Myelin Sheath

Myelin Sheath
Myelin sheaths are sleeves of fatty tissue that protect your nerve cells. These cells are part of your central nervous system, which carries messages back and forth between your brain and the rest of your body.

If you have multiple sclerosis (MS), a disease that causes your immune system to attack your central nervous system, your myelin sheaths can be damaged. That means your nerves won’t be able to send and receive messages as they should.

Because of this, MS can weaken your muscles, damage your coordination, and, in the worst cases, paralyze you. MS affects about 1 in every 750 people and usually shows up between the ages of 20 and 50. It’s not clear what causes it, and there’s no known cure.

Myelin and Your Nerves

Myelin Sheath

The myelin sheath wraps around the fibers that are the long threadlike part of a nerve cell. The sheath protects these fibers, known as axons, a lot like the insulation around an electrical wire.

When the myelin sheath is healthy, nerve signals are sent and received quickly. But if you have MS, your body’s immune system treats myelin as a threat. It attacks both the myelin and the cells that make it.

When that happens, the nerves inside the sheath can be damaged. That leaves scars on your nerves — known as sclerosis — and that makes it harder for them to carry the messages that tell your body to move.

Myelin Research

A lot of the research into MS is focused on boosting your body’s ability to repair damaged myelin. Scientists are looking into:

  • Ways to prevent the chemical reactions that lead to myelin damage
  • Drugs or experimental treatments that might prevent or fix multiple sclerosis
  • Which antibodies — the disease-fighting proteins your immune system makes when you get sick — attack myelin
  • If stem cells — which can grow into different types of tissues — can be used to reverse the damage caused by MS

Floor or Rhomboid Fossa

Floor or Rhomboid Fossa

Floor of fourth ventricle is formed by dorsal surfaces of pons and upper-half of medulla oblongata. It is called rhomboid fossa because it is rhomboid in outline. The area is outlined superolaterally by superior cerebellar peduncles and inferolaterally by inferior cerebellar peduncles. At the inferior angle, on either side of midline, floor is limited by gracile tubercle and superolateral to it lies cuneate tubercle. Whole area of rhomboid fossa is lined by ependyma, just beneath which lie different areas of gray matter, which are more precisely some cranial nerve nuclei.

Floor of fourth ventricle is divided by a vertically running midline sulcus called median sulcus. Each half of the floor is again subdivided into a medial part called medial eminence and a lateral part called vestibular area by a narrower sulcus limitans. Just above the horizontal line of pontomedullary junction, medial eminence presents a round elevation called facial colliculus. It is so called because, efferent facial nerve fibers from motor nucleus of facial nerve loop around abducens nucleus beneath this bulge. Above the level of facial colliculus, sulcus limitans presents a small depression called superior fovea.

Above the level of superior fovea, sulcus limitans becomes flattened and forms lateral limit of floor of fourth ventricle. This area is bluish gray in color and named locus coeruleus (to be pronouncedceruleus). Beneath this area, the group of neurons, containing melanin pigment, is called substantia ferrugenia. These neurons are rich in noradrenaline (norepinephrine).

Lateral to sulcus limitans, rhomboid fossa presents a wide triangular area known as vestibular area or vestibular triangle. Vestibular nuclei are situated beneath this area.

Just below the level of facial colliculus, fine strands of nerve fibers are found to pass beneath ependyma, in mediolateral direction, from median sulcus across medial eminence towards lateral angle. These are known as stria medullaris. These are efferent fibers from arcunate nucleus present on ventral aspect from pyramid. These fibers initially pass in ventrodorsal direction across whole thickness of medulla oblongata to reach rhomboid fossa, where they bend at right angle and cross the median sulcus to pass horizontally towards lateral angle. Finally the fibers reach opposite half of cerebellum via inferior cerebellar peduncle.

