5.3 Neural Control of Breathing

Question 1

What is the major impacter of blood gas homeostasis

Answer 1

Breathing

Question 2

What are the partial pressures of oxygen and carbon dioxide in the blood at rest

Answer 2

PO2 = 100±2 mm Hg
PCO2 = 40±2 mm Hg

Question 3

What are some things that can modulate breathing

Answer 3

-emotional events (e.g crying)
-vocalisation (e.g singing)
-reflexes (e.g coughing)
-volitional control (e.g holding breath)
-physiological factors (e.g sleep or exercise)

Question 4

What are the two types of receptors that send signals to the brain in order to regulate breathing

Answer 4

Chemoreceptors and mechanoreceptors

Question 5

What are the types of muscles that the brain then sends signals to to control breathing

Answer 5

Respiratory muscles (such as the diaphragm - via the phrenic nerve and intercostals) in order to produce the rhythmic breathing motion

Upper airways muscles (such as pharyngeal, laryngeal and tongue) in order to produce reflexes to keep the airways patent

Question 6

What are chemoreceptors

Answer 6

Sensory receptors that detect chemical changes in the surrounding environment. In the respiratory system, they detect changes in the partial pressure of oxygen, partial pressure of carbon dioxide and the pH. These changes are hypoxia (a decrease in pO2) or hypercapnia (an increase in pCO2, causing a decrease in pH)

Question 7

What are the two types of chemoreceptor

Answer 7

central or peripheral

Question 8

Describe peripheral chemoreceptors

Answer 8

Small highly vascularised bodies in the aortic arch or carotid sinus region. Peripheral chemoreceptors respond to a decrease in pO2 (hypoxia). They send signals down the glossopharyngeal and vagus nerves to the NTS in the brainstem. The NTS sends info to respiratory control centres which send signals down the phrenic nerves to respiratory muscles causing an increase in ventilation to restore pO2 levels.

Question 9

Describe the ventilatory response to hypoxia.

Answer 9

Progressive reductions in inspired oxygen have little effect on ventilation until pO2 goes below 60 mmHg, from this point there is progressive hyperventilation.

Question 10

Describe central chemoreceptors

Answer 10

Clusters of neurones in the brainstem that are activated when pCO2 increases (hypercapnia) or when pH decreases. Signals sent are then involved in generating breathing patterns to alter ventilation and restore pCO2.

Question 11

Describe the ventilatory response to hypercapnia

Answer 11

Unlike with O2, very small changes in CO2 have larger effects on ventilation

Question 12

One a moment to moment basis, which gas level has more of a control on breathing

Answer 12

CO2

Question 13

Describe mechanoreceptors

Answer 13

Mechanoreceptors are sensory receptors that detect changes in pressure, movement and touch. In the respiratory system, mechanoreceptors detect movement in the lung and chest wall during inspiration and expiration. Inflation of the lungs activate mechanoreceptors which send signal down the vagus nerve to the NTS in the brain stem and ventilation is adjusted accordingly.

Question 14

Give 2 examples of mechanoreceptors involved in breathing control

Answer 14

In airway smooth muscle, the mechanoreceptors are stimulated by the inflation/distension of airways and the resulting reflex is the termination of inspiration

In the airway epithelium, the mechanoreceptors are stimulated by rapid lung inflation/deflation or by oedema and the resulting reflex is a shortened expiration (a sigh)

Question 15

Describe the nucleus tractus solitarius (NTS)

Answer 15

The NTS is located in the 4th ventricle of the brainstem. Information is sent to the NTS from the mechanoreceptors and peripheral chemoreceptors. Information in the NTS is processed by clusters of respiratory neurones.

Question 16

What are the 2 types of respiratory neuron clusters in the NTS

Answer 16

Pattern generating neurons and rhythm generating neurons

Question 17

Describe pattern generating neurones

Answer 17

Pattern generating neurons are made up of inspiratory neurons (which are active during inspiration) and expiratory neurons (which are active during expiration). Pattern generating neurons are not continuously active and only fire in coordination with their part of the breath.

Question 18

Describe rhythm generating neurones

Answer 18

Rhythm generating neurons are a bilateral cluster of neurons with rhythm generating properties (that continue to generate a respiratory like rhythmic output even when isolated). Rhythm generating neurons fire continuously and the signals are sent down the spinal cord and exit vis the phrenic nerve at cervical 3-5. The phrenic nerve innervates the diaphragm and nerves exiting the thoracic spinal cord innervate the intercostals.

Question 19

What is volitional breathing

Answer 19

Conscious, voluntary control of breathing. For example holding your breath or taking a long exhale.

Question 20

How is volitional breathing controlled

Answer 20

Volitional breathing is controlled by the higher centres of the brain and initiated in the primary motor cortex (a site within the superior primary motor cortex corresponding to the thoracic muscles). Information is corroborated in the pontine respiratory group of the pons (above the NTS), info is sent between the PRG and higher brain centres such as the cerebral cortex, limbic system and hypothalamus. Volitional control of breathing is modulated via the corticospinal tract. The upper motor neurons originate from the primary motor cortex and descend within the corticospinal tract. They then synapse with the lower motor neurons (either directly or via interneurons) that are located in the anterior horn of C3, C4 and C5. These motor neurons then exit as part of the phrenic nerve to innervate the diaphragm

Question 21

What are the 2 sepearate nervous pathways controlling breathing

Answer 21

Autonomic and volitional

Question 22

Describe the structure of peripheral chemoreceptors

Answer 22

Very small carotid bodies (in the carotid arch) and aortic bodies (in the aortic arch). They have a high volume of blood going through them that is brought in through sinusoids. They have sympathetic and parasympathetic efferents going towards the brain to alter respiration rates. The vagus nerve connects the aortic bodies to the NTS and the glossopharyngeal nerve connects the carotide bodies to the NTS