Respiration regulation | Anatomy2Medicine

Respiration regulation

    • Control of Breathing
      • Breathing is controlled by centers in the brain stem with four components to control system:
        • chemoreceptors for O2 or CO2
        • mechanoreceptors in the lungs and joints
        • control centers for breathing in the brain stem (medulla and pons)
        • respiratory muscles, whose activity is directed by the brain stem centers

 

  • Voluntary control can also be exerted by commands from the cerebral cortex (e.g., breath-holding or voluntary hyperventilation), which can temporarily override the brain stem.
  • Brain stem control of breathing

 

      • The frequency of normal, involuntary breathing is controlled by three groups of neurons or brain stem centers
        • medullary respiratory center
        • the apneustic center
        • pneumotaxic center.
      • Medullary Respiratory Center

 

  • located in the reticular formation

 

        • composed of two groups of neurons
          • the inspiratory center (dorsal respiratory group) (MCQ)
          • expiratory center (ventral respiratory group). (MCQ)
        • Inspiratory center
          • controls the basic rhythm for breathing by setting the frequency of inspiration. (MCQ)
          • This group of neurons receives sensory input from
            • peripheral chemoreceptors via the glossopharyngeal (CN IX) and vagus (CN X) nerves (MCQ)
            • mechanoreceptors in the lung via the vagus nerve. (MCQ)
          • The inspiratory center sends its motor output to the diaphragm via the phrenic nerve.
          • The pattern of activity in the phrenic nerve includes a period of quiescence, followed by a burst of action potentials that increase in frequency for a few seconds, and then a return to quiescence.
          • Activity in the diaphragm follows this same pattern: Quiescence – action potentials rising to a peak frequency (leading to contraction of the diaphragm)-  Quiescence.

 

  • Inspiration can be shortened by inhibition of the inspiratory center via the pneumotaxic center (MCQ)

 

        • Expiratory center.
          • located in the ventral respiratory neurons (MCQ)
          • responsible primarily for expiration
          • Since expiration is normally a passive process, these neurons are inactive during quiet breathing.
          • During exercise when expiration becomes active, this center is activated.
      • Apneustic Center

 

  • Apneusis is an abnormal breathing pattern with prolonged inspiratory gasps, followed by brief expiratory movement

 

        • apneustic center is located in the lower pons (MCQ)
        • Stimulation of apneustic center apparently excites the inspiratory center in the medulla(MCQ)
        • apneustic center prolongs the period of action potentials in the phrenic nerve (MCQ)
        • it prolongs the contraction of the diaphragm. (MCQ)
      • Pneumotaxic Center
        • the pneumotaxic center is located in the upper pons (MCQ)
        • The pneumotaxic center turns off inspiration (MCQ)
        • It limits the burst of action potentials in the phrenic nerve (MCQ)
        • It limits the size of the tidal volume, and secondarily, it regulates the respiratory rate.
        • A normal breathing rhythm persists in the absence of this center.

 

  • Cerebral cortex

 

      • Commands from the cerebral cortex can temporarily override the automatic brain stem centers
      • a person can voluntarily hyperventilate (i.e., increase breathing frequency and volume).
      • The consequence of hyperventilation is a decrease in PaCO2, which causes arterial pH to increase.
        • Hyperventilation is self-limiting, however, because the decrease in PaCO2 will produce unconsciousness and the person will revert to a normal breathing pattern. Although more difficult, a person may voluntarily hypoventilate (i.e., breath-holding).

 

  • Hypoventilation causes a decrease in PaO2 and an increase in PaCO2, both of which are strong drives for ventilation
  • A period of prior hyperventilation can prolong the duration of breath-holding.

 

 

 

  • Chemoreceptors

 

      • Central Chemoreceptors
        • The central chemoreceptors, located in the brain stem, are the most important for the minute-to-minute control of breathing.
        • These chemoreceptors are located on the ventral surface of the medulla, near the point of exit of the glossopharyngeal (CN IX) and vagus (CN X) nerves and only a short distance from the medullary inspiratory center. (MCQ)

 

  • Thus, central chemoreceptors communicate directly with the inspiratory center.

