Neuroanatomy

Question 1

NEURAL CREST

Answer 1

The neuronal populations of the early epiblast become arranged in the medial region of the embryonic disc as the neural plate. Laterally, neural folds or crests indicate the transitional region between neural and
surface ectoderm. Along most of the neuraxis, the cells at the tips of the neural folds undergo an epithelial/mesenchyme transformation. They acquire migratory properties and leave the epithelium just prior to its
fusion with the contralateral fold in the dorsal midline. The migratory cells so formed are collectively termed the neural crest.
Neural crest populations arise from the neural folds as primary neurulation proceeds and simultaneously progresses rostrally and caudally. Crest cells migrate from the neural folds of the brain prior to tube closure. Caudally, from approximately somite 29, secondary neurulation processes produce the most caudal neural crest. Two distinct populations of neural crest cells are formed: a neuronal population produced throughout the brain and spinal cord, which gives rise to sensory and autonomic neurones and glia; and a non-neuronal mesenchymal population, which arises only from the brain. Melanocytes develop from a subpopulation of neural crest cells
derived from both the head and the trunk.

Question 2

Third ventricle and choroid plexus

Answer 2

The roof plate of the diencephalon rostral to the pineal gland, and continuing over the median telencephalon, remains thin and epithelial in character and is invaginated ­y the choroid plexuses of the third ventricle. Before the development of the corpus callosum and the fornix, it lies at the ­ottom of the longitudinal fissure, ­between and reaching the two cere­ral hemispheres. It extends as far rostrally as the interventricular foramina and lamina terminalis.
Choroid plexuses develop ­by the close apposition of vascular pia mater and ependyma without intervening nervous tissue. With development, the vascular layer is infolded into the ventricular cavity and develops a series of small villous projections, each covered
b­y a cu­boidal epithelium derived from the ependyma.
The early choroid plexuses secrete a protein-rich cere­rospinal fluid into the ventricular system, which may provide a nutritive medium for the developing epithelial neural tissues. As the latter ­ecome increasingly vascularized,
the histochemical reactions of the cu­boidal cells and the character of the fluid change to the adult type. Many regions of the lining of the third ventricle ­ecome highly specialized, and develop concentrations of tanycytes or other modified cells that are collectively termed the circumventricular organs, e.g. the su­fornical organ, the organum vasculosum (intercolumnar tu­ercle) of the lamina terminalis, the su­b-commissural organ, and the linings of the pineal, suprapineal and
infundi­bular recesses.

Question 3

Blood–brain barrier

Answer 3

Barriers ­between the ­blood supply and neural tissue, neurovascular units, function during development. Astrocytes are not required to induce the ­blood–­rain ­arrier. Pericytes are required for endothelial–astrocyte ­barrier formation during development, and disruption of pericyte–endothelial cell interaction may lead to ­barrier dysfunction.
The ventricular zone operates a ­brain–cere­rospinal fluid ­barrier during development ­but not once the ventricular zone has ceased as a dividing layer and the cells have differentiated to form ependyma. The choroid
plexuses, sites of a ­blood–cere­rospinal fluid ­barrier, function during development, controlling paracellular transport to the cere­brospinal fluid.

Question 4

CEREBRAL AQUEDUCT

Answer 4

The cerebral aqueduct is a small tube, roughly circular in transverse section and 1–2 mm in diameter. The length of the aqueduct in children, as measured at necropsy.
The aqueduct extends throughout the dorsal quarter of the midbrain in the midline and is surrounded by the periaqueductal (central­ grey matter). Rostrally, it commences immediately below the posterior commissure, where it is continuous with the caudal aspect of the third ventricle. Caudally, it is continuous with the lumen
of the fourth ventricle at the junction of the midbrain and pons. The superior and inferior colliculi are dorsal to the aqueduct and the midbrain tegmentum is ventral.

