Control of Breathing Flashcards
Medullary respiratory centre (MRC)
is the central integrator for breathing control. It is a system of neurons situated in the brainstem (medulla oblongata and pons).
The medullar respiratory centre consists of a number of groups of neurons, the most important of which are:
(1) Dorsal respiratory group (DRG)
(2) Ventral respiratory group (VRG)
Dorsal respiratory group (DRG)
‘Inspiratory centre’. Composed entirely of inspiratory neurons.
Ventral respiratory group (VRG)
‘Expiratory’ centre. Composed of inspiratory and expiratory neurons.
Quiet breathing
Primarily involves the DRG and rhythm generating neurons in the upper part of the VRG (pre- Botzinger complex).
Efferent signals from the DRG intitiate contraction within the inspiratory muscles, principally the diaphragm (phrenic nerve). Impulses are ramped.
Expiration is passive requiring only the cessation of signals from the DRG and subsequent relaxation of the inspiratory muscle, allowing elastic recoil to occur.
Forced breathing
Forced breathing involves the VRG and DRG.
Recruitment of the VRG inspiratory neurons reinforces those of the DRG during forced inspiration.
Expiration entails not only inhibition of all respiratory neurons but also activation of VRG expiratory neurons.
Forced expiration involves contraction of expiratory muscles.
Pontine respiratory centre
The pons contains two important control centres that modify the output from the medullary respiratory centre and fine-tune respiratory rhythm.
(1) The pneumotaxic centre in the upper pons determines the length of the inspiratory phase of breathing.
(2) The apneustic centre in the lower pons prolongs inspiratory activity
The pneumotaxic centre
In the upper pons, determines the length of the inspiratory phase of breathing.
Increased activity cuts inhibits inspiratory neurons.
Smooth transition for inspiration to expiration.
The apneustic centre
In the lower pons, prolongs inspiratory activity.
The pneumotaxic centre usually keeps the apneustic centre suppressed.
Factors which alter ventilation
(1) Higher brain centres
(2) Peripheral chemoreceptors (aortic and carotid bodies)
(3) Stretch receptors (Herring-Breuer reflex)
(4) Irritant receptors
(5) Proprioceptors in muscles and joints
Medullary respiration centre
The output control automatic breating
Cerebral cortex
Exerts voluntary control over breathing (breath holding, hyperventilation, singing, speech)
While the cerebral cortex modifies brainstem control to some degree, it exerts voluntary control of breathing by a separate neural pathway to the respiratory motoneurons (therefore, limited control)
Hypothalamus
Emotional stimuli.
Temperature control centres in the hypothalamus also affects the rate of breathing.
Thalamus
Areas entrain breathing to body movements e.g. during exercise.
Receptors in the airways and lungs
Receptors are located throughout the respiratory tract and lung tissue from the nasal cavity to the alveoli.
(1) Irritant receptors
(2) C-fibre endings
(3) Peripheral sensory receptors in upper airways
(4) Stretch receptors
Irritant receptors
Rapidly adapting pulmonary receptors (RARs)
Lie between airway epithelial cells (conducting airways).
Chemoreceptive - mediate the cough reflex/bronchoconstriction
C- fibre endings
Located in the pulmonary interstitial space, close to circulation.
Play a role in asthma/allergic reaction.
Sensitive to histamine, prostagandins, bradykinin, smoke, noxious gases, capsaicin.
Peripheral sensory receptors in upper airways
Within respirtaory mucosa from nasal cavity to the larynx.
Respond to mechanical and chemical stimuli.
Stretch receptors (slowly adapting pulmonary stretch receptors - SARs)
Lie among airway smooth muscle cells.
Mechanoreceptors
Mediate the Hering-Breuer reflex
Hering-Breur reflex
When stretch receptors in the small airways and lung tissues are sufficiently stimulated they shorten the inspiratory phase of breathign and inhibit further inflation of the lung.
This reflex only operates in adult humans when tidal volume becomes very high (>1L) e.g. strenuous exercise.
In babies, less than 1 year of age, the H-B reflex is an important determinant of resting tidal volume (chemical drive for breathing is not as sensitive in newborns).
Proprioceptors and breathing
Proprioceptors in the form of stretch receptors in muscles and various receptors in joints provide a powerful stimulatory influence to breathing.
These inputs, especially from the limbs and chest wall, become particularly important at the onset of exercise.
Chemical control of breathing
Chemoreceptors are among the most important inputs to the brainstem control centres:
(1) Centrally located in the medulla
(2) Periphally located in the aortic and carotid bodies
Central chemoreceptors
Detect PCO2 and H+.
They are located near the surface of the ventral medulla (separate from, but close to) the medullary respiratory centre.
These receptors are readily accessible to CO2 which diffuses easily out of the cerebral capillaries into the surrounding tissues and into the CSF.
Ions such as H+ and HCO3- are repelled by the blood brain barrier.
Central chemoreceptors: PCO2 and H+
The direct effect of PCO2 on the central chemoreceptors is only transitory. To be fully effective, CO2 must be hydrated in the CSF to yield H+.
Because the CSF has much less protein than plasma it buffers H+ relatively poorly. Therefore, H+ generated from CO2 are able to diffuse to the receptor and intensify the effect.
Peripheral chemoreceptors
Peripheral chemoreceptors are located in or close to the wall of major arteries in the neck (carotid bodies) and upper thorax (aortic bodies) and receive a rich blood supply. The largest concentration is in the carotid body.
Each carotid body is innervated by a branch of the carotid sinus nerve, which in turn forms a branch of the glossopharyngeal cranial nerve (IX) which projects to the medulla.
