Cynthia K. Kirkwood, PharmD, BCPP
- Executive Associate Dean for Academic Affairs
- Professor, Department of Pharmacotherapy & Outcomes Science, School of Pharmacy, Virginia Commonwealth University, Richmond, Virginia
When a light stimulus is placed in the receptive field of a photoreceptor treatment xdr tb discount olanzapine 5 mg amex, it produces a hyperpolarizing response that is transmitted to the bipolar cell natural pet medicine generic 2.5mg olanzapine overnight delivery. Some of the bipolar cells show a depolarizing potential that is excitatory to the ganglion cells medicine 20th century buy 7.5mg olanzapine visa, whereas other cells show a hyperpolarizing potential that is inhibitory treatment lower back pain buy generic olanzapine from india. Based on their morphology and function medications held before dialysis buy olanzapine online pills, the bipolar cells have been classified into 12 types. The cells show oncenter and offcenter response like the ganglion cells, as described below. The photoreceptors also give input to the horizontal cells, which respond with a hyperpolarizing mem brane potential. Also, the dendrites of the horizontal cells spread laterally and produce inhi bition of the bipolar cells on which they synapse. The amacrine cells spread laterally and synapse on the ganglion cells to which they provide excitatory input. They are classified into 29 types based on their connec tions with the ganglion cells in the inner plexiform layer. Thus in the retina, the bipolar cells, horizontal cells and amacrine cells comprise the neural network layer, in Bipolar Cell 1. Chapter 145: Visual Pathway and Visual Cortex 1183 which considerable processing of visual information takes place. The cells form gap junctions with the adjacent cells, so that signals spread in the form of electrotonic poten tials rather than propagated action potentials. They integrate the information received from bipolar and the amacrine cells and respond with a depola rizing potential that propagates along the axon (optic nerve). The response shown by the ganglion cells is of two types and accordingly, there are two types of cells: (i) oncenter ganglion cells and (ii) offcenter ganglion cells. Receptive field of a ganglion cell is the area of the visual field in which presence of a light stimulus modulates the discharge pattern of a ganglion cell. The central retinal neurons have smaller receptive fields, whereas peri pheral retinal neurons have larger receptive fields. The oncenter ganglion cell increases its spike rate in response to a central small circular light stimulus. In the offcenter ganglion cell, spike rate decreases when the central stimulus is turnedon and its spike rate increases in response to the surround illumination. Thus, the on-center ganglion cell is stimulated in response to an excitatory center with an inhibitory surround, whereas an off-center ganglion cell is stimulated in response to an inhibitory center with an excitatory surround. Due to this centersurround antagonism, the ganglion cells have been called as contrast detectors. During the offresponse of the oncenter ganglion cell, the annulus of light activates a ring of photoreceptors that stimulates the horizontal cells (present on both sides of the receptor circle) on which they synapse. The horizontal cells in turn inhibit the nearby photore ceptors by lateral inhibitory inputs. In this manner, the centrally present photoreceptors get inhibited by the surrounding horizontal cells. Thus, decrease in activity of the neighboring cells by stimulation of a specific cell is known as afferent or lateral inhibition. This is an important component of information processing during which, signal transmission through a particular pathway is allowed, whereas, propagation of action potential through the adjacent pathways is inhibited. In the retina, this improves the sharpness at the periphery of an image and increases contrast. Based on size, there are two types of ganglion cells present in the retina: large ganglion cells (magno or M cells), and small ganglion cells (parvo, or P cells). The firing rate of on-center ganglion cell increases in response to a central small circular light stimulus in its receptive field (on-response) and when an annulus of light around the center is turned on, its firing rate decreases (off-response). In the off-center ganglion cell, spike rate decreases by central illumination and increases in response to surround illumination. The P cells analyze and subtract response from one type of cone from response from another. Input to the retina is received by the photoreceptors and the ganglion cells are the output cells. The responses of the cells of the neuronal network layer are integrated in the ganglion cell layer. The photoreceptors and the ganglion cells are sponta neously active in the resting state, so that, any change in visual stimulus brings in a response in them. The ventrally placed layers 1 and 2 have large cells and accordingly are called magnocellular that receive input from the M ganglion cells. Layers 3,4,5,6 are called parvocellular