Peripheral features & malformations of the nervous system

During the fourth week of development, the nuclei for the 12 pairs of cranial nerves are formed. Except for the olfactory (I) and optic (II) nerves, which originate elsewhere, the cranial nerves originate in the brainstem. The oculomotor nerve (III) is the only one that originates outside the rhombencephalon. The neuroepithelium at the level of the rhombencephalon contains proliferation centers that give rise to eight distinct segments called rhombomeres. These rhombomeres are responsible for the formation of the motor nuclei of cranial nerves IV, V, VI, VII, IX, X, XI, and XII. The presence of mesoderm in the somitomeres beneath the neuroepithelium influences this segmental pattern. Motor neurons of cranial nerve nuclei are located in the brainstem, while sensory ganglia are located outside the brain. Although the organization of cranial nerves is similar to that of spinal nerves, not all cranial nerves have both motor and sensory components.

The sensory ganglia of cranial nerves originate from ectodermal placodes and neural crest cells. Ectodermal placodes include the otic placodes, nasal placodes, and four epibranchial placodes, which are ectodermal thickenings located dorsal to the pharyngeal (branchial) arches. The epibranchial placodes contribute to the formation of the ganglia associated with the pharyngeal arches (V, VII, IX, and X). The parasympathetic ganglia (visceral efferents) are derived from neural crest cells, and their neuron axons are part of cranial nerves III, VII, IX, and X.

The autonomic nervous system consists of two components: the sympathetic component located in the thoracolumbar region and the parasympathetic component located in the cephalic and sacral regions.

The sympathetic nervous system begins with cells originating from the neural crest in the thoracic region. In the fifth week, these cells migrate along the spinal cord towards the region behind the dorsal aorta. This migration forms two chains of sympathetic ganglia with segmental disposition, connected by longitudinal nerve fibers. The ganglia make up the sympathetic trunks, which are arranged bilaterally along the vertebral column. Neuroblasts from the thoracic region also migrate to the cervical and lumbosacral regions, causing elongation of each sympathetic trunk. However, the initial segmental arrangement of the ganglia is lost, particularly in the cervical region, due to interganglionic fusion processes. Some of the sympathetic neuroblasts migrate anterior to the aorta and form the preaortic ganglia, such as the celiac and mesenteric ganglia. Other sympathetic cells migrate to the heart, lungs, and gastrointestinal tract, forming the visceral sympathetic (autonomic) plexuses.

Once the sympathetic trunks are formed, nerve fibers from the thoracolumbar spinal cord segments (T1-L1, L2) enter the sympathetic ganglia. These nerve fibers, known as preganglionic fibers, synapse in the sympathetic ganglia at the same level or join the sympathetic trunks and synapse in the preaortic ganglia or collateral sympathetic ganglia. Preganglionic fibers are myelinated and form white communicating branches along their course from the spinal nerves to the sympathetic ganglia.

White branches are present only at the levels where the visceral efferent cell column extends, which is from the first thoracic segment to the second or third lumbar segment of the spinal cord. These branches consist of unmyelinated postganglionic fibers from neurons in the sympathetic ganglia. These fibers have a course either to other regions of the sympathetic trunk or to organs with sympathetic innervation, such as the lungs, heart, and intestinal tract.

On the other hand, gray communicating branches are found at all levels of the spinal cord and originate from the sympathetic trunk. They join the spinal nerves and distribute to peripheral blood vessels, hair follicles, and sweat glands.

Moving on to the parasympathetic nervous system, preganglionic parasympathetic fibers are formed by the axons of neurons in the brainstem and sacral region of the spinal cord. The fibers originating from brainstem nuclei join nerves such as the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. Postganglionic fibers, which originate from neurons (ganglia) derived from neural crest cells, are responsible for innervating specific structures such as viscera, salivary glands, and the iris musculature that controls pupillary diameter.

The adrenal gland is made up of two regions: the cortical region and the medullary region. The cortical region originates from mesodermal cells, while the medullary region originates from ectodermal cells. In the fifth week of development, mesothelial cells located between the mesentery root and gonads start to multiply and enter the underlying mesenchyme. These cells then differentiate into large acidophilic cells, forming the fetal cortex (or primitive cortex) of the adrenal gland. Another group of smaller mesothelial cells penetrates the mesenchyme and surrounds the acidophilic cells. These cells eventually develop into the definitive cortex of the adrenal gland. After birth, the fetal glandular cortex regresses rapidly, except for the outer layer that becomes the reticular zone. The characteristic structure of the cortical zone in adults begins to form during puberty.

Meanwhile, during the formation of the fetal cortex, cells from the sympathetic nervous system (neural crest cells) invade the medial region of the adrenal gland. These cells arrange themselves in cords and groups, forming the medullary region. When treated with chromium salts, these cells stain yellow-brown and are known as chromaffin cells. Chromaffin cells are present throughout the entire organism during embryonic development, but in adults, they persist only in the medullary regions of the adrenal glands.

Congenital brain and meningeal malformations can manifest in various ways. One such malformation is microcephaly, which is characterized by an underdeveloped and small-sized brain. Hydrocephalus, on the other hand, occurs when there is an excess of cerebrospinal fluid or a deficiency in its absorption. Exencephaly is a condition where the brain is only covered by the skin.

Agenesis of the corpus callosum can be either total or partial. In the case of Arnold-Chiari malformation, the brainstem and cerebellum elongate and herniate into the occipital hole and spinal canal. Spina bifida, a congenital defect, occurs when the spinal column does not fully close, resulting in a range of neurological issues.

These defects in the closure of the neural tube that can lead to such malformations can be prevented by administering folic acid to the mother, especially in the months prior to the conception as well as in the first trimester of pregnancy. The American Center for Disease Control states that all women of reproductive age should consume 400 micrograms (mcg) of folic acid daily, in addition to getting folate from a diverse diet, to help in preventing NTDs (neural tube defects).

Anencephaly is a severe birth defect where parts of the brain and skull are missing. Holoprosencephaly is another condition where the brain fails to divide into distinct hemispheres, leading to facial and neurological abnormalities. Neural tube defects, a group of malformations, affect the development of the brain and spinal cord and occur early in pregnancy.

Cerebral palsy is a non-progressive neurological disorder that affects movement and posture. It is often caused by brain damage during pregnancy or childbirth. Down syndrome, a chromosomal disorder, can also impact the nervous system and result in intellectual disability and developmental delays.

Spinal muscular atrophy is a genetic disorder that progressively weakens and atrophies muscles, affecting motor function. Tourette syndrome, on the other hand, is a neurological disorder characterized by repetitive, involuntary movements and vocalizations known as tics. Lastly, structural epilepsy is a chronic neurological disorder characterized by recurrent seizures of varying severity and type.

References

1. Anatomia Omului, Volumul I: Embriologie; Semestrul I; Prof. univ. dr. med. Andrei Motoc; Editura „Victor Babeș”, Timișoara, 2018

2. Embriologie - Sinteze didactice; Romeo Chiriac, Anca Indrei ;Editura Fundatiei Chemarea, Iasi, 1997

3. Embriologie Umană; Lidia Chircor, Loredana Surdu;Editura Ex Ponto, Constanța, 2014

Custom citation style optimized for readability:
Title -> Authors -> Source -> Date

Reset