Ventilation - the neural control of breathing

Question 1

location of the primary respiratory centre

Answer 1

‘Centres’ located in medulla oblongata and pons
• Collect sensory information about O2 and CO2 levels in blood
• Determines signal sent to respiratory muscles which leads to alveolar ventilation

Question 2

• Four major sites responsible for the adjustment:

Answer 2

– Respiratory control centre (source of central pattern generator)
– Central chemoreceptors
– Peripheralchemoreceptors
– Pulmonary mechanoreceptors

Question 3

where are the dorsal and ventral respiratory groups located and where does it receive sensory supply input from

Answer 3

nucleus tractus solitarius and organs of thorax and abdomen

Question 4

what is the character of these neurones

Answer 4

• Neurons in this group emit repetitive bursts of inspiratory neuronal action potentials
• Cause of repetitive bursts not known
• Involves respiratory ramp for 2 seconds
followed by cessation for 3 seconds

Question 5

• Ramp can be altered by

Answer 5

• Controlling rate of increased of ramp (heavy breathing, ramp increases rapidly so lungs fill rapidly)
  – Controlling limiting point at which ramp suddenly stops (control rate of respiration)

Question 6

Pneumotaxic & Apneustic Centres

Answer 6

Centres modulate, but are not essential for, normal respiratory output

Question 7

Pneumotaxic centre

Answer 7

located dorsally in nucleus parabrachialis medialis of upper pons

• 1o effect is to control switch-off point of inspiratory ramp (so controls filling phase of lung cycle)
• Strong pneumotaxic signal - inspiration may last for less than 0.5 second while a weak pneumotaxic signal - inspiration may last for 5 or more seconds

Question 8

• Ultimate goal of ventilation

Answer 8

to maintain proper levels of PO2, PCO2 & pH (H+)

Question 9

Hypercapnia

Answer 9

(↑PCO2) and acidosis (↓pH) detected by central respiratory centre

Question 10

Hypoxia

Answer 10

(↓PO2) detected by peripheral chemoreceptors in carotid and aortic bodies, also detects Hypercapnia (PCO2) and acidosis (pH)

Question 11

Mechanism of Action of Central Chemoreceptors

Answer 11

• Chemosensitive area located bilaterally beneath ventral surface of the medulla
• Neurons very sensitive to H+ ions (may be only important direct stimulus)
• H+ ions do not cross blood brain barrier very well, however, CO2 crosses easily

Question 12

• increases in blood PCO2 causes

Answer 12

PCO2 to increase in interstitial fluid of medulla and CSF

• CO2 combines with H2O to form H+ ions by action of carbonic anhydrase

Question 13

The Carotid Body

Answer 13

• Bodies have multiple highly characteristic glandular-like cells (Glomus cells) that
synapse directly or indirectly with nerve endings
• Both sympathetic & parasympathetic NS innervate carotid body

Question 14

• Senses decreased arterial PO2

Answer 14

– Low PO2, but normal PCO2 and pH
– increase in firing rate of carotid sinus nerve
– At normal values of PCO2 and pH a decrease of PO2 causes progressive increase in firing rate

Question 15

• Can sense increases in arterial PCO2

Answer 15

– Results show graded icnreases in PCO2 at a fixed blood pH (7.45) and fixed PO2 (80mmHg), produced graded increases in firing rate of carotid sinus

Question 16

• Can sense decreases in arterial pH (e.g. metabolic acidosis)

Answer 16

– Blood pH (7.25) and fixed PO2 (80mmHg), firing rate of carotid sinus nerve is greater over all PCO2 values

Question 17

Respiratory system receives input from 2 other sources:

Answer 17

– Stretch and chemical/irritant receptors

– Higher CNS centres that control non-respiratory activity

Question 18

• Slowly adapting pulmonary stretch receptors

Answer 18

– Hering-Breuer reflex (1868)
– Helps to prevent over-inflation of the lungs
– Stretch receptors located in muscular portions of walls of bronchi and bronchioles
– Send signals thro’ vagal nerves to DRG neurons when lungs overstretched
– Feedback response initiated that ‘switches off ‘ inspiratory ramp
– In humans reflex not activated until tidal volume es to about 3 times normal (i.e. 1.5L / breath)

Question 19

Modulation of Respiratory Output

• Rapidly adapting pulmonary stretch (Irritant) receptors

Answer 19

– Epithelium of trachea, bronchi and bronchioles contains sensory nerve endings,
pulmonary irritant receptors
– Responsible for coughing and sneezing

Question 20

Modulation of Respiratory Output

• C-fibre receptors (J Receptors)

Answer 20

– Receptors in alveoli and conducting airways close to capillaries
– Respond to chemical and mechanical stimuli
– Stimulated during conditions like pulmonary oedema, congestion, pneumonia, Also from endogenous chemicals such as histamine
– Induces apnea

Question 21

Cough Reflex

Answer 21

• Nerve endings of vagus and/or visceral afferent fibres are activated by irritation of trachea or bronchi
• Action potentials travel to medulla and spinal cord respectively

Question 22

three phases of the cough reflex

Answer 22

– Preparatory inspiration
– Compressive phase
• Glottis closed by vagal efferent activity
• Forced expiration against a closed glottis
• Pressure increases
– Expulsive phase
• Glottis suddenly opens and trapped air is expelled at high speed by contraction of internal intercostals and abdominal muscles

• Result is to dislodge mucous covering airways and carry irritant away to mouth

Question 23

Higher Brain Centre Activity

Answer 23

Some cortical neurons send axons to respiratory centres in medulla
Some cortical premotor neurons send axons to motor neurons controlling respiratory muscles