Regulation of the Respiration

Respiratory Center and Formation of the Respiratory Rhythm
1 Respiratory Center

Respiratory Centers

Two respiratory nuclei in medulla oblongata
Expiratory center (ventral respiratory group, VRG)
involved in forced expiration
Inspiratory center (dorsal respiratory group, DRG)
more frequently they fire, more deeply you inhale
longer duration they fire, breath is prolonged, slow rate

Respiratory Centers in Pons
Apneustic center (lower pons)
Sends continual inhibitory impulses to inspiratory center of the medulla oblongata,
As impulse frequency rises, breathe faster and shallower
Stimulation causes apneusis
Integrates inspiratory cutoff information
Pneumotaxic center (upper pons)

Respiratory Structures in Brainstem

2. Rhythmic Ventilation (Inspiratory Off Switch)
Starting inspiration
Medullary respiratory center neurons are continuously active (spontaneous)
Center receives stimulation from receptors and brain concerned with voluntary respiratory movements and emotion
Combined input from all sources causes action potentials to stimulate respiratory muscles

Increasing inspiration
More and more neurons are activated
Stopping inspiration
Neurons receive input from pontine group and stretch receptors in lungs.
Inhibitory neurons activated and relaxation of respiratory muscles results in expiration.
Inspiratory off swithch.

3. Higher Respiratory Centers
Modulate the activity of the more primitive controlling centers in the medulla and pons.
Allow the rate and depth of respiration to be controlled voluntarily.
During speaking, laughing, crying, eating, defecating, coughing, and sneezing. ….
Adaptations to changes in environmental temperature --Panting

II Pulmonary Reflex
Chemoreceptor Reflex

Two Sets of Chemoreceptors Exist
Central Chemoreceptors
Responsive to increased arterial PCO2
Act by way of CSF [H+] .
Peripheral Chemoreceptors
Responsive to decreased arterial PO2
Responsive to increased arterial PCO2
Responsive to increased H+ ion concentration.

Central Chemoreceptor Location
Rostral
Medulla
Caudal
Medulla
Ventral Surface

Central Chemoreceptor Stimulation

Peripheral Chemoreceptor Pathways

Peripheral Chemoreceptors
Carotid bodies
Sensitive to: PaO2, PaCO2, and pH
Afferents in glossopharyngeal nerve.
Aortic bodies
Sensitive to: PaO2, PaCO2, but not pH
Afferents in vagus

Carotid Body Function
High flow per unit weight:
(2 L/min/100 g)
High carotid body VO2 consumption:
(8 ml O2/min/100g)
Tiny a-v O2 difference:
Receptor cells see arterial PO2.
Responsiveness begins at PaO2 (not the oxygen content) below about 60 mmHg.

Carotid Body Response
Critical
PO2
Hypercapnea
Acidosis
Hypocapnea
Alkalosis

Carbon Dioxide, Oxygen and pH Influence Ventilation (through peripheral receptor)
Peripheral chemoreceptorssensitive to PO2, PCO2 and pH
Receptors are activated by increase in PCO2 or decrease in PO2 and pH
Send APs through sensory neurons to the brain
Sensory info is integrated within the medulla
Respiratory centers respond by sending efferent signals through somatic motor neurons to the skeletal muscles
Ventilation is increased (decreased)

Effects of Hydrogen Ions (through central chemoreceptors)
pH of CSF (most powerful respiratory stimulus)
Respiratory acidosis (pH < 7.35) caused by failure of pulmonary ventilation
hypercapnia (PCO2) > 43 mmHg
CO2 easily crosses blood-brain barrier, in CSF the CO2 reacts with water and releases H+, central chemoreceptors strongly stimulate inspiratory center
corrected by hyperventilation, pushes reaction to the left by “blowing off ” CO2
CO2 (expired) + H2O ← H2CO3 ← HCO3- + H+

Carbon Dioxide
Indirect effects
through pH as seen previously
Direct effects
↑ CO2 may directly stimulate peripheral chemoreceptors and trigger ↑ ventilation more quickly than central chemoreceptors
If the PCO2 is too high, the respiratory center will be inhibited.

Oxygen
Direct inhibitory effect of hypoxemia on the respiratory center
Chronic hypoxemia, PO2 < 60 mmHg, can significantly stimulate ventilation
emphysema, pneumonia
high altitudes after several days

Overall Response toPco2, Po2 and pH

2. Neuroreceptor reflex

Hering-Breuer Reflex or Pulmonary Stretch Reflex
Including pulmonary inflation reflex and pulmonary deflation reflex
Receptor: Slowly adapting stretch receptors (SARs) in bronchial airways.
Afferent: vagus nerve
Pulmonary inflation reflex:
Terminate inspiration.
By speeding inspiratory termination they increase respiratory frequency.
Sustained stimulation of SARs: causes activation of expiratory neurons

Significance of Hering-Breuer
Normal adults. Receptors are not activated at end normal tidal volumes.
Become Important during exercise when tidal volume is increased.
Become Important in Chronic obstructive lung diseases when lungs are more distended.
Infants. Probably help terminate normal inspiration.

Brainstem Transection

Factors Influencing Respiration