Overview
This is a revision session preparing students for the brainstem dissection lab, tying together material from earlier sensory and motor lectures. It covers how the brainstem coordinates conjugate eye movements, where each cranial nerve emerges at successive brainstem levels (midbrain, pons, medulla) and what nuclei are found at each level, how the corticobulbar tract distributes to brainstem motor nuclei (with the resulting upper motor neuron lesion patterns for the face, accessory nerve, and hypoglossal nerve), the four functional components of the facial nerve, and three brainstem reflexes (jaw jerk, corneal, gag) used clinically to test cranial nerve integrity. The lecture closes with a note that model answers for the Assisted Self-Learning “blanks” are posted on Moodle, and a suggestion to pre-label the brainstem lab sections beforehand.
Eye Movement Coordination by the Brainstem
Worked example: “Look left and up”, initiated by the right frontal eye field.
- To move both eyes left, the lateral gaze centre (PPRF, paramedian pontine reticular formation) must be activated to drive the left lateral rectus (via left VI) and the right medial rectus (via right III).
- To move both eyes up, both vertical gaze centres must be activated so both superior recti and both inferior obliques are active.
- Together this achieves conjugate eye movement (both eyes moving together in the same direction).
Diagram pathway: the right frontal eye field descends through the pretectal area and the medial longitudinal fasciculus (MLF) in the midbrain, connecting to the relevant cranial nerve nuclei — III (oculomotor: right SR, right IO, right MR), IV (trochlear: left SR, left IO), PPRF, and VI (abducens: left LR) — continuing down to VIII and spinal cord level. For “look left and up,” the pathway crosses to drive the left PPRF/left lateral rectus (VI) and right medial rectus (III), plus bilateral activation of both superior recti and inferior obliques for the “up” component.
Cranial Nerve Emergence from the Brainstem
Olfactory (I) and Optic (II) do NOT enter the brainstem.
Grouped by region of emergence:
- Midbrain: Oculomotor (III), Trochlear (IV)
- Pons: Trigeminal (V), Abducens (VI), Facial (VII), Vestibulocochlear (VIII)
- Medulla: Glossopharyngeal (IX), Vagus (X), Accessory (XI), Hypoglossal (XII)
Brainstem Levels in Cross-Section
Working rostral to caudal, each level’s key nuclei:
- Midbrain, level of the superior colliculus (SC): Edinger-Westphal nucleus (parasympathetic) and oculomotor nucleus (motor) together give rise to CN III, which emerges anteriorly.
- Midbrain, level of the inferior colliculus (IC): Trochlear nucleus (IV) lies posteriorly; its fibres decussate (cross the midline) before exiting the brainstem — the only cranial nerve to fully cross before emerging.
- Mid pons: Sensory trigeminal nuclear complex lies posteriorly, trigeminal motor nucleus more anteriorly; sensory (s) and motor (m) roots of V exit anteriorly.
- Caudal pons / pontomedullary junction: posterior to anterior — facial colliculus, vestibular/cochlear nuclei (VIII), abducens nucleus (VI), facial motor nucleus (VII); nerves VI, VII, VIII emerge anteriorly on both sides.
- Open medulla: posterior to anterior — dorsal motor nucleus of X, nucleus of the solitary tract, nucleus ambiguus, hypoglossal nucleus; combined IX + X/cranial XI roots and the XII root emerge bilaterally.
Corticobulbar Tract and Upper Motor Neuron Lesion Patterns
The corticospinal/corticobulbar pathway begins at the motor homunculus of the cortex (leg, arm, face/head regions, in the neocortex’s frontal and parietal lobes) and converges via the internal capsule into a single descending tract:
- Leg region neurons contribute to the corticospinal tract.
- Arm region neurons contribute to the corticospinal tract.
- Face region neurons form the corticobulbar tract.
The descending tract passes, in order: pontine reticular formation → basis pedunculi → motor root of the trigeminal nerve → trigeminal motor nucleus (face fibres exit here, “to face”) → facial motor nucleus → facial nerve → pontine/medullary reticular formation → nucleus ambiguus → vagus nerve → hypoglossal nerve → pyramid of medulla → pyramidal decussation → continues as the lateral and ventral corticospinal tracts to ventral roots supplying “to arm” and “to leg” (lumbosacral).
Facial innervation pattern: bilateral (evenly innervated) for the upper face; crossed for the lower face.
A UMN lesion causes paralysis of the lower face with a normal (unaffected) upper face, because the upper face's facial nucleus subunit still receives bilateral cortical input even though the lesioned side's input is lost; the lower face subunit receives only crossed input and loses it entirely.
