Midbrain & Pons Nuclei & Tracts
In concluding the midbrain & pons series, this article will focus on the nuclei and tracts found at different cross-sectional levels of both these brainstem structures. In addition to that, clinically relevant signs and symptoms associated with damage to the midbrain and pons will also be discussed.
Before discussing the nuclei and tracts, there are some nomenclatures associated with the midbrain that must be clarified. The midbrain can be divided into three parts, the tectum, tegmentum and the basis pedunculi (crus cerebri). The tectum is the region of the midbrain that is posterior to the cerebral aqueduct of Sylvius. It contains the nuclei of the superior and inferior colliculi. Between the cerebral aqueduct and the pars compacta of the substantia nigra, is the tegmentum. The anterior most aspect of the midbrain, inclusive of both layers of the substantia nigra, is the crus cerebri. Collectively, the tegmentum and the basis pedunculi are referred to as the pedunculus cerebri, otherwise called the cerebral peduncles.
Midbrain: Cross Section through the Superior Colliculi
The nuclei of the superior colliculi are the most dorsal structures in the midbrain. They communicate with each other via the commissure of the superior colliculus and with the lateral geniculate body (LGB) via the superior brachium. As a result of these connections, the superior colliculi are involved in pupillary and optic reflexes. Additionally, impulses from the inferior colliculi are sent to the superior colliculi to support audio-visual reflexes. The cerebral aqueduct of Sylvius can be observed in the dorsal part of the midbrain. It permits passage of cerebrospinal fluid (CSF) between the third and fourth ventricles. It is surrounded by a cluster of grey matter known as the periaqueductal grey matter. The periaqueductal grey matter also houses the nucleus of locus coeruleus, the enkephalinergic dorsal tegmental nucleus (pain mediation), the seratonergic dorsal nucleus of raphe and the mesencephalic nucleus and its tract (bilateral to the cerebral aqueduct).
Anterior and bilateral to the cerebral aqueduct are the paired CN III (oculomotor) nuclei. Its fibers continue anteriorly, passing through the red nucleus and exiting the brainstem through the lateral walls of the interpeduncular fossa. These nerves continue through the cranial vault, gaining access to the cavernous sinus on its way to the orbit, where it will innervate four of the six (medial rectus, superior rectus, inferior rectus and inferior oblique) extraocular muscles. The accessory CN III nucleus, also known as the Edinger-Westphal nucleus is located between the cerebral aqueduct and the main CN III nucleus. It has the responsibility of relaying parasympathetic innervation to the sphincter muscles of the iris by way of the ciliary ganglion. Also anterolateral to CN III nucleus is the medial longitudinal fasciculus (MLF). The MLF pathway has the responsibility of integrating the activity of CN III, CN IV (trochlear) and CN VI (abducent) with the inputs of the vestibular tract of CN VIII (vestibulocochlear) to regulate movements of the eyes.
The lemniscal tracts are located in the lateral parts of the tegmentum on both sides of the brainstem. The trigeminal lemniscus (tactile sensation from the head) and the spinal lemniscus (ventral and lateral spinothalamic tracts carrying pain, temperature sensation and light touch) are both located laterally in the anterior part of the tegmentum. Anterior to the trigeminal & spinal lemnisci is the medial lemniscus (carrying dorsal column sensation of proprioception, vibration sense and tactile discrimination).
In the midline of this section, there is a bilateral pair of red, round nuclei known as the red nucleus. This grey matter collection gains its color from its rich blood supply. Its resulting fibers decussate in the midline to give rise to the rubrospinal tract. These fibers control muscle flexor tone.
Between the tegmentum and the basis pedunculi is a heavily pigmented band of fibers known as substantia nigra. This band of neurons can be further separated into a posterior strip called the pars compacta and an anterior strip called pars reticularis. These fibers transmit excitatory and inhibitory signals between the caudatoputamen via the nigrostriatal and the striatonigral (striato/striatal = striatum, which is made up of the caudate nucleus and the putamen) pathways. Additionally, the mediodorsal and ventrolateral thalamic nuclei receive non-dopaminergic inputs via the nigrothalamic tracts.
