3. Organisation of the Brainstem and Cranial Nerves Flashcards
Define the brainstem.
That part of the CNS, exclusive of the cerebellum that lies between the cerebrum and spinal cord.
What are the thee major division of the brainstem (from top to bottom)?
- Midbrain
- Pons
- Medulla Oblongata
What do the 4 lumps on the diagram mark?
This marks the roof of the midbrain
What is the pons the floor of?
The 4th ventricle.
What is the function of the Pineal gland and identify it on the diagram.
- Releases melatonin and is important in regulating the circadian rhythm
- Light can entrain the pineal gland but we’ve got a thick skull so light doesn’t reach the pineal gland
- Instead we train it with vision through the retina
Describe the colliculi of the brainstem.
- Colliculus = low lying hills
- Superior Colliculus - important in the coordination of eye and head movements at the same time (think about watching tennis)
- Inferior Colliculus - auditory reflexes - if there is a loud bang you tend to look in the direction of the bang immediately
Where does the trochlear nerve (CN IV) emerge from and what is its function?
The Trochlear Nerve (CN IV) is the only cranial nerve that emerges from the back of the brainstem
- It supplies one of the extrinsic muscles of the eye
- It is one of the three nerves that control eye movement
What defines the medulla in the dorsal aspect?
- The dorsal columns
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Involved in 2 main sensory pathways
- Touch
- Proprioception
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Involved in 2 main sensory pathways
Identify the pons, olfactory nerves, optic nerve and optic chiasm
The pons is very clear because you get the transverse fibres going across it - above this is the midbrain and below is the medulla oblongata
CN I is the olfactory nerves
CN II is the optic nerve
Right behind the optic chiasm is the pituitary stalk
Describe the Structure of the Midbrain (starting from the top)
- At the top: optic chiasm (one of the midline structures) - where the optic nerves converge and 50% of the fibres cross to the contralateral side of the brain
- Immediately behind the optic chiasm: pituitary stalk (infundibulum)
- Behind the infundibulum: you can see the inferior part of the hypothalamus- mammillary bodies (which become dark in alcoholism) - part of the limbic system
- Just above the pons: Oculomotor nerve (CN III) emerge from in betwwen the cerebral peduncles (main motor fibres coming from the motor cortex down to the spinal cord). Cerebral penducle also hold the cerebrum onto the brainstem
Describe the cranial nerve of the pons.
- There is only one cranial nerve that emerges from the lateral part of the pons - trigeminal (CN V)
- Touch and sensation throughout the head and neck
- There is a very small root next to the large one because the trigeminal also has motor function - supplies the mandible (involved in mastication)
Name the three cranial nerves that emerge at the ponto-medullary junction. Explain their function.
- Most medially (long one in the middle) - Abducens (CN VI) supplies the lateral rectus muscle (the last extrinsic muscle of the eye) and causes abduction from the midline
- Moving laterally (nerve the arrow is pointing to) - Facial nerve (CN VII) - innervated the facial muscles involbed in facial expressions
- Bell’s Palsy - dysfunction of the facial nerve causing loss of facial muscle tone
- Most lateral - vestibulocochlear (CN VIII) - involved in balance and hearing
Describe the structure of the medulla oblongata
- 3 different cranial nerves emerge from the lateral medulla
- Glossopharyngeal (CN IX) - sensory and motor innervation of the tongue and pharynx
- Vagus (CN X) - main parasympathetic nerve that projects down in the viscera
- Accessory (CN XII) - supplies the sternocleidomastoid (Allows turning of the head) and the trapezius
- The hypoglossal nerve (CN XII) emerges separately to the other three from the medulla - supplies the musculature of the tongue
- There are pyramids on the anterior surface of the medula - these are the fibres that emerge from the cortex through the peduncles and disappear behind the pons because of the transverse fibres
- 90-95% of fibres cross over at the base of the medulla at a point called the pyramidal decussation
Name the 4 functional subtypes of the cranial nerves.
