Ventilation - the neural control of breathing Flashcards

1
Q

location of the primary respiratory centre

A

‘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

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2
Q

• Four major sites responsible for the adjustment:

A

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

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3
Q

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

A

nucleus tractus solitarius and organs of thorax and abdomen

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4
Q

what is the character of these neurones

A

• 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

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5
Q

• Ramp can be altered by

A
  • 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)
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6
Q

Pneumotaxic & Apneustic Centres

A

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

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7
Q

Pneumotaxic centre

A

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
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8
Q

• Ultimate goal of ventilation

A

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

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9
Q

Hypercapnia

A

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

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10
Q

Hypoxia

A

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

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11
Q

Mechanism of Action of Central Chemoreceptors

A
  • 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
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12
Q

• increases in blood PCO2 causes

A

PCO2 to increase in interstitial fluid of medulla and CSF

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

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13
Q

The Carotid Body

A

• 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

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14
Q

• Senses decreased arterial PO2

A

– 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

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15
Q

• Can sense increases in arterial PCO2

A

– 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

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16
Q

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

A

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

17
Q

Respiratory system receives input from 2 other sources:

A

– Stretch and chemical/irritant receptors

– Higher CNS centres that control non-respiratory activity

18
Q

• Slowly adapting pulmonary stretch receptors

A

– 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)

19
Q

Modulation of Respiratory Output

• Rapidly adapting pulmonary stretch (Irritant) receptors

A

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

20
Q

Modulation of Respiratory Output

• C-fibre receptors (J Receptors)

A

– 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

21
Q

Cough Reflex

A
  • 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
22
Q

three phases of the cough reflex

A

– 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

23
Q

Higher Brain Centre Activity

A

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