Floor or Rhomboid Fossa

Below the level of stria medullaris, medial eminence presents a triangular area with apex directed downward. This area is known as hypoglossal triangle beneath which lies nucleus of hypoglossal nerve. Lateral to hypoglossal triangle, lower end of sulcus limitans presents a small depression called inferior fovea.
Below inferior fovea, lateral to apical part of hypoglossal triangle, a smaller triangular area is present with the apex directed upward. This is called vagal triangle as beneath this area lies dorsal nucleus of vagus.
Inferolateral to vagal triangle, just above the upper end of central canal of medulla oblongata, anarrow area is called area postrema. This narrow area contains some neurons covered by thickened ependyma.
Area postrema is separated from vagal triangle by a ridge of ependyma called funiculus seperans. Lower angle of floor of fourth ventricle looks like a pen’s nib for which it is known as calamus scriptorius.

Following features are not parts of floor of fourth ventricle, but are closely related to it.

  1. Inferolateral boundary of rhomboid fossa, which is formed by inferior cerebellar peduncle is crossed by tranverse ridge of white matter called tinea.
  2. Tinea from both sides converge inferomedially towards the lower apex of fourth ventricle to form a thin fold called obex. It forms the roof of lower apex of fourth ventricle.

Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented)


Relations of Brainstem

Relations of Brainstem

Brainstem is the tubular stalk-like part of the brain made up of midbrain, pons and medulla oblongata from above downward. It is so called because it is like stem of a tree. Main mass of the brain, cerebrum with cerebellum rests on the brainstem and through it, is connected to spinal cord below. Long axis of brainstem is oblique, directed downward and backward.

  • Extent: Above, upper end of brainstem (midbrain) is continuous with diencephalon of forebrain.
  • Below: Lower end of brainstem (medulla oblongata) passes out of cranial cavity through foramen magnum to become continuous with spinal cord at the level of upper border of first cervical vertebra.

Relations of Brainstem

  • With cranial cavity: Brainstem lies in posterior cranial fossa of skull and rests on the slope of clivuswhich is formed by posterosuperior surface of basilar parts of sphenoid and occipital bones.
  • With tentorium cerebelli: Tentorium cerebelli is a crescentic horizontal shelf of dura mater of brain lying between posterior part of cerebrum (occipital lobe) and cerebellum. It posseses peripheral convex border. In front of concave anterior border (tentorial notch), brainstem passes downwards. Midbrain is the supratentorial part and, pons with medulla oblongata is the infratentorial part of brainstem lying above and below the tentorium cerebelli respectively.
  • With cerebrum and cerebellum: Cerebrum with thalamus (diencephalon) is above and, cerebellum is behind the brainstem. Ventral compact partof midbrain, composed of bundle of descending fibers connects the brainstem (midbrain) above with cerebrum. It is called cerebral peduncle having right and left identical halves. Cerebellum is connected to midbrain, pons and medulla oblongata of brainstem by three pairs of compact bundle of white matter. These are called superior, middle and inferior cerebellar peduncles respectively.
  • With fourth ventricle of brain: Fourth ventricle is the cavity of hindbrain. It is related anteriorly to pons and medulla oblongata and posteriorly to cerebellum.

Cavity related to brainstem is of different shapes and natures at different level as follows:

  • Midbrain– A narrow linear slit known as aqueduct of Sylvius.
  • Pons and upper part of medulla oblongata: A wide tent shaped space forming cavity of hindbrain called fourth ventricle of brain.
  • Lower part of medulla oblongata: A narrow central canal of medulla continuous below with central canal of spinal cord.

 

Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented)


Organization of Internal Structure at Different Level of Brainstem

Organization of Internal Structure at Different Level of Brainstem

Central cavity of brainstem show different characteristics and names at different level. At lower end of medulla it is a narrow canal continuous below with central canal of spinal cord. At the level of pons and upper half of medulla oblongata, it becomes wide to form the cavity of 4th ventricle of brain. At the level of midbrain it is a narrow slit called aqueduct of Sylvius.
Fundamentally, neurons of basal plate are motor and those of alar plate are sensory in function. Throughout the whole length of developing brainstem, initially, many neurons of both basal as well as alar plate will form number of continuous columns of cells which are as follows:
In basal plate (from medial to lateral)

  1. Somatic efferent
  2. Branchial efferent (special visceral efferent)
  3. General visceral efferent.

In alar plate: From medial to lateral in closed part of brainstem, i.e. midbrain and lower end of medulla oblongata and, from lateral to medial in open part, i.e. pons and upper part of medulla oblongata.

