Regulation of breathing

Cerebrum function (2)

• largest part of brain

• Voluntary breathing

Medulla Oblongata function (3)

• Involuntary breathing

• Rhythmicity centres

• Respiratory control centres

Motor Innervation of Muscles Involved in Breathing (2)

• Breathing involves the contraction and relaxation of specific muscles controlled by somatic motor neurons in the spinal cord

• These neurons receive inputs from both voluntary and involuntary pathways

Motor Innervation of Muscles (3)

• Diaphragm

• Muscles of the Rib Cage

• Abdominal Muscles

Diaphragm - Motor Innervation (2)

• Nerve: Phrenic nerve

• Cell Body Location: Cervical level of the spinal cord (gray matter)

Muscles of the Rib Cage - Motor Innervation (2)

• Includes external intercostals, internal intercostals, and parasternal intercostals

• Cell Body Location: Gray matter at various levels of the thoracic spinal cord

Abdominal Muscles - Motor Innervation

Cell Body Location: Thoraco-lumbar parts of the spinal cord

Somatic Motor Neurons (3)

• responsible for controlling respiratory muscles

• located in the gray matter (central H-shaped region) of the spinal cord

• controlled by descending tracts from Medulla Oblongata + Cerebral Cortex

Descending (Tracts) pathways (2)

• Voluntary Pathway

• Involuntary Pathway

Voluntary Pathway / Breathing (3)

• Origin: Cerebral Cortex

• Path: Axons descend in the corticospinal tracts

• Allows conscious control of respiration (e.g., holding breath)

Involuntary Pathway / Breathing (3)

• Origin: Respiratory control centers in the Medulla Oblongata (brainstem)

• Path: Axons descend in the lateral (4) and ventral (9) white matter of the spinal cord

• Responsible for rhythmic + automatic control of breathing (e.g., during sleep)

Involuntary, Automatic Breathing (3)

• Controlled by neurons in the ventrolateral region of the medulla oblongata

• Rhythmicity Centre generates the rhythm

• Pacemaker neurons in the Rhythmicity Centre responsible for the rhythmic firing of inspiration and expiration neurons

Pacemaker neurons (3)

• Located within the Rhythmicity Centre of the medulla oblongata

• Responsible for the rhythmical firing of inspiration (I) and expiration (E) neurons

• Show spontaneous, cyclical changes in membrane potential, similar to the pacemaker cells of the heart

Ventrolateral (Rhythmicity Centre) components (2)

• I neurons (inspiration): 4 types

• E neurons (expiration): 2 types

Ventrolateral Centre of the medulla oblongata parts (2)

• Dorsal Respiratory Group

• Ventral Respiratory Group

Dorsal Respiratory Group

Stimulates spinal motor neurons of the phrenic nerve to innervate the diaphragm

Ventral Respiratory Group (2)

• I neurons stimulate spinal interneurons activating spinal motor neurons

• E neurons inhibit motor neurons of the phrenic nerve during expiration

Respiratory Control Centres in Pons (2)

• Apneustic Centre

• Pneumotaxic Centre

Apneustic Centre

Promotes inspiration by stimulating I neurons in the medulla oblongata

Pneumotaxic Centre

Inhibits inspiration by antagonizing the apneustic centre

Chemoreceptors (2)

• sensory receptors that respond to chemical stimuli in the environment

• changes in the concentration of certain molecules such as carbon dioxide (CO₂), oxygen (O₂), and hydrogen ions (H⁺)

Sensory feedback from Chemoreceptors sensitive to (5)

• Arterial blood:

  • PCO₂

  • PO₂

  • pH

• PCO₂ + pH changes in Brain interstitial fluid + Cerebrospinal fluid

Central chemoreceptors + Peripheral chemoreceptors similarity (3)

• sensitive to PCO₂ + PO₂ + pH in Arterial blood

• detect changes in blood gas levels (PCO₂, PO₂) and pH

  • thus regulating ventilation to maintain homeostasis

Central Chemoreceptors location

Medulla Oblongata, near the exit of the IX and X cranial nerves, on the ventrolateral surface

Central Chemoreceptors Stimulated by (2)

