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Bavidra kulendrarajah How can breathing rate be altered by chemicals?
Automatic centers in the brainstem activate the respiratory muscles Breathing is primarily controlled by intrinsic activity of the neurones of the brainstem but if voluntary control occurs the neurons in the cortex overrides the activity of the neurons in the brainstem. The two key areas of the brainstem that control the alternate cycle of inspiration and expiration are pons and the medulla. The neurons in the dorsal area of the medullary respiratory centre control inspiration whilst the neurons in the ventral area control expiration. The dorsal, inspiratory neurons have an intrinsic activity which fire action potentials with a specific pattern. At first there is no initial firing but over time the frequency of action potentials slowly increases and after reaching a peak the action potentials stop. This cyclic process of action potential generation regulates the length of a breath. An active inspiratory neuron stimulates the phrenic nerve which innervates the diagphragm and the intercostals nerves to the intercostals muscles. The contraction of these muscles leads to contraction which increases the thoracic volume and leads to inspiration. When the inspiratory neurons are inactive the stimuli to these muscles are inhibited and this causes the muscles to relax and expiration occurs as the lungs and the chest wall return to their original position. The rate of breathing can be altered by modulating the activity of the inspiratory neurons in the dorsal area of the medulla. This is done through impulses from the vagal and glossophrayngeal nerves. In contrast the ventral area which controls expiration is often inactive during quiet breathing and is only activated when more forceful breathing is required. The other main area of the brainstem that controls breathing is the pons and this area is involved in the fine tuning in the rate of breathing. The activation of the apneustic site situated in the lower pons results in the stimulation of the inspiratory area of the medulla which increases the duration of action potentials from the medulla to the respiratory muscles. Whereas activation of the pneumotaxic centre in the upper pons inhibits inspiration and the main purpose of this site is to control the inspiration volume and respiratory rate. Central chemoreceptors
The breathing rate is regulated in relation to the metabolic activity of the body which is measured by partial pressures of mainly carbon dioxide. The role of chemoreceptors is to detect changes in the partial pressures of either oxygen or carbondioxide and to relay this information to the nerves that are responsible in innervating the respiratory muscles. The central chemoreceptors are a group of specialised neurones, which detect changes in partial pressures of carbondioxdie and are involved in the minute by minute control of ventilation. These receptors respond to changes in the concentration of protons and are found 200-400 micrometres below the ventral
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