Organisation of the Brainstem and Cranial Nerves

Question 1

Where does the brainstem lie? What are its major divisions?

Answer 1

Brainstem = a part of the CNS exclusive of the cerebellum which lies between the cerebrum and spinal cord.

Major divisions:

• Medulla oblongata
• Pons
• Midbrain

Question 2

Label this.

Answer 2

Question 3

Where are the colliculi found and what are their functions?

Answer 3

Roof of the midbrain - 4 bumps (“low lying hill”)

• Superior colliculi – important for the coordination of eye and head movements at the same time
• Inferior colliculi – auditory stimulus reflexes (for survival) e.g. turning your head in the direction of a loud noise

Question 4

Describe the location of the pons relative to the ventricular system.

Answer 4

The pons makes up the floor of the 4th ventricle

Question 5

Name an important unpaired, midline structure on the posterior aspect of the brainstem.

Answer 5

Pineal gland

Everything in the brainstem is bilateral except the pineal gland.

Question 6

What is the role of the pineal gland?

Answer 6

It produces melatonin, which is involved in regulating the circadian rhythm

• Disrupted in people who work shifts
• Melatonin release is controlled by connections from retina to the pineal gland

Question 7

Which cranial nerve emerges from the back of the brainstem?

Answer 7

Trochlear nerve CNIV - the only cranial nerve to emerge posteriorly from the brain stem

Question 8

What is the role of CNIV?

Answer 8

It supplies the superior oblique muscle – one of the extrinsic muscles of the eye

It is one of the three nerves supplying the eyes

Question 9

What structure defines the medulla in the dorsal aspect and what pathways are found within this structure?

Answer 9

Dorsal Columns – sensory pathways –fine touch and proprioception

Question 10

Label this.

Answer 10

Question 11

What three significant structures can be seen superior to the pons when viewing the brainstem from an anteroinferior view?

Answer 11

Optic Chiasm

Pituitary Stalk (infundibulum)

Mammillary Bodies

Question 12

What are the mammillary bodies? What causes them to become dark?

Answer 12

“little breats” - involved in housekeeping, homeostasis and memory

They are the inferior part of the hypothalamus, part of the diencephalon – it is part of the limbic system

• Become dark due to chronic alcoholism causing neovascularisation.

Question 13

Which cranial nerve emerges in the midline between the cerebral peduncles?

Answer 13

Oculomotor Nerve (CN III)

Question 14

What are the cerebral peduncles? What is the significance of the word ‘peduncle’?

Answer 14

Peduncle is specifically a structure that has a functional (are the descending motor tracts coming from the motor cortex) AND structural role (it holds the cerebrum onto the brainstem)

Question 15

Name the cranial nerve that emerges from the lateral aspect of the pons. What are its 3 branches?

Answer 15

Trigeminal (CN V) (motor and sensory) - ophthalmic, maxillary and mandibular nerves.

Question 16

What is the role of this nerve? (CNV)

Answer 16

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

Question 17

Which three nerves emerge at the pontomedullary junction (from medial to lateral)?

Answer 17

Abducens, Facial, Vestibulocochlear (CN6,7,8)

Question 18

Briefly state the role of each of these cranial nerves CN VI, VII, VIII (which emerge at the pontomedullary junction.

Answer 18

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

Question 19

Which three nerves emerge from the lateral medulla?

Answer 19

Glossopharyngeal, Vagus and Accessory - CN9, 10, 11

Question 20

Briefly state the role of each of these cranial nerves - CN IX, X, XI (which emerge from the lateral part of the medulla)

Answer 20

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

Question 21

State the last cranial nerve – CN XII. What is the function?

Answer 21

Hypoglossal Nerve – supplies the musculature of the tongue

Question 22

The motor fibres coming down from the motor cortex come via the cerebral peduncles then disappear behind the transverse fibres of the pons. What structure do they re-emerge as, inferior to the transverse fibres?

Answer 22

Pyramids = continuation of the corticospinal tract which re-emerges

Question 23

What percentage of motor fibres cross to the contralateral side of the body in the brainstem and where does this changing of sidestake place?

Answer 23

90-95%

Decussation (crossing over) occurs at the most caudal end of the pyramids.

Question 24

What are the four functional subtypes of the cranial nerves and what are their actions?

Answer 24

• General Somatic Afferent (GSA)
  
  • Sensation from the skin and mucous membranes
• General Visceral Afferent (GVA)
  
  • Sensation from the GIT, heart, vessels and lungs
• General Somatic Efferent (GSE)
  
  • Muscles for eye and tongue movements
• General Visceral Efferent (GVE)
  
  • Preganglionic parasympathetic

Question 25

What are the special subtypes of cranial nerves and what are their actions?

