What do motor nuclei in the brainstem consist of? (what fiber types, where do they go)
What order neurons are sensory nuclei in the brainstem? Where do they project to? What role do they play in reflexes?
Motor Nuclei: consist of 1) efferent lower motor neurons (both GSE and BE/SVE) to skeletal muscles and 2) preganglionic neurons (GVE) for parasympathetic outflow to smooth muscle and glands.
Cells in Sensory nuclei receive afferent fibers and serve as secondary neurons projecting up to the thalamus and cortex, as well as connecting locally to motor neurons for brainstem reflexes (e.g. pupillary light, corneal blink, jaw jerk and gag reflexes).
In the brainstem, motor nuclei are located more ____ and sensory nuclei are located more _____.
What is the embryologic origin of motor nuclei? Sensory nuclei?
In the brainstem, motor nuclei are located more medially (MM) and sensory nuclei are located more laterally.
• Motor nuclei medial (Basal plate derivatives)
• Sensory nuclei lateral (Alar plate derivatives)
What separates motor from sensory nuclei?
Nuclei with what fiber types (GSA, etc.) are autonomic nuclei (both sensory and motor) located btwn?
State the order of classification of nuclei (GSA, SSA, etc.) from lateral to medial in the brainstem.
Nuclei with what fiber types migrate to more anterolateral regions during development?
1. The sulcus limitans separates motor from sensory nuclei.
2. Autonomics are located between Somatic motor (GSE) and Somatic sensory (either GSA or SSA). see attached pic
3. Lateral to medial (see attached pic): SSA, GSA, GVA, GVE, BE, GSE
4. During development some BE and GSA neurons migrate to more anterolateral areas (see attached pic)
State what part of the brainstem each cranial nerve is associated with.
Midrain: III and IV
Pons: V, VI, VII, VIII
Medulla: IX, X, XI, XII
In the brainstem, state where each of the following pathways are (medial, lateral, ventral/anterior, etc)
lateral spinothalamic tract
If applicable, state what pathways join within the brainstem.
1) Corticospinal and Corticobulbar pathways are always ventral/anterior
2) Medial lemniscus changes position from midline caudally to more lateral at rostral levels
3) Lateral spinothalamic tract stays lateral and is joined by the Medial lemniscus rostrally
What parts of the brainstem (medial, lateral) does the vertebral-basilar-PCA stem supply?
The cerbellar branches? (PICA, AICA, SCA)
Specifically state what parts of the brainstem the PICA, AICA, and SCA supply. (midbrain, pons, medulla)
1) The vertebral-basilar-PCA stem supplies medially placed structures via small penetrating feeder branches
2) Cerebellar branches ( PICA, AICA, SCA) supply the lateral brainstem, each at a different level
PICA: lateral medulla
AICA: lateral pons
SCA: dorsolateral midbrain
What CNs have GSE fiber types?
General Somatic Efferents (GSE)
• Cranial Nerves III, IV, VI, and XII
In what part of the midbrain is the oculomotor (III) nucleus?
What is the position of the oculomotor nucleus in relation to the cerebral aqueduct and periaqueductal gray?
What is the position of the IIIrd nucleus in relation to the superior colliculi?
What is the function of CN III?
Oculomotor Nucleus (III) – ventral to cerebral aqueduct and periaqueductal gray at level of superior colliculus in rostral midbrain
CN III is the only motor nucleus at this level. Note that it exits btwn the cerebral peduncles.
Nerve Fibers innervate levator palpebrae (muscle which holds the eyelid open), and four of the six extraocular muscles (excluding the superior oblique and lateral rectus). Controls adduction and elevation of the eyeball.
In what part of the midbrain is the trochlear (IV) nucleus?
What is the position of the IVth nucleus in relation to the periaqueducatal gray?
What is the position of the IVth nucleus in relation to the inferior colliculi?
Explain the pathway CN IV takes to exit the midbrain.
Does it exit ventrally or dorsally?
What is the function of CN IV?
Trochlear Nucleus (IV) : beneath the PAG at the level of the inferior colliculus (caudal midbrain).
Fibers cross midline and exit dorsally, passing around the brainstem to innervate the superior oblique muscle : medially rotates (intorts) and moves eyeball down.
In attached pic, note that there is less substantia nigra than in the rostral midbrain and that the cerebral peduncles are closer together
In what part of the pons is the nucleus of CN VI located?
What is the position of the nucleus of the abducens nerve in relation to the 4th ventricle?
What is the function of CN VI?
Abducens Nucleus: CN VI - located in the midline of the caudal pons, just beneath the floor of the 4th ventricle
CN VI innervates the lateral rectus muscle which abducts the eyeball for lateral gaze
In what part of the medulla is the hypoglossal nucleus located?
Where does CN XII exit the medulla?
What is the function of CN XII?
