Brainstem and cranial nerves

Question 1

What are the two main sections of the brain stem and what is in them?

Answer 1

Midbrain

Hindbrain = PONS and medulla oblongata

Question 2

What sits on top of the brain stem?

Answer 2

The thalamus

Question 3

What does the diencephalon consist of?

Answer 3

Thalamus above and hypothalamus below

Question 4

What are the main 6 functions of the brain stem?

Answer 4

1. Conduit for ascending and descending pathways. (Pathways going to the brain from the spinal cord and motor fibres from brain go to the spinal cord).
2. Major connections for cerebellum
3. Most of the cranial nerve nuclei are housed here
4. Centre for chemoreception (taste or blood co2), salivation, mastication, swallowing
5. Reticular formation - arousal, cardiovascular and respiratory centres, vital life-supporting role
6. Houses the raphe (extend alone the midline of brain stem, for all serotonin pathways), locus coerulueus nuclei (level of PONS for mood and sleep, all noradrenaline pathways) and the substantial nigra (neurons at level of midbrain for movement control, dopamine pathways)

Question 5

When a ventrabra is myelin stained, what does each matter change colour to?

Answer 5

White matter (axons) goes black

Grey matter (call bodies) goes white

Question 6

Explain the cross section of the medulla oblongata

Answer 6

Dorsally:
White matter (stained black due to myelin). This is the gracile faciculi and the cuneatus faciculi present in the spinal cord that have ascended (carrying fine touch and vibration). They synapse at the level of the post-synaptic cell target that sit in the grey matter. 

Gracile faciculi = contain axons synapsing in gracile nuclei
Cuneate facilculi = axons synapsing in the cuneate nuclei

Ventrally:
More white matter (appears dark here due to myelin).
Have 2 medullary pyramids at bottom containing axons of the cortiocospinal tract (from cortex to spinal cord so a motor descending pathway).
Corticospinal tract is the main pathway concerned with voluntary movement of the limbs. Each side of brain moves the other side of body mensing information has to cross the midline (does it in the medullary pyramids).
Axon descended on the right side of brain cross over to the left in the spinal cord to affect left limbs.

Centre:
Grey matter. CSF filled central canal of medulla here that is continuous with the fourth ventricle above and the central canal of the vertebrae below.
Laterally, we have two big bits of grey matter. These are nuclei of granule nerves (dont need to know this). Bigger block for accessory nerve nuclei and smaller block on right part of the trigeminal nucleus.

Question 7

Explain the cross section through PONS

Answer 7

Big ventral bulge seen.
Dorsally is a small hole which is the start of the cerebral aqueduct.

Tracts taking motor information from PONS nuclei to the cerebellum. They are pontocereballa fibres.

Fragmented axon tracts here are axons of the cortiocospinal tract. They gather towards the medulla, through the PONS, towards the medullary pyramids. They terminate on ventral horns of the spinal cord.
The cortiobulbar tract runs along with this which are motor fibres but they dont terminate in spinal cord as this is too far away. They terminate in cranial nerve motor nuclei. Helps move muscles of head and neck.

Dorsally - around the cerebral aqueduct is some grey matter called the periacquductal grey. (Grey matter around aqueduct). Important in pain control.

Question 8

What is the only nerve which exists the CNS on the dorsal side?

Answer 8

Trochlear nerve (4)

Question 9

Explain the cross-section through the mid-brain

Answer 9

Dorsally - two bumps called inferior colliculus.

Also superior colliculus above this.

The inferior coliculous moves the eye in response to sound to move towards the sound.
This movement is due to connections between the auditory cortex and the inferior coliculous.

Superior coliculous is also responsible for reflex eye movement but due to visual stimuli. Makes eye move to something thats moving.

Ventrally - two big amounts of white matter called the cerebral peduncles. They attach the cerebrum to the brainstem. Mainly descending axons through here including the cortiospinal and cortiobulbar tracts.

Bulk of midbrain is a pale area. This must be cell bodies. This is the substantial nigra (neurons at level of midbrain for movement control, dopamine pathways).

Question 10

Write all the cranial nerve names

Answer 10

1 - olfactory 
2 - optic 
3 - oculomotor 
4 - trochlear 
5 - trigeminal 
6 - abducens 
7 - facial
8 - vestibulo-cochlear
9 - glossopharyngeal
10 - vagus
11 - spinal accessory
12 - hypoglossal

Question 11

Give details on where the cranial nerves come out the brain?

Answer 11

Ventral surface of brain. They all come out the ventral horns except the trochlear (4). This comes from across surface of PONS, crosses midline behind PONS and then emerges at the ventral surface of brain.

Question 12

Compare the trigeminal and abducens nuclei occupation in the brain stem

Answer 12

Trigeminal goes from the midbrain to medulla but the abducens nucleus occupy a small part of brain.

Question 13

What might lots of lesions of the cranial nerves in the brain stem suggest?

Answer 13

If we see lots of lesions and the cranial nerve nuclei are close together, we may suspect this part of the brain stem is compromised.

