Human Anatomy: A Course of Lectures - Kostylenko Yu.P. 2015

General Anatomy of the Nervous System. The Central Nervous System

Lecture Plan:

8.1. General characteristics of The Nervous System.

8.2. General Overview of Spinal Cord Anatomy.

8.3. General Overview of Brain Anatomy.

8.4. Functional Anatomy OF the Reticular Formation.

8.5. MAIN STAGES OF Nervous System Development.

8.1 General CHARACTERISTICS OF THE Nervous System

The nervous system ensures the internal coordination and continuous interaction of individual parts and Organs within the Organism, enabling it to function as a unified living system in its relationships with the external environment. The nervous system is built from Nervous Tissue, The properties of which include The ability to perceive various types of information from the external or internal environment, transmit the received impulses, and respond to them accordingly. The specific cellular elements of nervous tissue are neurocytes, or Neurons — Cells capable of perceiving stimuli, generating nerve impulses, conducting them, and transmitting them to other cells or tissues.

The neuron represents the fundamental structural unit of the nervous system. Neurons are extremely diverse in shape and size. The number of neurons in the human nervous system is estimated at 10¹⁰-10¹¹. They form numerous connections with one another — interneuronal synapses.

A characteristic structural feature of neurocytes is the presence of processes through which nerve impulses propagate. Two Types of processes are distinguished: dendrites — shorter, branching tree-like structures that bring nerve impulses toward The Cell body of the nerve cell; and the neurite (axon) — typically long and unbranched, which carries impulses away from the nerve cell body.

The neurite, the length of which can reach 1.5 m, is always singular in a nerve cell, whereas there may be two or more dendrites. Depending on the number of processes, neurons are classified into pseudounipolar (processes emerge as a single trunk and subsequently divide), bipolar (having one axon and one dendrite), and multipolar (having one axon and several dendrites). Based on their functional significance and certain morphological features, neurons include sensory (afferent), motor (efferent), autonomic (motor, secretory), interneurons, association neurons, etc.

Afferent neurons receive nerve impulses from the external and internal environment via sensory nerve endings (receptors) and dendrites. Neurons of this type are typically bipolar, sometimes pseudounipolar.

Efferent (motor) neurons receive impulses generated in other neurons, which are transmitted via synapses to the dendrites or the cell body of the motor neuron. Through the axon of the efferent neuron, the impulse reaches the innervated organ, producing a specific (motor or secretory) effect. These neurons are most frequently multipolar. Association neurons provide The connection between different groups of Nerve Cells.

Both dendrites and neurites are commonly referred to as nerve fibers. In organs and tissues, nerve fibers form sensory and motor nerve endings. The former, also called receptors, ensure the perception of stimuli from the external and internal environment and convert the energy of stimuli (mechanical, thermal, acoustic, etc.) into a Nerve Impulse transmitted along sensory fibers to the Central Nervous System. Motor nerve endings transmit excitation from the nerve fiber to the organ.

The activity of the nervous system is inherently reflex-based. A reflex is defined as the organism's response reaction to a stimulus, mediated by the nervous system. The anatomical substrate of a reflex is the reflex arc.

A simple reflex arc is formed by the connection of two neurons — an afferent and an efferent one (monosynaptic arc). The stimulus perceived by the receptor is relayed from the afferent neuron to the efferent neuron, and the latter sends an impulse to the effector organ. Typically, interneurons are located between the afferent and efferent neurons (bisynaptic, three-neuron reflex arc).

According to the topographical principle, the nervous system is conventionally divided into central and peripheral divisions. The central nervous system includes the BRAIN AND SPINAL cord, while the Peripheral Nervous System comprises all other components — nerve roots, ganglia, plexuses, nerves, and peripheral nerve endings.

