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USC BISC 307L - Synaptic Transmission and Autonomic Figs
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BISC 307L 2nd Edition Lecture 10 Current LectureSound LocalizationHow good is this mechanism?In a carefully conducted trialwhere the only sound iscoming from a specific soundsource, if you play a tone atvaried locations in yourazimuth (plane of hearing),you would be able to detect a1-2 degree change in sound,half of the time. Thiscorresponds to a very shortdifference in arrival time of the inputs, about 1 microsecond for a 90-degree difference. Are these axons perfectly symmetrical and precisely regulated? No – neither the conduction velocity, path length, nor trajectory of the inputs are standardized. But due to synaptic plasticity during development, the system learns this degree of precision and is self-correcting for its own sloppiness.Sound Localization in the Medial Superior OliveTo the right is a model of acircuit in the MSO of a cat. Thereare thousands of arrays of placeencoding neurons(see above),and across the one dimension ofthe nucleus of this MSO, thesedifferent arrays map out thefrequencies of the sounds. Eacharray is maximally sensitive to aparticular frequency, and theyare systematically arranged such that with distance in this dimension, it maps out along the log of this frequency.There are not five neurons in each array(like in the simplified schematic above), but something like 10,000. Cat’s can still move their ears. The funnel shaped pin of the external ear can collect sound, and amplify the intensity. To tune in the most sensitive part of the ear, because the ear is not perfectly symmetrical and some positions are better for hearing, people adjust themselves to focus their attention on sound. Owls have two ears, but unlike most mammals the ears are not on the same plane – they are displaced. They are in front of the face, under the eyes, and are vertically displaced which gives them the ability to place sound in the vertical plane as well. The Autonomic Nervous SystemGeneral properties:-Provides critical homeostatic mechanisms-Reflexes are mediated by local circuits in spinal cord and brainstem, and don’t involve the brain. Reflexes like regulation of blood pressure and respiration rate are all mediated by the ANS. For example, sensors carrying information about blood pressure and distention in stretch sensitive nerves in aortic bodies would cause blood vessels to be dilated, contraction strength tobe weakened, or beating to be slowed if blood pressure was too high. The brain would not even be aware of these processes. -An effector and sensory* system-Effector system. Efferent = carrying (commands) away from CNS. Commands are called effectors because they bring about an effect. Ex – motor commands-Sensory system. Afferent = carrying towards CNS, typically sensory information. This hastwo subdivisions – somatosensory which gives information about things like touch, pressure, and position of body parts, and the visceral afferents that bring sensory information from the viscera(organs in the body cavities), information like blood pressure, contents of the duodenum,and any other information necessary to maintain homeostasis basically. -In the efferent part of the ANS, there are 2 (or 3) Divisions: 1. Sympathetic2. Parasympathetic 3. (Enteric) – made up of sensory and effector nerves embedded in the walls of the digestive tract. Most of the ANS is in the digestive tract.Responses to /Recovery from EmergenciesThe responses on the left are for the sympathetic nervous system prepare the body for fighting or fleeing:-Increased respiratory and heart rate = oxygenation of blood and blood pressure and profusion of organs. -Vasodilation of limb muscles, which would be needed to get more blood flow to the muscles to run or fight, and blood vessels in the brain would dilate. -Vasoconstriction in visceral organs that are not necessary to deal with emergencies, like intestines, digestive organs, the skin, and reproductive organs. -Dilation of lung airway; there are springy cartilage-y rings around some parts of the small airways in the lungs. In addition, the elasticity of the lungs tends to pull things apart. If you are not restricting the bronchiole muscles, then the cartilaginous skeleton of the larger airways and the general pull of the surrounding lung tissue which will open up the airway and cause bronchodilation = more air into lungs + increased respiratory rate, which will increase blood oxygenation. -Decrease in secretion of saliva is part of digestive shutdown.-Increase in general alertness goes along with increased blood flow to the brain. -Those are just some of the other responses. Relaxation of the muscles in the iris (so eyes open up to get more light). When danger is passed, we have rest and digest response on the right. Parasympathetic activity is not just the cessation of sympathetic activity.-Digestive secretion returns to normal, and alertness may either return to normal or even go down. These shifting from one to the other is not onebeing active and the other not, but is a balancebetween the two. One doesn’t have to shut offfor the other to have an effect.CNS Centers regulating the ANSThese ANS reflexes are extremely important. There are many things that can go wrong –acid reflux, high blood pressure, asthma, and other diseases/conditions that involve malfunction at one or more of these systems. Therefore, drugs which target the autonomic synaptic transmission mechanisms by blocking receptors, increasing and decreasing transmitter release,affecting secondary messenger systems downstream from the transmitters, are all heavily researched for therapeutic intervention. There are small clusters of nerve cells and nuclei in the CNS that control the ANS. These control centers are mostly in the brainstem at an important area called the hypothalamus, which is jam packed with small nuclei that are not well defined – in that organ is packed the control of a whole bunch of important things like eating, water balance in the body, thirst, kidney function, control of temperature, all the “appetitive” behaviors (things we have an appetite for: food, drinking, sex). It contains a shockingly small number of neurons; only a few thousand are responsible for all these bodily functions.The pons and medulla are also important parts of the brainstem; a lot of these ANS control nuclei are located there as well. So these centers all tend to be vital - you can get derangement in behavior with damage to the hypothalamus, for example. And damage to


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