control of ventilation

Question 1

which nerves stimulate the muscles required for inspiration?

Answer 1

the phrenic nerves innervate the daiphragm causing it to contract and bring about inspiation.
the intercostal nerves innervate the external interostal muscles which contract to bring about inspiration

Question 2

what controls ventilation/

Answer 2

ill defined centres in the pons and the medulla

Question 3

which stimulation of ventilation has the “last word”?

Answer 3

the brainstem, it’s stimulation can be overidden by voluntary signals from the brain but not completely (you cant hold your breath until you die)

Question 4

what is the function of the repiratory centres in the pons and medulla?

Answer 4

1. set an automatic rhythm for breathing

2. adjust this rhythm according to stimuli

Question 5

what is the function of the DRG (dorsal respiratory group) ?

Answer 5

controls the inspiratory muscles

Question 6

what is the function of the VRG (ventral respiratory group)?

Answer 6

controls the expiratory muscles and to a lesser extent some inspiratory muscles and the pharynx, larynx and tongue muscles

Question 7

what inputs to the respiratory centres adjust respiraory rate?

Answer 7

1. emotion (via limbic system in the brain)
2. voluntary over-ride (via higher centres in the brain)
3. Mechano-sensory input from the thorax ( e.g. stretch reflex)
4. chemical composition of the blood (PCO2, PO2 and pH) by chemoreceptors

Question 8

what is the purpose of stretch receptors in the intercostal muscles?

Answer 8

they prevent damage to the lungs by over inflation as they send an inhibitory signal back to the brain when the external intercosal muscles are contracting too much

Question 9

how do the central chemoreceptors in the medulla control carbon dioxide levels in blood?

Answer 9

detect changes in the carbon dioxide concentration in blood, it does this by detecting the hydrogen ion concentration in the CSF. A high hydrogen ion concentration is indicative of a high CO2 level. receptors detect this rise and send signals to the respiratory control centres which causes an in crease in ventilation. it is the opposite for a decrease in carbon dioxide concentration

Question 10

how do peripheral chemoreceptors in the aortic and carotid bodies control oxygen levels in the blood?

Answer 10

cells in the chemoreceptors contain oxygen receptors on their surface, binding of oxygen to these receptors closes potassium channels. This causes depolarising the cell. This causes exocitosis of dopamine which binds to dopamine receptors on a sensory neuron, if the amount of dopamine is large enough an action potential is sent to the respiratory receptors which causes an increase in ventilation.
these chemoreceptors also respond to hydrogen ion concentration.

Question 11

when do peripheral chemoreceptors increase ventilation?

Answer 11

when PP of oxygen is low and concentration of hydrogen ions is high (low pH)

Question 12

What happens during alkalosis?

Answer 12

the peripheral chemoreceptors detect the decreases in hydrogen ion concentraion and inhibit ventilation

Question 13

what happens during acidosis?

Answer 13

the peripheral chemoreceptors detect the increase in hydrogen ion concentration and stimulate ventilation

Question 14

are central chemoreceptors more sensitive to changes in carbon dioxide level than peripheral chemoreceptors are to oxygen level?

Answer 14

yes

Question 15

do peripheral chemoreceptors respond to changes in oxygen content of blood?

Answer 15

no, just oxygen PP

Question 16

what happens to chemoreceptors in patients with chronic advanced lung disease?

Answer 16

the central chemoreceptors become desensitised to carbon dioxide as it is always at such a high level. they become dependent on peripheral receptors.

Question 17

why is it not possible to voluntarily hyperventilate?

Answer 17

ventilation is reflexly inhibited by an increase in arterial PP of oxygen or a decrease in arterial PP of carbon dioxide or hydrogen ion conc.

Question 18

what is the action of opiods and barbiturates?

Answer 18

they depress the respiratory centre

Question 19

what do gaseous anaesthetic agents do?

Answer 19

increase respiratory rate and decrease tidal volume so decrease alveolar ventilation

Question 20

why is nitrous oxide dangerous in sufferers of chronic advanced lung disease?

Answer 20

they have a hypoxic drive and nitrous oxide blocks the action of the peripheral chemoreceptors so they cannot detect a decrease in PO2 or an increase in PCO2. Giving oxygen aggravates the situation

Question 21

what is hypercapnea

Answer 21

driven by a raised level of PCO2 (as in the central chemoreceptors)