Answer 25

* 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

Question 26

What structure separates the sensory nuclei from the motor nuclei in the spinal cord and brainstem?

Answer 26

Sulcus limitans

Question 27

What happens in the formation of the rhombencephalon and how does this affect the arrangement of the nuclei?

Answer 27

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 * (Generally they remian in a classical organisation with the GSE remaining more medial and the GVE more lateral)

Question 28

Describe the arrangement of the different groups in columns within the brainstem.

Answer 28

* 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

Question 29

State the nuclei that are in the GSE (general somatic efferent) group and describe their location within the brainstem.

Answer 29

**_Midbrain_** – **oculomotor**; **trochlear**. **_Pons_** – **abducens** (emerges in pontomedullary junction, so fibres move down before emerging up) * *These are all the eye movement nerves.* **_Medulla_** – **hypoglossal**

Question 30

State the nuclei that are in the SVE (special visceral efferent) group and describe their location within the brainstem.

Answer 30

**_Pons_** – **trigeminal** motor (muscles of mastication); **facial** **_Medulla_** – nucleus **ambiguus** (involved in laryngeal movements) **_Cervical spinal cord_** – **accessory** 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

Question 31

State the nuclei that are in the GVE (general visceral efferent) group and describe their location within the brainstem.

Answer 31

This is the parasympathetic **_Midbrain_** – Edinger Westphal nucleus (follow the oculomotor nerve and provides parasympathetic innervation to the eye) **_Ponto-medullary border_** – salivatory (3 of these nuclei- prompt salivation) **_Medulla_** – vagus motor (gut motility)

Question 32

State the nuclei that are in the SSA (special somatic AFFERENT) group and describe their location within the brainstem.

Answer 32

**_Pons and medulla_** - **Vestibulocochlear** (balance and hearing; most lateral)

Question 33

State the nuclei that are in the GSA (general somatic AFFERENT) group and describe their location within the brainstem.

Answer 33

**_Midbrain(mesencephalon), pons, medulla and cervical spinal cord,_** - **trigeminal** = (in all 3 parts and spinal cord)

Question 34

State the nuclei that are in the GVA and SVA (general visceral AFFERENT and special visceral AFFERENT) groups and describe their location within the brainstem.

Answer 34

**_Medulla (and a bit in th pons)_**- nucleus solitarius for taste sensation

Question 35

Label this cross-section of the midbrain.

Answer 35

Question 36

Describe the appearance of a cross-section of the midbrain. What are the key features seen?

Answer 36

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_**

Question 37

What is the substantia nigra? Describe its clinical significance.

Answer 37

* The substantia nigra is a group of dopaminergic neurons * In their normal metabolism they produce **_neuromelanin_**, which gives the black colour of the substantia nigra * **Parkinson’s disease** is caused by loss of these dopaminergic neurons so patients with Parkinson’s will have a pale substantia nigra

Question 38

Describe the appearance of a cross-section of the pons. What are the key features seen?

Answer 38

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_**

Question 39

What is the difference between the peduncles seen in the midbrain and the ones seen in the pons?

Answer 39

Midbrain –cerebral peduncles Pons – cerebellar peduncles

Question 40

Describe the appearance of a cross-section of the medulla. What are the key features seen?

Answer 40

**_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

Question 41

What is the role of the inferior olivary nucleus?

Answer 41

It is involved in fine tuning motor function

Question 42

Describe the appearance of a cross-section of the lower medulla. What are the key features seen?

Answer 42

It will be more **_round_** - looks more like spinal cord The **_central cana_**l will be seen in the middle The **_dorsal columns_** will be seen on the dorsal side The **_pyramidal decussation_** may be seen - transverse fibres all cross at pyramidal decussation

Question 43

Name the two columns that make up the dorsal columns.

Answer 43

Gracilis – more medial – sensory information from the lower limb Cuneatus – more lateral – sensory information from the upper limb

Question 44

What is lateral medullary syndrome? List the symptoms.

Answer 44

It is a constellation of symptoms caused by an occlusion in the vertebral arteries or the posterior inferior cerebellar arteries (PICA) It causes: * Horner's Syndrome * Vertigo * Ipsilateral loss of pain/thermal sensation on the face * Contralateral loss of pain/thermal sensation on the trunk and limbs * Ipsilateral cerebellar ataxia * Hoarseness, difficulty swallowing

Question 45

Describe the reasons behind the symptoms of lateral medullary syndrome.

Answer 45

* **Horner's Syndrome–** disturbing the sympathetic tract which passes here * **Vertigo–** because of 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

Question 46

What is Horner's syndrome?

Answer 46

Loss of sympathetic drive to the face

Question 47

What are the symptoms of Horner’s Syndrome?

Answer 47

Ptosis Loss of sweating around the eye Hoarseness Difficulty swallowing