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General Somatic Afferent (GSA)
- Sensation from skin and mucous membrane
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General Visceral Afferent (GVA)
- Sensation from GI tract, heart, vessels and lungs
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General Somatic Efferent (GSE)
- Muscles for eye and tongue movements
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General Visceral Efferent (GVE)
- Preganglionic parasympathetic (vagus)
Name the special cranial nerves.
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Special Somatic Afferent
- Vision, hearing and equilibrium (only the cranial nerves)
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Special Visceral Afferent
- Smell (CN I) and Taste (comes from 3 cranial nerves that all go back to the nucleus solitarius)
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Special Visceral Efferent
- Muscles involved in chewing, facial expression, swallowing, vocal sounds and turning the head.
Describe the Embryonic Rhombencephalon
- In the brainstem, the alar plate opens up and you have a ventricle here
- Because of the separating of the alar plate, the afferent nuclei tend to be more lateral and the efferent nuclei tend to be more medial
- MOTOR = MEDIAL
- SENSORY = LATERAL
Describe the arrangement of the nuclei of cranial nerves.
- The nuclei are arranged in functional columns
- On diagram:
- Motor is blue
- Sensory is red
- On diagram:
- Motor is medial and sensory is lateral
- The trigeminal and accessory move down to the cervical spinal cord as well
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General Somatic Efferent = general somatic motor nuclei
- Oculomotor = most rostral
- Trochlear = immedialy under it
- Abducens = cranial nerve emerges at the ponto-medullary junction but the nucleus is within the pons
- Hypoglossal = in the medulla
Describe the appearance of the midbrain.
- The midbrain has a typical MICKEY MOUSE appearance
- If you can see the cerebral aqueduct (drains 3rd and 4th ventricles) then you’re in the midbrain
- The cerebral peduncle will vary in appearance
- You’ll see the inferior colliculus which is low down in the midbrain
- The most obvious sign is the SUBSTANTIA NIGRA
- These are dopaminergic neurones
- Project up to the striatum
- In normal metabolism they produce neuromelanin which is a pigment that gives its BLACK colour
- As you go through life it gets blacker and blacker
- NOTE: In Parkinson’s disease you lose these dopaminergic neurones so a pale substantia nigra could be a sign of Parkinson’s disease
Describe the structure of the pons.
- The pons is the region of the 4th ventricle - just underneath the cerebellum
- The most obvious feature are the transverse fibres
- You get cerebral peduncle holding the cerebrum onto the brainstem
- There are also cerebellar peduncles holding the cerebellum onto the back of the brainstem
- The main peduncle is the middle cerebellar peduncle
Descibe the structure of the medulla
- The medulla changes a lot as you go down the brainstem
- At the top you still have the 4th ventricle but the rest of the shape is very different to the pons
- Main feature = pyramids start to be seen
- There is a bit of a bulge in the side of the medulla that’s a weird shape called the inferior olivary nucleus - this is connected to the cerebellum and is involved in fine tuning motor movements
- If you see the inferior olivary nucleus you are in the medulla
Describe the structure of the lower medulla.
- At the junction with the spinal cord, the cross section is very round
- The dorsal columns can be seen here (touch and proprioception)
- The smaller of the columns is the gracilis - sensory information from the lower limb
- More laterally, you have the cuneatus - sensory information from the upper limb
- The central canal can be seen in the cross-section of the lower medulla
- You also see the crossing over of the fibres at the pyramidal decussation in the lower medulla
Define lateral medullary syndrome and its consequences.
Caused by thrombosis of the vertebral artery of the posterior inferior cerebellar artery.
Can result in:
- Vertigo - due to disturbing of the vestibular nucleus
- Ipsilateral cerebellar ataxia - problem with gait on the same side of the body as the lesion
- Ipsilateral loss of pain/thermal sense
- Horner’s Syndrome - loss of sympathetic innervation to the head and neck
- Ptosis
- Lack of sweating aroung the eye
- Hoarseness
- Difficulty swallowing - N.Ambiguus
- Contralateral loss of pain/thermal sense in the trunk and limbs - spinothalamic tract
- Contralateral because it has already crossed over
