13. Neuronal Control of Breathing

• minute ventilation = tidal volume x respiratory frequency
• respiratory cycle is due to alternating activity in inspiratory and expiratory muscles which brings about changes in volume
• at rest the diaphragm is the main inspiratory muscle and is innervated by the phrenic nerve

  • if extra effort is needed the external intercostals, scalene and sternocleidomastoid muscles help
• during resting breathing - expiration occurs due to relaxation of the diaphragm

  • if there are higher levels of ventilation - the internal intercostals and the oblique muscles of the abdomen help
• pons and medulla usually function as a single unit, but the medulla is more important for rhythm generation
• DRG = dorsal respiratory group on dorsal surface of brainstem

  • discharge action potentials before inspiration
  • present in the nucleus tractus solitarius (NTS)
  • receives afferent inputs from chemo and mechanoreceptors
• VRG = ventral respiratory group on ventral surface of brainstem

  • affect accessory muscles in both inspiration and expiration
• prebotzinger complex is pivotal is rhythm generation
• inspiration:

  • DRG projects down to motor neurones in spinal cord
  • diaphragm contracts
• expiration:

  • DRG silenced
  • diaphragm relaxes
• in forceful inspiration more action potentials are generated from DRG

  • VRG activity to increase activity of accessory inspiration muscles
• in forceful expiration

  • BOTZ neurones help prevent inspiration by inhibiting inhibitory neurones in either DRG or VRG
  • causes diaphragm and any other muscles to relax
  • activates accessory expiratory muscles to forcefully contract
• pontine group inhibits DRG inspiratory neurones
• neurones in the hypothalamus project down to DRG - temperature and fight/flight affect breathing

  • emotional stimuli from limbic system can affect breathing
• 4 types of mechanoreceptors:

  • lung and airways
  • nose and upper airways
  • joints and muscles
  • arterial baroreceptors
• lung and lower airway receptors are the most influential

  • pulmonary stretch receptors (PSRs) are sensory vagal fibres in the smooth muscle of trachea and conducting airways
  • breathing in -> stretching of the airways -> stimulates the receptors
  • slowly adapting receptors
  • bring inspiration to an end and promote expiration
• hering-breuer reflex prevents the lungs from over expansion

  • present during sleep and more powerful in babies
• lung irritant receptors are stimulated by noxious gases, smoke, dust and cold air

  • rapidly adapting receptors
  • cause bronchoconstriction to prevent noxious substances entering the lung
• c and j receptors are rapidly adapting receptors that are especially sensitive to oedema

  • distorted by fluid coming out of the vessels
  • evoke shallow rapid breathing or apnoea
• nasal mucosa is rich in trigeminal nerve afferents that are stimulated by chemical and mechanical irritants - followed by a sneeze

  • stimulation causes the diving reflex
• larynx and trachea can be excited by chemical and mechanical irritants to initiate a cough
• proprioceptors respond to position and movement - allow for increased force of inspiration and expiration

  • responsible for dyspnoea
• increase in blood pressure leads to reflex hypoventilation or apnoea

  • decrease in blood pressure leads to reflex hyperventilation