Comprehensive Study Notes on the Introduction to the Nervous System and Organogenesis

Fundamental Definition and Role of the Nervous System

The nervous system is defined as the totality of structures ensuring the reception, integration, transformation, and transmission of information originating from the organism's environment and from the organism itself. Its primary role is to ensure the regulation of the major functions of the body. The specific study of the diseases affecting this system is termed neurology. Structurally, the nervous system is divided into two distinct parts: an integrating part known as the central nervous system (CNS), and a receiving and effector part known as the peripheral nervous system (PNS). The functional loop of the system typically begins with a stimulus interacting with a sensory or sensitive receptor, which sends information to a nerve center (such as the brain/encephalon or the spinal cord), which then triggers an action or reaction via an effector organ, such as a muscle or a gland.

Organogenesis: The Formation of the Neural Tube

The development of the nervous system begins during gastrulation, which is the transition from a didermic disc to a tridermic embryo composed of the epiblast (ectoderm), mesoderm, and hypoblast (endoderm). Specifically, at day $18$ of embryonic development, an inductive influence from the notochord (chorde dorsale) causes a thickening of the ectoderm on the dorsal region of the embryo, forming the neural plate. This plate evolves into a neural groove and then into the neural tube by the $21^{st}$ day. The neural tube closes at its extremities, known as the rostral and caudal neuropores, on the $24^{th}$ day. Once closed, the nervous system appears as a tubular structure. The caudal portion remains narrow and cylindrical, destined to become the spinal cord (moelle spinale), while the rostral portion becomes wider, destined to become the encephalon. The walls of this neural tube are organized into four layers from the superficial surface to the deep internal canal: 1. The peripheral marginal layer, composed of neuroblast extensions derived from neuroepithelial cells, which gives rise to the white matter (substance blanche); 2. The pallial layer, consisting of neuroepithelial cells that will form the gray matter (substance grise); 3. The ependymal layer, which becomes the ependymal membrane and the epithelium of the choroid plexuses responsible for manufacturing cerebrospinal fluid (LCS); and 4. The neural canal, which will develop into the ventricular cavities of the central nervous system.

Migration of Neural Crests and Peripheral Development

During the closure of the neural tube, as the banks of the neural groove fuse, specific cells known as neural crest cells depart from the neuroectoblast. These cells migrate and incorporate into the underlying mesoblast. They form two longitudinal cords that subsequently undergo segmentation—corresponding to the segmentation of the paraxial mesoblast into bilateral bands called somites—to form the spinal ganglia (ganglions rachidiens). The surrounding mesoblast is categorized into several regions: the paraxial mesoblast, intermediate mesoblast, somatopleure, splanchnopleure, entoblast, and the internal coelom near the yolk sac (vésicule vitelline).

Development of the Encephalon: Primary and Secondary Vesicles

The encephalon derives from the cranial portion of the neural tube. By the end of the $4^{th}$ week, the tube presents three primary cranial dilatations or encephalic vesicles: the prosencephalon (forebrain), the mesencephalon (midbrain), and the rhombencephalon (hindbrain). At this stage, the tube bends ventrally, creating the cephalic flexure (courbure céphalique) between the prosencephalon and mesencephalon, and the cervical flexure (courbure cervicale) between the rhombencephalon and the spinal cord. During the $5^{th}$ week, these vesicles further divide. The prosencephalon splits into the telencephalon and the diencephalon. The mesencephalon remains the mesencephalon. The rhombencephalon divides into the metencephalon and the myelencephalon. A third curvature, the pontine flexure (courbure pontique), develops between the metencephalon and myelencephalon with a posterior concavity.

Adult Derivatives of the Encephalic Vesicles and Cavities

The secondary vesicles evolve into specific adult brain structures. The telencephalon produces two lateral swellings, the primitive cerebral hemispheres, and adult structures including the cerebral cortex, basal nuclei (such as the caudate nucleus), olfactory bulbs, amygdala, hippocampus, corpus callosum, anterior commissure, and the fornix (trigone). The diencephalon organizes into various nuclei to form the hypothalamus, thalamus, epithalamus (pineal gland/épiphyse), and pituitary gland (hypophyse); it also gives rise to the optic nerve, optic chiasm, retina, and mammillary bodies (corps mamillaires). The mesencephalon becomes the cerebral peduncles, the red nucleus, and the corpora quadrigemina (tubercules quadrijumeaux), which consist of the superior and inferior colliculi. The metencephalon forms the pons (pont de Varole/protubérance annulaire) and the cerebellum. The myelencephalon gives rise to the medulla oblongata (moelle allongée/bulbe rachidien) and the olivary nuclei. Simultaneously, the neural canal evolves: the telencephalon and diencephalon cavities become the lateral ventricles (I and II) and the third ventricle, respectively, with the lateral ventricles communicating with the third via the interventricular foramina (foramens interventriculaires or trous de Moruro). The mesencephalic cavity becomes the cerebral aqueduct (aqueduc de Sylvius), and the rhombencephalic cavity becomes the fourth ventricle.

