Physiological Control when Breathing

Average breaths per day

~20,000 taken unconsciously

How does unconscious contraction of respiratory muscles occur?

Respiratory muscles are skeletal (voluntary) muscles.

However, their contractions are controlled within the respiratory centre of the brain stem.

• Medulla oblongata & pons

Medulla neurons:

2 groups

• dorsal respiratory group (DRG)

• ventral respiratory group (VRG)

Medulla neurons:

• dorsal respiratory group (DRG)

Maintain the rhythm of normal, quiet breathing

When inhaling:

Increase DRG activity → contraction of diaphragm/external intercostals.

When exhaling:

Decrease DRG activity → relaxation of diaphragm/external intercostals.

Medulla neurons:

• ventral respiratory group (VRG)

Active during forced, controlled breathing.

When inhaling & exhaling:

Innervates accessory respiratory muscles

Pons neurons:

1 group

• pontine respiratory group (PRG)

Pons neurons:

• pontine respiratory group (PRG)

Modifies activity of the DRG based on inputs from various receptors/brain regions (sensory information received)

What information is sent to the brain to control breathing?

• Chemoreceptors

• Higher brain centres

  • Conscious control of things

  • Varying emotions (being frightened)

• Irritant receptors in lungs

  • senses irritants, can cause coughing

• Stretch receptors in lungs

  • Reason why after holding breath for too long, feels like lungs are being stretched too far (not actually stretching)

• Proprioceptors in muscles/joints

Chemoreceptors in lungs:

2 types

• Central chemoreceptors (in brainstem)

• Peripheral chemoreceptors (carotid arteries & aortic arch)

Central chemoreceptors

Brainstem

• Respond to changes in pH/pCO₂ in cerebrospinal fluid (CSF)

• Detects changes in H⁺ ions

  • Not sensitive to changes in pO₂

• If pCO₂ goes up, H⁺ ions go up, pH decreases.

  • Signals for more breathing to occur, so CO₂ exits the body

Peripheral chemoreceptors

Carotid arteries & aortic arch

• respond to changes in pCO₂, pH and pO₂ in blood

• We are much less sensitive to changes in pO2 and it can drop a long way before peripheral chemoreceptors detect a problem (needs to drop below ~60 mmHg)

  • Stimulates brain to have more breathing occur

• Mainly because RBCs/Hb are so good at binding/delivering O₂, they can do it efficiently even if pO₂ drops

• Peripheral chemoreceptors are STILL more responsive to increases in CO₂ than drops in O₂

Does the brain moniter pO₂ or pCO₂ more attentively?

pCO₂ is the primary determinant of breathing rate

Why?

Because the Carbonic anhydrase reaction:

CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃^-

Changes H⁺ ion concentration, which controls pH.

The body reacts strongly to changes in pH, a certain level of pH must be maintained for optimal activity.

Haemoglobin is very forgiving which changes in pO₂ (can drop from 100 mmHg to 75 mmHg without a steep decline in haemoglobin saturation). changes in pO₂ are much more forgiving than changes in pCO₂