as they contain small cells and receive input from the P ganglion cells. Inputs from nasal retina of contralateral eye projects to layers 1,4 and 6; and inputs from temporal retina of ipsilateral eye project to layers 2, 3 and 5. The visual cortex inputs provide necessary feedback regarding perception of orientation and movement of an object. From the cell bodies in layers 1 and 2, the fibers form the magnocellular path way that is concerned with detection of movement, depth and flicker. The fibers form the cell bodies in layers 3-6, carries the impulses regarding color, texture, shape and finer detail. From there, the visual signal is transmitted to a single point in the right visual cortex. It is also known as the visualsensory area as it receives the sensory information regarding vision. The geniculocalcarine tract fibers mainly terminate on the medial part of occi pital cortex, above and below the calcarine fissure. The painted area shows the projection of retinal fibers into the calcarine cortex. The cortical area devoted to receive afferents from macula is much greater than other areas. This layer receives information regarding movement, location, orientation, texture, shape and color of an object. The cells of layers 2,3 are rich in the enzyme cytochrome oxidase and form clusters, known as blobs that receive information regarding color of an object. The neurons that have their cell bodies in layers 2 and 3, project to the other cortical regions and to the neurons in layer 5. The neurons that have their cell bodies in deeper layer 5, project to the superior collic ulus, pulvinar and other brainstem nuclei. But unlike these cells that respond to a point stimulus, the neurons of visual cortex respond to a linear stimulus like lines, edges or bars of light. Thus, the stimulus can be a bar of light against dark background or a dark bar against a light background. The neurons of layer 4 of the visual cortex respond to a stimulus that is positioned at any angle i. But, the cells in other layers are responsive to the orientation of the bar stimulus. Based on this, the cells are classified into simple, com plex and hypercomplex cells. The complex cells show a low resting discharge rate, whereas the simple cells discharge only on stimulation. The complex cells respond best when a bar stim ulus moves laterally without change in its orienta tion. Thus, the complex cells are concerned with movement and velocity of the stimulus and less with its central location. They receive visual information from primary visual cortex and are concerned with interpretation of the visual stimulus. They help in appreciating the finer attributes of the objects like perception of form, texture, shape, depth, location and orientation of the object. Lesions of the association areas produce impairment of these higher visual functions. There are also hypercomplex cells that respond best to a moving bar with a precise orientation but also with a defined length. Some respond better to a stimulus having a specific angle (for example, a Lshaped or tongueshaped stimulus) or having two borders. Thus, the visual cortex receives information regarding orientation, movement, depth, velocity as well as color of an object and help in depth perception and stereopsis (binocular depth perception). Usually, the simple cells project to the complex cells and the hypercomplex cells receive inputs from the complex cells. So, the receptive fields of the simple cells are smaller than those of the complex cells and receptive fields of the hypercomplex cells are larger than those of the complex cells. When a microelectrode is inserted perpendicularly into the visual cortex, the cells that come across have the same orientation (orientation column). All cortical neurons have their receptive fields in the contralateral visual field. Also, the cells receiv ing input from one eye alternate with the cells receiv ing input from the other eye. When radioactive dye is injected into one eye, the dye reaches visual cortex by axoplasmic transport. Many of the simple and complex cells of the visual cortex receive input from both the eyes. Convergence, divergence and near response is influenced by activity in an area anterior to the frontal eye fields. The superficial layers receive inputs from the optic tract fibers and visual cortex and in turn project to various thalamic nuclei like anterior thalamic nucleus, pulvinar and lateral geniculate body. The neurons of the superficial layer receive inputs from both the eyes and they discharge in response to a stimulus moving quickly in a particular direction. Thus, the superior colliculi coordinate simultaneous bilateral eye movements, like saccades and convergence and keep the eyeball in focus. The deep layers of superior colliculi receive visual inputs from its superficial layers, inputs from the auditory fibers and somatosensory inputs from different body parts. The sensory stimuli are integrated in the deep layers for various reflex activities that involve appro priate head movement and change in eye opposition. Fibers from the optic tract project