Relatively evenly (bilaterally) innervated nuclei/muscles:
- Facial nucleus (upper face)
- Nucleus ambiguus (pharynx and larynx)
- Spinal accessory nucleus innervating sternocleidomastoid
Nuclei/muscles with contralateral predominance (mostly crossed innervation):
- Trigeminal motor nucleus (muscles of mastication)
- Facial nucleus innervating lower face (completely crossed)
- Spinal accessory nucleus innervating trapezius
- Hypoglossal nucleus (tongue movements)
Accessory Nerve (XI): Cranial vs Spinal
- XI provides motor supply to trapezius and sternocleidomastoid, arising from the spinal accessory nucleus.
- Cranial XI arises from the nucleus ambiguus in the medulla and joins the vagus nerve; carries mostly branchial/autonomic motor functions.
- Spinal XI arises from the accessory nucleus in the ventral horn of spinal cord segments C1–C5, ascends, and exits via the jugular foramen.
- The internal ramus carries fibres to the muscles of the larynx and pharynx; the external ramus carries fibres to trapezius and sternocleidomastoid.
- Sensory component: from the mucous membrane of the pharynx and larynx, and from the back of the ear.
- Runs to the periphery alongside the vagus (X) and is indistinguishable from it there.
- UMN lesion effect: contralateral weakness of trapezius (contralesional weakness, since trapezius has contralateral predominance), with relative sparing of sternocleidomastoid (bilaterally/evenly innervated, similar to the upper face) — sternocleidomastoid is “equally affected” on both sides.
Hypoglossal Nerve (XII)
Controls tongue movements; has contralateral predominance (mostly crossed corticobulbar innervation). Clinical example given: a right-sided UMN lesion affects the left hypoglossal nucleus, causing slight tongue deviation/weakness on protrusion (deviation toward the weak side).
The Facial Nerve (VII): Four Functional Components
The facial nerve carries branchial efferent, general visceral efferent, general somatic afferent, and taste (special visceral afferent) components.
1. Branchial motor: the nerve to stapedius branches off first (dampens the stapes), then the nerve exits via the stylomastoid foramen and branches to the auricularis muscles (superior, posterior, occipitalis), and via cervical/posterior belly branches to platysma, stylohyoid, and the posterior belly of digastric (functions: superficial neck, swallowing, floor of mouth — the last shared with V) — and to the muscles of facial expression.
2. General visceral (secretomotor) efferent: the nervus intermedius carries fibres from the superior salivatory and lacrimal nuclei via two routes:
- Greater petrosal nerve → (joins) via maxillary V → pterygopalatine ganglion → lacrimal gland.
- Chorda tympani nerve → submandibular ganglion → submandibular gland and sublingual gland.
3. General somatic afferent: cutaneous sensation from the concha, back of the ear, external acoustic meatus, and tympanic membrane, carried via the auricular branch of the nervus intermedius. Somatic sensation overall is carried primarily by V but also by VII, IX, X, and cranial XI, all of which descend together in the spinal trigeminal tract to synapse in the nucleus of the spinal tract of trigeminal (spinal trigeminal nucleus).
4. Taste (special visceral afferent): receptors on the tongue and palate.
- Afferents: VII carries taste from the anterior 2/3 of the tongue; IX carries the posterior 1/3; X/cranial XI carries the epiglottis.
- Cell bodies for the VII taste afferents are in the geniculate ganglion.
- These synapse in the rostral nucleus of the tractus solitarius, then relay via the salivatory nuclei and a central taste pathway (running with the medial lemniscus) up to the postcentral gyrus and insula of the cortex.
Brainstem Reflexes
| Reflex | Synaptic type | Tests | Stimulus | Response |
|---|---|---|---|---|
| Jaw jerk | Monosynaptic | Trigeminal motor pathway integrity | Tap chin, mouth slightly open | Masseter contracts, jaw closes |
| Corneal | Polysynaptic (both eyes close) | Trigeminal sensory + facial motor pathway integrity | Light touch to cornea (e.g. cotton wisp) | Bilateral orbicularis oculi contraction (blink) |
| Gag | Polysynaptic (both levator palatini elevate) | Glossopharyngeal sensory + vagal motor (nucleus ambiguus) pathway integrity | Light touch to posterior pharyngeal wall | Pharyngeal elevation + contraction of pharyngeal muscles (± retching) |
Jaw jerk pathway: muscle spindle in masseter → V3 → mesencephalic nucleus → motor nucleus of V → V3 → masseter contraction.
Corneal reflex pathway: afferent (sensory) limb travels via the ophthalmic division (V1) of the trigeminal nerve; fibres enter the brainstem and descend to the nucleus of the spinal tract of trigeminal, join an interneuron that projects bilaterally to both facial motor nuclei, which drive orbicularis oculi → eyelid closure.
