Brain stem and cranial nerves

Question 1

Brainstem: identify structures in the brainstem and recall their function

Answer 1

Definition of Brainstem

‘That part of the CNS, exclusive of the cerebellum that lies between the cerebrum and the spinal cord.’

Three major divisions of the brainstem from the top to the bottom:

1. Midbrain
2. Pons
3. Medulla Oblongata

Posterior view of the brainstem (image)

• The area with the four lumps at the top is the roof of the MIDBRAIN
• The pons is the floor of the 4th ventricle
• Nearly everything in the brainstem is bilateral but there are a couple of structures that are midline
• PINEAL GLAND - midline - 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
• 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
• There is only ONE cranial nerve that emerges from the BACK of the brainstem: TROCHLEAR NERVE (CN IV)
• This supplies one of the extrinsic muscles of the eye
• The DORSAL COLUMNS defines the medulla in the dorsal aspect
• Dorsal columns are involved in two main sensory pathways:

1. Touch
2. Proprioception

Anteroinferior view of the brainstem:

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

Question 2

Distinguishing the different parts of the brainstem

Answer 2

• If you can see the cerebral aqueduct then you’re in the midbrain
• The cerebral peduncle will vary in appearance (mickey ears)
• You’ll see the inferior colliculus which is low down in the midbrain (adjacent to foramina)
• The most obvious sign is the SUBSTANTIA NIGRA
  
  • These are dopaminergic neurones
  • 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

Question 3

Pons

Answer 3

• The pons is in the region of the 4th Ventricle - just underneath the cerebellum
• The most obvious feature are the TRANSVERSE FIBRES
• You get cerebral peduncles holding the cerebrum onto the brainstem
• There are cerebellar peduncles holding the cerebellum onto the back of the brainstem
• The main peduncle is the middle cerebellar peduncle where the fibers actually join to the cerebellum

Question 4

Medulla

Answer 4

• 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

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

Lateral Medullary Syndrome

• Caused by thrombosis of the vertebral artery or the posterior inferior cerebellar artery.
• A blockage of either of these arteries results in:

1. Vertigo
2. Ipsilateral Cerebellar Ataxia - problem with gait on the same side of the body as the lesion (broad-based gate - they tend to shuffle)
3. Ipsilateral loss of pain/thermal sense (face)
4. Horner’s Syndrome - loss of sympathetic innervation to the head and neck
   
   • Ptosis
   • Lack of sweating around the eye
   • Hoarseness
   • Difficulty swallowing
5. Contralateral loss of pain/thermal sense in the trunk and limbs

• PICA supplies the shaded section of the medulla
• Disturbing the vestibular nucleus causes the vertigo
• Disturbing the inferior cerebellar peduncle (usually involved in providing proprioceptive information coming up from the spinal cord) - this causes the balance problem

Sympathetic Tract - sympathetic fibres pass through here so disrupting this causes Horner’s Syndrome

Spinothalamic Tract - pain and sensory information coming from the body

• This has been knocked out and because it has already crossed over, the deficit is contralateral
• Damage to this is so catastrophic because everything is packed very close together in the medulla

Question 5

Cranial nerve origins: explain the origins of the cranial nerves

Answer 5

MIDBRAIN

• At the top you get the 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 is the pituitary stalk (infundibulum)
• Behind the infundibulum you can see the inferior part of the hypothalamus - mammillary bodies
• The mammillary bodies are part of the limbic system
• Emerging midline just above the transverse fibres of the pons is the oculomotor nerve (CN III)

Oculomotor nerve - main nerve involved in eye movements

• The midbrain consists of the cerebral peduncle - the main motor fibres coming from the motor cortex down to the spinal cord
• NOTE: peduncle - a fibre tract that has a structural AND functional role (it holds the cerebrum onto the brainstem)

PONS

• There is only one cranial nerve that emerges from the lateral part of the pons - TRIGEMINAL (CN V)