to the pretectal nucleus and cause activation of the EdingerWestphal nucleus. These fibers mediate pupillary reflexes and are involved in the regulation of visual fixation. It is a small nucleus in the medial hypothalamus, located just above the optic chiasm. It receives fibers from the optic nerve and synchronizes sleepwake cycle, secretion of hormones like cortisol, and other circadian rhythms with lightdark cycle. Information from the visual receptors regarding light intensity during diurnal cycle gets transmitted to the suprachiasmatic nucleus and entrains its biological clock activity. Apart from these areas, some other areas are also activated, such as parts of inferior temporal cortex, posteroinferior parietal cortex, amygdale, caudate nucleus, putamen and claustrum. Effect of lesions at various levels of visual pathway is asked as a Short Question. Name the areas or pathways for processing of visual signal, Macular sparing, What is the on-center and offcenter response of the ganglion cells, What are important features of retinal neurons, Cortical visual areas. Name the visual field defects produced by lesion at different levels of visual pathway. The short est distance, by which two objects can be separated and still be visualized as two different objects, is known as the minimum separable distance. Visual acuity expresses the resolving power of the eye, that is, the extent to which the eye can perceive the details and contours of an object. There are different tests for testing visual acuity for distant vision and near vision. Visual acuity increases as the size of the object increases and distance of the object from eye decreases. But the contrast between the object and the back ground plays a more important role than the colorper se. Factors Affecting Visual Acuity Optical Factors these factors mainly determine the degree of visual acuity. The rows are numbered as 60, 36, 24, 18, 12, 9, 6 and 5 from top to bottom; and the size of the letters in each row gradually decreases. As the cones are concentrated more at the fovea, Chapter 146: Visual Acuity, Visual Field, Light and Dark Adaptations, and Visual Reflexes 1189. The gaps between the lines as well as the width of each line subtend an angle of 1 minute at the nodal angle. The number below each row depicts the distance in meters from which the letters can be read by a normal eye. The subject is asked to read the chart with each eye separately from a distance of 6 meters.
Clinical toxicology is a dynamic field of medicine; new treatment methods are developed regularly medicine used for adhd generic olanzapine 5 mg amex, and the effectiveness of old as well as new modalities is subject to constant critical review medicine 2632 2.5mg olanzapine otc. There is general agreement that prevention of pesticide poisoning remains a much surer path to safety and health than reliance on treatment medications bad for your liver quality olanzapine 5 mg. In addition to the inherent toxicity of pesticides symptoms schizophrenia purchase 7.5mg olanzapine, none of the medical procedures or drugs used in treating poisonings is risk-free treatment 2 go discount 5 mg olanzapine mastercard. In fact, many antidotes are toxic in their own right, and such apparently simple procedures as gastric intubation incur substantial risk. Clinical management decisions have to be made promptly and, as often as not, on the basis of limited scientific and medical information. The complex circumstances of human poisonings rarely allow precise comparisons of alternative management. In no sense, then, are the treatment recommendations in this book infallible guides to successful outcomes. They are no more than consensus judgments of the best available clinical management options. This manual deals almost entirely with short-term (acute) harmful effects of pesticides. Although obviously important, the subject of chronic effects is too complex to deal with exhaustively in a manual designed as guidance for emergency management. Nonetheless, appropriate treatment of serious exposures to pesticides represents an important step in avoiding chronic as well as acute disease. The pesticides and commercial products mentioned in this manual do not represent the universe of pesticide products in existence. They were selected based on frequency of use and exposure, severity of toxicity, and prior experience with acute poisonings. The amount of pesticide absorbed is a critical factor in making treatment decisions, and estimation of dosage in many circumstances of pesticide exposure remains difficult. The terms "small amount" and "large amount" used in this book are obviously ambiguous, but the quality of exposure information obtained rarely justifies more specific terminology. Exposure to spray drift properly diluted for field application is not likely to convey a large dose unless exposure has been prolonged. Spills of concentrated technical material onto the skin or clothing may well represent a large dose of pesticide unless the contamination is promptly removed. Brief dermal exposure to foliage residues of