20 - Organization of Brainstem Flashcards
The brainstem can be organized based on three types of cranial nerve nuclei
1 - Motor
2 - Both (mixed)
3 - Sensory
Motor cranial nerve nuclei fiber type
GSE
- General Somatic Efferent (GSE): Fibers of motor neurons which innervate skeletal muscles (derived from somites)
Location of motor cranial nuclei
Located in the midline of the brainstem
Which cranial nerves are motor?
3, 4, 6, 12
III oculomotor
IV trochlear
VI abducens
XII hypoglossal
Mixed (both motor and sensory) nuclei fiber type
There are 4
SVA
- Special Visceral Afferent
- Special senses of taste and smell
GVA
- General Visceral Afferent
- Pressure and pain sensation from visceral structures (i.e. wall of the GI tract)
SVE
- Special Visceral Efferent
- Innervate skeletal muscles derived from embryological pharyngeal arches
- Muscles of mastication, facial expression, larynx, pharynx
GVE
- General Visceral Efferent
- Autonomic fibers for visceral motor innervation to smooth muscle or cardiac muscle and secretions from glands
Location of mixed cranial nuclei
In between the motor (midline) and sensory (dorsal) nuclei
These cranial nerve nuclei (4 different types) are intermediate columns that run throughout the pons and medulla of the brainstem in relation to the pure motor and pure sensory cranial nerve nuclei
What CNs are mixed (both sensory and motor fibers)
5, 7, 9, 10
V trigeminal
VII facial
IX glossopharyngeal
X vagus
There is only one pure sensory nucleus and nerve located in the brainstem. What is it?
Cranial nerve VIII - Vestibulocochlear
What type of fibers are found in CN VIII?
SSA
- Special Somatic Afferent (SSA)
- Fibers associated with the special senses of sight, hearing and equilibrium
Where is CN VIII located in the brainstem
Dorsolaterally in the caudal (lower) pons and rostral (upper) medulla
Recall that the motor neurons are found medially, the mixed neurons are found after that and the sensory neurons are found dorsally or dorsal-laterally
What is the “exception to the rule” when it comes to sensory nuclei of the brainstem
CN V - trigeminal
CN V is a mixed nerve (sensory and motor fibers), but three of its nuclei are located in the most lateral aspect of the brainstem
What are the three nuclei of the trigeminal located near the purely sensory area of the brainstem (lateral aspect)
Trigeminal (CN V)
- Spinal nucleus
- Principal nucleus
- Mesencephalic nucleus
What type of fibers run in the lateral nuclei of the trigeminal nerve?
GSA
- General Somatic Afferent
- Fibers that carry touch, pressure, general proprioception, pain, or temperature sensation from somatic structures (ex: skin, muscles, joints)
Four embryologic structures of the brainstem
All found in the gray matter
- Alar plate
- Basal plate
- Central canal
- Sulcus limitans
Alar plate
AKA “roof”
- The alar plate is part of the dorsal side of neural tube, that involves the communication of somatic and visceral SENSORY systems
- The caudal part later becomes SENSORY axon part of the spinal cord
When you think alar plate, think SENSORY nuclei
Which sensory nuclei develop from the alar plate?
Sensory nuclei of cranial nerves V, VII, VIII, IX, and X
Basal plate
The basal plate is a part of the neural tube located near the alar plate and eventually gives rise to the basis pontis (little lines on the pons), pyramids, crus cerebri, etc.
MOTOR nuclei are found in the basal plate
When you think basal plate, think MOTOR nuclei
Central canal
- The central canal is a cerebrospinal fluid-filled space that runs longitudinally through the length of the entire spinal cord
- The central canal is continuous with the ventricular system of the brain (lateral ventricles, third ventricle, etc.)
- The fourth ventricle narrows at a region called the obex to become the central canal of the spinal cord
Note: The central canal represents the adult remainder of the central cavity of the neural tube. It generally occludes (closes off) with age.
What type of cells line the central canal? What is their role?
