Neural Pathways — C6 Transection

Cardiovascular

Innervation

• ANS
• Afferents
  • Visceral afferent nerves from the heart via the vagal nerve to the NTS
  • Baroreceptors via vagal (aortic arch) and glossopharyngeal (carotid sinus)
• Sympathetic system
  • Efferents to the heart from the post-ganglionic sympathetic fibres synapsing from T2~5 pre-ganglionic sympathetic fibres
  • Positive inotropy, chronotropy, lusitropy and dromotropy
  • Efferents to vascular smooth muscle via pre-ganglionic fibres from the thoracolumbar spinal roots synapsing on the sympathetic chain
• Parasympathetic system
  • Efferents from the vagal nerve
  • Negative inotropy, chronotropy, lusitropy and dromotropy
  • Unknown significance of muscarinic innervation of the vasculature

Effects of transection at C6

• Immediate
  • Sympathetic stimulation from catecholamine release from adrenal medulla
    • Hypertension
    • Tachycardia
• Acute
  • Afferents from heart and baroreceptors in tact
  • Central processing of vasomotor centre in tact
  • Efferent sympathetic innervation to heart will be lost
    • Resting sympathetic tone of heart lost
      • HR will decrease from ~60 to 50bpm
    • Baroreceptor response to hypotension will be lost
      • No reflex tachycardia or positive inotropy
  • Efferent sympathetic innervation to vasculature will be lost
    • Resting sympathetic tone of blood vessels lost → vaso and venodilatation → decreased MAP and decreased venous return
      • Decreased cardiac output
    • Decreased baroreflex vasoconstriction in response to hypotension
  • Efferent parasympathetic innervation in tact (via vagus nerve).
    • Parasympathetic tone of resting heart rate will remain
    • Reflex negative inotropy and chronotropy in response to hypertension will remain

Respiratory

Innervation

• Afferents from central and peripheral chemoreceptors
• Meduallary respiratory centre
• Sympathetic
  • Unknown innervation and significance although β₂ adrenoceptors are present within respiratory system
  • Postulated response to circulating catecholamine from adrenal medulla
• Parasympathetic
  • Vagal innervation of respiratory system causing bronchoconstriction and mucous secretion
• Motor innervation of respiratory muscles
  • Phrenic nerve – C3~5
    • Diaphragm
  • Thoracic spinal nerve roots
    • Intercostal muscles
      • External (inspiration)
      • Internal (expiration)
      • Innermost
  • Accessory muscles of respiration
    • Accessory nerve innervation of sternocleidomastoid and trapezius
    • Brachial plexus
      • Pec. major/minor
      • Nerve to subclavius
      • Serratus anterior
    • Thoracic spinal roots
      • External, internal, innermost intercostal muscles
    • Abdominal wall muscles

Effect of C6 transection

• Sympathetic
  • Decreased adrenaline secretion from adrenal medulla → decreased circulating adrenaline → unopposed muscarinic stimulation of lung → increased likelihood of bronchoconstriction
  • Vasodilation leading to pooling of blood in pulmonary circulation → decreased lung compliance
• Parasympathetic
  • Innervation will remain → bronchoconstriction
• Motor innervation
  • Phrenic nerve function in tact
    • Diaphragm – Tidal inspiration will largely remain
  • Intercostal muscle function lost
    • External – Loss of elevation of ribs to increase AP diameter – required in inspiration
    • Internal – Loss of lowering of ribs to decrease AP diameter – required in expiration
  • Accessory muscles
    • Variable effects
      • Sternocleidomastoid and trapezius function in tact (forced inspiration)
      • Scalenus anterior, medius, posterior in tact (forced inspiration)
      • Pec. major and minor in tact (forced inspiration)
    • Abdominal muscle function lost
      • Required in forced expiration, cough
  • Loss of expiratory reserve volume
  • Decreased vital capacity

Sakurai 2016

Answer by Sakurai