cholinesterase-inhibiting pesticides is not likely to lead to poisoning, but prolonged exposures may well do so. Suicidal ingestions almost always involve "large amounts," requiring the most aggressive management. Except in children, accidental pesticide ingestions are likely to be spat out or vomited. The therapist usually must base clinical management decisions on "worst case" assumptions of dosage. Childhood poisonings are still further complicated by the greater vulnerability of the very young, not only to pesticides themselves, but also to drugs and treatment procedures. The nature of neurological development in children entails an additional level of risk that is not present in adults. Some adult groups such as farmwrokers with poor nutrition and high exposure may also be at increased risk. Death has occasionally resulted from this complication, even following ingestions of substances having relatively low toxic potential. In poisonings by agents that depress central nervous system function or cause convulsions, early placement of a cuffed endotracheal tube (even when this requires light general anesthesia) may be life saving. Maintenance of adequate pulmonary gas exchange is another essential element of poisoning management that deserves constant reemphasis. Gastric intubation, with aspiration and lavage, remains a useful method for removing poisons from the stomach shortly after they have been swallowed, but the time after ingestion during which lavage is likely to be beneficial is shorter than many clinical toxicologists have thought. Rarely are significant amounts of swallowed toxicants recovered more than 1-2 hours after ingestion, and, in many instances, the bulk of swallowed material passes into the duodenum and beyond in 15-30 minutes. In addition, the majority of controlled studies evaluating the effectiveness of gastric emptying procedures are done for ingestions of solid material (pills) rather than liquids. Full advantage should be taken of new highly adsorbent charcoals that are effective in binding some pesticides in the gut. Unfortunately, charcoal does not adsorb all pesticides, and its efficiency against many of them is not known. In poisonings caused by large intakes of pesticide, hemodialysis and hemoperfusion over adsorbents continue to be tested as methods for reducing body burdens. Overall effectiveness appears to depend not only on efficiency of clearance from the blood, but also on the mobility of toxicant already distributed to tissues before the extracorporeal blood-purification procedure is started. The volume of distribution and avidity of tissue binding are important considerations in making such decisions. The critical determinant of success in using these systems may well be the speed with which they can be put into operation before tissuedamaging stores of toxicant have accumulated. There remains a need for systematic reporting of pesticide poisonings to a central agency so that accurate statistics describing the frequency and circumstances of poisoning can be compiled, and efforts to limit these occurrences can be properly directed. In some countries there has been an increase in the use of pesticides as instruments of suicide and even homicide. Producers are now devoting considerable effort to modifying formulation and packaging to deter these misuses. This work is important because suicidal ingestions are often the most difficult pesticide poisonings to treat successfully. The list is based on symptomatic cases classified as minor, moderate, major, or fatal outcome for unintentional cases involving a single product. Numbers of cases are reported for both children under six years of age and for adults and older children. Cases listed as organophosphates (and the other categories as well) may also include other insecticides such as carbamates and organochlorines in a single product. Source: American Association of Poison Control Centers, Toxic Exposure Surveillance System, 1996 data. Approximately 90% of symptomatic cases involve only minor symptoms of the type that could typically be treated at home with dilution or just observation. However, seven of the top ten categories listed in the table above (organophosphates, pyrethrins/pyrethroids, hypochlorite disinfectants, carbamates, organochlorines, phenoxy herbicides, and anticoagulant rodenticides) are much more likely to require medical attention. This list cannot be considered representative of all symptomatic poisonings because it only shows cases reported to Poison Control Centers. However, it does give a sense of the relative frequency and risk of poisoning from various agents or classes of agents. The relative frequency of cases generally reflects how widely a product is used in the environment. Denominator information on the population at risk (numbers exposed) would be needed to better understand the relative risk of different pesticides. However, the main purpose of these tables is to give physicians a sense of what types of cases they are most likely to see in their practice. Although suicide cases make up roughly 3% of pesticide-related calls