3 - Brainstem and cranial nerves Flashcards
Define the brainstem
that part of the CNS, exclusive of the cerebellum, that lies between the cerebrum and the spinal chord
What are the 3 divisions of the brainstem?
midbrain
pons
medulla oblongata
Where are the colliculi found and what are their functions?
Roof of the midbrain
Superior – important for coordination of eye, head and neck movements at the same time
Inferior – auditory reflexes – turning your head in the direction of a loud noise
Describe the location of the pons relative to the ventricular system.
The pons is the floor of the 4th ventricle
Nearly everything in the brainstem is bilateral, but there are a few structure that are midline.
Name an important unpaired, midline structure on the posterior aspect of the brainstem.
Pineal gland
What is the role of the pineal gland?
It produces melatonin, which is involved in regulating the circadian rhythm
Name the ONE cranial nerve emerging from the BACK of the brainstem
Trochlear nerve
What is the role of the trochlear nerve?
It supplies the superior oblique muscle – one of the extrinsic muscles of the eye
What structure defines the medulla in the dorsal (posterior) aspect and what pathways are found within this structure?
Dorsal Columns – sensory pathways – touch and proprioception
What three significant structures can be seen superior to the pons when viewing the brainstem from an anteroinferior view?
Optic Chiasm
Pituitary Stalk (infundibulum)
Mammillary Bodies
What are the mammillary bodies?
They are the inferior part of the hypothalamus – it is part of the limbic system
important in homeostasis
(midline structure)
What is significant about the optic chiasm?
the optic nerves converge and 50% of them cross to the contralateral side of the brain
Which nerve cannot be seen when looking at the brainstem?
Where is it located?
CN I - olfactory nerve
sits in the mucosa at the top of the nose and synapses on the olfactory bulb
(midline structure)
Which cranial nerve emerges in the (ventrally) midline just above the transverse fibres of the pons (at the level of the midbrain)?
Oculomotor Nerve (CN III)
What are the cerebral peduncles?
What is the significance of the word ‘peduncle’?
These are the descending motor tracts coming from the motor cortex.
Peduncle is specifically a structure that has a functional AND structural role – it holds the cerebrum onto the brainstem
Name the one cranial nerve that emerges from the lateral aspect of the pons.
Trigeminal (CN V)
What is the role of the trigeminal nerve?
(‘THE HEAD AND NECK SENSORY NERVE)
Touch and sensation throughout the head and neck
It has a small root next to the larger one as it emerges out of the transverse fibres – this is the motor root providing motor innervation of the muscles of mastication
The trigeminal nerve has a small and a large root. Why is this?
(small root) the nerve also has motor function (but mainly sensory)
Which three nerves emerge at the pontomedullary junction (from medial to lateral)?
Abducens (VI)
Facial (VII)
Vestibulocochlear (VIII)
Briefly state the role of each of the cranial nerves emerging from the pontomedullary junction
(abducens, facial and vestibulocochlear)
Abducens – innervates the lateral rectus which is involved in abducting the eye
Facial – innervates the muscles of facial expression and is involved in taste sensation for the anterior 2/3 of the tongue
Vestibulocochlear – involved in balance and hearing
Which three nerves emerge from the lateral medulla?
Glossopharyngeal (IX)
Vagus (X)
Accessory (XI)
Briefly state the role of each of the cranial nerves emerging from the lateral medulla (glossopharyngeal, vagus and accessory)
Glossopharyngeal – sensory and motor innervation of the tongue and pharynx
Vagus – main parasympathetic nerve descending down to the viscera
Accessory – supplies the trapezius and sternocleidomastoid
State the last cranial nerve – CN XII.
Hypoglossal Nerve – supplies the musculature of the tongue
What are the pyramids? What happens to form them?
(contain motor fibers of the corticospinal and corticobulbar tracts)
The motor fibres coming down from the motor cortex come via the cerebral peduncles then disappear behind the transverse fibres of the pons. They then re-emerge inferior to the transverse fibres, as the pyramids.
What is the name of the point where (90-95%) motor fibres cross to the contralateral side of the body in the brainstem?
the pyramidal decussation
What are the four functional subtypes of the cranial nerves and what are their actions?
General Somatic Afferent • Sensation from the skin and mucous membranes General Visceral Afferent • Sensation from the GIT, heart, vessels and lungs General Somatic Efferent • Muscles for eye and tongue movements General Visceral Efferent • Preganglionic parasympathetic
What are the special subtypes of cranial nerves and what are their actions?
Special Somatic Afferent
• Vision, hearing and equilibrium
Special Visceral Afferent
• Smell and Taste (comes from three nerves that converge on the nucleus solitarius)
Special Visceral Efferent
• Muscles involved in chewing, facial expression, swallowing, vocal sounds and turning the head
What structure separates the sensory nuclei from the motor nuclei in the spinal cord and brainstem?
Sulcus limitans
What happens in the formation of the rhombencephalon and how does this affect the arrangement of the nuclei?
