Comprehensive Study Notes on the Nervous System and Spinal Anatomy

Topographical and Functional Classification of the Nervous System

The nervous system is classified according to two primary perspectives: topographical location and overall function. Topographically, the system is divided into the Central Nervous System (SNC) and the Peripheral Nervous System (SNP). The Central Nervous System consists of the spinal cord (măduva spinării) and the encephalon (encefalul). The encephalon is composed of several critical regions: the brainstem (trunchiul cerebral), which is subdivided into the medulla oblongata (bulbul rachilian), the pons (puntea lui Varolio), and the midbrain (mezencefalul); the cerebellum (cerebelul), often referred to as the "small brain"; the diencephalon (diencefalul), which includes the thalamus, hypothalamus, epithalamus, metathalamus, and subthalamus; and the cerebral hemispheres (emisferele cerebrale), known as the "large brain." The Peripheral Nervous System consists of the nerves that extend from the central structures, including $31$ pairs of spinal nerves (totaling $62$ individual nerves) and $12$ pairs of cranial nerves (totaling $24$ individual nerves).

Functionally, the nervous system is categorized into the somatic nervous system and the vegetative (autonomic) nervous system. The somatic nervous system is associated with the "life of relation," managing interactions with the external environment. The vegetative nervous system (SNV) regulates internal functions and is further divided into the sympathetic (sy) and parasympathetic (psy) branches. Within these systems, nerve tissue is organized into white matter (substanșă albă), which consists of nerve fibers or prolongations, and gray matter (substanșă cenușie), which is comprised of the neuronal cell bodies.

Classification and Cellular Morphology of the Neuron

The neuron is the fundamental unit of the nervous system, consisting of a cell body (corp neuronal) and nerve prolongations (prelungiri nervoase), which include dendrites and the axon. Neurons are classified based on the number of these prolongations and their specific functions. By the number of processes, neurons are categorized as unipolar, possessing a single nerve prolongation (the axon); bipolar, possessing two prolongations consisting of one dendrite and one axon; multipolar, which feature multiple dendrites and one axon; and pseudounipolar, which possess a single process that bifurcates or splits over a certain portion within the spinal ganglion.

Based on function, neurons are categorized into four types. Sensory (afferent) neurons carry information toward the central nervous system. Motor (efferent) neurons carry information away from the central nervous system to the effectors. Vegetative neurons are those associated with the sympathetic and parasympathetic systems. Association (intercalary) neurons serve as links between other neurons. Additionally, nerves can be described as mixed when they contain both sensory and motor neurons.

Structural Components of the Neuron and Terminal Button

The neuronal cell body (corp celular) is enclosed by a cell membrane called the neurilemma. The interior consists of unstructured cytoplasm known as neuroplasma. It contains common organelles found in most cells, such as the rough and smooth endoplasmic reticulum (REN/RER), ribosomes, mitochondria, lysosomes, and the Golgi apparatus. It also contains specific organelles: Nissl bodies (also called tigroid bodies or corpusculii Nissl) and neurofibrils. The cell body houses a single nucleus. Dendrites are similarly composed of the neurilemma and neuroplasma, but their specific organelles are limited to neurofibrils.

The axon is the long nerve fiber characterized by its own membrane, the axolemma, and an unstructured cytoplasm called axoplasma. Its common organelles include lysosomes and mitochondria, while its specific organelles are neurofibrils. The terminal portion of the axon is the terminal button (butonul terminal). Like the rest of the axon, it is defined by the axolemma and axoplasma and contains mitochondria and neurofibrils. Critically, the terminal button houses vesicles containing chemical mediators (vezicule cu mediator chimic), which are essential for signal transmission across synapses.

Specialized Axonal Sheaths: Myelin, Schwann, and Henle

The axon is typically surrounded by three distinct sheaths. The Myelin Sheath (teaca de mielină) is situated directly on the axolemma. It is a discontinuous sheath composed of lipoprotein lamellae arranged concentrically. The gaps between individual segments of the myelin sheath are known as the Nodes of Ranvier (strangulașii Ranvier). The second layer is the Schwann Sheath (teaca Schwann), which is also a discontinuous sheath. It is formed by Schwann cells, which play a role in secreting the lipoprotein lamellae that arrange themselves beneath this sheath. The outermost layer is the Henle Sheath (teaca Henle). Unlike the others, the Henle sheath is continuous and is composed of reticulin fibers, elastin fibers, and connective cells.

Anatomy and Protection of the Spinal Cord

The spinal cord (măduva spinării) is located within the vertebral canal of the spinal column. It extends from the first cervical vertebra ($CC_1$ or $C_1$) to the second lumbar vertebra ($L_2$). Below the level of $L_2$, the canal contains the cauda equina (coada de cal), which extends toward the second coccygeal vertebra ($Co_2$). The cauda equina is composed of the filum terminale and various spinal nerves. The spinal cord is protected within the vertebral canal by the meninges, which consist of three layers: the pia mater (the innermost layer), the arachnoid, and the dura mater (the outermost layer).

Radial Morphology of Spinal Nerves

There are $31$ pairs of spinal nerves. Each spinal nerve is structured with two roots, one trunk, and two branches. The sensory root (rădăcină senzitivă) is posterior or dorsal and utilizes dendrites and neuronal cell bodies to bring information in. The motor root (rădăcină motoare) is anterior or central and consists of axons carrying signals out. These roots join to form the nerve trunk (trunchi), which is mixed. From the trunk, the nerve splits into two branches: the anterior branch and the posterior branch. The anterior branch is described as central and motor-oriented, following the axon, while the posterior branch is described as sensory and dorsal, following the dendrite.

Internal Micro-Architecture: Gray Matter and White Matter

The internal structure of the spinal cord is organized into centrally located gray matter and peripherally located white matter. The gray matter (substanșă cenușie), composed of neuronal bodies, is arranged into horns. The posterior horns contain sensory neurons (somatosensory). The lateral horns contain both viscerosensory neurons and visceromotor neurons. The anterior horns contain motor neurons (somatomotor). Connecting the sides is the gray commissure (comisura cenușie), which surrounds the central canal (canal ependimar) filled with cerebrospinal fluid (LCR/CSF). Toward the exterior of the gray matter, specifically in the lateral funiculus, lies the medullary reticular substance (substanșa reticulată medulară).

The white matter (substanșă albă) surrounds the gray matter and consists of nerve prolongations. It is organized into three pairs of columns or funiculi: the posterior columns (cordoane posterioare), the lateral columns (cordoane laterale), and the anterior columns (cordoane anterioare).

Surface Topography and Fissures of the Spinal Cord

The surface of the spinal cord is marked by several longitudinal grooves or fissures. On the posterior side, there is the median posterior sulcus (șanș median posterior). On the anterior side, there is a deeper groove known as the anterior median fissure (fisura mediană anterioară). Additionally, there are four lateral sulci: two posterior lateral sulci (șanșuri laterale posterioare) located between the posterior horn and the lateral horn, and two anterior lateral sulci (șanșuri laterale anterioare).