Furthermore, based on functional criteria, the human nervous system is conventionally divided into two parts:

1. Autonomic (involuntary) nervous system — provides innervation of viscera, Skin Muscles, The Heart, and Blood Vessels;

2. Somatic (voluntary) nervous system — regulates the activity of voluntary musculature (skeletal muscles).

8.2 General Overview of Spinal Cord Anatomy

The spinal cord (medulla spinalis, myelos) is the most ancient division of the central nervous system in vertebrates. In lower animals, it is more developed compared to the brain. As the central part of the nervous system progressively evolved, the size ratio between the spinal cord and the brain shifted in favor of the latter. The general structural patterns of the central nervous system are most clearly manifested in the Anatomy of the spinal cord.

The spinal cord is located within the vertebral canal and has an irregular cylindrical shape; its length is about 45 cm in men and 41-42 cm in women. The mass of an adult spinal cord averages 34-38 g. The Termination of the spinal cord within the vertebral canal corresponds to the second lumbar vertebra (LII). To avoid spinal cord injury, this circumstance must be taken into account when performing a lumbar puncture for therapeutic and diagnostic purposes.

In the thoracic region, the spinal cord has a transverse diameter of about 10 mm and a sagittal dimension of about 8 mm. Throughout its length, the spinal cord features cervical and lumbosacral enlargements, located correspondingly to the innervation centers of the upper and lower limbs. In the cervical enlargement, the transverse diameter of the spinal cord reaches 13-14 mm, and the sagittal dimension is 9 mm. In the lumbar enlargement, the thickness of the spinal cord is about 12 mm, and the sagittal dimension is about 9 mm.

Along the midline throughout the entire length of the spinal cord, the anterior median fissure runs anteriorly, while the posterior median sulcus runs posteriorly, dividing the spinal cord into two halves (right and left). On each half, additionally, the posterior and anterior lateral sulci are distinguished, which delimit the anterior, lateral, and posterior funiculi (cords) of the spinal cord, respectively.

The spinal cord consists of White matter located peripherally, which predominantly contains Cytology/practical/65.html">Myelinated nerve fibers (nerve cell processes), and an inner core of Gray matter. The latter is formed mainly by nerve cell bodies.

In the WHITE MATTER OF the spinal cord, anterior, posterior, and lateral funiculi are distinguished; these funiculi contain associative, commissural, and projection Neural Pathways.

Associative pathways are represented by fiber bundles that run along the periphery of the gray matter across all spinal cord funiculi, connecting spinal cord segments within the same half of the spinal cord.

Commissural pathways are formed by fibers connecting both halves of the spinal cord gray matter, creating the white commissure located between the gray matter and the anterior median fissure.

Projection Pathways connect the spinal cord with the brain. They are divided into ascending (afferent) and descending (efferent) pathways.

Ascending pathways are represented by axons of neurons from the spinal ganglia, posterior horns, and the intermediate zone of the spinal cord gray matter. They pass through the posterior and lateral funiculi. The posterior funiculus contains the gracile and cuneate fasciculi, which serve as pathways for conscious proprioceptive and tactile sensitivity.

Phylogenetically older ascending pathways run within the lateral funiculus (anterior and posterior spinocerebellar tracts, lateral spinothalamic tract).

Descending pathways transmit impulses from the Cerebral Cortex, subcortical nuclei, and Brainstem nuclei to the spinal cord neurons. They are located in the lateral and anterior funiculi.

In cross-section, the gray matter of the spinal cord forms a figure resembling the letter "H" or a butterfly with open wings. The anterior and posterior horns of the gray matter are distinguished (when viewing the spinal cord in longitudinal section, they form the anterior and posterior columns, respectively), and in the thoracic and lumbar regions, lateral horns also protrude. The shape of the horns varies along the length of the spinal cord. The volume of the spinal cord gray matter is about 5 cm³ (17.8% of the total spinal cord volume), and the number of neurons it contains is approximately 13.5 million.

The anterior horns of the spinal cord contain the bodies of predominantly efferent (motor) neurons grouped into several nuclei. The neurites of these cells emerge from the spinal cord as rootlets and participate in The formation of Spinal Nerves.

Nerve Cells of the posterior columns (interneurons), which also form nuclei, process various types of sensitivity. They form synapses with the axons of pseudounipolar neurons from the spinal ganglia, which penetrate the substance of the spinal cord as part of the rootlets of the posterior (sensory) roots (Fig. 8.1).