  1. Somatic afferent
  2. Branchial afferent (special visceral afferent)
  3. General visceral afferent.

Ultimately, neurons of all these columns will persist in some level and disappear in some level. So they will no longer be present in the form of continuous cell column althrough. These cell groups will form different motor and sensory nuclei of 3rd to 12th (last 10) cranial nerves.

Migration of neurons of alar lamina: Apart from formation of sensory (afferent) nuclei of cranial nerves, neurons of alar plate will migrate from its original position either ventrally or further dorsally to form some other named nuclei in different level of brainstem (described below). This nuclei, as migrated, will intermingle with the components (white matter) of marginal zone.
Derivatives of marginal zone: It is already understood that, marginal zone is composed of processes of nerve cells of mantle zone. These processes will form different groups of bundles of nerve fibers which are basically of following two types:

  1. Vertical: These are either ascending (afferent) or descending (efferent) tracts of nerve fibers connecting spinal cord with various higher centers.
  2. Horizontal: These are fiber bundles connecting various centers of central nervous system with cerebellum in both direction, passing through 3 cerebellar peduncles.

Organization of Internal Structure at Different Level of Brainstem

Migration of cells of alar plate to form various nuclei: As already stated, neurons of alar plate form various sensory neclei of last 10 pairs (3rd–12th) of cranial nerves. Besides, neurons from alar plate migrate either ventrally or further dorsally to form various nuclei in different levels of brainstem as follows.

  1. At the level of lower closed part of medulla oblongata: Cells of alar plate migrate further dorsally on either side of posterior median sulcus to form two nuclei.
    1. Medial: Nucleus gracilis
    2. Lateral: Nucleus cuneatus.
  2. At the level of upper half of medulla oblongata:
    Cells of alar plate migrate ventrally in the peripheral plane of marginal zone in the form of
    following nuclei.

Medulla oblongata at its lower end (at the plane of motor decussation)

Medulla oblongata at its lower end (at the plane of motor decussation)

Structural characteristics

  1. At this level structure of medulla oblongata is almost similar to the structure of spinal cord, with centrally positioned gray matter and peripheral white matter.
  2. Ventral horn of gray matter gets separated from main mass due to decussation of pyramidal tract fibers which pass backwards and laterally to approach lateral white column before passing downwards to the spinal cord.

Structural detail
Gray matter:

  1. Central gray matter is traversed by more dorsally pushed central canal lined by ependyma.
  2. Apex of posterior horn of spinal cord is represented at this level by nucleus of spinal tract of trigeminal nerve. On either side it is directed backwards and laterally with further abduction.
  3. Medial to nucleus of spinal tract of trigeminal nerve, gray matter shows, on either side, two small bulge of gray matter, nucleus gracilis (medial) and nucleus cuneatus (lateral) which receive the fibers of fasciculus gracilis and fasciculus cuneatus respectively, which are the ascending tracts in posterior column of white matter.
  4. Anterior gray horn becomes detached from main mass of gray matter by decussating fibers of corticospinal (pyramidal) tract.

Topographically, cells of anterior horn is a part of gray matter of medulla oblongata. But functionally, these are upwards continuation of cells of anterior horn of upper cervical segments of spinal cord. These cells form following two nuclei.

  1. Supraspinal nucleus of first cervical nerve: It is the upward continuation of anterior horn cells of first cervical segments of spinal cord. Axons of these neurons pass downward and are distributed along the ventral root of first cervical nerve.
  2. Ascending nucleus: It is the upward continuation of spinal nucleus of accessory nerve which is continuous below up to fifth cervical segment of spinal cord. Above it is continuous with nucleus ambiguous.