 

        • The brain stem chemoreceptors are exquisitely sensitive to changes in the pH of cerebrospinal fluid (CSF).
          • Decreases in the pH of CSF produce hyperventilation(MCQ)
          • increases in the pH of CSF produce hypoventilation
        • The medullary chemoreceptors respond directly to changes in the pH of CSF and indirectly to changes in arterial PCO2
        • In the blood, CO2 combines reversibly with H2O to form H+ and HCO3- by the familiar reactions.
        • Because the blood-brain barrier is relatively impermeable to H+ and HCO3-, these ions are trapped in the vascular compartment and do not enter the brain.
          • CO2, however, is quite permeable across the blood-brain barrier and enters the extracellular fluid of the brain. (MCQ)
          • CO2 also is permeable across the brain-CSF barrier and enters the CSF. (MCQ)
        • In the CSF, CO2 is converted to H+ and HCO3- .
          • Thus, increases in arterial PCO2 produce increases in the PCO2 of CSF, which results in an increase in H+ concentration of CSF (decrease in pH). (MCQ)
          • The central chemoreceptors are in close proximity to CSF and detect the decrease in pH. (MCQ)
          • A decrease in pH then signals the inspiratory center to increase the breathing rate (hyperventilation).
      • Peripheral Chemoreceptors
        • There are peripheral chemoreceptors for O2, CO2, and H+ in the carotid bodies located at the bifurcation of the common carotid arteries and in the aortic bodies above and below the aortic arch
        • Information about arterial PO2, PCO2, and pH is relayed to the medullary inspiratory center via CN IX and CN X, which orchestrates an appropriate change in breathing rate. (MCQ)
        • Changes in arterial blood composition detected by peripheral chemoreceptors that produces an increase in breathing rate:
        • Decreases in arterial PO2.
          • peripheral chemoreceptors are relatively insensitive to changes in PO2
          • They respond when PO2 decreases to less than 60 mm Hg
          • Thus, if arterial PO2 is between 100 mm Hg and 60 mm Hg, the breathing rate is virtually constant (MCQ)
          • However, if arterial PO2 is less than 60 mm Hg, the breathing rate increases in a very steep and linear fashion.

 

  • In this range of PO2, chemoreceptors are exquisitely sensitive to O2; in fact, they respond so rapidly that the firing rate of the sensory neurons may change during a single breathing cycle. (MCQ)

 

        • Increases in arterial PCO2.
          • The peripheral chemoreceptors also detect increases in PCO2 (MCQ)
          • Increased PCO2 effect is less important than response to decrease in PO2.
          • Detection of changes in PCO2 by the peripheral chemoreceptors also is less important than detection of changes in PCO2 by the central chemoreceptors.
        • Decreases in arterial pH.
          • Decreases in arterial pH cause an increase in ventilation, mediated by peripheral chemoreceptors for H+.
          • This effect is independent of changes in the arterial PCO2
          • mediated only by chemoreceptors in the carotid bodies (not by those in the aortic bodies). (MCQ)
          • Thus, in metabolic acidosis, in which there is decreased arterial pH, the peripheral chemoreceptors are stimulated directly to increase the ventilation rate (MCQ)
      • Other receptors

 

  • In addition to chemoreceptors, several other types of receptors are involved in the control of breathing, including lung stretch receptors, joint and muscle receptors, irritant receptors, and juxtacapillary (J) receptors. (MCQ)

 

        • Lung stretch receptors.
          • Mechanoreceptors

 

  • present in the smooth muscle of the airways

 

          • Hering-Breuer reflex(MCQ)
            • When stimulated by distention of the lungs and airways, mechanoreceptors initiate a reflex decrease in breathing rate called the Hering-Breuer reflex.
            • The reflex decreases breathing rate by prolonging expiratory time. (MCQ)
        • Joint and muscle receptors.
          • Mechanoreceptors located in the joints and muscles
          • detect the movement of limbs
          • instruct the inspiratory center to increase the breathing rate.

 

  • Information from the joints and muscles is important in the early (anticipatory) ventilatory response to exercise (MCQ)

 

      • Irritant receptors.
        • Irritant receptors for noxious chemicals and particles
        • located between epithelial cells lining the airways.
        • Information from these receptors travels to the medulla via CN X  (MCQ)
        • causes a reflex constriction of bronchial smooth muscle
        • causes an increase in breathing rate.
      • J receptors.
        • Juxtacapillary (J) receptors are located in the alveolar walls  are near the capillaries. (MCQ)
        • Engorgement of pulmonary capillaries with blood and increases in interstitial fluid volume may activate these receptors
        • produce an increase in the breathing rate.
        • For example, in left- sided heart failure, blood “backs up” in the pulmo- nary circulation, and J receptors mediate a change in breathing pattern, including rapid shallow breathing and dyspnea (difficulty in breathing). (MCQ)