Human Anatomy, Part 2 - K. A. Diubenko, A. K. Kolomiitsev, Yu. B. Chaikovskyi 2008

Special Part
Nervous system, systema nervosum - General overview

The Nervous System (Fig. 120) regulates and coordinates physiological processes at the level of Organs and body systems, ensuring an optimal level of vital activity.

As previously mentioned in the "Nervous Tissue" section, the most crucial biological process underlying the leading role of the nervous system in regulating the Functions of other organs and systems is the Nerve Impulse (excitation) propagating along Neurons. The nerve impulse is the primary carrier of information within the nervous system. Its relative speed of propagation through Nerve Cells allows the nervous system to be considered the body's primary information system.

The primary functional act of the nervous system that ensures interaction between body parts and organs is the reflex. Reflexes (from Latin reflexus - reflection) are the body's response reactions to The stimulation of sensory nerve endings (receptors), mediated by the Central nervous system (CNS). The pathway along which excitation generated in receptors is transmitted to the effector organ is called the reflex arc, which is the functional unit of the nervous system (Fig. 121). It consists of:

Receptors are sensory nerve endings that perceive stimuli from the external and internal environments of the body, transform them into nerve impulses, and propagate them along nerve pathways. Based on their Location, receptors are classified into external and internal. External receptors, or exteroceptors, perceive stimuli from the surrounding environment. They are located in the Skin, mucous membranes, and Sensory Organs. Internal receptors, or interoceptors, are nerve endings specialized in receiving information from Internal Organs, and proprioceptors* are a type of sensory nerve endings that perceive the stretching or compression of tissues in the muscular or articular apparatus.

- Sensory neurons or protoneurons (sensory, receptor, ganglionic, afferent, pseudounipolar). A protoneuron is always located in the Peripheral Nervous System, within the sensory ganglia of cranial or Spinal Nerves; its dendrite terminates in a receptor, and its axon ends in a synapse. The protoneuron is the first neuron to receive information.

- Interneurons or deuteroneurons (interneurons) - Relay, intermediate, or associative neurons; located in the central nervous system, their dendrites and axons terminate in synapses. The interneuron transmits information. Interneurons make up 90% of the total number of neurons; it is from interneurons that the Cerebral Cortex, cerebellar cortex, subcortex, cerebellar nuclei, as well as the sensory and autonomic nuclei of the BRAIN AND SPINAL cord are formed.

*Proprioceptor [syn. proprioreceptor] from Latin proprius - one's own, and recipere - to receive.

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Fig. 120. The human nervous system (diagram)

- Efferent neurons or motoneurons - motor, effector, or efferent neurons. A motoneuron is primarily located in the central nervous system, in the motor nuclei of Cranial Nerves and the motor nuclei of the anterior horns of the Spinal Cord. However, visceromotor neurons are located in the peripheral nervous system, specifically in autonomic ganglia. The dendrite of a motoneuron terminates in a synapse, and its axon ends in an effector. The motoneuron triggers Muscle contraction.

Fig. 121. Simple reflex arc (diagram)

- Effector organs whose function is realized by the reflex act. Effector organs include: Cytology/practical/58.html">Striated Skeletal Muscle fibers, cardiomyocytes, smooth myocytes, and glandular cells.

The simplest reflex arc is the two-neuron (monosynaptic) arc. It consists of a sensory and a motor neuron. Such reflex arcs are relatively rare. In most reflex arcs, the impulse from the receptor neuron is transmitted to interneurons and only then to effector neurons. The functional role of interneurons is to vary the pathways of impulse propagation within the CNS, which enables The complexity of reflex responses.

The number of interneurons can range from a few cells to millions and billions of neurons if the nerve centers are located above the Medulla Oblongata.

The transmission (switching) of nerve impulses from interneurons to efferent neurons occurs in nerve centers. The latter are complexes of functionally linked neurons located in one or more Regions of the CNS that regulate a specific body function. Since nerve centers are connected to various PARTS OF THE CNS, they integrate (unify) The activity of different brain structures to achieve a beneficial outcome for the Organism.

According to the LOCATION OF THE nerve centers, reflexes are classified as spinal (centers located in the spinal cord), bulbar (centers located in the medulla oblongata and Cerebellum), mesencephalic (centers in the Midbrain), diencephalic (centers in the Diencephalon), and cortical (involving the cerebral cortex).

The concept that the higher Divisions of the CNS also function on a reflex principle was substantiated by I. M. Sechenov in his book "Reflexes of the Brain" (1863).

I. P. Pavlov established a method for the experimental Study of the reflex activity of the higher divisions of the CNS through his discovery of conditioned reflexes, and on this basis developed The Doctrine of Higher Nervous Activity, which provides individual behavioral adaptation of humans and higher animals to changing environmental conditions.