Gag reflex pathway: afferent (sensory) limb travels via the IX nerve, entering the brainstem and synapsing in the nucleus of tractus solitarius (NTS); an interneuron in the NTS projects bilaterally to both nucleus ambiguus, which output via X/cranial XI to levator palatini and other pharyngeal muscles. The transcript separately notes a sensory-discrimination question (“Could you feel that?”) that tests a distinct pathway: glossopharyngeal → nucleus of the spinal tract of trigeminal → ventral trigeminothalamic tract → VPM (thalamus) → cortex — this is a sensory perception check, separate from the gag reflex arc itself.
The lecture's final slide (23) is administrative only: it notes that model answers to the Assisted Self-Learning "blanks" are posted on Moodle, and encourages pre-labelling the brainstem lab sections before the lab — no additional anatomical content.
Self-test
- Which two cranial nerves do not enter/emerge from the brainstem?
- List the cranial nerves that emerge from the midbrain, the pons, and the medulla respectively.
- Describe the full pathway and bilateral coordination required to produce a conjugate “look left and up” eye movement.
- At the level of the superior colliculus, which two nuclei combine to form cranial nerve III, and what is the functional difference between them?
- What happens to the trochlear nerve (IV) fibres before they exit the brainstem, and at what level is this seen?
- Describe the arrangement of the trigeminal nuclei at mid-pons level and which components of CN V they give rise to.
- List the nuclei present at the pontomedullary junction and the cranial nerves that emerge there.
- List the four nuclei visible at the level of the open medulla and the cranial nerve(s) associated with each.
- Distinguish which cortical regions contribute to the corticospinal tract versus the corticobulbar tract.
- Explain why a UMN lesion causes paralysis of the lower face while sparing the upper face.
- List the nuclei/muscles that are relatively evenly (bilaterally) innervated versus those with a contralateral predominance (mostly crossed innervation).
- A patient has a right-sided UMN lesion. On tongue protrusion, which way does the tongue deviate, and why?
- Distinguish cranial XI from spinal XI in terms of nucleus of origin and course to the periphery.
- A patient with a UMN lesion has contralateral trapezius weakness but normal sternocleidomastoid strength on both sides. Explain this pattern using the innervation of the spinal accessory nucleus.
- Describe the four functional components of the facial nerve.
- Describe the branchial motor distribution of VII, from its first branch to its final targets.
- Describe the secretomotor (general visceral efferent) pathway of VII, including the two ganglia involved and the glands each supplies.
- Which structures make up the general somatic afferent territory of VII, and where do these afferents (along with those of V, IX, X and cranial XI) synapse?
- Describe the taste pathway from tongue/palate/epiglottis to cortex, including which cranial nerves carry taste from each region.
- Distinguish the jaw jerk reflex from the corneal and gag reflexes in terms of synaptic complexity, and explain why the difference exists.
- Describe the full corneal reflex arc, including the cranial nerves involved in the afferent and efferent limbs.
- Describe the full gag reflex arc, including the cranial nerves involved in the afferent and efferent limbs.
- Integrative: a patient loses the corneal reflex to light touch on one side but can still blink voluntarily and has normal facial movement. Using the reflex arc, which limb (afferent or efferent) and which cranial nerve/nucleus is most likely affected?
Answers
Reveal answers
- Olfactory (I) and optic (II).
- Midbrain: oculomotor (III), trochlear (IV). Pons: trigeminal (V), abducens (VI), facial (VII), vestibulocochlear (VIII). Medulla: glossopharyngeal (IX), vagus (X), accessory (XI), hypoglossal (XII).
- The right frontal eye field’s descending pathway crosses to activate the left PPRF (lateral gaze centre), driving the left lateral rectus (VI) and right medial rectus (III) for the “left” component; both vertical gaze centres are also activated bilaterally so both superior recti and both inferior obliques contract for the “up” component. Together this produces conjugate movement of both eyes.
- The Edinger-Westphal nucleus (parasympathetic) and the oculomotor nucleus (motor) combine to form CN III; Edinger-Westphal supplies the parasympathetic (e.g. pupillary) fibres while the oculomotor nucleus supplies the motor fibres to the extraocular muscles.
- The trochlear (IV) fibres decussate (cross the midline) before exiting the brainstem; this is seen at the level of the inferior colliculus, where the trochlear nucleus lies posteriorly.
- The sensory trigeminal nuclear complex lies posteriorly and the trigeminal motor nucleus lies more anteriorly; sensory (s) and motor (m) roots of V exit anteriorly, giving rise to the sensory and motor components of CN V respectively.
