K1i: Anatomy of CN relevant to brain stem reflexes

  • Brainstem = central trunk of the brain which continues downwards to form the SC

Anatomy

3 parts

Function

  • Relays signals from CNS ⮂ peripheries
  • Control centre for functions vital to life

Brain Death Definition

  • The irreversible loss of all brain function, including the brainstem:
  • 3 essential findings:
    1. Coma (unresponsiveness)
    2. Absent brainstem reflexes
    3. Apnoea

* Preconditions must be met*

Brainstem Reflexes

Pupillary Light Reflexes

    • CN II OPTIC
    • CN III OCCULOMOTOR
  • Afferent Limb
    • Light shined simulates retinal photoreceptors
    • Retinal ganglion cells transmit to Optic N whose cell bodies lie in retina
    • CN II enters skull via optic canal
    • At optic chiasm the nasal retinal fibres cross to contralateral optic tract
    • Temporal fibres stay in ipsilateral optic tract
    • Fibres terminate in PRETECTAL NUCLEUS
    • Pretectal nuclei project fibres to ipsi & contralateral EDINGER WESTPHAL NUCLEUS
  • Efferent Limb
    • EWN projects pre-ganglionic parasympathetic fibres which travel along CN III & synapse in CILIARY GANGLION onto post-ganglionic parasympathetic fibres
    • Post ganglionic parasympathetic fibres innervate sphincter m. of pupils à PUPILS CONSTRICT

= Direct & consensual light reflex

Third Cranial Nerve

Corneal Reflex

→ CN V TRIGEMINAL N (OPTHALMIC BRANCH) Afferent

→ CN VII FACIAL N Efferent

  • Light touch receptors in conjunctiva & cornea stimulated
  • Pass via OPTHALMIC BRANCH of TRIGEMINAL N & through Trigeminal Ganglion to Main Sensory Nucleus of Trigeminal n.
  • Interneurons transmit to Motor Nuclei of Facial n.
  • Facial n. supplies ORBICULARIS MUSCLE
  • Closure of eyelids

Reflex Response to Pain in Trigeminal Distribution

  • CN V TRIGEMINAL N
  • CN VII FACIAL N

(Apply pain to supraorbital region (V) → No facial movement (VII))

    • Main Sensory Nucleus lies in pons & receives light touch sensation
    • Postganglionically Trigeminal N. divides into OPHTHALMIC, MAXILLARY & MANDIBULAR NERVES
    • Interneurons transmit to Motor Nuclei of Facial N (in Pons near border with Medulla)
    • Facial N. carries motor control to all facial m. except eye muscles

Vestibulo-Ocular Reflex (VOR)

  • CN III OCULOMOTOR N
  • CN IV TROCHLEAR N (Vertical response)
  • CN VI ABDUCENS N (Horizontal response)
  • CN VIII VESTIBULOCOCHLEAR N

Vor Test Basis

  • Activation of vestibular movement causes eye movements
  • Reflex to stabilise visual field on retina
  • Move head → eyes move opposite direction
  • VOR does not require visual input & can be elicited by caloric (cold/hot) stimulation of inner ear & in total darkness!

Caloric Testing

  • Ice cold (<30°C) water is irrigated into external auditory canal
  • Temperature difference b/w body & water creates a convective current in endolymph of semicircular canal
  • If water COLD the endolymph falls

→ ↓rate of vestibular firing → mimics head turn to contralateral side

→ eyes turn to ipsilateral side (fast movement) & nystagmus (slow movements) to correct towards contralateral ear

COWS = Cold opposite, warm same

  • WARM water → endolymph rises → ↑firing vestibular afferent n. → mimics head turn to ipsilateral side → both eyes turn to contralateral ear (fast) & ipsilateral ear (slow = nystagmus) to correct
  • ∴ VOR elicits = PHYSIOLOGICAL NYSTAGMUS
  • Nystagmus = rhythmic abnormal eye movements
    • Slow movement driving eye off target
    • Fast follow up movement bringing eye back to target
  • ∴Absence of VOR = Brainstem not working

Vor Test

  • Cold H2O causes endolymph to fall
  • ↓Vestibular N (CN VIII) afferent firing
  • Mimics head turn to CONTRALATERAL SIDE of H2O irrigation
  • Transmitted via Vestibular ganglion to vestibular nucleus in brainstem
  • Vestibular nuclei fibres cross to Contralateral Abducens Nucleus (CN VI)
  • In the abducens nuclei, they synapse at 2 pathways:
    1. Direct to IPSILATERAL Lateral Rectus Muscle via Abducens n (VI)
    2. To CONTRALATERAL Medial Rectus Muscle via Oculomotor N (III)

Also inhibitory pathways in IPSILATERAL abducens nucleus so that ipsilat. (to cold stimulus) the lateral rectus & contralateral medius rectus are inhibited

    • Slow movement eyes towards IPSILAT
    • Fast movement (NYSTAGMUS) to CONTRALAT
  • Abnormal HORIZONTAL (III, VI, pons)
  • Abnormal VERTICAL (III, IV, Midbrain)
Vor Test
Vor Test 1

Pharyngeal (GAG) Reflex

    • CN IX GLOSSOPHARYNGEAL Afferent
    • CN X VAGUS Efferent
  • Pharyngeal wall stimulated
  • Afferent sensory carried by CN IX to Nucleus Ambiguus
  • Nucleus Ambiguus contains motor of CN IX, X, XI
  • Efferent output through CN IX & X to pharyngeal constrictor muscles
  • Pharyngeal elevation to expel foreign object
Gag Reflex

Cough Reflex

    • CN X VAGUS Afferent & Efferent
  • Irritant receptors on posterior wall trachea, pharynx & tracheal carina
  • Via Internal Laryngeal N. branch of the Superior Laryngeal n. of the Vagus (X) to Medulla
  • Motor signals transmitted via Superior Laryngeal n. of Vagus (X) to glottis, external IC & diaphragm
  • Diaphragm (phrenic) & external IC (intercostal n’s) contract
  • Negative intrathoracic P
  • Air rushes into lungs
  • Glottis shuts (Recurrent Laryngeal N. of Vagus X) & vocal cords shut
  • Abdominal m. & resp m. contract simultaneously generating v. high P > 300mmHg
  • ↑intrathoracic P
  • VC relax → glottis opens → air expelled @ 100mph
  • Clearing irritants as air sweeps up