Medulla oblongata at its lower end (at the plane of motor decussation)

White matter: Pattern of three white columns (funiculi) of spinal cord, namely anterior, lateral and posterior, is grossly maintained.

  1. Anterior column: On either side of ventral median fissure, area of anterior white column mainly presents the bundle of pyramidal tract fibers which shows decussation of fibers at this level. Through anterior column, also traverse tectospinal tract, vestibulospinal tract, anterior spinothalamic tract.
  2. Lateral Column:
    1. Peripherally: Dorsal and ventral spinocerebellar tracts.
    2. Centrally:
    1. Lateral corticospinal tract which is formed at this level after decussation of fibers of pyramid.
    2. At the center of lateral white column, a scattered group of nerve cells intermingled with nerve fibers form brainstem reticular formation.
    3. Lateral spinothalamic tract.
  3. Posterior column: It present upward continuation of fasciculus gracilis and fasciculus cuneatus of posterior white column of spinal cord. As already mentioned earlier, these two tracts will relay in next order of neurons in nucleus gracilis and nucleus cuneatus which are seen to appear at this level of medulla oblongata, ventral to the corresponding tracts.

Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented) (2014)


Medulla oblongata at its middle

Medulla oblongata at its middle

Structural characteristics

  1. There is no more existance of gray matter area which is homologous to anterior horn.
  2. Gray matter of posterior horn presenting nucleus gracilis, nucleus cuneatus and spinal nucleus of trigmenial nerve gets detached from central gray matter. This detachment is because of the arched fibers arising from nucleus gracilis and nucleus cuneatus which decussate ventrally to form ascending fiber tract which is called medial lemniscus.
  3. Central canal surrounded by central gray matter is pushed more dorsally. Central gray matter presents appearance of cranial nerve nuclei.
  4. It is the plane of medulla oblongata from where upward typical relationship of central gray matter and peripheral white matter of spinal cord is lost. It results intermingling of gray and white matters.

Structural details

  1. On either side of ventral median fissure bulge of pyramid presents sections through descending (efferent) fibers of pyramidal (corticospinal) tract.
  2. Lateral to fibers of pyramid, inferior olivary nucleus starts appearing. It looks like a small irregularwalled sac whose cavity opens backwards and medially. Inferior olivary nucleus is the most prominent part of olivary nuclear complex of human brain. Rudimentary components are dorsal and medial olivary nuclei which together are known as accessory olivary nuclei.
  3. Ascending (afferent) tracts, e.g. dorsal and ventral spinocerebellar tracts, lateral and anterior spinothalamic tracts are found to be in corresponding positions as noticed in previous section of medulla oblongata.
  4. Nucleus gracilis and nucleus cuneatus are seen to be more prominent in this section. These nuclei receive fibers from fasciculus gracilis and fasciculus cuneatus which carry conscious proprioceptive sensation and sense of tactile discrimination from lower and upper halves of body respectively.
  5. Dorsolateral to nucleus cuneatus, a smaller accessory cuneate nucleus is seen. It receives fibers of fasciculus cuneatus which carry same sensations from uppermost part (head-end) of body. Cuneocerebellar tract from this nucleus end in cerebellum as spinocerebellar pathway above T1 spinal cord segment.
  6. Central core of the section presents scattered nerve cells and reticulum (network) of fibers to form brainstem reticular formation.
  7. Posterior gray horn separated from central gray matter is represented by spinal nucleus of trigeminal nerve which is capped on the surface by fibers of sensory root of trigeminal nerve carrying pain and temperature sensation, called spinal tract of trigeminal nerve.

Nucleus gracilis and nucleus cuneatus are the medial and lateral mass of gray matter on either side of posterior median septum. These are also the components of posterior gray horn which are detached from central gray matter.
Reason for separation of spinal nucleus of trigeminal nerve, nucleus gracilis and nucleus cuneatus from central gray matter is due to followingcharacteristic of structure of medulla oblongata at this level.

