3. Organisation of the brainstem and cranial nerves Flashcards
Define the brainstem
Part of the CNS, exclusive of the cerebellum, that lies between the cerebrum and the spinal cord
What are the three major divisions of the brainstem from the top to the bottom?
- Midbrain
- Pons
- Medulla Oblongata
How can you identify the roof of the midbrain?
- Area with four lumps
* Colliculi - “low lying hills”
What is the ventral floor of the 4th ventricle?
Pons
What is the pineal gland?
- One of the few midline structures (not bilateral like most others in the brainstem)
- Releases melatonin
- Regulation of circadian rhythm (24hr cycle)
- Can be entrained by light - skull too thick so trained with vision
What is the Superior Colliculus for?
Midbrain nucleus for coordination of eye and head movements at the same time
What is the Inferior Colliculus for?
Midbrain nucleus for auditory reflexes e.g. looking in the direction of a bang
Which cranial nerves emerge from the back of the brainstem
Only trochlear nerves (IV) - supplies superior oblique muscle of the eyes
Where are the dorsal columns involved in?
- Touch
- Proprioception (space perception)
- Sensory from lower limbs medial to sensory from upper limbs
Why is it easy to identify the pons?
Transverse fibres going across it
What structure is at the top of the midbrain?
Optic chiasm
What structure is immediately behind the optic chiasm?
Pituitary stalk (infundibulum)
What can you see behind the infundibulum?
Inferior part of the hypothalamus - mammillary bodies (part of limbic system)
What happens to mammillary bodies in chronic alcoholics?
- Becomes dark
* Neovascularisation
Which nerves emerge just above the the transverse fibres of the pons?
Oculomotor nerves (III)
What is the cerebral peduncle?
- Structure above the transverse fibres of the pons
- Main motor fibres coming from motor cortex to the spinal cord
- Also has structural role - holding cerebrum onto the brainstem
What does Pons mean, and what does it do?
- Pons = bridge
* Joins two halves of cerebellum together
Which nerves emerge from the lateral parts of the pons?
• Trigeminal (V) • Divide into: - Opthalmic - Maxillary - Mandibular
What is the function of the Trigeminal nerve?
- Touch and sensation throughout the head and neck
* Very small root next to large one as Trigeminal also has a motor function - mastication
Which nerves emerge below the pons (ponto-medullary junction) from medial to lateral?
- Abducens (VI) - supplies the lateral rectus muscle, causing abduction from midline
- Facial (VII) - innervates muscles involved in facial expression
- Vestibulocochlear (VIII) - balance and hearing
What is Bell’s Palsy?
- Dysfunction of facial nerves
- Loss of facial muscle tone
- May be due to post-infection inflammation
- Usually temporary
Which nerves emerge from the lateral medulla?
- Glossopharyngeal (IX) - sensory and motor innervation of tongue and pharynx
- Vagus (X) - main parasympathetic nerve that projects down into the viscera
- Accessory (XI) - supplies the sternocleidomastoid and trapezius
Which nerves emerge more anteriorly/medially from the medulla?
- Hypoglossal nerves (VII)
* Supplies the musculature of the tongue
Describe the pyramids of the medulla?
- Structures on the anterior surface of the medulla
- Continuation of corticospinal tracts
- Fibres come down from the cortex through the peduncles, behind the transverse fibres, and re-emerge as pyramids
- 90-95% of fibres cross over at “pyramidal decussation” at the base of the medulla - contralateral motor control
What are the four functional subtypes of the cranial nerves?
- General Somatic Afferent (GSA) - sensation from skin and mucous membranes (V)
- General Visceral Afferent (GVA) - sensation from GI tract, heart, vessels and lungs
- General Somatic Efferent (GSE) - muscles for eye and tongue movements (III, IV, VI, XII)
- General Visceral Efferent (GVE) - pre-ganglionic parasympathetic: Edinger Westphal - input into eye, 3 pairs of salivatory nuclei at ponto-medullary border, Vagus (X)
What are the 3 “special” subtypes of cranial nerves?
- Special Somatic Afferent: vision, hearing and equilibrium - optic and vestibulocochlear
- Special Visceral Afferent: Smell (olfactory) and taste (anteriorly facial, posteriorly glossopharyngeal, Nucleus Solatarius)
- Special Visceral Efferent: muscles involved in chewing (V), facial expression (VII), swallowing, vocals (nucleus ambiguus for IX + X) and head turning (XI)
How does information enter and leave the embryonic spinal cord?
- Enters via dorsal root
* Leaves via ventral root
How are the afferent and efferent nuclei arranged in the embryonic rhombencephalon (brainstem)?
• As the alar plate opens up creating a ventricle…
- afferent nuclei are more lateral (sensory)
- efferent nuclei are more medial (motor)
Describe the general appearance of the midbrain (cross-section), and what structures can you see here?
• Mickey mouse appearance • Cerebral aqueduct always visible • Cerebral peduncle varies in appearance • Inferior colliculus low down in midbrain • Most obvious sign - Substantia Nigra - dopaminergic neurones - normally produce neuromelanin - black pigment - gets darker through life (pale - sign of Parkinson's)
Describe the identifiable features pons?
- 4th ventricle
- Transverse fibres
- Cerebral peduncles - holding cerebrum onto brainstem
- Cerebellar peduncles - holding cerebellum onto back of brainstem (main one is middle cerebellar peduncle)
As you go down, the features change:
• Pyramids start to be seen
• Bulge in the sides - inferior olivary nuclei (connected to cerebellum - fine tuning motor)
Describe the cross-section and identifiable features of the lower medulla?
• Junction with the spinal cord is very round
• Dorsal columns seen
- gracillis (smaller and more dorsal/medial - sensory from lower limb)
- cuneatus (larger and more lateral - sensory from upper limbs)
• Central canal
• Crossing over of fibres at pyramidal decussation
What is Lateral Medullary Syndrome?
Thrombosis of vertebral artery or posterior inferior cerebellar artery (PICA), resulting in:
• vertigo
• ipsilateral cerebellar ataxia: problem with gait on the same side of the body as the lesion
• ipsilateral loss of pain/thermal sense (face)
• Horner’s Syndrome - loss of sympathetic innervation to the head and neck
- ptosis (droopy eye lid)
- lack of sweating around the eye
- hoarseness
- difficulty swallowing
• Contralateral loss of pain/thermal sense in the trunk and limbs
Why can thrombosis of the PICA cause vertigo?
Vestibular nucleus in medulla is disturbed
Why can thrombosis of the PICA cause a loss of balance?
Inferior cerebellar peduncle (involved in providing proprioceptive information coming up) is disturbed
Why can thrombosis of the PICA cause a loss sensation on the face and contralateral side of rest of body?
Spinal nucleus of trigeminal in medulla is disturbed
Why can thrombosis of the PICA lead to Horner’s syndrome?
Sympathetic fibres in medulla are disrupted
Why can thrombosis of the PICA lead to hoarseness and difficulty swallowing?
Nucleus ambiguus is disrupted
Why can thrombosis of the PICA lead to contralateral loss of pain/thermal sense in the trunk and limbs?
- Spinothalamic tract is disturbed
* Has already crossed over before reaching medulla - therefore contralateral
