Overview
This lecture builds a scheme for understanding the brainstem: which cranial nerves emerge at which brainstem level, why a nerve and a nucleus are different things, and how development explains where the nuclei sit. The organising idea is that the embryonic dorsal (sensory) to ventral (motor) arrangement of the neural tube is flattened into a medial (motor) to lateral (sensory) arrangement in the brainstem, producing six functional columns whose position predicts function. That column map is then used to describe the longitudinal extent of each nucleus, the functions of the motor and sensory columns, and the classification of nerves as pure motor, pure sensory or mixed.
Approach to learning brainstem neuroanatomy
The lecture sets out a seven-step roadmap, and states plainly that the material is difficult and requires the student to revise and work out a scheme for themselves (working through each lecture in the Assisted Self-Learning document on Moodle).
- Learn the names of the cranial nerves and where they emerge from the brainstem.
- Understand the anatomical and functional difference between a cranial nerve and a brainstem nucleus.
- Understand the embryological origins of brainstem nuclei positions.
- Know the broad functions of each cranial nerve (motor, sensory, mixed).
- Know the broad idea of where a nerve runs within the brainstem level.
- Appreciate the longitudinal extent of each brainstem nucleus.
- Know the blood supply of each part of the brainstem, the likely targets of stroke in each area, and their effects.
Resources given: Young, Young & Tolbert 2nd edition, Ch 3 pp25-35 and Ch 24 pp300-2; key diagrams in the Assisted Self Learning document pp1-4. Prior learning: HUBS191 Lecture 20; also lab class 3 (CSF/brainstem) and class 8 (brainstem).
Where the cranial nerves emerge
Grouped by brainstem level on the ventral view:
- Do not enter the brainstem at all: olfactory (I) and optic (II). The optic chiasm and optic tract lie at the rostral end of the specimen but are not brainstem emergences.
- Midbrain: oculomotor (III), trochlear (IV).
- Pons: trigeminal (V), abducens (VI), facial (VII), vestibulocochlear (VIII).
- Medulla: glossopharyngeal (IX), vagus (X), accessory (XI), hypoglossal (XII).
Warning
The transcript flags that the exact left/right emergence point of each rootlet on the specimen photograph could not be confirmed at the available resolution. The level grouping above and the nerve identities are as labelled on the slide.
Cranial nerves versus brainstem nuclei
This is a distinction of organising principle, not of anatomy alone.
- Nerves are highways to a particular area. A single nerve may carry fibres serving more than one function. NERVES = to a common location.
- Nuclei are groups of cell bodies subserving a common function. A single nucleus may contribute fibres to several different nerves going to different areas. NUCLEI = common function.
- The numbers do not match: 10 cranial nerves reach the brainstem, but there are 15 brainstem nuclei.
The 15 named brainstem nuclei: oculomotor, Edinger-Westphal, trochlear, trigeminal motor, sensory trigeminal nuclear complex, abducens, facial, superior salivatory, inferior salivatory, vestibulocochlear, hypoglossal, nucleus ambiguus, dorsal motor nucleus of vagus, nucleus of the solitary tract, spinal accessory.
Embryological basis of nuclear position
In the spinal cord, the neural tube is organised dorsoventrally about the sulcus limitans:
- Alar lamina (dorsal) gives sensory structures. A sensory neuron in the dorsal root ganglion projects via the spinal nerve to the alar side.
- Basal lamina (ventral) gives motor structures. The motor neuron sits in the basal lamina and projects out via the spinal nerve.
During development the brainstem flexes (in the 6-week embryo: cephalic flexure, pontine flexure at the metencephalon, and cervical flexure at the junction with the spinal cord, along the axis telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon, spinal cord). The neural tube flattens as it opens out around the developing fourth ventricle, and the original dorsal-to-ventral sensory-to-motor axis is reoriented into a medial (motor) to lateral (sensory) axis. This is the embryological reason for the mediolateral column arrangement seen in the adult brainstem.