Medulla oblongata at its middle

Fasciculus gracilis and fasciculus cuneatus are the two ascending tracts of posterior column of spinal cord which carry sense of conscious proprioception and tactile discrimination from lower and upper halves of body respectively. Reaching the medulla oblongata upto this level, fibers of these two tracts relay in corresponding nuclei lying ventrally. Processes of next order of neurons in nucleus gracilis and nucleus cuneatus, before ascending further upwards to relay in thalamus, decussate to cross the midline. During decussation, these fibers presents following three characteristics.

  1. Fibers of both nucleus gracilis and nucleus cuneatus pass forwards arching along the lateral aspect of central gray matter horizontally in a curved fashion that is why they are called internal arcuate fibers.
  2. After decussation, the fibers form a compact bundle just behind the bulge of pyramid, before this compact bundle of fibers ascend upwards to reach thalamus. This bundle is known as medial lemniscus (Plural – Lemnisci).
  3. During formation of medial lemnisci, fibers from nucleus gracilis (carrying sensations from lower half of body) are positioned anterior to the fibers from nucleus cuneatus (carrying sensation from upper half of body).
    Behind medial lemniscus, pass tectospinal tract medial longitudinal fasciculus.

Central gray matter: It encircles the central canal of medulla oblongata which is pushed more posteriorly. It presents following cranial nerve nuclei which are interrelated ventrolaterally.

  1. Hypoglossal nerve nucleus (XII): It is the nucleus of somatic efferent column, lying ventral to central canal of medulla oblongata.
  2. Nucleus ambiguous (IX, X, XI): It

Source: Easy and Interesting Approach to Human Neuroanatomy (Clinically Oriented)


Medulla oblongata at the level of olive

Medulla oblongata at the level of olive

he medulla oblongata (medulla) is one of the three regions that make up the brainstem. It is the most inferior of the three and is continuous above with the pons and below with the spinal cord. The medulla houses essential ascending and descending nerve tracts as well as brainstem nuclei.

In this article, we shall look at the anatomy of the medulla – its external features, internal anatomy, and blood supply.

External Anatomy of the Medulla

The medulla is conical in shape, decreasing in width as it extends inferiorly. It is approximately 3cm long and 2cm wide at its largest point.

The superior margin of the medulla is located at the junction between the medulla and pons, while the inferior margin is marked by the origin of the first pair of cervical spinal nerves. This occurs just as the medulla exits the skull through the foramen magnum.

Anterior Surface

There are several structures visible on the anterior surface of the medulla – namely the three fissures/sulci, the pyramids, the olives, and five cranial nerves.

In the midline of the medulla is the anterior median fissure, which is continuous along the length of the spinal cord. However, it is interrupted temporarily by the decussation of the pyramids (see below). As we move away from the midline, two sulci are visible – the ventrolateral sulcus and the posterolateral sulcus.

The pyramids are paired swellings found between the anterior median fissure and the ventrolateral sulcus. Information on the pyramids can be found here. The olives are another pair of swellings located laterally to the pyramids – between the ventrolateral and posterolateral sulci.

Arising from the junction between the pons and medulla is the abducens nerve (CN VI). Extending out of the ventrolateral sulcus is the hypoglossal nerve (CN XII). In the posteriolateral sulcus, three more cranial nerves join the medulla (CN IX, CN X, and CN XI).

Posterior Surface

Unlike the anterior surface of the medulla, the posterior surface is largely obstructed from view and is relatively devoid of features. In order to appreciate the posterior surface, the cerebellum must be removed.

Similar to the anterior surface, the posterior surface has a midline structure – the posterior median sulcus – which is continuous below as the posterior median sulcus of the spinal cord. Above, the sulcus ends at the point in which the fourth ventricle develops.

As we move lateral from the midline, the fasciculus gracilis and fasciculus cuneatus are seen, separated by the posterior intermediate sulcus.