The lateral horns of the spinal cord house the centers of the Sympathetic division of the Autonomic nervous system.

Throughout the entire length of the spinal cord, rootlets enter the posterior sulcus and emerge from the anterior sulcus, grouping into anterior and posterior roots. On the sides of the spinal cord, the anterior and posterior roots unite to form spinal nerves. Just before their union, each posterior ROOT includes a spinal ganglion containing pseudounipolar afferent nerve cells.

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Fig. 8.1 Structure OF THE spinal cord:

1 - anterior median sulcus; 2 - posterior median fissure; 3 - anterior funiculi; 4 - lateral funiculi; 5 - posterior funiculi; 6 - anterior horns; 7 - posterior horns; 8 - posterior roots; 9 - anterior roots.

The structure of the spinal cord is characterized by segmentation; it can be represented as consisting of repeating, similar parts—segments—each connected by neural pathways to a specific body segment.

There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal segment in the spinal cord. Externally, the segmentation of the spinal cord is manifested by the regular emergence of alternating anterior and posterior roots that form the spinal nerves.

Thus, a spinal cord segment should be understood as a region of gray matter developed from a single neurotome, corresponding to a pair of anterior and posterior roots, and giving rise to a single pair of spinal nerves.

8.3 General Overview of Brain Structure

The brain is located within the cranial cavity and generally conforms to the internal contours of the Skull. Three major parts can be distinguished in the brain: the Telencephalon (cerebrum), the Cerebellum, and the brainstem.

The brainstem, in turn, includes the Medulla Oblongata, Pons, Midbrain, and Diencephalon (Fig. 8.2). However, it should be noted that not all scientists agree with this Classification. For instance, V. H. Cherkasov does not include the diencephalon in the brainstem, while other authors, notably N. P. Mikheiev, include only the medulla oblongata and the pons in this concept.

Fig. 8.2 Brainstem and cerebellum:

1 - medulla oblongata;

2 - pons;

3 - midbrain;

4 - diencephalon;

5 - cerebellum.

Medulla oblongata (medulla oblongata, myelencephalon). Phylogenetically, the medulla oblongata is one of the oldest structures of the brain in Chordates. It is a vital region of the vertebrate central nervous system, housing the centers for Respiration, Blood Circulation, and swallowing.

The medulla oblongata is a direct continuation of the spinal cord. Its shape resembles a young onion bulb; hence, in clinical practice, pathological processes affecting the structures of the medulla are referred to as "bulbar disorders" (from the Latin bulbus - bulb).

The boundary separating the medulla oblongata from the spinal cord lies at the level of the upper edge of the posterior arch of the atlas (corresponding to The Emergence of the first pair of spinal nerves), while superiorly and anteriorly it is demarcated from the pons (the next segment of the brainstem) by a deep transverse sulcus.

The external anatomy of the medulla oblongata largely resembles that of the spinal cord. Specifically, the medulla features anterior and posterior surfaces. An anterior median fissure runs along the anterior surface, serving as a continuation of the spinal cord's fissure of the same name. Flanking this fissure are massive, roller-like longitudinal elevations known as the pyramids (corresponding to the anterior funiculi of the spinal cord), which are delimited by the anterolateral sulci.

Lateral to the pyramid (on each side) lies the olive (corresponding to the lateral funiculi of the spinal cord). The posterior surface of the medulla oblongata features the posterior median fissure, a continuation of the corresponding spinal cord sulcus. Lateral to it on each side is the posterior funiculus.

The posterior intermediate sulcus divides the posterior funiculus into two bundles, which continue the corresponding bundles of the spinal cord: the medial gracile fasciculus and, lateral to it, the cuneate fasciculus. Ascending superiorly, the posterior funiculi diverge, forming the lower BOUNDARIES OF THE lower half of the Rhomboid fossa, and together with a portion of the lateral funiculi, they form the inferior cerebellar peduncles.

