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42 Neural ControlJanuary 14 2008 Control centers receptors pathways Organisationand so it is there that we must look for theUltimately the control of breathing lies within the central nervous systemorganisation Consider for example that breathing is affected by many factors including o voluntary controlspeech from the motor cortex o Chemoreceptorsthe chemical control of breathing o posture affects the intercostal and abdominal muscles o startling eventsprovoke respiratory changes horrific gasp o emotions fear anxiety sorrow cause changes in breathing o exercise produces an immediate increase in breathing more later o Pain increases breathingLets start at the muscles of breathing They cause the rhythmic contraction and relaxation of the diaphragm and intercostal muscleso These muscles are driven by signals from motoneurons in the spinal cord whose axons make up the phrenic nerve which drive the diaphragm are found in cervical o Phrenic motoneuronsspinal cord segments C3 C4 and C5 are found in the thoracic spinal o The motoneurons for the intercostal nerves which drive the intercostal muscles cord segments T1 to T12 These motoneurons are thereforeAll of the factors that affect breathing must do so via the spinal respiratory motoneuronsthe final common pathway for all systems attempting to control the respiratory musclesRemember the breathholding experiment The battle between the two opposing forces was fought for control of the respiratory motoneurons from the following sources VoluntaryvsChemoreflex Drive motor cortex medulla oblongataNOTE These two influences reach the respiratory motoneurons via separate pathways down the spinal cord o If one pathway is injured spinal cord trauma the other may still worko Patients may be able to breathe on command but their chemical control of breathing doesnt work and viceversa The diagram below illustrates the various respiratory influences and their pathways1
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