Ependymal cells
- Ependyma is the thin epithelium-like lining of the ventricular system of the brain and the central canal of the spinal cord
- Ependyma is one of the four types of neuroglia in the central nervous system (CNS)
- It is involved in the production of cerebrospinal fluid (CSF)
Sulcus limitans
In the floor of the fourth ventricle, the sulcus limitans separates the cranial nerve motor nuclei (medially/ventrally) from the sensory nuclei (lateral/dorsally)
It is a landmark inside the ventricular system near the tegmentum of the brainstem
Describe the “rule of fours” and what it helps us to divide and organize
It is a system that helps students to remember which cranial nerves arise from which areas of the brainstem
There are exceptions to the rules, so pay attention to details here
First group of four cranial nerves
CN I - Olfactory - Cerebrum
CN II - Optic - Diencephalon
CN III - Oculomotor - Midbrain
CN IV - Trochlear - Midbrain
The first group is supposed to be the “midbrain group” but as you can see it is more of the “midbrain and up” group since the first two arise above the midbrain
Second group of four cranial nerves
CN V - Trigeminal - Pons
CN VI - Abducens - Pons
CN VII - Facial - Pons
CN VIII - Vestibulocochlear - Pons and medulla
Why does the vestibulocochlear arise from the pons AND the medulla? Because it has two parts. The vestibular part arises from the rostral medulla where it crosses the pontomedullary junction and cocochlear part arises from the pons
Third group of four cranial nerves
CN IX - Glossopharyngeal - Medulla
CN X - Vagus - Medulla
CN XI - Accessory - Medulla and spinal cord
CN XII - Hypoglossal - Medulla
The accessory is located in both the spinal cord and the medulla, which is the only exception in this group
How does the position of the sensory nuclei change during embryological development in the region of the pons and rostral (“open”) medulla?
Sensory nuclei are moved from a dorsal position to a dorsolateral position
How can you identify this level?
The presence of the 4th ventricle
You will see this in the pons and “open” part of the rostral (upper) medulla
How does the position of the motor system change during embryological development in the region of the pons and rostral (“open”) medulla?
The motor systems are not affected, therefore they stay in the midline
What is the cerebral aqueduct?
- A structure within the mdibrain (mesencephalon) which contains cerebrospinal fluid (CSF)
- It connects the third ventricle in the diencephalon to the fourth ventricle within the region of the midbrain (mesencephalon) and pons/cerebellum (metencephalon)
What is the diencephalon?
- In adults, the diencephalon appears at the upper end of the brain stem, situated between the cerebrum and the brain stem
It is made up of four distinct components:
- Thalamus
- Subthalamus
- Hypothalamus
- Epithalamus
Describe the position of the sensory system on its way to the VPL/VPM of the thalamus in the embryologic medbrain
VPL = Ventral posterolateral nucleus VPM = Ventral posteromedial nucleus
These are just nuclei that are found in the thalamus
The sensory systems that are heading to these nuclei take a dorsolateral position
Recall that in the rostral (upper) medulla and pons, the sensory nuclei moved from the dorsal to dorsolateral position… They are continuing in that position as they go further up
Describe the position of the motor system in the embryologic midbrain
The motor systems stay in the midline position
Quick review… What are the tectum and the colliculi of the midbrain?
Recall what the colliculi and tectum are…
- The tectum (“roof”) is a region of the brain, specifically the dorsal part of the midbrain. This is contrasted with the tegmentum, which refers to the region ventral to the ventricular system. The tectum is responsible for auditory and visual reflexes.
- The corpora quadrigemina (“quadruplet bodies”) are the four colliculi—two inferior, two superior—located on the tectum of the dorsal aspect of the midbrain. They are respectively named the inferior and superior colliculus. The corpora quadrigemina are reflex centers involving vision and hearing.
Describe the position of the pure sensory relays that are heading to the colliculi or tectum in the embryologic midbrain
Both the tectum and the colliculi are located on the DORSAL aspect of the midbrain, so the pure sensory relays that travel to these areas are found dorsally on the midbrain
Now we are moving on to the adult brainstem…
Just an FYI
We can divide the medulla up into the parts. What are they?
- Caudal “closed” medulla (lower)
- Rostral “open” medulla (upper)
The caudal “closed” medulla has four zones. What are they?
1 - Medial zone
2 - Dorsal zone
3 - Lateral zone
4 - Intermediate zone
Note: these zones divide the cross section view of the medulla - the slides are helpful for visualizing this, but they show a “dorsolateral zone” instead of a separate dorsal zone and lateral zone, FYI
What four structures will you find in the medial zone of the caudal “closed” medulla?
