary terms, and two new chapters, on the teenage brain and thinking and decision-making. Two examples of the eight Core Concepts and their symbols featured in the new book. The book serves as a companion publication to BrainFacts. On the site, the Core Concepts are explained with short videos, an interactive activity, and a related reading; a 3-D brain model lets visitors examine the brain up-close; and articles from SfN and outside sources cover important topics in neuroscience, such as aging and language.
There are even materials for educators who want to cover neuroscience in their classrooms! For more information, see our blog post on the launch of the recently redesigned BrainFacts. Both the book and website are great resource for Brain Bee participants , high school students, teachers, or anyone who wants a basic introduction to the world of neuroscience.
You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email.Recently, SfN launched the eighth edition of the book, which was scientifically reviewed by nine members of the Dana Alliance, among others, to make sure the information is as credible and up-to-date as possible.
They also may be responsible for long-term changes in the nervous system. Dec 14, Siobhan rated it really liked it. Semantic memory is a form of declarative knowledge that includes general facts and data. Other research suggests that genes important for brain development may also play a role in susceptibility to autism spectrum disorders.
The way the brain responds to hormones indicates that the brain is very malleable and capable of responding to environmental signals. However, if one twin gets the disease, the probability the other will also be affected is between 30 percent and 60 percent, indicating that there are environmental factors at play as well. Working memory depends on the prefrontal Different areas and systems of the brain are responsible for different kinds of memory.