Anatomy of the brain

Anatomy of the brain

Development of the brain

In the beginning, the brain resembles a simple tube. Early during the developmental stage the neural tube tube is divided into three vesicles which are separated from each other by ring-like bands. These vesicles are called, correspondingly, the forebrain vesicle, the midbrain vesicle and the hindbrain vesicle. They later develop into the three main regions of the brain:

Figure 1. A schematic representation of the three primary brain vesicles.

1) The forebrain -- prosencephalon; 2) The midbrain -- mesencephalon; 3) The hindbrain -- rhombencephalon.

These three vesicles then divide afain, in the end forming five secondary brain vesicles.

Figure 2. Five secondary brain vesicles.

From the forebrain vesicle, the secondary forebrain brain is formed. It divides into the diencephalon and the telencephalon (located at the cranial end of the tube). The frontal part of the neural tube is designated lamina terminalis. Because the subsequent development of telencephalon is facilitated by the regions located laterally to the lamina terminalis, which is located at the midline, this vesicle develops on the both sides of the neural tube. Therefore, the telencephalon is now represented by two vesicles which later develop into the hemispheres of the brain.

Figure 3. Brain of a 5-week-old human embryo (after W.His).

Disproportional development of different parts of the rhombencephalon leads to its division into two parts -- the metencephalon and the myelencephalon. Metencephalon is separated from the midbrain by a narrow band called isthmus (s. isthmus rhombencephali), while the myelencephalon continues into the spinal cord.

Figure 4. A schematic representation of the development of the brain.

To summarize, the neural tube develops into 6 regions:

1) Telencephalon; 2) Diencephalon; 3) Mesencephalon; 4) Isthmus s. isthmus rhombencephali; 5) Metencephalon; 6) Myelencephalon (medulla oblongata).

The rate of later development greatly varies between different regions of the brain, and some regions develop rapidly while the others are outpaced by their neighbors. Prosencephalon and mesencephalon are collectively called the cerebrum.

Figure 5. Brain of a 5-month-old human embryo (after W.His).

The brainstem, truncus cerebri, contains????? the basal ganglia and consists of the trunk of telencephalon, diencephalon, mesencephalon, isthmus, pons cerebri and myelencephalon.

The central canal of the spinal cord continues into the posterior part of the myelencephalon vesicle. The cavity of the anterior part of the myelencephalon vesicle and the metencephalon vesicle form the fourth ventricle of the brain, while the cavity of the mesencephalon vesicle forms the cerebral aqueduct, aquaeductus cerebri (Sylvii).

Figure 6. Further development of the five secondary brain vesicles (after W.His).

The cavity of the diencephalon vesicle forms the third ventricle of the brain, which connects to the lateral ventricles of the hemisphere vesicles via interventricular foramina of Monro (foramen interventriculare). Each cavity is filled with cerebrospinal fluid, liquor cerebrospinalis.

Development of the spinal cord

The part of the neural tube that develops into the spinal cord is oval in cross-section. The central canal of the spinal cord projects on the dorsoventral plane as a longitudinal fissure. It is confined by the walls of the neural tube, which are thicker in the lateral parts and thinner in the dorsal and ventral parts.

The thinner dorsal and ventral walls of the neural tube develop into the posterior and anterior commissures. The dorsal (posterior) commissure is also called roof plate, and the anterior (ventral) commissure is also called floor plate. During subsequent development these two plates exhibit little growth. The lateral parts, however, expand rapidly, especially in the ventral direction, forming ventral protrusions on both sides. The floor plate is pushed inward, which eventually leads to the formation of the anterior median sulcus of the spinal cord. In a similar process on the dorsal side the roof plate is pushed inward to disappear in the posterior median sulcus of the spinal cord.

Figure 7. A cross-section of the spinal cords of a 4.5-week-old and a 3-month-old embryos (after W.His).

Up to the fourth week of embryonal development, the spinal cord spans all the length of the spinal canal, maintaining almost equal thickness. Its end, however, remains underdeveloped and forms the medullary cone, conus medullaris.

Because the spinal cord develops slower than the spine itself, its length is shorter than that of the spinal canal. The medullary conus rises from the sacral into the lumbar region of the spine, and eventually the medullary conus elevates to the level of L1-L2. The end of the medullary conus, called ascensus medullae spinalis, tapers into a thin filament called the termial thread, filum terminale. It continues to the end of the spinal canal. Together the medullar conus and the terminal thread form the cauda equina.

This process defines the topology of the nerves originating from the spinal cord, which are laid out diagonally due to the mismatch between the lengths of the spinal cord and the spinal canal. In the cervical region of the spine this effect is less pronounced, and the nerves branch out horizontally. They begin to shift in the diagonal direction in the thoracic region, while in the lumbar and especially in the sacral regions the nerves are forced to move straightly downwards.

Eventually, two regions of the spinal cord reach a significant level of development. These regions are called, respectively, the cervical enlargement, intumescencia cervicalis, and the lumbar enlargement, intumescencia lumbalis.

Figure 7. The schematic frontal view of the spinal cord.

Telencephalon.

Telencephalon consists of the following parts:

  • Cerebral hemispheres, haemisphaerium;
  • Pars optica hypothalami.

Cerebral hemispheres consist of:

  • Pallium;
  • The olfactory brain, rhinencephalon;
  • The telencephalic trunk and the basal ganglia.

Pars optica hypothalami consists of:

  • Lamina terminalis;
  • Optic chiasma, chiasma opticum;
  • Tuber cinereum;
  • Infundibulum;
  • Hypophysis.

The surface of the palluium is divided by the continuous deep grooves (fissurae) and the more superficial grooves (sulci) into several lobes. The four cerebral lobes are as follows:

  • Frontal lobe, lobus frontalis;
  • Parietal lobe, lobus parietalis;
  • Temporal lobe, lobus temporalis;
  • Occipital lobe, lobus occipitalis.

The surface of each lobe is covered with ridges (gyri cerebri), which are separated by the grooves. The ridges often interconnect in the depth of the grooves via gyri profundi. Shallow grooves with irregular localization that branch from the deeper grooves and cut into the ridges, causing them to branch or double up, are called incisures, incisurae, or notches.

Sulci and gyri of the dorsolateral surface

Consider the basal surface of the hemisphere. It is divided into the anterior and posterior parts by a deep groove called vallecula lateralis s. fossa cerebri lateralis (Sylvii) and located to the outside of the anterior perforated substance. From there, a deep groove called the lateral (Sylvian) fissure (fissura cerebri lateralis (Sylvii) continues upwards on the dorsolateral surface of the hemisphere. In the beginning this groove is designated truncus fissurae lateralis. It then splits into three branches: a shorter anterior horizontal branch (ramus anterior horizontalis), which goes forward; an almost as short anterior ascending branch (ramus anterior ascendens), which goes almost directly upwards, and a longer posterior branch (ramus posterior), which serves as a continuation of the anterior horizontal branch and goes backwards and diagonally upwards, then making a Y-shaped split into the ascending branch (ramus ascendens) and the descending branch (ramus descendens).

From approximately the middle of the dorsal end of the pallium another groove extends downwards and to the front in the direction of the anterior part of the posterior branch of the lateral fissure. It is called the central sulcus, or the Rolandic fissure (sulcus centralis (Rolandi). Typically, it forms two bends: the first between the upper and the middle thirds of the sulcus, and the second between the middle and the lower thirds of the sulcus. Sulcus centralis usually spans the upper bound of the pallium.

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Author: radiologist, Candudate of Medical Sciences, M.D. Evgeniy Vlasov