py3002 integrative physiology neural control of ventilation

1. PY3002 Integrative Physiology Dr Derek Scott d.scott@abdn.ac.uk See Chapter 10 of �The Respiratory System� by Davies & Moores or A more advanced account in Chapter 31 of �Medical Physiology by Boron & Boulpaep

3. Neural control of respiration Neural control of respiration is an ancient physiological process shared by many organisms. Neural control reacts in less than a second, whilst chemical control takes minutes/seconds. Unconscious act but can be altered voluntarily to take part in other actions e.g. speaking, lifting heavy weight. Normally produces level of minute respiration to control levels of 02, CO2 and [H+] in blood. Neural control determines this pattern of respiration. Minimise work and allows resonation.

4. Where does neural control originate? Breathing originates in brainstem Brainstem consists of medulla and pons. Also contains central pattern generator which has outputs to respiratory muscles, modulated by various sensory afferent inputs. Impulses leave CNS via phrenic and intercostal nerves. Causes contraction of diaphragm and intercostal muscles. Other nerves to accessory muscles (e.g. larynx) synchronize their contractions with phases of breathing.

5. Brainstem respiratory �centres�

6. Rhythm Generator Rhythmic process of breathing produced by central pattern generator, which gives �rough and ready� pattern of breathing. Modified/improved by other parts of brain, using afferent sensory inputs from lung/chest receptors to give pattern that�s efficient and responds to conditions. Pattern originates at area of brainstem (medulla/pons) which links brain to spinal cord. If you cut above medulla � NORMAL BREATHING If you cut between medulla and spinal cord � BREATHING CEASES. Hanging people kills them not by asphyxiation, but by snapping spinal cord � stops breathing by cutting off output from medulla to phrenic nerve and diaphragm.

9. Oscillator model of breathing

10. Neural Activity

11. Off-switch model of breathing

12. Conscious/voluntary control of breathing Control of breathing usually automatic and does not rely on brain areas above pons. Unanaesthetised human breathing is affected involuntarily and voluntarily. Involuntarily � emotion, hyperthermia, exercise Voluntarily � speaking, blowing, whistling, pushing, defecation. Voluntary control is bilateral � cannot contract half of diaphragm or larynx. Voluntary control probably from motor cortex which bypasses PRG and rhythm generator in brainstem to descend via pyramidal tracts. Can destroy voluntary control without losing involuntary control i.e. stroke Reverse of this is rare � Ondine�s curse

13. Respiratory Muscle Innervation Diaphragm controlled directly by ? motor neurons � �C3,4 & 5 keeps the diaphragm alive�. Motor neurons take turns to stimulate different groups of muscle fibres active to minimise fatigue. Poor supply of muscle spindles, so control comes from dorsal and ventral respiratory group. Explains why rare to feel fatigue in diaphragm.

14. Larynx and Expiratory Muscles Larynx movements are synchronized with breathing. Superior and recurrent laryngeal nerves are branches of vagus nerve. Expiration � vocal cords together. Inspiration � vocal cords apart. Automatic rhythm that we can override consciously. Expiratory muscles � abdominal and internal intercostals. Abdominals � forced expiration during exercise or coughing. Helps breathing/vocalisation when moving, lifting or in different postures. Opera singers try and maximise control of breathing using these muscles.

15. Vagal Reflexes Control of breathing affected profoundly by vagus nerves (Cranial Nerve X). Two of these run down either side of neck and torso, near trachea. Take info from lots of places, but the ones which are most involved in regulating respiration are: Slowly-adapting pulmonary stretch receptors (PSR�s) Rapidly-adapting (irritant) receptors (RAR�s) C-fibre receptors (J receptors)

16. Adaptation in respiratory receptors

17. Pulmonary Stretch Receptors (PSR�s) Signals travel via large diameter myelinated fibres. Over-inflation increases PSR activity and operates the inspiratory off-switch to halt inspiration. This is the HERING-BREUER INFLATION REFLEX. Not pronounced in resting adults (due to speech?), but may determine breathing rate and depth in neonates or adults when tidal volume is more than 1 L e.g. exercise.

18. Irritant Receptors (RAR�s) Signals travel via small diameter myelinated fibres. Respond to inhaled irritants (e.g. smoke, gases) or distortion of lung (e.g. pneumothorax). Respond more to �rate of change of lung volume� than volume itself. Receptors exist near branching points of airways at epithelial surface. Can trigger Rapid, shallow breathing by shortening expiration. Long, deep, augmented breaths where inspiration is 2 x longer. Two opposite types of breathing occurs because of refractory period after each augmented breath, so rapid breathing can occur. RAR�s help newborns inflate their lungs � aid inflation. Also triggered during lung disease where the structure of the lungs is damaged � explains dyspnoea, changed breathing patterns, reflex bronchoconstriction and increased mucous in many of these diseases.

19. C-Fibre Receptors (J Receptors) Signals travel via small diameter unmyelinated fibres (C-fibres). Some of them in pulmonary vagus nerves are near pulmonary capillaries (i.e. juxtapulmonary capillary/ J receptors). Also some in bronchial walls. Stimulated by oedema, histamine, bradykinin, prostaglandins during lung damage/allergy. Cause apnoea, then rapid,shallow breathing, hypotension, bradycardia, laryngospasm and relaxation of skeletal muscles. Consider � what would you want to do if you had sever lung damage? Is there a role for pulmonary C fibres in normal breathing? � UNKNOWN.

20. Other Reflexes Sneezing Aspiration reflex Swallowing Chest wall � load detection Cough Somatic pain � rapid, shallow breathing Visceral pain - inhibits breathing Cold water � diving reflex � facial immersion Exercise � controversial � increase ventilation due to sensory stimulation from limbs?