At the upper ends of the gracile and cuneate fasciculi lie quite distinct elevations: the tubercle of the gracile Nucleus and the tubercle of the cuneate nucleus, inside of which lie the nuclei of the same name.

As in the spinal cord, the Internal Structure of the medulla oblongata comprises gray and white matter. However, unlike in the spinal cord, the gray matter of the medulla is not a continuous, single formation; instead, it is represented by isolated clusters of nerve cell bodies—nuclei. The nuclei of the medulla oblongata developed in connection with the emergence of centers regulating and coordinating movements (the olivary nucleus, the reticular formation), centers for respiration, circulation, and METABOLISM, as well as centers controlling the Functions derived from the branchial apparatus and HEAD movements (the nuclei of the glossopharyngeal, vagus, and hypoglossal nerves).

Thus, the gray matter of the medulla oblongata is represented by the olivary nucleus, the nuclei of the reticular formation, the gracile and cuneate nuclei, and the nuclei of cranial nerve pairs IX–XII.

The white matter of the medulla oblongata consists of the fibers of the pyramids (most of which decussate, forming the pyramidal decussation), as well as the lateral and posterior funiculi. Both groups of nerve fibers pass through the medulla oblongata, connecting the spinal cord with higher centers of the brain. In addition, descending fibers run from the olivary nucleus and the reticular formation to the spinal cord, while ascending fibers project to the cerebellum. From the nuclei of the gracile and cuneate fasciculi, the majority of fibers form a decussation and enter the medial lemniscus, while a portion proceeds to the cerebellum.

At this point, it is appropriate to digress slightly from the main topic and say a few words about the structure of the medial lemniscus. The medial lemniscus (lemniscus medialis) is the collector of sensory pathways within the brainstem. It gathers second-neuron fibers for practically all sensory modalities (gustatory, vestibular, proprioceptive, nociceptive, thermal, and tactile), which then ascend to the thalamus.

The medial lemniscus lies in the medulla oblongata and pons dorsal to the pyramids, and in the midbrain dorsal to the substantia nigra. The bulk of the fibers of the medial lemniscus terminate in the lateral Nucleus of the thalamus, while a portion also reaches the medial nucleus.

Consequently, the medulla oblongata is linked—either directly (via Cranial Nerves) or indirectly (via projection PATHWAYS OF THE central nervous system and the reticular formation)—with the periphery and other PARTS OF THE central nervous system.

Pons. The next segment of the brainstem is called the pons (or pons Varolii). It likely received this name because it serves as a connecting bridge between the medulla oblongata, the midbrain, and the cerebellum.

The pons is situated on the ventral surface of the brain as a broad, transversely striated prominence. Anteriorly, it borders the cerebral peduncles; posteriorly, it meets the medulla oblongata; and laterally, it transitions into the middle cerebellar peduncles. Along the midline of the ventral surface lies the basilar sulcus, traversed by the Basilar artery of the same name (which originates from the subclavian Arterial System and participates in the Blood supply to the brain).

In cross-section, the pons is conventionally divided into ventral (anterior) and dorsal (posterior) parts, the boundary between which is formed by a band of transverse nerve fibers known as the trapezoid body, which is part of the Auditory pathway.

The ventral part of the pons consists primarily of longitudinal and transverse nerve fibers. The longitudinal fibers are formed by the corticopontine tract, connecting the pons with the cerebral cortex, and by the fibers of the pyramidal tract.

The transverse fibers of the pons, which make up the bulk of the ventral part, connect the pons to the cerebellum by running within the middle cerebellar peduncles. Interspersed among these transverse fibers are numerous pontine nuclei, where the corticopontine pathways terminate and the pontocerebellar tract originates.

The dorsal part of the pons contains the following structures: the reticular formation, which is a continuation of that in the medulla oblongata; the nuclei of cranial nerve pairs V–VIII; ascending fibers of the medial and lateral lemnisci, and the spinotectal tract; fibers of the spinal and mesencephalic tracts of the Trigeminal nerve along with their decussation; fibers of the medial longitudinal fasciculus, which originate from the vestibular nuclei and project to the nuclei of the oculomotor, trochlear, abducens, and accessory nerves; and descending fibers of the rubrospinal, tectospinal, and reticulospinal tracts, which terminate on the motor neurons of the anterior horns of the spinal cord's gray matter.