Question 14

Give some details on the olfactory nerve

Answer 14

• Comes out the cribiform plate
• Sense of smell
• Synapses with the olfactory bulb that sits on ventral surface of brain
• The nerve itself is outside the cranial vault
• Olfactory epithelium is the sensory endings of C1 projecting up through a perforated cribriform plate and synapses with neves in the olfactory bulb
• The post-synaptic nerve carries information along axons in olfactory tract to the olfactory cortex.

Question 15

Give some details on the optic nerve

Answer 15

• Function of vision
• Optic nerve is found between the retina and optic chiasma
• The nerve arises in the retina and then axons are projected back to the optic chiasma. Most synapse in the lateral geniculate nucleus of thalamus
• Temporal retina stay on the same side (no crossing). The nasal retina (closest to midline) do cross over. This is because light coming in from right will hit light on the temporal retina in left eye and the nasal retina in right eye so these need to come together
• A lesion between the optic chiasma and the lateral nucleus (optic tract) would be different to one between the retina and optic chiasma

Question 16

Give information on the three nerves causing eye movement

Answer 16

Oculomotor (2)

• moves almost all extra ocular muscles
• important parasympathetic role to constrict the papillary muscles in bight light reflex and constrict the ciliary muscles that hold the eye lens

Trochlear (4)
- innervates the superior oblique muscle acting to depress eyeball and move it laterally downwards

Abducens (6)
- innervates the lateral rectus and moves the eyeball away from the midline

Question 17

Give some roles of the trigeminal nerve

Answer 17

Brings in sensory information to head and neck and has an important motor output to muscles of mastication.
Sensory information in can be pain or temperature to the spinal nucleus of the trigeminal.
If the sensory information is touch or balance, it goes to the principal nucleus (equivalent to dorsal columns).
Unconscious reflexes goes to the mesencephalic nucleus. This is up by the midbrain.

Sensory information that comes in goes via the thalamus and then off to the cortex of brain.

Has 3 main branches. Opthalmic branch to forehead, maxillary branch to maxilla+cheek and mandibular branch to mandible and tongue.

Trigeminal ganglia = where cell bodies of sensory part of trigeminal nerves are. (Equivalent to dorsal root ganglia seen in spinal cord)
They synapse in the brain stem with second order neurons in the trigeminal nucleus.

Question 18

Give details on the facial nerve

Answer 18

Functions:
Motor to muscles of facial expression.
Sensory to skin and ear drum.
Special sensory for anterior 2/3 of tongue for face.
Parasympathetic for innervation of submandibular and sublingual salivary glands. Parasympathetic to lacrimal glands for tears and eye moisture

Lesion of the cranial nerve itself (cut for example). After it has left the motor nucleus, the patient has a facial palsy. Patient can’t frown or smile. This is a peripheral facial palsy.

Facial nucleus will receive information from motor cortex supplying the face. A lesion here (in upper motor neurons) would mean the patient can still move upper part of face.

This is because upper part of face receives information from both hemispheres of the brain (so other hemisphere can compensate) but the lower part of the face only receives information from the opposite hemisphere of brain mending the whole side of the face is affected.

Question 19

Give some information on the vestibulo-cochlear nerve

Answer 19

Special sensory functions.
Two nerves coming together.
Vestibular brings balance and cochlear monitors hearing.

The two branches exit from ear together into PONS.
One comes from vestibule and one from the cochlea.

Question 20

How does the glossopharyngeal nerve function?

Answer 20

(Mops up that the facial has missed)
Special sensory to posterior 1/3 of tongue.
Parasympathetic to the parotid gland.
Motor to upper pharynx.
Some sensory information from back of tongue and oropharynx.
Chemoreception of blood pH but monitoring the carotid body. This is a life saving function.

Comes from PONS.

Question 21

Give some information on the vagus nerve

Answer 21

Responsible for life sustaining function.
Has a sensory function going to respiratory tree and gut. Also to linings of the pharynx.
Has a motor supply to upper oesophagus.
Parasympathetic component to heart, smooth muscle of gut and the major airways.

Question 22

What does the accessory nerve do?

Answer 22

It innervates the shoulder and for head movement.

Goes through to the trapeziums for elevating shoulder.
Sternocleidomastoid is for moving neck.
It arises from the C1 spinal cord down to C5.

They exit the cord as rootlets, travel through the foramen magnum and then carry information to skeletal muscles of neck and upper back.

Question 23

What is the role of the hypoglossal nerve?

Answer 23

Motor nerve.
Innervates intristisce muscles of tongue and moat extrinsic muscles of tongue.

It comes from the medulla through the hypoglossal canal.

Question 24

What are the two main special nuclear groups?

Answer 24

Some nerves come together to form discrete nuclei in brains stem.

Nucleus Amniguus (nerves 9 and 10)

• Motor to larynx, pharynx, upper oesophagus
• Vomiting, swallowing, modulation of phonation

Nucleus Tractus Solitarius (nerves 7, 9 and 10)

• Visceral sensory: tongue, carotid body, carotid sinus, trachea
• Chemoreception, blood pressure monitoring