to Poison Control Centers, they may account for nearly 10% of the cases seen in a health care facility. The leading types of products involved in suicidal cases include anticoagulant rodenticides (20% of total suicide attempts), pine oil disinfectants (14%), organophosphates (11%), pyrethrins/pyrethroids (6%), unknown rodenticides (5%), carbamate insecticides (4%), and phenol disinfectants (3%). Poison Control Centers are best at capturing pesticide exposures which occur in residential environments. The table above presents the number of occupationally-related cases in California reported from 1991 through 1995 where a pesticide was considered a probable or definite cause of the resulting illness. Pesticide combinations, where the primary pesticide responsible for the illness could not be identified, are not included in this table. Among persons who encounter pesticides in the course of their occupational activities, dermal and eye injuries, rather than systemic poisonings, are more common. However, many different agents commonly require similar procedures in treating poisonings and it is not practical to repeat these protocols in every chapter. General principles for management of pesticide poisoning, including skin and eye decontamination, gastrointestinal decontamination, and control of convulsions are considered in Chapter 2, General Principles. Changes in this reformatted edition include: tabular listings of Commercial Products in each chapter, the addition of a new chapter on Disinfectants (Chapter 19), and the addition of a chapter on Environmental and Occupational History (Chapter 3), which places pesticide poisonings in the context of other environmental and occupational exposures, provides questionnaires designed to elicit exposure information, discusses resources available to the practitioner, and provides a list of governmental and non-government contacts and Web sites for more information. In addition, each chapter is referenced to key references in readily accessible current literature. Most references were selected as primary references in peer review journals, although some review papers are also included. The contents of this book have been derived from many sources: published texts, current medical, toxicological, and pesticide product literature, and direct communications with experts in clinical toxicology and pesticide toxicology and environmental and occupational health specialists. Ensminger Oak Ridge National Laboratory, 1980 Clinical Toxicology of Agricultural Chemicals Sheldon L. Where special considerations and treatments are required for a particular pesticide, they are addressed separately in the appropriate chapter. Skin Decontamination Decontamination must proceed concurrently with whatever resuscitative and antidotal measures are necessary to preserve life. Shower patient with soap and water, and shampoo hair to remove chemicals from skin and hair. If there are any indications of weakness, ataxia, or other neurologic impairment, clothing should be removed and a complete bath and shampoo given while the victim is recumbent. The possibility of pesticide sequestered under fingernails or in skin folds should not be overlooked. If eye irritation is present after decontamination, ophthalmologic consultation is appropriate. Persons attending the victim should avoid direct contact with heavily contaminated clothing and vomitus. Shoes and other leather items cannot usually be decontaminated and should be discarded. Note that pesticides can contaminate the inside surfaces of gloves, boots, and headgear. Decontamination should especially be considered for emergency personnel such as ambulance drivers at the site of a spill or contamination. Latex and other surgical or precautionary gloves usually will not always adequately protect from pesticide contamination, so only rubber gloves are appropriate for this purpose. In severe poisonings, it may be necessary to mechanically support pulmonary ventilation for several days. Note on Specific Pesticides: There are several special considerations with regard to certain pesticides. In organophosphate and carbamate poisoning, adequate tissue oxygenation is essential prior to administering atropine. As important, in paraquat and diquat poisoning, oxygen is contraindicated early in the poisoning because of progressive oxygen toxicity to the lung tissue. Gastrointestinal Decontamination A joint position statement has recently been released by the American Academy of Clinical Toxicology and the European Association of Poisons Centres and Clinical Toxicologists on various methods of gastrointestinal decontamination. A summary of the position statement accompanies the description of each procedure. Gastric Lavage If the patient presents within 60 minutes of ingestion, lavage may be considered. If the patient is neurologically impaired, airway protection with a cuffed endotracheal tube is indicated prior to gastric lavage. Lavage performed more than 60 minutes after ingestion has not proven to be beneficial and runs the risk of inducing bleeding, perforation, or scarring due to