A student of I. P. Pavlov, P. K. Anokhin, evaluating The Significance of the reflex in regulating various body functions, substantiated The Theory of the essential role of feedback (retrograde afferentation) in achieving the beneficial effect of a reflex reaction. This means that during its execution, information from the receptors located in the functioning organ (or organs) is sent back to the CNS regarding the progress of the function, allowing for its evaluation and, if necessary, adjustment to achieve a beneficial result. THE PRINCIPLE OF self-regulation of reflex acts served as the basis for P. K. Anokhin's theory of functional systems, which are dynamic, self-regulating organizations that selectively integrate different levels of nervous and humoral regulation to ensure the achievement of results beneficial to the organism. The primary purposes of functional systems are to maintain Homeostasis (the relative constancy of The chemical composition of the body's internal environment) and to self-regulate physiological functions and behavioral responses.

The same functional system can include not only organs belonging to a single body system (for example, cardiovascular, respiratory, digestive, excretory, etc.) but also organs that are geographically distant from one another. Therefore, METABOLISM/2.html">THE CONCEPT OF a "functional system" of the organism is broader than the morphological one.

Thus, the reflex arcs of functional systems provide not only the delivery of excitatory or inhibitory stimuli to effector organs but also feedback (retrograde afferentation) regarding the results of the ongoing actions. Reactions resembling reflexes are observed even in Ciliates, where cilia perform the function of receptors.

In phylogenesis, the nervous system passes through several stages (E. K. Sepp, 1949). Coelenterates (for example, hydra) possess a diffuse nervous system, represented by a subectodermal nerve plexus and nerve cells in the wall of the gastrovascular cavity. Such animals are characterized by total, generalized motor acts, resulting in a standard contraction response when any part of the body is stimulated.

With The Emergence of new types of invertebrates, nerve cells became concentrated in clusters (ganglia), giving rise to the ganglionic nervous system. Its appearance was associated with body segmentation. In Annelids, There are two ganglia in each segment, connected by both transverse and longitudinal interganglionic connectives. In such a nervous system, The connection between sensory and motor neurons is mediated by interneurons, which increased the volume of information Processing. In vertebrates, from the earliest stages of embryonic development, features of ganglionation are absent. The nervous system is laid down as a continuous neural tube (tubular nervous system), which gives rise to various brain regions and initiates The Development of the peripheral nervous system ganglia.

The human nervous system develops from the ectodermal epithelium of the medullary tube, i.e., the outer germ layer (ectoderm). Ectodermal cells in the dorsal regions of the embryo differentiate to form the medullary (neural) plate (Fig. 122 A). The edges of the medullary plate begin to rise slightly above the level of the embryo's notochord, while the medullary plate itself thickens due to the rapid growth of its cells. As a result, a groove is formed in the plate, the bulge of which faces dorsally, and the groove itself deepens (Fig. 122 B). The edges of the medullary plate approach each other and eventually fuse. Thus, the medullary plate closes and takes the shape of the neural (medullary) tube (Fig. 122 C).

Fig. 122 A, B, C. Early stages of human nervous system Embryogenesis.

Formation of the neural tube

During The formation of the neural tube, a group of cells detaches from it to form lateral (ganglionic) crests on both sides of the tube, which subsequently segment and give rise to the intervertebral ganglia. In the early Selection/3.html">Stages of development, the neural tube consists of a single layer of elongated columnar cells, which then rapidly divide to form three layers in the neural tube wall: the inner (ependymal), middle (mantle), and outer layers. The inner layer gives rise to the ependyma, which lines the central canal of the spinal cord, the cerebral ventricles, and the cerebral aqueduct; the middle (mantle) layer forms the Gray matter of the brain; the outer layer consists of nerve Cell processes that form the White matter.

During differentiation, the Cells of the medullary tube primarily give rise to Two Types of cellular elements: neuroblasts, which subsequently develop into nerve cells, and spongioblasts, which differentiate into neuroglial cells.

Thus, the entire human nervous system develops from the neural tube. The cranial (HEAD) end of the medullary tube expands into a sac-like shape (at the end of the third week) to form the brain, while the caudal (trunk) end represents the primordium of the spinal cord, medulla spinalis.

The ongoing cephalization of the nervous system led to the emergence of the brain. In mammals, the neocortex emerged, the cerebral hemispheres significantly increased in volume, and the cortical surface area expanded, accompanied by a dramatic increase in the number of neurons. The human cortex contains approximately 10–14 billion nerve cells. Based on their functional characteristics, cortical nerve cells are grouped into "nuclei" or the cortical ends of analyzers. Through these cortical analyzers of the human cerebral cortex, the analysis and synthesis of incoming environmental signals are performed, forming the first signaling system.

Alongside this, as a result of speech (words), writing, and labor, a higher signaling system emerged, which is unique to the activity of the human cerebral cortex—the second signaling system, according to I. P. Pavlov.

Functionally, the nervous system is conventionally divided into the somatic (animal) part and the vegetative (autonomic) part, pars autonomica.

The somatic nervous system innervates the soma (body), including striated Muscles and skin, and mediates the body's interaction with the environment.

The vegetative (autonomic) nervous system controls the visceral Functions of the body (motility, secretion of digestive organs, Blood pressure, metabolic processes, etc.). Based on Anatomical and physiological characteristics, the Autonomic nervous system is divided into two parts: the Sympathetic part (pars sympathetica) and the Parasympathetic part (pars parasympathetica).

Topographically, the nervous system is divided into the central part, pars centralis, and the peripheral part, pars peripherica.

The central part has its own subdivisions (see table).