Trigeminal Nerve - touch and sensation throughout the head and neck

1. There is a very small root next to the large one because the trigeminal also has motor function
2. All the muscles involved in mastication are innervated by the trigeminal
3. Most cranial nerves are multifunctional

• THREE cranial nerves emerge at the ponto-medullary junction

1. Most medially (the 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
2. Moving laterally (the nerve the arrow is pointing to) - FACIAL (CN VII) - innervates the facial muscles involved in facial expression
3. Most lateral of the three - VESTIBULOCOCHLEAR (CN VIII) - involved inbalance and hearing

NOTE: Bell’s Palsy - dysfunction of the facial nerve causes a loss of facial muscle tone

MEDULLA OBLONGATA

• THREE different cranial nerves emerge from the lateral medulla

1. Glossopharyngeal (CN IX) - sensory and motor innervation of the tongue and pharynx
2. Vagus (CN X) - main parasympathetic nerve that projects down into the viscera
3. Accessory (CN XI) - 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
• The hypoglossal nerve supplies the musculature of the tongue
• There are pyramids on the anterior surface of the medulla
• Fibres come down from the cortex through the peduncles and disappear from view behind the pons because of the transverse fibres
• These fibres re-emerge below the pons as pyramids

NOTE: extra-pyramidal symptoms are motor symptoms that aren’t linked to this direct pathway

• 90-95% of fibres cross over at the base of the medulla at a point called the PYRAMIDAL DECUSSATION

There are four functional subtypes of the cranial nerves

1. General Somatic Afferent (GSA)
   
   • Sensation from skin and mucous membranes
2. General Visceral Afferent (GVA)
   
   • Sensation from GI tract, heart, vessels and lungs
3. General Somatic Efferent (GSE)
   
   • Muscles for eye and tongue movements
4. General Visceral Efferent (GVE)
   
   • Preganglionic parasympathetic

Functional Classification II

1. Special Somatic Afferent

Vision, hearing and equilibrium (only the cranial nerves)

2. Special Visceral Afferent

Smell (CN I) and Taste (comes from THREE cranial nerves that all go back to the nucleus solitarius)

3. Special Visceral Efferent

Muscles involved in chewing, facial expression, swallowing, vocal sounds and turning the head

Question 6

Discuss how cranial nerves relate to development of the brainstem

Answer 6

Embryonic Spinal Cord

• Dorsally, there are sensory afferent areas. Ventrally, there are motor efferent fibres.
• Information goes into the spinal cord via the dorsal root and leaves the spinal cord via the ventral root
• This is different to the embryonic brainstem

Embryonic Rhombencephalon

As the brainstem develops with the formation of the 4th ventricle, the alar plate opens up, thus spreading the functional columns into a particular distribution…

The motor roots tend to lie towards the midline

The sensory fibres tend to lie more laterally

Question 7

Arrangement of the nuclei of cranial nerves

Answer 7

The nuclei are arranged in functional columns

• MOTOR is BLUE
• SENSORY is RED
• Remember from the last slide that the motor tends to be medial and the sensory tends to be lateral

NOTE: this is obvious but the motor is efferent and the sensory is afferent

• Trigeminal and Accessory move down to the cervical spinal cord as well

General Somatic Efferent = general somatic motor nuclei

• Oculomotor = most rostral
• Trochlear = immediately behind it (these are involved in eye movement)
• Abducens = cranial nerve emerges at the ponto-medullary junction but the nucleus is within the pons
• Hypoglossal = in the medulla

Special Visceral Efferent

• Trigeminal = first one - it is in the pons
• Facial = also in the pons
• Ambiguus = involved in swallowing and phonation - in the medulla Accessory = innervates the sternocleidomastoid and trapezius - located in the cervical spinal cord

General Visceral Efferent

• Edinger Westphal - provides parasympathetic input to the eye - in midbrain
• Salivatory - there are THREE sets of salivatory nuclei at the ponto- medullary border
• Vagus - in the pons

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