30. Cerebrum (forebrain) – description, embryonic development. Exterior of the hemispheres – sulci, gyri.

SULCI and GYRI

The telencephalon (endbrain) gives rise to the cerebral hemisphere wich consists of the superficial cerebral cortex, convoluted to form gyri & sulci.

sulci - dipressions

gyri - elevated force

The sulci & gyri increase the surface area.

The sulci & gyri divide the hemisphere into 5 lobes:

1. Frontal
2. parietal
3. Occipital.
4. temporal
5. limbic

• + hidden insula

Major sulci separating the lobes of the cerebral hemisphere:

1. Central sulcus: It start at the superomedial border just behind the midpoint between frontal & occipital poles

• It descends on the lateral surface separating the frontal lobe from the parietal lobe

2. Lateral sulcus:

• It has stem and 3 rami
• It seperates frontal and parietal lobes from the temporal lobe

3. Parieto-occipital sulcus:

• It appears small in the supero-lateral surface while it is mainly present on the medial surface.
• Seperates the parietal and temporal lobes from the occipital lobe.

Sulci & Gyri of the superolateral surface:

1) Frontal lobe: it has:

• Precentral sulcus: parallel to the central sulcus.
• Precentral gyrus between central and precentral sulci.
• From the precentral sulcus: 2 parallel sulci:
• The superior and the inferior frontal sulci.
• So 3 gyri are formed: superior, middle, and inferior frontal gyri.
• Notice the inferior frontal gyrus which is divided by the stem and the rami of lateral sulcus into 3 parts: orbital, triangular, and opercular.

2) Parietal lobe:

• Postcentral sulcus: parallel to the central sulcus.
• Postcentral gyrus: between central and postcentral sulci.
• From the postcentral sulcus: one intraparietal sulcus.
• Superior and inferior parietal lobules above and below the intraparietal sulcus respectively.
• Notice the supramarginal gyrus “around the end of lateral sulcus” and the angular gyrus “around the end of superior temporal sulcus”.

3) Temporal lobe:

• 2 sulci parallel to the lateral sulcus: superior and inferior temporal sulci.
• So 3 gyri are formed: superior, middle, and inferior temporal gyri.

4) Occipital lobe:

• It shows the end of the calcarine sulcus, with the surrounding lunate sulcus.

N.B: The insula:

• Present in the floor of the lateral sulcus.
• Surrounded by a circular sulcus except its apex “limen insulae”
• Divided by central sulcus into large anterior and small posterior parts.
• Anterior part having 3-4 short gyri.
• Posterior part having a long gyrus.

Its function:

• Anterior part: smell, taste, and autonomic areas.
• Posterior part: 2nd somatosensory area.