Fourth ventricle of the brain

The fourth (IV) ventricle (ventriculus quartus) is a derivative of the cavity of the rhombencephalon. The medulla oblongata, pons, cerebellum and isthmus of the rhombencephalon participate in the formation of the walls of the IV ventricle. The shape of the cavity of the IV ventricle resembles a tent, the bottom of which has the shape of a rhombus (rhomboid fossa) and is formed by the posterior (dorsal) surfaces of the medulla oblongata and pons. The border between the medulla oblongata and the pons on the surface of the rhomboid fossa are the medullary stripes (IV ventricle) [striae medullares (ventriculi quarti)]. They originate in the region of the lateral angles of the rhomboid fossa, go in a transverse direction and plunge into the median groove.

The roof of the fourth ventricle (tegmen ventriculi quarti) hangs over the rhomboid fossa in the form of a tent. The superior cerebellar peduncles and the superior medullary velum (velum medullare craniale, s. superius) stretched between them participate in the formation of the anterior superior wall of the tent.

The posteroinferior wall is more complex. It consists of the inferior medullary velum (velum medullare caudate [inferius, s. posterius]), which is attached to the sides of the flocculus. From the inside, the inferior medullary velum, represented by a thin epithelial plate (a remnant of the dorsal wall of the third cerebral vesicle - the rhombencephalon), is adjacent to the vascular base of the fourth ventricle (tela choroidea ventriculi quarti). The latter is formed by the invagination of the soft membrane of the brain into the gap between the lower surface of the cerebellum above and the inferior medullary velum below.

The vascular base, covered on the side of the cavity of the fourth ventricle by an epithelial plate, forms the choroid plexus of the fourth ventricle (plexus choroidea ventriculi quarti). In the posteroinferior wall of the fourth ventricle there is an unpaired median aperture (apertura medidna ventriculi quarti; Magendie's foramen). In the lateral sections, in the area of the lateral recesses of the fourth ventricle, there is a paired lateral aperture (apertura lateralis ventriculi quarti; Luschka's foramen). All three apertures connect the cavity of the fourth ventricle with the subarachnoid space of the brain.

Diamond fossa

The rhomboid fossa (fossa rhomboidea) is a diamond-shaped depression whose long axis is directed along the brain. It is bounded on the sides in its upper section by the superior cerebellar peduncles, and in its lower section by the inferior cerebellar peduncles. In the posteroinferior angle of the rhomboid fossa, under the lower edge of the roof of the fourth ventricle, under the obex, is the entrance to the central canal of the spinal cord. In the anterosuperior angle there is an opening leading to the midbrain aqueduct, through which the cavity of the third ventricle communicates with the fourth ventricle. The lateral angles of the rhomboid fossa form lateral recesses (recessus laterales). In the median plane, along the entire surface of the rhomboid fossa, from its upper to the lower angle, there is a shallow median groove (sulcus medianus). On the sides of this groove there is a paired medial eminence (eminentia medianus), limited on the lateral side by the limiting groove (sulcus limitans). In the upper parts of the eminence, related to the bridge, there is the facial tubercle (colliculus facialis), corresponding to the nucleus of the abducens nerve (VI pair) located in this place in the thickness of the brain and the genu of the facial nerve that encircles it, the nucleus of which lies somewhat deeper and more laterally. The anterior (cranial) parts of the limiting groove, somewhat deepening and widening upward (anteriorly), form the superior (cranial) fossa (fovea cranialis, s. superior). The posterior (caudal, lower) end of this groove continues into the inferior (caudal) fossa (fovea caudalis, s. inferior), barely visible on preparations.

In the anterior (upper) sections of the rhomboid fossa, slightly to the side of the median eminence, a small area is sometimes noticeable on fresh brain preparations, which differs from the others in its bluish color, which is why it is called the bluish place (locus caeruleus). In the lower sections of the rhomboid fossa, related to the medulla oblongata, the median eminence gradually narrows, turning into the triangle of the hypoglossal nerve (trigonum nervi hypoglossi). Lateral to it is the smaller triangle of the vagus nerve (trigonum nervi vagi), in the depth of which lies the vegetative nucleus of the vagus nerve. In the lateral angles of the rhomboid fossa lie the nuclei of the vestibulocochlear nerve. This area is called the vestibular (vestibular) field (area vestibularis). The cerebral stripes of the fourth ventricle originate from this area.

Projection of the nuclei of the cranial nerves onto the rhomboid fossa. The gray matter in the area of the rhomboid fossa is located in the form of separate clusters, or nuclei, which are separated from each other by white matter. To understand the topography of the gray matter, it should be remembered that the neural tube in the area of the medulla oblongata and pons opened on its posterior (dorsal) surface and turned in such a way that its posterior sections turned into the lateral parts of the rhomboid fossa. Thus, the sensory nuclei of the rhomboid brain, corresponding to the posterior horns of the spinal cord, occupy a lateral position in the rhomboid fossa. The motor nuclei, corresponding to the anterior horns of the spinal cord, are located medially in the rhomboid fossa. In the white matter between the motor and sensory nuclei of the rhomboid fossa are the nuclei of the autonomic (vegetative) nervous system.

The nuclei of the cranial nerves (pairs V-XII) are located in the gray matter of the medulla oblongata and pons (in the rhomboid fossa). The nuclei of the V, VI, VII, VIII pairs of cranial nerves are located in the region of the superior triangle of the rhomboid fossa.

The 5th pair, the trigeminal nerve (n. trigeminus), has 4 nuclei.

