Essay On How Can Breathing Rate Be Altered By Chemicals Notes

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 surface of the medulla where they are surrounded by brain extracellular fluid. The composition of the extracellular fluid around the receptors is determined by the cerebrospinal fluid, local cerebral blood flow and local metabolism. Both the brain extracellular fluid and the cerebrospinal fluid are separated by the blood brain barrier which is relatively impermeable to H+ and HC03- but are highly permeable to CO2 which diffuses from the capillaries into the surrounding fluid. When partial pressures of carbondioxide increase in the blood, greater amount of C02 diffuses into the cerebal spinal fluid and this causes the right handed shift in the following equilibrium reaction CO2 + H2O H2CO3 - H+ + HCO3- Due to this there is an increase in the concentration of proton ions in the brain extracellular fluid and the cerebral spinal fluid. The cerebral spinal fluid contains less protein than the blood so it has less of a buffering ability so change in the pH due to change in partial pressures of carbondioxide is greater in the cerebral spinal fluid than the blood. The decrease in external pH caused by arterial hypercapnia is detected by the chemoreceptors and this causes in an increase in the depth and rate of inspiration. The presence of these chemoreceptors and their action was shown by Leusen who perfused an acidic solution in cerebral ventricles of dogs which had denervataed peripheral chemoreceptors. He found that dog shortly after the perfusion hyperventilated and this led to respiratory alkalosis in the blood. This shows that action of the central chemoreceptors in response to the elevated proton concentration.

The central chemoreceptors located in the brain parenchyma near the ventrolateral medulla are made up of pH sensitive neurons. Inhibitory neurons, which release the neurotransmitter GABA, are inhibited by the decrease in pH and this results in a decrease in the inhibition of breathing. In contrast excitatory neurons which release serotonin are stimulated by acidosis and cause an increase in breathing rate. The serotonergic neurons in the ventrolateral medulla are ideally positioned closely to the basilar artery and this allows the neurons to accurately and immediately detect changes in arterial changes in partial pressures of C02 through rapid changes in proton concentration in the extracellular fluid surrounding the neurons. The importance of these neurones is shown in sudden infant death syndrome which is often suffered by infants that have insufficient numbers of serotonergic neurons so are unable to respond their breathing rate in response to changes in partial pressures of carbon dioxide. The activated serotonergic neurons trigger an increase in the strength and duration of neuronal impulses from the inspiratory centre in the dorsal area of the medulla. This leads to an increase rate and depth of inspiration which results in the excess C02 being blown out of the lungs and when the partial pressures of C02 return to...

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