The hypoglossal nucleus (XII) is located in the midline of the rostral to middle medulla and supplies musculature of the tongue. The nerve fibers exit anterior to the inferior olive (in the pre-olivary sulcus).
Lesions of the hypoglossal nerve cause tongue deviations to what side (with respect to the lesion)?
• LMN lesion to Hypoglossal nerve following Lymph node removal
• Same effect with damage to CN XII nucleus
• tongue deviates to the side of the lesion (ipsilateral) when asked to protrude
• Tongue muscles are weak on that side and overpowered by those on the opposite side
What CNs contain BE fibers? What structures (generall) do they innervate?
Branchial Efferent (BE) Review
• Innervate muscles of mastication (V), facial expression (VII), and muscles of the pharynx (IX, X), larynx (X), and soft palate (V, X)
• Derivation from pharyngeal or branchial arches; therefore innervation is classified as BE rather than GSE (which refers to an embryologic derivation from a somite)
Arches 1,2,3, and 4-6 corespond to CN V, VII, IX, and X, respectively.
In what part of the brainstem is the motor nucleus of CN V located?
What do the fibers from the motor nucleus of the trigeminal nerve travel with?
What does CN V innervate?
The motor nucleus of cranial nerve V is located in the pons. These motor fibers will course together with the sensory fibers of V as they exit the pons. The motor fibers of cranial nerve V innervate the muscles of mastication, anterior belly of digastric, mylohyoid, tensor veli palatini (Arch 1 derivatives).
In what part of the pons is the facial motor nucleus located?
What is the facial colliculus? What is it created by?
What does CN VII innervate?
What will damage to the facial nerve or its nucleus cause?
The facial motor nucleus is in the caudal pons. Axons wrap around Abducens nucleus creating a bump called the facial colliculus. Innervate muscles of facial expression. Damage to CN VII or its nucleus results in Bell’s palsy: paralysis on the entire ipsilateral half of the face.
Where in the brainstem is the nucleus ambiguus?
What CNs have nerve fibers stemming from this nucleus? What kind of fibers?
What structures do these fibers innervate?
Where do these fibers exit?
The Branchial Efferent fibers (BE) of cranial nerves IX and X are derived from a single nucleus, the Nucleus Ambiguus (N.A.). These fibers will innervate skeletal muscles of the pharynx, larynx, and palate. Fibers from this nucleus exit in the post-olivary sulcus.
Note that we will not have to ID this nucleus on a picture of the medulla.
What is the name for the parasympathetic nucleus of CN III?
Where is the parasympathetic nucleus of CN III located?
Are these fibers pre or postganglionic?
What is the function of fibers from this nucleus?
Located in the rostral midbrain is the Edinger-Westphal nucleus of cranial nerve III (pupillary constriction and ciliary muscle contraction for accommodation of the eye for near vision). This is located just dorsal to the main oculomotor nucleus in the rostral midbrain.
There is a pontomedullary cell column of nuclei that send GVE fibers to CNs VII, IX, and X.
What is the more rostral portion of this column called? What do fibers arising there innervate?
What is the more caudal portion of this column called? What do fibers arising there innervate?
General visceral efferents are preganglionic paraympathetic neurons that supply cranial outflow of this portion of the ANS. The orignate in:
The pontomedullary cell column that sends fibers to cranial nerves VII, IX, and X. The rostral segment of this column in the pons is sometimes designated as the salivatory nucleus because the fibers arising here will travel to the salivary glands and the lacrimal gland (as well as nasal, oral and pharyngeal mucosa). The more caudal portion of this nucleus in the medulla is the dorsal motor nucleus of the vagus (DMN) and is the source of preganglionic parasympathetic fibers in the vagus nerve (for innervation of the thoracic and abdominal viscera and mucosa of pharynx/larynx). Note that the DMN is just lateral to the hypoglossal nucleus.
Note that fibers from the DMN join with fibers from the NA (nucleus ambiguus)
The motor nuclei of CNs are LMNs. What tract contains the UMNs for CNs?
Where does this pathway begin? explain its pathway.
Where do the fibers of this tract decussate?
Explain why dysfunction after unilateral UMN lesions is not obvious upon exam. Which CN is an exception to this?
The motor nuclei of cranial nerves (which are LMNs) are driven by descending corticobulbar fibers and local reflex circuits. The corticobulbar inputs are upper motor neurons (UMNs) from the precentral gyrus (head and face area in inferior portion) which descend via the medial portion of the crus cerebri into the brainstem (medial to the corticospinal tract). These fibers decussate at approximately the level of their target nuclei. The cranial nerve motor nuclei (with ONE exception-CN VII) receive bilateral inputs from these UMNs so dysfunction after unilateral UMN lesions is not obvious.
Explain what part of the face would be paralyzed for an UMN lesion of the corticobulbar tract?
Why is this the case?
What are the places in which this tract could be lesioned?
What is "Central VII"?