The trapezoid body incorporates auditory pathway fibers emerging from the cells of the cochlear root nuclei of the Vestibulocochlear nerve and from the cells of the trapezoid body nuclei. These auditory fibers within the trapezoid body subsequently form a decussation known as the lateral lemniscus.

Here, it would be worthwhile to say a few words about the Fourth ventricle, which represents the remnant of the rhombencephalic cavity and whose formation involves the cerebellum, pons, and medulla oblongata. It should be noted that each region of the brain contains a ventricular cavity that represents the remnant of its corresponding embryonic brain vesicle.

In terms of shape, the IV ventricle resembles a tent set on its edge, with its floor (the anterior wall) formed by the rhomboid fossa, which is a depression on the posterior surface of the pons and medulla oblongata. The IV ventricle, just like the other ventricles of the brain, circulates CEREBROSPINAL FLUID (CSF)—a fluid whose function and significance will be detailed below. Thus, the IV ventricle communicates with the cerebral aqueduct, and through it with the III ventricle, the central canal of the spinal cord, and three apertures (paired lateral and unpaired median) with the subarachnoid spaces of the brain.

Excess CSF produced in the ventricles flows through these apertures into the subarachnoid space, where it is absorbed into the venous segments of the microcirculatory bed. Any disruption in the CSF passage (outflow) from the IV ventricle leads to a pathological condition known as Hydrocephalus (Water on the brain), which can be complicated by the herniation of the medulla oblongata into the foramen magnum, resulting in death due to the destruction of vital centers located in the medulla.

The midbrain (mesencephalon). The midbrain emerged at Early stages of vertebrate evolution in connection with the visual and, partially, auditory analyzers. In Embryogenesis, it develops from the third secondary brain vesicle. This segment of the brainstem is located between the pons inferiorly and the diencephalon superiorly. The midbrain consists of the tectum of the midbrain and the cerebral peduncles, between which lies the midbrain cavity—the cerebral aqueduct (aqueduct of Sylvius).

The tectum of the midbrain appears as a plate of white matter featuring two pairs (two superior and two inferior) of elevations known as colliculi. They are separated by intersecting transverse and longitudinal grooves. Externally, the colliculi of the midbrain are covered by a thin layer of white matter, beneath which lies a cluster of gray matter. The gray matter of the colliculi has diverse functional significance: the gray layer of the superior colliculi belongs to the subcortical visual centers, while the nuclei of the inferior colliculi serve as one of the subcortical auditory centers.

The cerebral peduncles appear as two thick, longitudinally oriented bundles that diverge anteriorly and enter the hemispheres of the telencephalon. Each peduncle is divided into two parts: the tegmentum and the crus cerebri (base), with the boundary between them marked by a pigmented layer of gray matter visible on a longitudinal section—the substantia nigra.

On a frontal section of the midbrain, structures forming its white and gray matter can be distinguished. The latter includes the aforementioned substantia nigra (Soemmerring's substance), which got its name due to the presence of the pigment melanin in its constituent neurons, giving it a black color.

The tegmentum of the peduncles also contains various functionally significant nuclei. The largest of these is the paired red nucleus, an elongated structure located between the substantia nigra and the central gray matter surrounding the aqueduct.

The red nuclei serve as an important Relay center for the pathways of the brainstem. They receive fibers of the extrapyramidal system originating from the basal nuclei of the telencephalon, as well as fibers coming from the cerebellum.

The nuclei of the oculomotor (III pair) and trochlear (IV pair) cranial nerves, located in the tegmentum beneath the floor of the aqueduct, innervate the Muscles of the Eyeball and participate in the REFLEX REGULATION OF Eye Movements.