Development of the Spinal Cord

The spinal cord derives from the caudal part of the neural tube. Cell proliferation in the pallial layer creates ventral and dorsal thickenings. The dorsal or alar plates (lames dorsales ou alaires) give rise to the dorsal horns (sensory), while the ventral or fundamental plates (lames ventrales ou fondamentales) give rise to the ventral horns (motor). The dorso-lateral and ventro-lateral plates become the lateral horns. The marginal layer produces the spinal cords (cordons spinaux). A central longitudinal groove, the sulcus limitans, marks the boundary between the ventral motor area and the dorsal sensory area. The thin median regions (dorsal and ventral) serve as pathways for association fibers and are known as commissures. Centered within is the ependymal canal.

Composition of the Peripheral Nervous System (PNS)

The PNS consists of neurons and support structures. Sensory neurons originate from ganglion neuroblasts that emit two types of extensions: centripetal extensions that enter the dorsal region of the neural tube to terminate in the alar plate or ascend via the marginal layer to the encephalon, and centrifugal extensions that join the fibers of the fundamental plate to form the spinal nerve. Neurolemmocytes (Schwann cells) derive from neurolemmoblasts from the neural crests. Motor neurons originate from neuroblasts in the fundamental plate, extending toward peripheral structures. Connective tissue and neuroglia (made of gliocytes) derive from the mesenchyme; these tissues have no conductive function but serve to bind neurons together.

Morphological and Systematic Subdivisions of the Nervous System

Morphologically, the system is split into the CNS and PNS. The CNS includes the spinal cord (moelle épinière) housed in the vertebral canal, the brainstem (tronc cérébral)—which consists of the medulla oblongata, the pons, and the mesencephalon—the cerebellum (cervelet), and the brain (cerveau). The term "Encephalon" specifically refers to the parts of the neuraxis within the cranium (brain, brainstem, and cerebellum). The PNS consists of nerves and nerve ganglia. A nerve is an assembly of neurofibers visible to the naked eye. Nerves are classified by distribution (Cranial nerves emerging from the encephalon, Spinal nerves from the spinal cord, and Vegetative nerves for viscera and smooth muscles) or by function (Motor nerves for centrifugal impulses, Sensory nerves for centripetal sensory impulses, and Mixed nerves containing both). A nerve ganglion consists of a fibrous capsule continuous with the epineurium and a stroma containing neuron bodies; it is the site of synaptic articulations.

Classification and Numbering of Nerves

There are $12$ pairs of cranial nerves: I (olfactory), II (optic), III (oculomotor), IV (trochlear), V (trigeminal), VI (abducens), VII (facial), VIII (vestibulocochlear), IX (glossopharyngeal), X (vagus), XI (accessory), and XII (hypoglossal). There are $31$ pairs of spinal nerves: $8$ cervical (C), $12$ thoracic (T), $5$ lumbar (L), $5$ sacral (S), and $1$ coccygeal (Co). Naming convention: up to C7, nerves are named after the vertebra below them. Nerve C8 emerges between vertebrae C7 and T1. From T1 onwards, nerves are named after the vertebra above them.

Functional and Structural Subdivisions

Functionally, the system is divided into the sensory-motor system (vie de relation/cérébro-spinal) and the vegetative or autonomous system (vie végétative). The vegetative system regulates the internal environment (humoral, secretory, vasomotor, and visceral) and consists of the orthosympathetic and parasympathetic systems. Structurally, the CNS shows three types of tissues: 1. Gray matter (substance grise), formed by clumps of neuron bodies, making up the cortex or deep basal nuclei in the brain/cerebellum and the internal gray column in the spinal cord. 2. White matter (substance blanche), consisting of myelinated axons and glial cells, serving as conduction pathways. It is peripheral in the spinal cord/brainstem and intermediate in the brain/cerebellum. 3. Reticular formation, a mixture of gray and white matter. In the spinal cord, it is in the lateral horn of cervical segments; in the encephalon, it extends from the medulla to the diencephalon. It coordinates vegetative functions (swallowing, blood pressure, thermoregulation), links cerebellar nuclei with the hypothalamus and brainstem, and controls posture, endocrine activity, biological rhythms, sleep-wake states, and emotional reactions.