For upper motor neuron (UMN) lesions the contralateral lower face is paralyzed. This is because LMNs which innervate muscles in the lower face receive upper motor innervation from the contralateral cortex only. LMNs which innervate muscle in the upper face receive bilateral innervation from UMNs in the cortex.
UMN lesions are supranuclear (above the nucleus of the facial nerve) and result in contralateral paralysis of the lower face only. These lesions could occur in the cortex, internal capsule, or crus cerebri and are referred to as a “Central VII”.
e.g. A person with a stroke affecting the inferior part of the precentral gyrus (middle cerebral artery territory) would be unable to smile or “bare teeth” on the side contralateral to the location of the lesion, but could blink or raise eyebrows on this side. See lack of nasiolabial fold and drooping of lower face on side contralateral to lesion.
What is "Peripheral VII"? How does it differ from central VII?
For lower motor neuron (LMN) injury to the VIIth nerve OR its nucleus in the brainstem, the entire ipsilateral face is paralyzed on the side of the lesion. This is “Bell’s palsy”. A LMN lesion of VII could be at the nucleus of the facial nerve in the caudal pons or to the peripheral nerve itself. In a Bell’s palsy, both the ability to smile and blink is lost ipsilateral to the location of the lesion. Bell’s palsy is often referred to as a “Peripheral VII” clinically.
The trigeminal nerve (opthalmic, maxillary, and mandibular division) are peripheral nerves that have their cell bodies of origins in which ganglion?
Where do central processes enter the sensory root of the trigeminal nerve (in the brainstem)?
What nucleus do trigeminal nuclei of the brainstem send fibers to? Explain the travel of these neurons.
The trigeminal nerve provides sensory innervation to the head. It has three main divisions, the ophthalmic (V1), maxillary (V2), and mandibular (V3) which convey sensory information from the periphery back to the CNS. The peripheral processes within these divisions have their cell bodies of origin in the semilunar ganglion. The central processes enter the sensory root of the trigeminal nerve at the mid-lateral pons.
Different modalities of sensation are distributed to three trigeminal nuclei at different levels of the brainstem. These nuclei send fibers to the ventral posterior medial (VPM) nucleus of the thalamus. Thalamocortical fibers then project from the VPM through the internal capsule to the appropriate region of the inferior post-central gyrus.
Where in the pons is the main sensory nucleus of V?
What kind of information (sensory modality) is conveyed to this nucleus?
After synapsing in this nucleus, where do fibers go next? Explain the pathway.
The main sensory nucleus of V is located lateral to the motor nucleus of V.
Main Sensory Nucleus of V: Discriminatory touch (2 point discrimination, deep pressure and vibration)
The incoming primary fibers synapse within this nucleus and the secondary fibers project to the contralateral VPM nucleus of the thalamus via the medial portion of the medial lemniscus.
Where is the spinal nucleus of V?
What kind of information (sensory modality) is conveyed to this nucleus?
After synapsing in this nucleus, where do fibers go next? Explain the pathway.
What other CNs also have inputs to this pathway?
Pain and temperature afferents of V enter the spinal nucleus of the trigeminal nerve. This nucleus begins at the lateral pons but extends as far caudally as the spinomedullary junction. Note that this tract widens inferiorly. Fibers synapse at multiple levels of this tract (similar to the lateral spinothalamic tract in the spinal cord). Fibers of the associated “spinal tract” are primary fibers that descend alongside the nucleus to connect along its length in a somatotopic fashion. The inferior portions of the spinal tract in the lower pons and medulla receive pain inputs from cranial nerves VII, IX, and X. The cells of the spinal nucleus are secondary neurons that send their axons to the contralateral VPM nucleus of the thalamus.
Identify the spinal tract and nucleus of V in this section of the mid-pons.
Identify the spinal tract and nucleus in this section of the caudal pons.
Identify the spinal tract and nucleus of V in this section of the rostral to middle medulla.
Identify the spinal tract and nucleus of V in this section of the spinomedullary junction/caudal medulla.
Where in the brainstem is the mesencephalic nucleus of V? Be specific.
What kind of information is conveyed in fibers from this nucleus?
Are the cell bodies in the mesencephalic nucleus primary, secondary, or tertiary neurons?
What reflex do nerves from this nucleus participate in? Explain.
Proprioceptive fibers from ALL head and face muscles travel to the mesencephalic nucleus of V located in the rostral and caudal midbrain (lateral borders of periaqueductal gray). They are an exception in that the primary cell bodies migrated into the midbrain during development and are not found in the semilunar ganglion. Mesencephalic neurons associated with the jaw musculature (i.e. the masseter) constitute the afferent limb of the jaw jerk reflex. They connect directly to motor neurons in the motor nucleus of V which control muscles of mastication. Remember that fibers for conscious proprioception relay through secondary neurons in the main sensory nucleus on their way to the contralateral VPM of the thalamus.