7. Breathlessness and Control of Breathing (Awake) Flashcards
Why do we breathe?
The purpose of control of breathing muscles include:
o Appropriate gas exchange to maintain metabolic homeostasis
o Defence of lung and airways, i.e. through reflex protective behaviours (e.g. coughing, sneezing and yawning)
o Other functions (non-metabolic) include communication (speech, singing), expressing emotions, non-respiratory behaviours (including defecation and posture)
Provide an overview of breathing control
There are two separate ‘controllers’ in the brain:
o ‘automatic’ bulbopontine controller (found in the brainstem, within the medulla and pons)
o ‘behavioural’ suprapontine controller (widely distributed, but all superior to the pons)
These have a common motor output from a spinal motor neurone pool which leads to lung inflation and alveolar ventilation
What are the 3 factors of input to consider in terms of breathing control?
Chemoreceptors
Mechanoreceptors
Sensory input
Outline chemoreceptors as an input mechanism to breathing control
Peripheral chemoreceptors are found in the bifurcation of the common carotids (respond to pH, PaCO2, hypoxia), and the aortic
arch (respond to PaCO2, hypoxia)
Central chemoreceptors are located on the surface of the
medulla, respond to PaCO2, but not pH or hypoxia
What is PaCO2?
The partial pressure of carbon dioxide in the arterial blood; arterial carbon dioxide concentration or tension
It is usually expressed in millimeters of mercury (mmHg)
The normal range is 38-42mmHg
What is PO2?
The partial pressure of oxygen in there arterial blood
It is normally expressed in millimetres of mercury (mmHg)
The normal range is 80-100mmHg
Outline mechanoreceptors as an input mechanism to breathing control
Within the lung:
o Slowly adapting pulmonary stretch receptors (which respond via inhalation reflex or Hering-Breuer reflex)
o Rapidly adapting pulmonary stretch receptors
o J receptors (bronchial C fibre receptors)
o Irritant receptors
Within the chest wall:
o Joint receptors
o Golgi tendon organs
o Muscle spindles
Outline the Hering-Breur reflex
The Hering–Breuer inflation reflex is a reflex triggered to prevent over-inflation of the lungs
Pulmonary stretch receptors present in the smooth muscle of the airways respond to excessive stretching of the lung during large inspirations
Once activated, they send action potentials through large myelinated fibers of the vagus nerve to the inspiratory area in the medulla and apneustic center of the pons
In response, the inspiratory area is inhibited directly and the apneustic center is inhibited from activating the inspiratory area
This inhibits inspiration, allowing expiration to occur
Outline sensory input as an input mechanism to breathing control
From the nose - trigeminal (V) nerve
From the pharynx - glossopharyngeal (IX) and vagus (X) nerves
From the larynx - vagus (X) nerve
From the lungs – vagus (X) nerve
From the chest wall – spinal nerves
Outline neural output in terms of breathing control
From the diaphragm: Phrenic nerve and Cervical plexus (C3 - C5)
From the intercostal muscles: T1 - T12
From the abdominal muscles: T6 - L1
What can the pons be divided into?
The pons can be divided into two different centres; the pneumotaxic centre and the apneustic centre:
o The pneumotaxic centre, found in the rostral dorsal lateral pons, antagonises the apneustic centre cyclically, inhibiting inspiration by sending a ‘switch off’ signal to the dorsal respiratory group within the medulla
o The apneustic centre, found in the lower pons, promotes inspiration by stimulations of the dorsal respiratory group of neurons in the medulla
What are the two sets of respiratory groups of neurones in the medulla?
The dorsal respiratory group (DRG) consists of inspiratory neurones
The ventral respiratory group (VRG) consists of expiratory neurones (which receive input from the DRG) and the pre-Botzinger complex (pre-Bot C) which is a complex of rhythm-generating neurones
Define bulbopontine, rhombencephalon, tegmentum and pons
Bulbopontine; relating to the rostral part of the rhombencephalon composed of the pons and overlying tegmentum
The rhombencephalon is the portion of the brain developed the most caudal to the three primary vesicles of the embryonic neural tube
The tegmentum is the posterior part of the mesencephalon, and is also a term for a ‘covering structure’
The pons varolii or pons cerebelli; the part of the brainstem between the medulla oblongata caudally and the mesencephalon rostrally, composed of the basilar part of pons and the tegmentum of pons
Outline rhythm generation in terms of the automatic bulbopontine controller
Output from the DRG and VRG –> spinal motor neurons –> muscles –> inspiration/expiration (depending upon the origin of input)
Ventilation of the lung –> stimulation of the lung stretch receptors, which then inhibit the inspiratory neurons of the DRG and the Apneustic centre within the pons
Ventilation also alters the blood gas partial pressures, which stimulates arterial chemoreceptors, which result in stimulation/inhibition of DRG (depending upon gas partial pressures)
Neuron activity is cyclical/rhythmic due to the synaptic interaction between groups of neurons
Outline automatic reflex drive
Ventilator response to increased PaCO2 is linear, due to an immediate response from arterial chemo and mechanoreceptors
However the ventilatory response to reduced PaO2 is not immediate or linear