Lying beneath The Nucleus of the Oculomotor nerve is the accessory oculomotor nucleus, which includes an unpaired nucleus (of Perlia) and paired nuclei (of Yakubovich, Westphal, and Edinger). These nuclei ensure the contraction of the smooth muscles of the eyeball (the sphincter pupillae and ciliary muscles). The tegmentum of the midbrain also houses the sensory mesencephalic nucleus of the trigeminal nerve. The midbrain is not only a site where important Reflexes are integrated, but it also performs a significant conduction function.

Separated from the tegmentum by the substantia nigra, the Base of the peduncles consists exclusively of descending pathways connecting the cerebral cortex with the pons and the spinal cord.

The diencephalon (interbrain). Anatomically and functionally, the diencephalon serves as a connecting link between the cerebral hemispheres and the lower levels of the central nervous system. It is subdivided into the thalamic and hypothalamic regions. The Thalamic region, in turn, is divided into the thalamus, epithalamus, and metathalamus. The cavity of the diencephalon is the III ventricle.

The thalamus is a complex of nuclei separated by medullary laminae of white matter.

Considering the diverse functional roles of the thalamic nuclei, their main groups can be distinguished as follows.

1. Anterior nuclei (anterodorsal, anteroventral, anteromedial).

2. Median nuclei (anterior and posterior paraventricular, rhomboid, reuniens).

3. Medial nuclei (dorsomedial).

4. Intralaminar nuclei, located along the internal medullary laminae of the thalamus (centromedian, paracentral, parafascicular, lateral central, medial central).

5. Ventral nuclei (ventral posterolateral, ventral anterolateral, ventroanterior, ventrointermediate, ventromedial, ventroposterior, ventroposteromedial).

6. Paratenial nucleus.

7. Posterior nuclei (pulvinar nuclei, lateral geniculate body, medial geniculate body).

8. Reticular nuclei.

9. Subthalamic nucleus.

The thalamus acts as the primary relay station for afferent impulses. Here, initial Processing of information from all Sensory Organs (except Olfaction) takes place, followed by its transmission to the CEREBRAL CORTEX AND subcortical nuclei. Each group of nuclei is connected to specific areas of the cerebral cortex. This connection is reciprocal, meaning that alongside ascending thalamocortical fibers, there are descending corticothalamic fibers.

The Role of the thalamus is not limited to the simple relay and switching of incoming impulses. In the thalamus, afferent signals acquire an affective, visceral coloring, whereas the cerebral cortex performs fine discrimination of stimuli. It is believed that the sensation of pain is associated with the thalamus. Damage to the thalamus results in various sensory disorders, such as diminished or, conversely, heightened sensitivity, complete loss of pain sensations, or agonizing bouts of intense pain.

The metathalamus consists of the medial and lateral geniculate bodies. The bulk of the optic tract fibers terminate in the lateral geniculate body, from where visual impulses are relayed to the visual cortex. The nucleus of the medial geniculate body receives auditory impulses via the lateral lemniscus and projects them to the auditory cortex of the cerebrum.

The epithalamus is formed by the habenulae, interconnected by the habenular commissure, and the Pineal Gland (epiphysis). The habenulae contain nuclei that belong to the subcortical nuclei of the limbic system. They receive afferent fibers from the olfactory brain, thalamus, and basal nuclei via the medullary stria of the thalamus. Efferent fibers from the habenular nuclei extend to the reticular Formation of the midbrain and the interpeduncular nucleus. This pathway provides the connection between the Limbic System and the brainstem.

The pineal gland is an endocrine gland. For centuries, the pineal body has captured the attention of researchers. The first mentions of it date back more than 2,000 years to ancient India, where the pineal gland was likened to an eye turned toward the inner world. Through this "third eye," ancient Indians believed, a person could retrieve images of long-past years of their life.

In the Middle Ages, attention to the pineal gland was revived thanks to the works of the French philosopher and naturalist René Descartes. "The pineal gland," he maintained, is the "seat of the soul" and THE PLACE OF "concentration of imagination and common sense." Therefore, dysfunction of the gland leads to mental disorders.

