Respiratory Physiology: Control of Ventilation and Neural and Chemical Mechanisms

Neural Control of Ventilation Overview

• Ventilation vs. Respiration: Ventilation is the physical process of moving air in and out of the lungs. It is distinct from respiration, which involves the exchange of gases.
• Automaticity: Ventilation is an automatic process. Under normal circumstances, it requires no conscious awareness. This allows humans to perform other tasks like driving or talking without focusing on breathing.
• Conscious Control: While automatic, the breathing pattern and volume can be voluntarily changed for activities such as:
  • Calming oneself down (deep breaths).
  • Singing.
  • Playing instruments.
  • Yelling.
• Rhythmic Cycle of Breathing: This originates in the brainstem, primarily from neurons within the medulla oblongata. Higher brain centers and various receptors modify the medulla's impulses to control the respiratory rate ($RR$) and depth of breathing, also known as alveolar ventilation.

Medullary Respiratory Centers

• Definition: The Medullary Respiratory Center consists of two main groups of neurons located in the medulla oblongata on the brainstem.
• Dorsal Respiratory Group (DRG):
  • Location: The posterior (back) portion of the medulla.
  • Function: Activates the primary inspiratory muscles.
  • Innervation: Receives impulses via the Ninth Cranial Nerve (Glossopharyngeal) and the Tenth Cranial Nerve (Vagus). It sends impulses down the phrenic nerve and external intercostal nerves to initiate inspiration.
• Ventral Respiratory Group (VRG):
  • Location: The anterior (front) and lateral portions of the medulla.
  • Function: Contains both inspiratory and expiratory neurons. It is primarily activated during times of stress, exercise, or forced breathing maneuvers.
  • Inspiratory Action: Specifically assists by sending impulses to laryngeal and pharyngeal muscles via the Vagus nerve to abduct vocal cords and increase the diameter of the glottis to allow more air in. It also stimulates the diaphragm and external intercostals for larger contractions.
  • Expiratory Action: Under normal conditions, expiration is passive. During forced expiration (e.g., exercise or a Pulmonary Function Test), the VRG sends impulses to the internal intercostals and abdominal muscles to contract.

Pontine Respiratory Centers

• Location: Found in the Pons, the upper portion of the brainstem. These centers help "fine-tune" the rhythmic transition between inspiration and expiration.
• Pneumotactic Center:
  • Location: Upper Pons.
  • Primary Function: Limits the length of inspiration (the "off switch"). It sends inhibitory signals to the DRG to halt inspiratory impulses, allowing for smooth transition to expiration.
  • Regulation: Receives signals from stretch receptors in the lungs.
  • Inspiratory Time ($T_I$):
    • Strong Signals: Abruptly stop inspiration, leading to a shorter $T_I$ and faster $RR$.
    • Weak Signals: Slowly inhibit inspiration, leading to a longer $T_I$ and slower $RR$.
• Apneustic Center:
  • Location: Lower Pons.
  • Function: Sends inspiratory signals to the DRG and VRG to trigger inspiration. Under normal conditions, it is held in check by the Vagus nerve and Pneumotactic center.
  • Apneosis: Only activated if the Pneumotactic center is damaged. This results in "apneosis," characterized by very long, gasping inspiratory efforts interrupted by occasional expiration.

Spinal Cord Anatomy and Injuries

• Phrenic Nerve: Arises from the cervical spinal cord at the $C3$ to $C5$ levels. It innervates the diaphragm.
  • Mnemonic: "C3, C4, C5 keep the diaphragm alive."
• Intercostal Nerves: Arise from the thoracic spinal cord at levels $T1$ to $T12$.
• Abdominal Muscles: Arise from the lumbar region, roughly $T7$ to $L1$.
• Impact of Trauma:
  • Above Pons: Breathing remains, but patterns become irregular due to loss of the Pneumotactic center's regulation.
  • Below Medulla/C-Spine: Complete transection leads to cessation of all spontaneous breathing efforts, as communication between the brainstem and respiratory muscles is severed. These patients require lifelong mechanical ventilation.

The Inspiratory Ramp Signal

• Definition: The interaction between medullary neurons and muscles where signals start low and gradually increase in strength.
• Purpose: Prevents gasping and creates a progressively stronger contraction for smooth lung filling.
• Exercise Modification: During exercise, peripheral proprioceptors in joints send signals to the DRG to increase the "steepness" of the ramp, filling the lungs more rapidly while maintaining smoothness.
• Timings: Inspiration typically lasts about two seconds before inhibitory signals switch it off, followed by approximately three seconds of passive expiration.