- Facial colliculus, vestibular/cochlear nuclei (VIII), abducens nucleus (VI), facial motor nucleus (VII); nerves VI, VII, and VIII emerge anteriorly on both sides at this level.
- Dorsal motor nucleus of X, nucleus of the solitary tract, nucleus ambiguus, and hypoglossal nucleus; the combined IX + X/cranial XI roots and the XII root emerge bilaterally.
- Leg-region and arm-region cortical neurons contribute to the corticospinal tract; face-region neurons form the corticobulbar tract.
- The upper face subunit of the facial nucleus receives bilateral cortical input, so a unilateral UMN lesion still leaves the opposite hemisphere’s input intact — upper face function is preserved. The lower face subunit receives only crossed (contralateral) input, so a UMN lesion abolishes its cortical drive entirely, causing paralysis of the lower face.
- Evenly/bilaterally innervated: facial nucleus (upper face), nucleus ambiguus (pharynx/larynx), spinal accessory nucleus for sternocleidomastoid. Contralateral predominance (mostly crossed): trigeminal motor nucleus (mastication), facial nucleus for lower face (completely crossed), spinal accessory nucleus for trapezius, hypoglossal nucleus (tongue).
- A right UMN lesion affects the left hypoglossal nucleus (contralateral predominance), causing weakness of the left side of the tongue; on protrusion the tongue deviates toward the weak (left) side.
- Cranial XI arises from the nucleus ambiguus in the medulla and joins the vagus nerve, carrying mostly branchial/autonomic motor functions. Spinal XI arises from the accessory nucleus in the ventral horn of spinal cord segments C1–C5, ascends, and exits via the jugular foramen; peripherally it runs alongside X and is indistinguishable from it.
- Trapezius has a contralateral predominance in its corticobulbar innervation, so a UMN lesion causes contralesional (contralateral) weakness. Sternocleidomastoid is relatively evenly/bilaterally innervated, so it is equally affected on both sides and appears clinically unaffected/normal.
- Branchial efferent (motor to muscles of facial expression and related muscles), general visceral efferent (secretomotor to lacrimal, submandibular and sublingual glands), general somatic afferent (cutaneous sensation from parts of the ear), and taste/special visceral afferent (anterior 2/3 of tongue).
- The branchial motor component first branches to the nerve to stapedius (dampens the stapes), then exits via the stylomastoid foramen and branches to the auricularis muscles (superior, posterior, occipitalis), platysma, stylohyoid, and posterior belly of digastric, and to the muscles of facial expression.
- Two pathways: (1) greater petrosal nerve, joining via maxillary V, to the pterygopalatine ganglion, supplying the lacrimal gland; (2) chorda tympani nerve to the submandibular ganglion, supplying the submandibular gland and sublingual gland.
- The concha, back of the ear, external acoustic meatus, and tympanic membrane (via the auricular branch). These afferents, along with those of V, IX, X and cranial XI, descend together in the spinal trigeminal tract and synapse in the nucleus of the spinal tract of trigeminal (spinal trigeminal nucleus).
- Taste receptors are on the tongue and palate. VII carries taste from the anterior 2/3 of the tongue (cell bodies in the geniculate ganglion), IX carries the posterior 1/3, and X/cranial XI carries the epiglottis. All converge and synapse in the rostral nucleus of the tractus solitarius, then relay via the salivatory nuclei and a central taste pathway (with the medial lemniscus) to the postcentral gyrus and insula.
- The jaw jerk is monosynaptic because it is a simple muscle-spindle stretch reflex with a single synapse between afferent and efferent trigeminal neurons. The corneal and gag reflexes are polysynaptic because they involve an interneuron that projects bilaterally to both the facial motor nuclei (corneal) or both nucleus ambiguus (gag), producing a bilateral motor response (both eyes close; both sides of the pharynx/levator palatini elevate) from a unilateral stimulus.
- Afferent: ophthalmic division (V1) of the trigeminal nerve, entering the brainstem and descending to the nucleus of the spinal tract of trigeminal. This synapses with an interneuron that projects bilaterally to both facial motor nuclei (efferent: facial nerve, VII), which drive orbicularis oculi to close both eyelids.
- Afferent: glossopharyngeal nerve (IX), entering the brainstem and synapsing in the nucleus of tractus solitarius (NTS). An interneuron in the NTS projects bilaterally to both nucleus ambiguus, which output via the vagus/cranial XI (X/CrXI) to levator palatini and other pharyngeal muscles (efferent).
- Since the motor (efferent) side is intact — voluntary blink and facial movement are normal, meaning the facial motor nuclei and nerve are working — the deficit lies in the afferent limb: the ophthalmic division (V1) of the trigeminal nerve or its central connection to the nucleus of the spinal tract of trigeminal.