Medulla oblongata at the level of olive

Internal Anatomy of the Medulla

The internal structures of the medulla must be viewed in cross section to understand the layout. Three levels of the medulla are typically discussed (inferior – superior):

  • Level of decussation of the pyramids
  • Level of decussation of the medial lemnisci
  • Level of the olives

The medulla itself is typically divided into two regions: the open and the closed medulla. This distinction is made based on whether the CSF-containing cavities are surrounded by the medulla (closed medulla) or not (open medulla). The medulla becomes open when the central canal opens into the fourth ventricle (see Fig. 3).

Some features are seen in all three cross sections. Anteriorly we can see the paired lumps representing the pyramids which are separated by the anterior median fissure. Centrally, the central canal can be seen as it rises to form the fourth ventricle in the final cross section.

Level of the Decussation of the Pyramids

This is the major decussation point of the descending motor fibres. Roughly 75% of motor fibres housed within the pyramids cross diagonally and posteriorly, and continue down the spinal column as the lateral corticospinal tracts.

At this level, the central portion of the medulla contains gray matter, while the outer portions consist of white matter. The posterior white matter contains the fasiculus gracilis and the more lateral fasiculus cuneatus. Corresponding portions of gray matter extend to these regions and are the nucleus gracilis and nucleus cuneatus respectively.

Unchanged from the spinal cord, the spinocerebellar tracts (posterior and anterior) are located laterally, with the lateral spinothalamic tract situated between them. The large trigeminal nucleus and tracts can be found posterior to these tracts. This is a continuation of the substantia gelatinosa of the spinal cord.

Level of Decussation of the Medial Lemniscus

This level marks the sensory decussation occurs of the medial lemniscus. (Fig. 5). Purple lines have been used to represent the internal arcuate fibres as they run from the nucleus gracilis and nucleus cuneatus around and anterior to the central gray matter to form the medial lemniscus.

Lateral to the medial lemniscus, the trigeminal nucleus and spinal tract can once again be seen, as can the spinocerebellar tracts and the lateral spinothalamic tract. Similarly, the posterior structures are much the same at this level.

Centrally, the hypoglossal nucleus and medial longitudinal fasciculus are seen. Moving laterally, the nucleus ambiguous can be seen. Between this structure and the pyramids is the inferior olivary nucleus.

Medulla oblongata at the level of olive

Level of the Olives

This level shows significant change in structure both externally and internally when compared with previous levels. The central canal has now expanded into the fourth ventricle and as such makes this region the open medulla.

The large inferior olivary nucleus is responsible for the external expansion of the olives. The related medial and dorsal accessory olivary nuclei can be seen medial and posterior to this structure respectively.

The large inferior cerebellar peduncles come into view and are surrounded by multiple nuclei. The two vestibular nuclei (medial and inferior) are both found towards the midline while the two cochlear nuclei are found somewhat above and below the peduncles. Now a much smaller structure, the trigeminal tract and nucleus is seen adjacent to the peduncle.

The nucleus ambiguous remains as it was previously, while the hypoglossal nucleus has migrated with the central canal posteriorly, joined by the medial longitudinal fasciulus. An additional cranial nucleus comes into view lateral to the hypoglossal – the dorsal vagal nucleus. Moving further lateral, the nucleus of tractus solitarius comes into view.

Centrally, the medial lemniscus hugs the midline posterior to the pyramids, as does the tectospinal tract.

Between the peduncle and the olivary nuclei resides the lateral spinothalamic tract and the more lateral anterior spinocerebellar tract.

Vasculature

The vasculature of the medulla is complex and is dependent on the level being viewed (Fig. 7). The following attempts to simplify this complexity. Despite this it may suffice the reader to know that the vessels that supply the medulla include: the anterior spinal, the posterior spinal, the posterior inferior cerebellar, the anterior inferior cerebellar, and vertebral arteries.

Throughout the medulla, the anterior spinal artery supplies a region beginning at the central canal (or anterior border of the fourth ventricle), and fans out to encompass the pyramids.