At 45 days the pattern is visible in section: sensory nuclei dorsal/lateral (alar derivatives), motor nuclei ventral/medial (basal derivatives), with somatic and visceral subdivisions on each side, and the developing cerebellum overlying the fourth ventricle. A further point made on the open medulla section is that some nuclei shift their location ventrally during development, so they no longer sit exactly where the basic embryonic scheme would place them.
Warning
The transcript flags that the colour-to-category mapping in the 45-day schematic has no legend on that slide; the colours there are described positionally only. The named colour code below comes from the open-medulla slide, which does give a key.
The six functional columns: position predicts function
This is the key schema of the lecture, drawn on a transverse section through the open medulla. Six functional zones run either side of the midline, in a fixed medial-to-lateral order.
Basal lamina, motor functions (from the midline outwards):
- General somatic efferent (green)
- Branchial efferent (purple)
- General autonomic (visceral) efferent (blue)
Alar lamina, sensory functions (continuing outwards):
4. General autonomic afferent plus special visceral sense, that is taste (yellow)
5. General somatic afferent (red)
6. Special somatic afferent, the special senses (orange)
Important
Take-home message from the slide: learn this schema, because you can predict the function of any nucleus from its position.
The master brainstem map: longitudinal extent
The master diagram plots the nuclei as ovals within their columns, on a vertical map running midbrain, pons, medulla, cervical spinal cord, with the columns mirrored either side of the midline and the headers reading across as Sensory | Motor | Sensory (Sensory: special somatic, general somatic, general visceral | Motor: general visceral, branchial efferent, general somatic | Sensory: general visceral, general somatic, special somatic). It shows both the mediolateral position and the rostrocaudal (longitudinal) extent of each nucleus. Labels as plotted:
- Midbrain: III, III/Oc, Ew in the medial motor columns; IV/Tr.
- Pons: V in the general somatic sensory column; VI/Ab and VII/Fa in the motor columns; VIII, mTr, sT in the lateral sensory columns.
- Medulla: VII, IX, X, XII/Am, Hy, dVg on the motor side; Ss, Is, Sol on the sensory side; Coc/Vest in the lateral sensory column.
- Cervical spinal cord: CrXI in the lateral column; SpXI, Ac in the medial motor columns.
Warning
The transcript flags that this diagram uses dense abbreviations (Oc, Ew, Tr, Ab, Fa, Am, Hy, dVg, Ss, Is, Sol, mTr, sT, Coc, Vest, CrXI, SpXI, Ac) with no on-slide key; a key is referenced in the handout but not shown in this deck. The abbreviations above are transcribed exactly as they appear and have not been expanded or guessed. The same diagram is reused unchanged on the following slides (the motor-column, sensory-column and nerve-classification slides), and the same flag applies there; the added boxes on the classification slide mark classification only and do not change the underlying labels.
Functions of the three motor columns
- General somatic efferent: skeletal muscle. Eye movements (III, IV, VI) and tongue movements (XII).
- Branchial efferent: special skeletal muscles of the face derived from the branchial arches. Mastication (V), facial expression (VII), throat and larynx (IX and X), sternomastoid and trapezius (spinal XI).
- General visceral efferent (parasympathetic): pupil constriction (III), salivary glands (VII and IX), thoracic and abdominal viscera (X).
Functions of the three sensory columns
- General visceral afferent, including taste: tongue (VII and IX) and epiglottis (X).
- General somatic afferent, somatic sensation: face (V); also VII, mouth and pharynx (IX), larynx (X), and cranial XI, which the slide defers with “see later”.
- Special somatic afferent: auditory and vestibular (VIII).
Warning
Cranial XI under general somatic afferent is marked “see later” on the slide, so its detail belongs to a subsequent lecture not covered here.
Pure motor, pure sensory and mixed nerves
- Pure somatic motor: IV, VI (eye movements) and XII (tongue).
- Pure sensory: I (olfactory), II (optic), VIII (vestibulocochlear).
- Mixed: all the rest, that is III, V, VII, IX, X, XI.
On the map, I and II are boxed off to the side because they do not enter the brainstem, IV and VIII are boxed as non-mixed, and III, V, VII, IX, X and XI (both CrXI and SpXI) are boxed as mixed.
