Study Note – Control of Breathing

Breathing can be both voluntary and involuntary.

• Voluntary control: Conscious control through the motor cortex.

• Involuntary control: Automatic regulation via the brainstem (pons and medulla oblongata).

Your body continues to breathe even when you stop thinking about it because of automatic brainstem control mechanisms.

Muscles Involved in Breathing

1. External intercostal muscles – between ribs; elevate the rib cage during inspiration.

2. Diaphragm – dome-shaped muscle; contracts downward during inspiration to expand the thoracic cavity.

When these muscles contract → thoracic cavity expands → air flows into lungs.
When they relax → thoracic cavity recoils → air is exhaled.

Neural Control of Breathing

1. Somatic Motor Neurons

• Control skeletal muscles (diaphragm & intercostals).

• Located in:

  • C3–C5 → form the phrenic nerve (controls diaphragm).

  • T1–T12 → control external intercostal muscles.

• When excited → release stimulatory neurotransmitters → muscles contract.

2. Voluntary (Conscious) Control

• Originates in Primary Motor Cortex (part of cerebral cortex).

• Upper motor neurons travel from cortex → brainstem → spinal cord → synapse with lower (somatic) motor neurons.

• You can start or stop breathing consciously (e.g., holding your breath).

3. Involuntary (Automatic) Control – Brainstem Centers

A. Medulla Oblongata

Contains two major centers:

a. Ventral Respiratory Group (VRG)

• Located on the ventral side of medulla.

• Controls rhythmic breathing:

  • Pre-Bötzinger Complex – acts as pacemaker for inspiration (fires ~12–20 times per minute).

  • Inhibitory neurons alternate firing to stop inspiration → allow expiration.

• Responsible for quiet (eupneic) breathing.

b. Dorsal Respiratory Group (DRG)

• Located on the dorsal side of medulla.

• Integrates sensory input and modifies VRG activity.

• Responds to feedback from:

  • Proprioceptors (in muscles): detect activity → increase breathing rate during exercise.

  • Nociceptors (pain receptors): detect pain → increase respiration (fight-or-flight).

  • Stretch receptors (in lungs): detect over-inflation → inhibit inspiration (via Hering-Breuer reflex).

B. Chemoreceptor Control

Located in the medulla (central chemoreceptors) and peripheral arteries (carotid & aortic bodies).

Central Chemoreceptors detect:

• ↑ CO₂ → increases respiratory rate

• ↓ pH (acidosis) → increases rate

• ↓ O₂ → less potent stimulus, but can also increase rate

Opposite conditions (↓CO₂, ↑pH, ↑O₂) → decrease respiratory rate.

C. Pons

Contains two modulatory centers:

1. Pontine Respiratory Group (PRG)

   • Fine-tunes breathing rhythm; modulates inspiration and expiration.

   • Can both stimulate or inhibit medullary centers.

2. Apneustic Center

   • Promotes prolonged inspiration (“gasping”) when active.

   • Works closely with PRG and VRG to balance inhalation and exhalation.

Hierarchy Analogy (Brainstem as a Company)

• VRG → Workers (directly control muscles).

• DRG → Middle management (monitors and adjusts based on sensory input).

• PRG & Apneustic Center → CEOs (fine-tune and coordinate rhythm).

• Motor Cortex → External override (you can consciously control or interrupt breathing).

If higher centers fail → automatic brainstem centers maintain basic respiration.
If VRG or DRG are damaged → respiration becomes severely compromised.

Key Terms Summary

Term	Function	Location
Phrenic nerve (C3–C5)	Diaphragm control	Cervical spinal cord
VRG (Ventral Respiratory Group)	Generates basic breathing rhythm	Medulla
Pre-Bötzinger Complex	Pacemaker for inspiration	VRG
DRG (Dorsal Respiratory Group)	Integrates sensory feedback	Medulla
PRG (Pontine Respiratory Group)	Modulates rhythm and transitions	Pons
Apneustic Center	Promotes inspiration	Pons
Central Chemoreceptors	Detect CO₂ and pH changes	Medulla
Proprioceptors	Detect muscle activity	Skeletal muscles
Stretch receptors	Prevent lung over inflation	Lungs

Summary

• Breathing = Voluntary (cortical) + Involuntary (brainstem)

• Medulla (VRG + DRG) sets basic rhythm.

• Pons (PRG + Apneustic Center) fine-tunes rhythm.

• Chemoreceptors, proprioceptors, and stretch receptors constantly provide feedback.

• The motor cortex can override automatic control when you consciously alter your breathing.