Brief Overview of Neuroembryology

The neural tube (and thus the central nervous system) forms entirely from ectoderm. (Recall that the three layers of the developing embryo are ectoderm, mesoderm, and endoderm.)

A longitudinal neural groove forms in the ectoderm, which gets deeper and deeper.

The edges of the groove fuse and the resulting neural tube detaches: this is the separation of neural ectoderm from surface ectoderm. (The former is now deep to the latter.)

• The neural tube then develops three vesicles at its rostral end:
  1. prosencephalon (most rostral)
  2. mesencephalon (middle)
  3. rhombencephalon (most caudal)
• Two of these primary vesicles subdivide, and one does not:
  1. prosencephalon divides into:
     • telencephalon (rostral part)
     • diencephalon (caudal part)
  2. mesencephalon does not divide
  3. rhombencephalon divides into:
     • metencephalon (rostral part)
     • myelencephalon (caudal part), which is continuous with the 'tail' of the neural tube -- the precursor of the spinal cord.
• All five of these structures form portions of the mature brain.

The telencephalon gives rise to the 2 cerebral hemispheres, which are comprised of:

• the cerebral cortex (grey matter)
• the basal ganglion (also grey matter)
• the white matter of the cerebral hemispheres

Recall grey vs. white matter:

• Grey matter is primarily nerve cell bodies and neuroglia, while white matter is is primarily axons, both myelinated and unmyelinated, and neuroglia.
• Also recall that white matter is called white because the high myelin content makes the tissue appear white in a fresh specimen.

The diencephalon gives rise to:

• the thalamus
• the hypothalamus
• the metathalamus, consisting of the medial and lateral geniculate bodies
• the epithalamus

The mesencephalon (midbrain) gives rise to:

• the crus cerebri
• the cerebral peduncles
• the substantia nigra
• the tectum

The metencephalon gives rise to:

• the pons
• the cerebellum

The myelencephalon gives rise to the medulla (full name: medulla oblongata.)

General CNS Anatomy

Deep to the cerebrum is the brainstem, which consists of:

• the medulla
• the pons
• the mesencephalon
• the diencephalon, according to some sources

Four important landmarks of the cerebral hemispheres that allow us to divide the cerebral hemisphere into lobes:

1. lateral sulcus (sulcus = trench, often discontinuous)
2. central sulcus
3. preoccipital notch, a depression more visible from the medial aspect than the lateral aspect
4. parieto-occipital sulcus, best visible from the medial aspect of the hemisphere

The lobes are named according to what bone of the skull they lie under and are bounded by the above landmarks:

• frontal lobe -- bounded dorsally by the central sulcus and inferiorly by the lateral sulcus
• parietal lobe -- bounded anteriorly by the central sulcus; and posteriorly by an imaginary continuation of the parieto-occipital sulcus to the preoccipital notch (let? call it the PO-PO line)
• temporal lobe -- bounded superiorly by the lateral sulcus
• occipital lobe -- bounded anteriorly by the PO-PO line

More landmarks of the cerebrum:

• precentral sulcus and postcentral sulcus, which lie parallel to the central sulcus
• precentral gyrus (gyrus = ridge), between the precentral and central sulci
  • Considered part of the frontal lobe
  • Is the primary motor cortex, which controls motor activity on the contralateral (opposite side) of the body (that is, the right precentral gyrus controls motion of the left side of the body).
• postcentral gyrus, between the central and postcentral sulci:
  • Considered part of the parietal lobe
  • Is the primary sensory cortex for the contralateral side of the body

Other aspects of the frontal lobe:

• superior frontal sulcus
• inferior frontal sulcus
• superior frontal gyrus (dorsal to superior frontal sulcus)
• middle frontal gyrus (between the two sulci)
• inferior frontal gyrus (below inferior frontal sulcus and above lateral sulcus)

Other aspects of the temporal lobe:

• superior temporal sulcus
• inferior temporal sulcus
• superior temporal gyrus (superior to the superior sulcus)
• middle temporal gyrus (between the two sulci)
• inferior temporal gyrus (inferior to the inferior sulcus)

Collectively, these two gyri are called the inferior parietal lobule:

• supramarginal gyrus, around the superior termination of the lateral sulcus in the parietal lobe
• angular gyrus, around the superior termination of the superior temporal sulcus

Important Brodmann's areas:

• precentral gyrus = area #4
• postcentral gyrus = areas #1, 2, and 3
• primary auditory cortex = areas #41 and 42
• visual system = areas #15, 17, 18, and 19

Origin of the cranial nerves -- they come off in numerical order, front to back, and VI-XII form a kind of circle.

1. olfactory -- passes through the olfactory tract to the olfactory bulb, and terminates in the olfactory epithelium
2. optic -- passes through the optic tract to the optic chiasm, where some of the fibers cross to the contralateral side, then continues to the retina
3. oculomotor -- originates from the mesencephalon
4. trochlear -- originates from the dorsal mesencephalon
5. trigeminal -- originates at mid-pontine level
6. abducens -- originates at the junction of the pons and medulla
7. facial -- originates at the cerebellopontine angle
8. vestibulocochlear -- originates just caudal to VII
9. glossopharyngeal -- originates in the medulla, just caudal to VIII
10. vagus -- originates just caudal to IX
11. accessory -- originates from the medulla and spinal cord just caudal to X
12. hypoglossal -- originates from the medulla in a series of rootlets, between the medullary pyramid (medial) and the inferior olive (lateral.)