additional trauma to already traumatized tissues. Studies of poison recovery have been performed mainly with solid material such as pills. Position Statement: Gastric lavage should not be routinely used in the management of poisons. Lavage is indicated only when a patient has ingested a potentially life-threatening amount of poison and the procedure can be done within 60 minutes of ingestion. Because magnesium citrate has not been studied as much, its use is not described here. It will increase gut motility to improve excretion of the charcoal-poison complex. Repeat doses of cathartics may result in fluid and electrolyte imbalances, particularly in children, and are therefore not recommended. Sorbitol is formulated in 70% and 35% solutions and usually packaged in 100 mL bottles. The gram dosage of sorbitol in a 100 mL bottle can be calculated by multiplying 100 (mL) x 0. Note on Specific Pesticides: Significant poisoning with organophosphates, carbamates, and arsenicals generally results in a profuse diarrhea. Position Statement: the administration of a cathartic alone has no role in the management of the poisoned patient. There are no definite indications for the use of cathartics in the management of the poisoned patient. Data are conflicting with regard to use in combination with activated charcoal, and its routine use is not endorsed. If a cathartic is used, it should be as a single dose in order to minimize adverse effects. There are numerous contraindications, including absent bowel sounds, abdominal trauma or surgery, or intestinal perforation or obstruction. It is also contraindicated in volume depletion, hypotension, electrolyte imbalance, or the ingestion of a corrosive substance. Activated Charcoal Adsorption Activated charcoal is an effective absorbent for many poisonings.
In experimental animals medicine mart buy olanzapine 2.5 mg with visa, the defense response is elicited by stimulation of hypothalamic areas that in cat elicits hissing in treatment 1-3 purchase olanzapine cheap, arching of the back medicine vile order olanzapine 7.5mg without prescription, piloerection and dilation of the pupil medications with dextromethorphan 7.5mg olanzapine sale. This is associated with autonomic cardiovascular responses consisting of a increased heart rate and blood pressure symptoms 9 weeks pregnant generic olanzapine 5mg fast delivery, and a large increase in skeletal muscle blood flow. Physiology of Emotions Emotion is a state of feeling that manifests mentally and physically through cardiovascular and other autonomic changes. Emotional activation and responses occur in three phases: stimulus recognition and evaluation, emo tional responses and emotional experience. Conditioned Response this is the emotional response attached to a conditioned stimulus. Most of our emotional responses are condi tioned emotional responses as they are usually evoked by a conditioned stimulus. The subsequent emotional reactions to the similar stimuli mostly depend on the knowledge of the previous experiences. Stimulus Recognition and Evaluation First, the stimulus should be perceived, which is called recognition or the awareness of sensation. Then, the next step is the stimulus evaluation, the process by which the output is compared with stored information. The inherited infor mation also helps in emotional integration, for example inherent fear in monkeys for snakes. Control of Emotional Responses Though the emotional responses are profoundly influ enced by external factors like social and cultural influ ences, they are mainly mediated by neural and hormonal mechanisms. The neural control mechanisms for emotion are divided into peripheral and central mechanisms. Emotional Responses Emotional response has three components: affect, cona tion and physical changes. However, emotional manifestations mainly depend on the type of emotional response. Emotional response may be a natural response, a defense response and a conditional response. Peripheral Control of Emotion the main output pathway for emotional responses is the autonomic nervous system. The emotion of fear and rage closely resembles the effects of injection of norepinephrine. Thus, it is clear that emotional responses of fear and anger are mediated by sympathetic activation. Central Control of Emotion Emotional responses have been experimentally seen to occur in the absence of cortex. Hypothalamus and other limbic structures are important for expression of emo tion. As suggested by Papez, for emotional expression and experience, the sensory information from environment passes through the thalamus to the hypothalamus. Natural Response Natural emotional response is the normal response of fear, anxiety or pleasure encountered in daily life. Though, the natural stimulus for emotion involves the neocortical perceivecognitive mechanisms like the nonemotional sensory processes, the natural emotion is associated Chapter 137: Limbic System 1133 Ascending Output: the ascending output reaches anterior thalamus via mammillothalamic tract and from there it projects to cingulate cortex. Descending