• Arterial PCO₂ (most)

• Cerebrospinal fluid PCO₂ and pH

Central Chemoreceptors Function (4)

• Responsible for 70-80% of the increase in ventilation in response to a sustained rise in arterial PCO₂

• Communicate with the rhythmicity centre neurons in the medulla

  • Their response to a rise in PCO₂ takes several minutes

• The immediate response to PCO₂ elevation is mediated by peripheral chemoreceptors

Peripheral Chemoreceptors Location (2)

• Carotid Bodies

• Aortic Bodies

Carotid Bodies (2)

• Small nodules at the bifurcation of the common carotid artery into the internal and external carotid arteries

• Send information via the glossopharyngeal nerve

Aortic Bodies (2)

• Small nodules around the aortic arch

• Send sensory information to the medulla oblongata via the vagus nerve

Peripheral Chemoreceptors Stimuli (3)

• Decrease in pH in arterial blood due to an increase in H⁺ ions

• Immediate response

• PCO₂ rise in hypoventilation produces a Fall (acidic) in pH which stimulates the peripheral chemoreceptors

PCO₂ and Respiratory Regulation

• PCO₂ is a stronger stimulus for the reflex control of ventilation

• Respiratory rate and depth are adjusted to maintain arterial PCO₂ at 40 mmHg

• PO₂ content in the blood fluctuates less with changes in ventilation because oxygen is attached to hemoglobin

• Blood PCO₂ and pH are affected more quickly by changes in ventilation

• Oxygenation of the blood occurs as a side product

Hypercapnia (2)

• Refers to a rise in PCO₂ due to hypoventilation

• Chemoreceptors respond by stimulating an increase in ventilation

Hypocapnia (2)

• Refers to a fall in PCO₂ due to hyperventilation

• Chemoreceptors respond by decreasing ventilation

Chemoreceptor Input

Chemoreceptor input to the brainstem modifies the rate and depth of breathing to maintain relatively constant levels of PCO₂ + pH + PO₂

The Effect of Arterial PO₂ on Ventilation (5)

• The effect of PO₂ on breathing is indirect

• PO₂ influences chemoreceptor sensitivity to changes in PCO₂

  • Low PO₂ increases chemoreceptor sensitivity to PCO₂

  • High PO₂ decreases chemoreceptor sensitivity to PCO₂

• Breathing 100% oxygen blunts the response to PCO₂ decreasing the ventilation rate

Hypoxic Drive (5)

• PO₂ in the blood falls below 70 mmHg (Hypoxemia)

• Ventilation increases significantly due to a direct effect on the carotid bodies

  • sensitive to plasma oxygen (not the oxygen bound to hemoglobin in red blood cells)

• Not normally occur at sea level

• Relevant in Chronic Obstructive Pulmonary Disease (COPD)

Chronic Obstructive Pulmonary Disease (COPD) - Hypoxic Drive (3)

• Chronic retention of PCO₂ due to inadequate ventilation results in blunting of the chemoreceptor response to increased PCO₂

• Over time, chronic hypoxia reduces the sensitivity of carotid bodies to low PO₂, and even hypoxic drive is no longer effective

• Breathing problems are exacerbated

Pulmonary Receptors

Influence brainstem respiratory control centres via sensory fibers in the vagus nerve

Pulmonary Receptors Types (3)

• Rapidly Adapting Receptors (Irritant receptors)

• Pulmonary Stretch Receptors

• Unmyelinated C fibres

Rapidly Adapting Receptors (Irritant receptors) (5)

• Found in the walls of the larynx and lungs

• Stimulated by an increase in pulmonary interstitial tissue fluid

• Cause coughing in response to irritants like smoke, inhaled fine particles, asthma, and smog

Pulmonary Stretch Receptors (3)

• Inhibit respiratory control centres during inspiration

• Prevent overdistention of lungs

• Involved in the Hering-Breuer reflex

Unmyelinated C fibres (2)

• Stimulated by capsaicin (chemical in hot peppers)

• Initial apnoea, followed by rapid shallow breathing

Motor responses to airway pulmonary receptors (photo!)