Self-test
- Distinguish a cranial nerve from a brainstem nucleus, giving the organising principle of each.
- State how many cranial nerves reach the brainstem and how many brainstem nuclei there are, and explain why the two numbers differ.
- List the cranial nerves that emerge at each brainstem level (midbrain, pons, medulla), and name the two that do not enter the brainstem at all.
- Explain how the developmental flattening of the neural tube changes the orientation of the sensory and motor territories in the brainstem.
- Name the two laminae separated by the sulcus limitans and state the functional class each gives rise to.
- List the six functional columns in order from the midline outwards, with the colour code given on the open medulla slide.
- Explain what the lecture means by the claim that you can predict the function of any nucleus from its position.
- List the motor functions and responsible cranial nerves of the general somatic efferent column.
- List the branchial efferent targets and the cranial nerve supplying each.
- Describe the general visceral efferent (parasympathetic) outputs and the cranial nerve responsible for each.
- Distinguish the general visceral afferent column from the special somatic afferent column by the modality and the cranial nerves carried.
- Classify cranial nerves IV, VIII and IX as pure motor, pure sensory or mixed, and give the reason for each.
- A nucleus is found in the medulla immediately lateral to the most medial motor column. Predict its functional class and name a muscle group it would supply.
- Explain how the embryological account and the master brainstem map together explain why a nucleus can contribute fibres to more than one cranial nerve while still having a single function.
Answers
Reveal answers
- A nerve is a highway to a common location and may carry fibres serving more than one function; a nucleus is a group of cell bodies subserving a common function and may contribute fibres to several nerves going to different areas. Nerves are defined by destination, nuclei by function.
- Ten cranial nerves reach the brainstem but there are 15 brainstem nuclei. They differ because nuclei are grouped by function rather than by destination, so one nerve can draw from several nuclei and one nucleus can feed several nerves.
- Midbrain: III and IV. Pons: V, VI, VII, VIII. Medulla: IX, X, XI, XII. Olfactory (I) and optic (II) do not enter the brainstem.
- In the spinal cord the alar (sensory) lamina is dorsal and the basal (motor) lamina is ventral. As the brainstem flexes and the neural tube flattens open around the fourth ventricle, that dorsoventral axis is reoriented into a mediolateral one, so motor territory lies medial and sensory territory lateral.
- Alar lamina, dorsal, sensory; basal lamina, ventral, motor.
- From the midline outwards: general somatic efferent (green), branchial efferent (purple), general autonomic/visceral efferent (blue), general autonomic afferent plus taste (yellow), general somatic afferent (red), special somatic afferent (orange).
- Because the six functional categories occupy fixed columns in a fixed medial-to-lateral order, knowing where a nucleus sits relative to the midline and the sulcus limitans tells you its functional class. The slide’s take-home message is to learn this schema for that reason.
- Skeletal muscle: eye movements via III, IV and VI, and tongue movements via XII.
- Mastication (V), facial expression (VII), throat and larynx (IX and X), sternomastoid and trapezius (spinal XI). These are special skeletal muscles derived from the branchial arches.
- Pupil constriction (III), salivary glands (VII and IX), thoracic and abdominal viscera (X).
- General visceral afferent carries visceral sensation including taste, from the tongue (VII and IX) and epiglottis (X); special somatic afferent carries the special senses of hearing and balance via VIII. The visceral afferent column lies medial to the somatic afferent column, and the special somatic afferent column is the most lateral.
- IV is pure somatic motor (eye movement only). VIII is pure sensory (vestibulocochlear). IX is mixed, since it contributes to branchial efferent (throat/larynx), general visceral efferent (salivary glands) and afferent (taste, mouth and pharynx) columns.
- Immediately lateral to the most medial motor column is the branchial efferent column, so the nucleus would be branchial efferent. At medullary level that predicts supply to throat and laryngeal muscles (IX and X).
- Development sorts cell bodies into continuous functional columns running rostrocaudally, so cells are grouped by modality rather than by the nerve they eventually join. The master map shows those columns extending over several brainstem levels, so a single functional column or nucleus can send fibres out along whichever nerves leave at those levels, keeping one function but multiple destinations.