Output: the descending output of the hypothalamus is directed to the brainstem and spinal cord centers from where autonomic fibers originate. Endogenous fear, anxiety, depression and euphoria Role of Limbic System Limbic system receives both exteroceptive and interocep tive sensory inputs. Exteroceptive inputs reach limbic system via two ways; first, through collaterals arising from ascending sensory projections before the fibers reach cortical areas, and second, form sensory cortex. Interoceptive inputs reach hypothalamic and amyg dalar nuclei of limbic system from brainstem especially from the nucleus tractus solitarius. The major output of limbic system to autonomic con trol areas is via its connections with brainstem and spinal cord. In addition, amygdala has direct connections with hypothalamus and brainstem autonomic areas. The Papez circuit forms the limbichypothalamic connections for emotional expression through autonomic pathways. Animal exhibits placidity and inability to recognize object visually inspite of good vision (visual agnosia), but will pick up almost all objects and explore them orally. Animal fails to ignore peripheral stimuli (hypermetamorphosis), and therefore, respond to every stimulus and explore everything. Applied Physiology Psychosurgery In 1935, John Fulton observed that frontal lobotomy in mon keys cures experimentally induced neurosis. Since then, selected lesions of the brain, especially of the limbic areas have been performed for the treatment of psychiatric disor ders, which are popularly known as psychosurgery. Psycho surgery is usually used for the treatment of psychiatric illness in which extremes of aggression is the main symptom. Emotional responses, autonomic reaction and visceral of the body are influenced by limbic system. Nucleus accumbens is the major reward center and dopamine is the key neurotransmitter for motivation. Limbic functions, Papez circuit, Motivation, Emotion, Addiction, Reward system, Avoidance system, Autonomic responses may come as Short Questions. Name the structures of limbic system, What are the limbic functions, What are the components of Papez circuit, Physiology of Motivation, Physiology of Emotion, Physiology of Addiction, Physiology of Reward system, Physiology of Avoidance system, Types of autonomic responses, Neurotransmitter systems and their functions, Causes and features of KluverBucy syndrome. This is because the process of learning involves the storage of new information in memory and its retrieval at appropriate time, and the pro cess of memory involves repeated acquirement of new knowledge (learning). It is known that frequent learning of newer facts increases the horizon of memory. Memory is the simple repetition of what has already been performed or stored, whereas learning involves more than the simple use of memory. It uses all less-understood processes like reasoning, cognitive processes and common sense: 1. The common sites of learning and memory in the human brain are the association areas of the cerebral cortex and sub-cortical structures in the temporal lobe, including the hippocampus and amygdala. The association cortical areas imbibe sensory information from the somatosensory cortex, and visual, auditory, and olfactory cortices. These information are integrated with previous experiences of learned skills and are then stored in the memory. Definitions Learning Learning is defined as acquirement of information or knowledge by experience that results in the alteration of behavior. It depends mostly on motivation that creates adequate interest and attention to promote learning. The stored information should be retrieved and utilized at any time in life whenever needed. Registration of memory that includes proper percep tion and attention: Failure of learning and memory occurs due to impaired perception and attention because the material to be learned is never registered and assimilated (Application Box 138. Recognition and recall: At the appropriate time and place, memory is recalled for proper use. Reutilization: Memory (the learned experience) is utilized for improvement of further learning. Long-term memory is the one that stores information for years together, and sometimes for life. Sensitization Explicit memory and many forms of implicit memory involve short-term and long-term memory. The implicit memory is not associated with awareness and is therefore also called as reflexive or nondeclara tive memory. For example, a learner of motorcycle riding initially remembers the steps of changing the gear (he changes gears with conscious knowledge) till it becomes a reflexive habit to do so (he changes gear without awareness). Priming Priming is the facilitation of recognition of words or objects by prior exposure to them. An example is improved recall of a word when presented with first few letters of it. Procedural Memory Includes skills and habits, which once acquired become unconscious and automatic. The explicit memory, also known as declarative memory is connected with awareness. It has two forms: the memories of events (episodic memory), and the