Question 5

CHOROID PLEXUS

Answer 5

The vascular pia mater in the roofs of the third and fourth ventricles, and in the medial wall of the lateral ventricle along the line of the choroid fissure, is closely apposed to the ependymal lining of the ventricles, without any intervening brain tissue. It forms the tela choroidea, which gives rise to the highly vascularized choroid plexuses from which CSF is secreted into the lateral, third and fourth ventricles. The body or stroma of the choroid plexus consists of many capillaries, separated from the ventricles by the pia mater and choroid ependymal cells.
In the lateral ventricle, the choroid plexus extends anteriorly as far as the interventricular foramen, through which it is continuous across the third ventricle with the plexus of the opposite lateral ventricle. From the interventricular foramen, the plexus passes posteriorly, in contact with the thalamus, curving round its posterior aspect to enter the inferior horn of the ventricle and reach the hippocampus. Throughout the body of the ventricle, the choroid fissure lies between the fornix superiorly and the thalamus inferiorly. From above, the tela choroidea is triangular with a rounded apex between the interventricular foramina, often indented by the anterior columns of the fornices. Its lateral edges are irregular and contain choroid vascular fringes. At the posterior basal angles of the tela, these fringes continue and curve on into the inferior horn of the ventricle.
When the tela is removed, a transverse slit (the transverse fissure­) is left between the splenium and the junction of the ventricular roof with the tectum. The transverse fissure contains the roots of the choroid plexus of the third ventricle and of the lateral ventricles. The choroid plexus of the third ventricle is attached to the tela choroidea, which is, in effect, the thin roof of the third ventricle as it develops during fetal life.
The choroid plexus of the fourth ventricle is similar in structure to that of the lateral and third ventricles. The roof of the inferior part of the fourth ventricle develops as a thin sheet in which the pia mater is in direct contact with the ependymal lining of the ventricle. This thin sheet, the tela choroidea of the fourth ventricle, lies between the cerebellum and the inferior part of the roof of the ventricle. The vertical (longitudinal­ limb is double,
flanks the midline and is adherent to the roof of the ventricle.

The blood supply of the choroid plexus in the tela choroidea of the lateral and third ventricles is usually via a single vessel from the anterior choroidal branch of the internal carotid artery and several choroidal branches of the posterior cerebral artery; the two sets of vessels anastomose to some extent. Capillaries drain into a rich venous plexus served by a single choroidal vein. The blood supply of the fourth ventricular choroid plexus is from the inferior cerebellar arteries. Physiological calcification of the choroid plexus and pineal gland are the most frequently described intracranial calcifications incidentally discovered during head computed tomography (CT­ examinations).

Question 6

CEREBROSPINAL FLUID

Answer 6

CSF is a clear, colourless liquid. Normal CSF contains small amounts of protein and differs from blood in its electrolyte content. It is not simply an ultrafiltrate of blood but is actively secreted by the choroid plexuses in the lateral, third and fourth ventricles. The choroid plexus epithelium constitutes a blood–CSF barrier. Choroid plexus epithelial cells have the characteristics of transport and secretory cells; their apical surfaces have microvilli, and their basal surfaces exhibit interdigitations and folding. Tight junctions (occluding junctions), zonulae adherentes­ at the apical ends of the cells are permeable to small molecules. Fenestrated capillaries lie just beneath the epithelial cells in the stroma of the choroid plexus. The ependymal lining of the ventricles and the extracellular fluid from the brain parenchyma are additional sources of CSF.

Question 7

SUBARACHNOID SPACE

Answer 7

The subarachnoid space lies between the arachnoid and the pia mater. It is continuous with the lumen of the fourth ventricle via the median aperture (foramen of Magendie­) and the paired lateral apertures (foramina of Luschka­). The subarachnoid space contains CSF, the larger arteries and veins that traverse the surface of the
brain, and the intracranial or intravertebral portions respectively of the cranial and spinal nerves.

Question 8

Facial nucleus

Answer 8

The facial (motor) nucleus lies in the caudal pontine reticular formation, posterior to the dorsal trapezoid nucleus and ventromedial to the spinal tract and nucleus of the trigeminal nerve. Groups of facial neurones form columns, which innervate individual muscles or which correspond to branches of the facial nerve. Neurones innervating muscles in the scalp and upper face are dorsal, and those supplying the lower facial musculature are
ventral.
Efferent fibres of the large motor neurones of the facial nucleus form the motor root of the facial nerve.
Clinically, upper and lower motor neurone lesions of the facial nerve can be differentiated because the former results in paralysis that is confined to the contralateral lower face (supranuclear facial palsy), whilst the latter
results in a complete ipsilateral paralysis (Bell’s palsy).