Respiratory Reflexes and Receptors

• Hering-Breuer Inflation Reflex:
  • Location: Stretch receptors in the visceral pleura and walls of the bronchi/bronchioles.
  • Function: Protective mechanism to prevent overinflation. When lungs reach a tidal volume ($V_T$) of $800\,mL$ to $1000\,mL$, inhibitory signals are sent to the DRG to cease inspiration.
  • Pathology: Patients with low lung compliance (restrictive disorders) activate this reflex sooner because increased inspiratory effort puts more stress on these receptors.
• Hering-Breuer Deflation Reflex:
  • Trigger: Sudden collapse of lung tissue or getting "the wind knocked out of you."
  • Result: Triggers hyperpnea (strong, rapid inspiratory effort) to reinflate the lungs.
• Head’s Paradoxical Reflex:
  • Occurs when the Hering-Breuer reflex is blocked. It allows for deep gasps (like yawning) on top of a breath that is already in the lungs.
  • Newborns: Believed to be responsible for a baby's first breath, providing the high pressure needed to pop open previously fluid-filled alveoli.
• Irritant Receptors:
  • Located in the epithelium of the larger airways (especially the Carina).
  • Triggers: Inhaling water, dust, or mechanical stimulation (suctioning/bronchoscopy).
  • Responses: Coughing, sneezing, reflex bronchoconstriction (to stop deep penetration of irritants), glottis narrowing, and "vagal down" (slowing of the heart rate).
• J-Receptors (Juxtacapillary Receptors):
  • Located in the lung parenchyma, near gas-exchange sites and the interstitial space between alveoli and capillaries.
  • Trigger: Responds to fluid accumulation (pulmonary edema, pneumonia).
  • Result: Triggers rapid, shallow breathing and the sensation of dyspnea.
• Peripheral Proprioceptors:
  • Located in joints, tendons, and skeletal muscles.
  • Function: Send positive impulses to the DRG during physical activity to increase the frequency of nerve firing, resulting in increased $RR$ and depth of breath.

Chemical Control of Ventilation

• Homeostasis: The body monitors partial pressures of oxygen ($P_aO_2$), carbon dioxide ($P_aCO_2$), and hydrogen ions ($H^+$).
• Central Chemoreceptors:
  • Location: Medulla oblongata/brainstem.
  • Stimulus: Bathed in cerebrospinal fluid ($CSF$). They respond to high levels of $H^+$ concentration.
  • Mechanism: While $H^+$ and bicarbonate cannot cross the blood-brain barrier ($BBB$) easily, $CO_2$ crosses readily. Once in the $CSF$, it undergoes the hydration reaction:         $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$
  • Response: For every $1\,torr$ increase in $P_aCO_2$, alveolar ventilation increases by about $2\text{ to }3\,L/min$. This is essentially an "instantaneous" response to blow off excess $CO_2$.
• Peripheral Chemoreceptors:
  • Location: Carotid bodies and Aortic arch.
  • Stimulus: Primarily sensitive to hypoxemia ($O_2$ levels). They also respond to changes in $H^+$ and $CO_2$.
  • Relation to Breathing: If minute ventilation ($V_E$) is doubled, $P_aCO_2$ is halved. This is a very powerful inverse relationship.

Questions & Discussion

• Question (Instructor): Why can you not increase your Total Lung Capacity ($TLC$) with max effort, even if offered a million dollars?
  • Answer: You have a fixed amount of lung tissue and thoracic cavity space. Alveoli only develop until age eight. $TLC$ only increases pathologically (e.g., air trapping in COPD).
• Discussion on David Blaine: The instructor mentions David Blaine as an example of "breath stacking" exercises, but notes humans cannot exceed anatomical limits.
• Clinical Application (Vagal Stimulation):
  • RTs stimulate the Vagus nerve through suctioning at the Carina. Negative effects include bradycardia, hypotension, or even cardiac arrest if overstimulated.
  • Vagal maneuvers (e.g., Valsalva maneuver, carotid massage) can be used to treat tachycardia by stimulating the parasympathetic "rest and digest" response.
• Max Heart Rate Formula:$220 - \text{age} = \text{Max Heart Rate}$
• Anemia and Buffering: The instructor discussed that if a patient is anemic (low red blood cells), they lack Carbonic Anhydrase, the enzyme located in RBCs that catalyzes the dissociation of $CO_2$. This puts them at risk for both hypoxia and difficulty buffering acid, leading to potential metabolic acidosis.
• Bridge Program: Brief mention of a Bachelor of Science in Cardiopulmonary Science ($BSRT$) program available online for practitioners looking to advance beyond an associate degree.