The alar plate opens up and a ventricle forms
This opening of the alar plate results in the motor nuclei being medial in the brainstem and the sensory nuclei are lateral
Describe the arrangement of the different groups in columns within the brainstem
Motor – Medial
Sensory – Lateral
Motor – arranged in columns from medial to lateral in this order: GSE, SVE, GVE
Sensory – arranged in columns from medial to lateral in this order: GVA/SVA, GSA, SSA
State the nuclei that are in the general somatic efferent group and describe their location within the brainstem
- Oculomotor = midbrain
- Trochlear = midbrain
- Abducens = pons (though it emerges at the ponto-medullary junction)
- Hypoglossus = medulla
State the nuclei that are in the special visceral efferent group and describe their location within the brainstem.
- Trigeminal = pons
- Facial = pons
- Ambiguus = medulla
- Accessory = cervical spinal cord
- NOTE: ambiguus is a group of large motor neurons situated deep in the medullary reticular formation. It contains cell bodies of nerves that innervate muscles of the soft palate, pharynx and larynx – strongly associated with speech and swallowing
State the nuclei that are in the general visceral efferent group and describe their location within the brainstem.
- Edinger Westphal = midbrain
- Salivatory = ponto-medullary border (there are THREE sets of salivatory nuclei)
- Vagus = pons
State the nuclei that are in the special somatic AFFERENT group and describe their location within the brainstem.
Vestibulocochlear = pons and medulla
State the nuclei that are in the general somatic AFFERENT group and describe their location within the brainstem.
Trigeminal = in all three parts and in the cervical spinal cord
State the nuclei that are in the general visceral AFFERENT and special visceral AFFERENT groups and describe their location within the brainstem.
Solitarius – mainly in the medulla (but a little bit in the pons)
Describe the appearance of a cross-section of the midbrain. What are the key features seen?
It has a distinctive ‘mickey mouse’ appearance
- Ears of Mickey Mouse are the cerebral peduncles
- At the point where the cerebral peduncles meet the rest of the midbrain you find the substantia nigra
- You will see the cerebral aqueduct in the middle (small diamond shape)
- The two rounded protrusion on the opposite side of the cerebral peduncles are the inferior colliculi
What is the substantia nigra? What is it made up of and what does it produce? (how does this affect the colour?)
The substantia nigra is a group of dopaminergic neurons that project up into the striatum.
In their normal metabolism they produce neuromelanin, which gives the black colour of the substantia nigra
What will the substantia migra look like in a patient with Parkinson’s?
Parkinson’s disease is caused by loss of these dopaminergic neurons (that make up the substantial nigra) so patients with Parkinson’s will have a pale substantia nigra
Describe the appearance of a cross-section of the pons. What are the key features seen?
The 4th ventricle will be seen at the dorsal aspect of the pons
The most distinctive feature are the transverse fibres
On either side you will see the middle cerebellar peduncles
What is the difference between the peduncles seen in the midbrain and the ones seen in the pons?
Midbrain – cerebral peduncles
Pons – cerebellar peduncles
Where is the cerebellum with reference to the brainstem?
inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
Describe the appearance of a cross-section of the medulla. What are the key features seen?
Pyramids will be seen on the ventral aspect
The inferior olivary nucleus will be found next to the pyramids
The 4th ventricle will still be visible at the very top of the medulla
What is the inferior olivary nucleus?
Where is it found?
What is its function?
a bulge on the side of the medulla that is connected to the cerebellum
involved in fine tuning motor movements
Describe the appearance of a cross-section of the lower medulla (@ junction with spinal chord). What are the key features seen?
It will be more round than the rest of the brainstem
The central canal will be seen in the middle
The dorsal columns will be seen on the dorsal side
The pyramidal decussation may be seen
*Name the two columns that make up the dorsal columns.
Gracilis – more medial – sensory information from the lower limb
Cuneatus – more lateral – sensory information from the upper limb
What causes lateral medullary syndrome?
thrombosis of the artery supply of the lateral medulla (i.e. the verterbral or posterior inferior cerebellar artery)
What are the symptoms of Horner’s Syndrome?
Ptosis
Loss of sweating around the eye
Hoarseness
Difficulty Swallowing
How is the pons easily identified?
it has traverse fibres going across it
Summarise what is contained in the midbrain
- optic chiasm
- pituitary stalk (infundibulum)
- mammillary bodies
- cerebral peduncles
- oculomotor nerve emerges between the cerebral peduncles
- trochlear nerve emerges dorsally
- pineal gland in the midline
- colliculi
Describe the contents of the pons
- facial nerve emerges from the lateral part of the pons
at the point-medullary junction
- abducens (medially)
- facial
- vestibulcochlear nerve
What is Bell’s Palsy?
dysfunction of the facial nerve causes a loss of facial muscle tone
What is the cerebral aqueduct?
part of the ventricular system
drains from the 3rd ventricle into the 4th
Describe and explain the symptoms of lateral medullary syndrome.
symptoms caused by an occlusion in the vertebral arteries or the posterior inferior cerebellar arteries (PICA:
- Horner’s Syndrome – disturbing the sympathetic tract
- Vertigo – disturbing the vestibular nucleus
- Ipsilateral loss of pain/thermal sensation on the face – disturbing the spinothalamic tract
- Contralateral loss of pain/thermal sensation on the trunk and limbs – disturbing the spinothalamic tract
- Ipsilateral cerebellar ataxia – disturbing the inferior cerebellar peduncle