1 - Corticospinal tract (pyramidal tract)
2 - Hypoglossal nucleus
3 - Internal arcuate fibers forming the medial lemniscus
4 - Ventral trigeminothalmic tract
Remember: “Mid chiv” - the middle child is always the most chivalrous
What feature of the corticospinal tract will we see in the caudal medulla?
Decussation
What type of fibers will we see in the hypoglossal nucleus?
It is a motor cranial nerve - we will see GSE fibers
GSE
- General Somatic Efferent
- Fibers of motor neurons which innervate skeletal muscles (derived from somites).
What type of fibers will we see in the trigeminothalamic tract?
- Second order neurons of the face
- These form the pain and temperature pathway of CN V
What structure will you find in the dorsal zone of the caudal “closed” medulla?
Posterior column - Medial lemniscus system
What two nuclei are found in the posterior column - medial lemniscus system?
- Gracile fasciculus/gracile nucleus
- Cuneate fasciculus/cuneate nucleus
What three structures will you find in the lateral zone of the caudal “closed” medulla?
1 - Spinothalamic (ALS) tract
2 - Spinal trigeminal nucleus
3 - Spinal trigeminal tract
What types of fibers would you find in the spinal trigeminal nucleus?
GSA
- General Somatic Afferent
- Fibers that carry touch, pressure, general proprioception, pain, or temperature sensation from somatic structures (ex: skin, muscles, joints).
What structures will you find in the intermediate zone of the caudal “closed” medulla?
Mixed cranial nerves
These are cranial nerves that have both sensory and motor functions
Which cranial nerve nuclei are found in the intermediate zone of the caudal “closed” medulla?
1 - Dorsal motor nucleus of CN X
2 - Nucleus ambiguus
3 - Nucleus solitarius
Describe the dorsal motor nucleus of CN X
Dorsal motor nucleus of the vagus nerve
- Contains GVE fibers
- Carries pre-ganglionic parasympathetic fibers to viscera
Recall…
- General visceral efferent fibers are autonomic fibers
Describe the nucleus ambiguus
Nucleus ambiguus
- Contains SVE fibers
- Supply motor innervation to the muscles of the larynx and pharynx
- Cranial nerves IX, X, XI contribute to the nucleus ambiguus
Recall…
- Special visceral efferent fibers are derived embryologically from the pharyngeal arches
Describe the nucleus solitarius (solitary nucleus)
Solitary nucleus
- Contains SVA and GVA fibers
- Provides taste and visceral sensations
- Cranial nerves IX and X contribute to the solitary nucleus
Recall…
- Special visceral afferent fibers are associated with the special senses of olfaction (smell) and gustation (taste)
- General visceral afferent fibers fibers that carry pressure or pain sensation
from visceral structures
-
The rostral “open” medulla has three zones. What are they?
1 - Medial zone
2 - Dorsolateral zone
3 - Intermediate zone
What five structures will you find in the medial zone of the rostral “open” medulla?
1 - Medial lemniscus and ventral trigeminothalamic tract 2 - Corticospinal tract 3 - Corticonuclear tract 4 - Medial longitudinal fasciculus (MLF) 5 - Hypogloassal nucleus (GSE)
Remember: MVCC w/ a MLF w/ tongue
What other terms does Dr. Matz use for the corticospinal and corticonuclear tracts?
Corticospinal = pyramidal tract
Corticonuclear = corticobulbar tract
What is the medial lemniscus?
- A large ascending bundle of heavily myelinated axons that decussate (cross) in the brain stem, specifically in the medulla.
- The medial lemniscus is formed by the crossings of internal arcuate fibers (axons of nucleus gracilis and nucleus cuneatus).
- The axons of the nucleus gracilis and nucleus cuneatus in the medial lemniscus have cell bodies that lie in the contralaterally.
- Lesion of the medial lemnisci causes an impairment of vibratory and touch-pressure sense.
What is the ventral trigeminothalamic tract?
- The ventral (or anterior) trigeminothalamic tract serves as a proprioception, touch, and vibration pathway from the face, head and neck.
What is the medial longitudinal fasciculus (MLF)?