memories of facts (semantic memory). The declarative memory is dependent on the hippocampus and other parts of the medial temporal lobes of the brain for its retention. Habituation is a simple form of learning in which repeated application of a neutral stimulus elicits less and less response. The response that was first studied was gill withdrawal in Aplysia when the gill is stroked. The withdrawal is an aversive response, and if the stroke is harmless, the animal gets habituated to it. Habituation implies learning and therefore can be studied for its cellular mechanisms. It can be shortterm, or it can be prolonged if exposure to the benign stimulus is repeated many times. Sensitization is the opposite reaction in which repeated application of stimulus evokes greater and greater response. Short-term memory is the memory that lasts for seconds to hours, during which processing in the hippocampus and elsewhere lays down long-term changes in synaptic strength. Working memory is a form of short-term memory that makes information available for a brief period. As a result of repeated training, short-term memory can be transferred into long-term memory, which depends on a process called consolidation. During short-term memory, the memory traces are subject to disruption by trauma and various drugs. Sensitization occurs especially when the stimulus to which habituation has developed is coupled with a pleasant or unpleasant stimulus. For example, application of noxious stimulus to gill results in greater withdrawal of gill (an increased responsiveness). Sensitization may occur as a transient response, or if it is reinforced by additional pairings of the noxious stimulus and the initial stimulus, it can exhibit features of short-term or long-term memory. He is popularly known for the conditioned experiments of salivation he carried on dogs. Associative Learning the organism learns about the relation of one stimulus with other. Types of Conditioned Reflexes There are two types of conditioned responses: classical conditioning and operant conditioning. Conditioned reflexes have two components that are associated with emotional responses and motor responses. The emotional responses are regulated by amygdala and the motor responses are controlled by cerebellum. In classic conditioning, in the beginning, there is a stimulus that normally elicits a specific innate response (the response which is already present without training). Inhibition of Conditioned Reflex: Conditioned reflexes can be inhibited in two ways: internal and external inhibitions: 1. Reinforcement of Conditioned Reflex: It is difficult to maintain conditioned reflex indefinitely: 1. Accordingly, there are two types of reinforcements: Positive and negative reinforcement. For example, heart and blood pressure can be decreased by appropriate conditioning. In this type of conditioning, animal is trained to carry out a task for either to receive a reward or to avoid a punishment. The animal, usually a rat is kept in the Skinner box, in which provision is made in such a way that pressing a bar results in delivery of food pallet, or prevention of an electric shock. However, later response occurs with greater probability as reward follows the response (animal learns that food is obtained by pressing the bar or the shock is prevented). Thus, the reinforcement may be a positive reinforcement (by pressing bar animal gets food), or a negative rein forcement (by pressing bar animal prevents electric shock). Therefore, this is also called conditioned avoidance reflexor aversion conditioning (Application Box 138. Sensitization occurs due to prolongation of action potential in the sensory endings that results in increase in intracellular calcium, which in turn increases neuro transmitter release. In the sea snail Aplysia, the noxius stimulus causes discharge of serotonergic neurons that end on presynaptic endings of sensory neurons. This facilitates voltage dependent calcium influx into the sensory terminal that increases release of transmitter by exocytosis. The long-term potentiation also involves protein synthesis and growth of the presynaptic and postsynaptic neurons and their connections. The basic mechanism involved is the prolongation of action potential that causes presynaptic facilitation. Molecular Mechanisms Learning and memory are initiated and established by several neurochemical changes like increased synaptic connection, neurotransmitter secretion, formation of intracellular second messenger, protein synthesis and gene activation. Sensitization and Habituation Habituation and sensitization occur due to change in neurotransmitter release at the sensory neuron terminals due to alteration in intracellular calcium. Habituation occurs due to decreased neurotransmit ter release from the presynaptic sensory ending in response to repeated application of a particular stimulus. The stimuli gradually inactivate calcium channel resulting in decreased calcium content at the presynaptic terminal that in turn inhibit neurotransmitter release.
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