The basis pedunculi acts as a conduit for corticopontine, corticobulbar and corticospinal fibers to relay signals to their respective centres. The anterior one-fifth of the basis pedunculi carries corticopontine and corticobulbar fibers. Corticospinal fibers take up the middle three-fifths and the remaining portion is occupied by corticopontine fibers.
Midbrain: Cross Section through the Inferior Colliculi
There are some structures present at the level of the superior colliculi that are not seen at the level of the inferior colliculi. The converse is also true. The location and composition of the basis pedunculi, substantia nigra, cerebral aqueduct, periaqueductal grey matter, mesencephalic nucleus of CN V and MLF is similar to that at the level of the superior colliculi.
The nuclei of the inferior colliculi occupy the tectum at this level. Similar to the nuclei of the superior colliculi, they communicate via the commissure of the inferior colliculus. Auditory impulses from the MGB arrive at the inferior colliculi auditory via the inferior brachium. The CN IV (trochlear) nucleus is located in the periaqueductal grey matter in a similar location to that of the CN III nucleus. Its fibers, however, course posteriorly and decussate at the commissure of the inferior colliculus, before leaving the midbrain at the frenulum of the superior medullary velum. The fibers then course around the cerebral peduncles and gains access to the cavernous sinus in order to enter the orbit to supply the superior oblique muscle of the eye.
The previously mentioned lemniscal tracts present at the superior colliculi are also present at the inferior colliculi in a similar location. They are, however, joined by the lateral lemniscus, which carries auditory impulses from both ears to the inferior colliculi via the MGB.
The tracts occupying the superior cerebellar peduncle make a significant decussation in the midline of the midbrain. Anterior to this decussation is the interpeduncular nucleus which, by way of the habenulointerpeduncular tract (fasciculus retroflexus of Meynert), communicates with the Habenular nucleus. The distribution of the tracts in the basis pedunculi differs slightly from that at the level of the superior colliculi. The fibers occupy equal thirds at this level, as opposed to unequal fifths mentioned earlier.
Pons: Cross Section through the Facial Colliculus
Since the pons and the midbrain communicate, there are similar structures that were previously described in the midbrain that are present in the pons. At the level of the facial colliculus, MLF assumes a position just deep to the floor of the fourth ventricle (which occupies a large space in this section). Division of the pons into two parts facilitates understanding its internal arrangement. The dorsal region containing the cranial nerves and fourth ventricle is the tegmentum, while the anterior part with the pontine nuclei and corticospinal and corticobulbar tracts is the base of the pons.
In the tegmentum, the genu of CN VII as it winds around the nucleus of CN VI can be observed bilaterally, and anterior to the MLF. The fibers of CN VI pass through the medial lemniscus, transverse pontine fibers, corticospinal and corticobulbar tracts to exit the brainstem. It travels toward the middle cranial fossa where it passes through the cavernous sinus to enter the orbit and supply the lateral rectus muscle of the eye. Both the nucleus and tract of the spinal trigeminal nerve are found at this level, lateral to CN VII. The nuclei of the vestibular component of CN VIII are present just lateral to the fourth ventricle, giving rise to the vestibular area of the rhomboid fossa. The medial lemniscus is a central structure at this level alongside decussation of the ventral cochlear nuclei of the CN VIII, which forms the trapezoid body. The pontine nuclei are dispersed along the midline of the base of the pons, just deep to the groove of the basilar artery. They communicate with the dentate nucleus of the cerebellum via the pontocerebellar fibers. Clusters of corticospinal and corticobulbar fibers also occupy the base of the pons.
Pons: Cross Section at CN V
The aforementioned cranial nerve nuclei are no longer visible at this level. Only the motor and primary sensory divisions of CN V are found in the tegmentum. CN VI, CN VII, spinal trigeminal nucleus, and the vestibular component of CN VIII are not present at this level. The trapezoid body, medial lemnisci and the aforementioned structures maintain their positions.