- MLF is the main central connection for the oculomotor nerve, trochlear nerve, and abducens nerve.
- Horizontal gaze coordination center
What four structures will you find in the dorsolateral zone of the rostral “open” medulla?
It is dorsolateral and not just dorsal because of the opening of the 4th ventricle
1 - Spinothalamic tract (ALS)
2 - Spinal trigeminal nucleus
3 - Spinal trigeminal tract
4 - Sensory cranial nerve (VIII)
Remember: the four S’s
What is the spinothalamic (ALS) tract
AKA anteriolateral tract
- A sensory pathway from the skin to the thalamus.
- The spinothalamic tract consists of two adjacent pathways: anterior and lateral.
- The anterior spinothalamic tract carries information about crude touch.
- The lateral spinothalamic tract conveys pain and temperature.
What is the spinal trigeminal nucleus responsible for?
Pain and temperature to the orofacial region
- CN V
- GSA fibers
Describe the CN VIII seen in the dorsolateral zone of the rostral “open” medulla
- Vestibular and cochlear nuclei are present
- SSA fibers
- Seen in the most rostral medulla and caudal pons
Reminder…
- Special Somatic Afferent
- Fibers associated with the special senses of sight, hearing and equilibrium
What would you find in the intermediate zone of the rostral “open” medulla?
Mixed cranial nerves
Both sensory and motor fibers
What are the four nuclei of mixed cranial nerves in the intermediate zone of the rostral “open” medulla?
1 - Dorsal motor nucleus of the vagus nerve
2 - Inferior salvatory nucleus
3 - Nucleus ambiguus
4 - Nucleus solitarius
Describe the dorsal motor nucleus of the vagus nerve
CN X
- General visceral efferent (GVE) fibers (autonomics)
- Pre-ganglionic parasympathetic fibers to the viscera
Describe the inferior salivatory nucleus
CN IX - Glossopharyngeal
- General visceral efferent (GVE) fibers (autonomics)
- Pre-ganglionic parasympathetic fibers to the parotid gland
- Secretomotor function
Describe the nucleus ambiguus
CN IX, X
- Special visceral efferent fibers (SVE)
- Motor supply to the muscles of the larynx and pharynx
-
Describe the nucleus solitarius
AKA solitary nucleus
- SVA and GVA fibers
- Provides taste and visceral sensations
- Includes CN IX and X
Recall…
- Special visceral afferent fibers associated with the special senses of olfaction (smell) and gustation (taste)
- General visceral afferent fibers that carry pressure or pain sensation from visceral structures (ex: wall of GI tract)
Onto the pons…
We are not going to distinguish between different levels of the pons (caudal vs rostral), but just have one cross-section to memorize. FYI
What three zones are found in the pons?
1 - Medial zone
2 - Dorsolateral zone
3 - Intermediate zone
What 6 structures will you find in the medial zone of the pons?
1 - Corticospinal tract 2 - Corticonuclear tract 3 - Medial lemniscus with ventral trigeminothalamic tracts 4 - Basis pontis 5 - Motor cranial nerve (VI) 6 - Medial longitudinal fasiculus (MLF)
What is the basis pontis?
The bulbar part of the pons
- Contains pontine nuclei
- Carries information from the primary motor cortex to the ipsilateral pontine nucleus in the ventral pons
What cranial motor nuclei is found in the medial pons?
Abducens nuclei
What 4 structures will you find in the dorsolateral zone of the pons?
Remember it is dorsolateral (and not just dorsal) because the 4th ventricle opens up to the dorsal side
1 - Spinothalamic (ALS) tract
2 - Spinal trigeminal nucleus
3 - Spinal trigeminal tract
4 - Sensory cranial nerve
What sensory cranial nerve is found here? What leel of the pons?
CN VIII
- Vestibular and cochlear nuclei
- Found in caudal pons
What will you find in the intermediate zone of the pons?
Mixed cranial nerves (both sensory and motor)
Six nuclei of mixed cranial nerves are found in the intermediate zone of the pons. What are they?