1. The motor nucleus of the trigeminal nerve (nucleus motorius nervi trigeminalis) is located in the upper parts of the rhomboid fossa, in the region of the superior (cranial) fossa. The processes of the cells of this nucleus form the motor root of the trigeminal nerve.
2. The sensory nucleus, to which the fibers of the sensory root of this nerve approach, consists of 3 parts:
   • The pontine nucleus of the trigeminal nerve (nucleus pontinus nervi trigeminalis) is located laterally and somewhat posteriorly to the motor nucleus. The projection of the pontine nucleus corresponds to the locus coeruleus;
   • the (lower) nucleus of the spinal tract of the trigeminal nerve (nucleus spinalis [inferior] nervi trigiinalis) is, as it were, a continuation of the previous nucleus. It has an elongated shape and lies along the entire length of the medulla oblongata, entering the upper (IV) segments of the spinal cord;
   • The nucleus of the midbrain tract of the trigeminal nerve (nucleus [tractus mesencephalici] nervi trigeminalis) is located cranially (upward) from the motor nucleus of this nerve, next to the aqueduct of the midbrain.

The VI pair, the abducens nerve (n. abducens), has one motor nucleus of the abducens nerve (nucleus nervi abducentis), located in the loop of the genu of the facial nerve, in the depths of the facial hillock.

The VII pair, the facial nerve (n. facialis), has 3 nuclei.

1. The facial nerve nucleus (nucleus nervi facialis) is motor, large, and lies quite deep in the reticular formation of the pons, lateral to the tubercle (hillock) of the same name. The processes of the cells of this nucleus form the motor root. The latter is directed in the thickness of the brain first in the dorsomedial direction, bends around the nucleus of the VI pair from the dorsal side, forming the knee of the facial nerve, and then goes in the ventrolateral direction.
2. The nucleus solitarius is sensitive, common to the VII, IX, X pairs of cranial nerves, lies deep in the rhomboid fossa, projects laterally to the border groove. The cells that make up this nucleus are found already in the tegmentum of the pons, slightly proximal to the level of the medullary stripes of the IV ventricle, and extend along the entire length of the dorsal sections of the medulla oblongata up to the first cervical segment of the spinal cord. The fibers that conduct impulses of taste sensitivity end on the cells of this nucleus.
3. The superior salivatory nucleus (nucleus salivatorius rostralis, s.superior) is vegetative (parasympathetic), located in the reticular formation of the pons, somewhat superficial (dorsal) and lateral to the motor nucleus of the facial nerve.

The VIII pair, the vestibulocochlear nerve (n. vestibulocochlearis), has 2 groups of nuclei: two cochlear (auditory) and four vestibular (vestibular), which lie in the lateral parts of the bridge and project into the vestibular field of the rhomboid fossa.

1. Anterior cochlear nucleus (nucleus cochlearis ventralis, s. anterior).
2. Posterior cochlear nucleus (nucleus cochlearis dorsalis, s. posterior). The processes of the neurons of the cochlear ganglion (spiral ganglion of the cochlea), forming the cochlear part of the nerve, terminate in synapses on the cells of these nuclei. These nuclei lie one ventral to the other and to the side of the vestibular nuclei.

The vestibular nuclei receive nerve impulses from sensory areas (ampullar ridges and macula) of the membranous labyrinth of the inner ear.

1. Medial vestibular nucleus (nucleus vestibularis, medialis; Schwalbö nucleus).
2. Lateral vestibular nucleus (nucleus vestibularis lateralis; Deiters' nucleus).
3. Superior vestibular nucleus (nucleus vestibularis rostralis, s. superior; Bechterew's nucleus).
4. Inferior vestibular nucleus (nucleus vestibularis caudalis [inferior]; Roller's nucleus).

The nuclei of the last four pairs of cranial nerves (IX, X, XI and XII) are located in the inferior triangle of the rhomboid fossa, formed by the dorsal part of the medulla oblongata.

The IX pair, the glossopharyngeal nerve (n. glossopharyngeus), has 3 nuclei, one of which (double, motor) is common to the IX and X pairs of cranial nerves.

1. The ambiguous nucleus (nucleus ambiguus), motor, is located in the reticular formation, in the lower half of the rhomboid fossa, and projects into the region of the inferior (caudal) fossa.
2. The nucleus of the solitary tract (nucleus solitarius) is sensory, common to the VII, IX and X pairs of cranial nerves.
3. The inferior salivatory nucleus (nucleus salivatorius caudalis, s. inferior) is vegetative (parasympathetic), located in the reticular formation of the medulla oblongata between the inferior olivary nucleus and the nucleus ambiguus.

The 10th pair, the vagus nerve (n. vagus) has 3 nuclei: motor, sensory and vegetative (parasympathetic).

1. The ambiguous nucleus (nucleus ambiguus) is motor, common to the IX and X pairs of cranial nerves.
2. The nucleus of the solitary tract (nucleus solitarius) is sensory, common to the VII, IX and X pairs of nerves.
3. The posterior nucleus of the vagus nerve (nucleus dorsalis nervi vagi) is parasympathetic and lies superficially in the region of the triangle of the vagus nerve.

The XI pair, the accessory nerve (n. accessorius), has a motor nucleus of the accessory nerve (nucleus nervi accessorii). It is located in the thickness of the rhomboid fossa, below the double nucleus, and continues in the gray matter of the spinal cord along the upper 5-6 segments (between the posterior and anterior horns, closer to the anterior).

The 12th pair, the hypoglossal nerve (n. hypoglossus), has one nucleus in the lower angle of the rhomboid fossa, in the depth of the triangle of the hypoglossal nerve (nucleus nervi hypoglossi). The processes of its cells participate in the innervation of the muscles of the tongue and, together with the nerves extending from the cervical plexus, in the innervation of the muscles of the anterior region of the neck (hyoid muscles).