Below the level of the olives the posterior half of the medulla is supplied by the posterior spinal artery. No other regions are supplied by this vessel. The remaining portions are supplied by the posterior inferior cerebellar and vertebral arteries.

In cross section through the olives both the posterior inferior cerebellar and vertebral arteries take on greater territories posterolaterally and anterolaterally respectively. They continue to do so as the medulla ascends.

At the highest point in the medulla, the anterior inferior cerebellar artery supplies the outermost portions of the posterior region.


Tegmental part at lower half of pons

Tegmental-part-at-lower-half-of-pons

Gray matter: Some cranial nerve nuclei and nuclei of pontine part reticular formation. l Abducent nerve nucleus: It is the nucleus of somatic efferent group. Fibers of abducent (VIth cranial) nerve arising from this nucleus supply lateral rectus muscle of eyeball which is developed from preoccipital myotome of paraaxial mesoderm. This nucleus is situated deep to a paramedian bulge adjacent to posterior median sulcus. The bulge is called facial colliculus because the surface of abducent nucleus is winded by fibers of facial nerve.

  • Motor nucleus of facial nerve: This is the nucleus of special visceral efferent column which supplies muscles developed from mesoderm of second branchial arch.efferent nucleus of facial nerve, situated lateral to motor nucleus of facial nerve. It has a component called lacrimatory nucleus. Parasympathetic secretomotor fibers from these nuclei are directed to supply to submandibular and sublingual salivary glands, and lacrimal gland.
  • Spinal nucleus of trigeminal nerve: This is exteroceptive variety of general somatic afferent nucleus of trigeminal nerve, which receives pain and temperature sensation from skin of face. Though called spinal nucleus, main part of this nucleus extends throughout whole length of medulla oblongata. Its lower end extends upto 2nd cervical segments of spinal cord and upper end extends to the lower half of pons. This nucleus is situated in the lateral part of tegmentum of lower end of pons. It receives sensory fibers of trigeminal nerve which caps dorsal aspect of the nucleus to form spinal tract of the nerve.
  • Vestibular nucleus of vestibulocochlear nerve:
    This is proprioceptive type of special somatic afferent nucleus of vestibulocochlear nerve. It is composed of superior, lateral, medial and inferior parts. Vestibular nucleus is situated partly in lower part of pons and upper part of medulla. It is placed in superficial plane at the lateral angle of pontomedullary junction. This nucleus receives afferent fibers which are nothing but vestibular fibers of VIIIth cranial nerve carrying sense of equilibrium or balance.

Tegmental-part-at-lower-half-of-pons
Efferent fibers are:

    1. Vestibulocerebellar fibers
    2. Vestibulospinal fibers
    3. Medial longitudinal bundle: Which connect vestibular nucleus with nuclei of IIIrd, IVth, VIth and XIth cranial nerves and anterior horn cells of upper cervical segments of spinal cord. It causes reflex movement of eyeball and head and neck in response to change of position body.
    4. Cochlear nucleus of vestibulocochlear nerve: It is exteroceptive type of special somatic afferent nucleus of cochlear component of vestibulocochlear nerve. It is made up of dorsal and ventral components lying dorsal and ventral to inferior cerebellar peduncle fibers at the level of pontomedullary junction.

    1. Connections of cochlear nuclei:
  • Afferent: Fibers of cochlear component of vestibulocochlear nerve carrying sense of hearing from receptors (organ of Corti) at internal ear relay in dorsal and ventral cochlear nuclei.
  • Efferent: Axons of cochlear nuclei will have to reach upto corresponding thalamic nuclei to carry impulse to sensory area of cerebral cortex. While ascending through central core of brainstem to reach the thalamus, at the level of lower end of pons, relayin a nucleus, called nucleus of trapezoid body. Before the relay, axons of both dorsal and ventral cochlear nuclei partly remain in the same side, partly cross the midline to relay in nucleus of trapezoid body of opposite side. In horizontal section, the fibers show a trapezoid shape, for which the decussating and nondecussating fibers are called trapezoid body, so the nucleus is also accordingly named.