1 - Facial nucleus 2 - Principal (main) sensory nucleus of CN V 3 - Mesencephalic nucleus of CN V 4 - Motor nucleus of CN V 5 - Superior salivatory nucleus 6 - Nucleus solitarius
Facial nucleus
SVE fibers
- Special visceral efferent (autonomics)
- The route of nerves emerging from the facial nucleus are unique
- The genu of nerves forms a facial colliculus
Principal (main) sensory nucleus of CN V
GSA fibers
- General somatic afferent
- General discriminative touch to the orofacial region
Recall…
- GSA: Fibers that carry touch, pressure, general proprioception, pain, or temperature sensation from somatic structures (ex: skin, muscles, joints)
Mesencephalic nucleus of CN V
GSA fibers
- Rostral pons enters the caudal midbrain, hence the name “mesencephalic”-pertaining to the midbrain.
- Proprioceptive nucleus for muscles of the head.
Motor nucleus of CN V
SVE fibers
- Special visceral efferent
- Motor fibers to muscles of mastication, mylohyoid, anterior belly digastric, tensor tympani and tensor veli palatini
Superior salivatory nucleus
GVE fibers of CN VII (Facial)
- General visceral efferent (autonomics)
- Pre-ganglionic parasympathetics
- Provides secretomotor innervation to the sublingual, submandibular and palatine glands
Nucleus solitarius (solitary nucleus)
SVA fibers from CN VII (facial)
- Special visceral afferent
- Taste (gustation)
Onto the midbrain…
Last section of the brainstem
What four zones do we find in the midbrain?
1 - Medial zone
2 - Dorsal zone
3 - Lateral zone
4 - Intermediate zone
What 7 structures do we find in the medial zone of the midbrain?
1 - Corticospinal tract 2 - Corticonuclear tract 3 - Substantia nigra 4 - Red nucleus 5 - Decussation (crossing) of SCP 6 - Motor cranial nerves (III and IV) 7 - Medial longitudinal fasiculus (MLF)
Describe the motor cranial nerves of the medial zone of the midbrain
Trochlear nucleus
- General somatic efferent (GSE) fibers
- Skeletal muscle innervation
Oculomotor nuclear complex
- GSE and GVE fibers
- General somatic efferent (skeletal muscle innervation)
- General visceral efferent (autonomic innervation)
- We call this the Edinger-Westphal nucleus
What two structures are found in the dorsal zone of the midbrain?
1 - Periaqueductal gray (PAG)
2 - Tectum - sensory relay
What is the periaqueductal gray (PAG)?
PAG
- The grey matter located around the cerebral aqueduct within the tegmentum of the midbrain
- it is the primary control center for descending pain modulation
What is the tectum - sensory relay?
Tectum is responsible for sensory reflexes and contains the superior and inferior colliculi
- Superior = vision
- Inferior = auditory
What two structures are found in the lateral zone of the midbrain?
1 - Spinothalamic (ALS) tract
2 - Medial lemniscus
What would you find in the intermediate zone of the midbrain?
Mesencephalic nucleus of CN V
Mesencephalic nucleus of CN V
GSA fibers
- General somatic afferent
- Provides proprioceptive innervation to the muscles of the head
- This nucleus is found where the rostral pons enters the caudal midbrain, hence the name “mesencephalic”
What would a lesion of the mesencephalic nucleus of CN V result in?
Ipsilateral motor deficits
Reading assignment and material…
Reticular formation
Pg. 241-251
Reticular formation function
- Regulation of the level of consciousness and cortical alertness
- Control of somatic motor movements
- Regulation of visceral motor or autonomic functions
- Control of sensory information
The reticular formation has different functions associated with many zones of the medulla, pons and midbrain
Medulla and pons are clumped together, midbrain is separate
Reticular formation - medial zone of medulla and pons
- Associated with sleep
- Modulate or suppress the transmission of pain
Reticular formation - paramedian zone of medulla and pons
- Feedback systems are associated with intricate patterns of movement
Reticular formation - medial zone of medulla and pons
- Connection to thalamus and hypothalamus to control arousal and the ANS
- Motor control of axial and proximal limb musculature
- Modulation of the transmission of sensory information
Reticular formation - lateral zone of medulla and pons
- Sensory or afferent zone
- Relays this information to the medial zone of the reticular formation
Reticular formation - median zone of midbrain
- Associated with sleep
- Modulate or suppress the transmission of pain
Note that this is the same as the median zone of the medulla and pons