Comparative anatomy data are also noteworthy, showing that in lower vertebrates, the pineal body plays the role of a third, parietal eye that has sunk deep into the brain during evolution. There are cyclostome Fishes and certain species of lizards in which the parietal eye, hidden beneath the skull roof, still perceives light. However, in most vertebrates, it no longer perceives light, having transformed into an endocrine organ with a different function.

According to modern concepts, the pineal gland is a central endocrine gland. Its main function is to ensure rhythmic fluctuations in the activity of the Hypothalamus and Pituitary gland. Consequently, the pineal gland regulates all processes occurring rhythmically in the body, as well as circadian rhythms (Changes in the functional activity of the organism throughout the day and seasons), which are presumably linked to changes in the secretory activity of the gland depending on illumination.

In addition, the pineal gland produces melatonin, a hormone that prevents premature sexual maturation, along with A number of other BIOLOGICALLY ACTIVE SUBSTANCES whose function remains not fully understood to this day.

The hypothalamus is the phylogenetically oldest part of the Forebrain. It includes the optic chiasm, optic tracts, tuber cinereum, infundibulum, mammillary bodies, and subthalamic nucleus. The hypothalamus contains more than 15 nuclei. Based on the localization of the nuclei, the dorsal, anterior, intermediate, and posterior hypothalamic regions are distinguished. These nuclei belong to the centers of the Autonomic (vegetative) nervous system.

The optic tracts contain fibers from the halves of both retinas (the right tract from the right halves, the left from the left). These fibers travel to the subcortical visual centers already familiar to us: the lateral geniculate body, the posterior nucleus of the thalamus, and the superior colliculus of the midbrain tectum. The nuclei of the anterior hypothalamic region lie close to the optic tract.

The tuber cinereum and infundibulum belong to the intermediate hypothalamic region; they contain numerous nuclei whose cells possess neurosecretory functions. The mammillary bodies are located in the posterior hypothalamic region; they serve as subcortical olfactory centers and belong to the limbic system.

The cerebellum (cerebellum). The cerebellum is located in the posterior cranial fossa and consists of an unpaired vermis and paired hemispheres. In mature age, the mass of the cerebellum ranges from 136 to 169 g, accounting for 10–12% of the total brain mass.

White and gray matter are distinguished in the cerebellum. The white matter lies in the core of the cerebellum. The gray matter forms the cerebellar cortex; the alternation of white and gray matter visible on a cerebellar section resembles the leaves of a thuja tree and was named the "tree of life" (*arbor vitae*) by Renaissance anatomists.

The cerebellar cortex contains 3 layers. Lying On the surface is the molecular layer, which contains basket and stellate neurons. The deep granular layer consists predominantly of very small granule cells, and positioned between these two layers is the layer of Purkinje cells (pear-shaped neurons). These cells send their dendrites into the molecular layer, where, together with other nerve fibers, they form a rectangular spatial network of branches. The axons of the Purkinje cells exit the cerebellar cortex and terminate in its nuclei.

The cerebellar nuclei are isolated paired clusters of gray matter embedded within the white matter. The dentate, emboliform, globose, and fastigial nuclei are distinguished. Each nucleus is connected to a specific zone of the cerebellar cortex.

From a phylogenetic standpoint, the cerebellum can be divided into three parts.

The archicerebellum (vestibulocerebellum) is represented by the nodulus in the vermis and the flocculus in the hemispheres; together they form the flocculonodular lobe. This part of the cerebellum is connected to the vestibular nuclei and the Reticular Formation and is involved in maintaining balance and spatial orientation of the body.

The paleocerebellum includes the inferior part of the vermis and adjacent areas of the hemispheres. This region of the cerebellum receives proprioceptive signals from the spinal cord and is connected to the olivary nucleus and various brainstem nuclei. Its function consists primarily in participating in the Regulation of Muscle tone.

The neocerebellum comprises the upper part of the vermis and the majority of the hemispheres. It is connected to the cerebral cortex via the pontine nuclei. Its functional significance lies in the control and coordination of voluntary movements, especially of the limbs.

The cerebellum is connected to the brainstem by three pairs of cerebellar peduncles. The inferior cerebellar peduncles arise from the medulla oblongata and contain primarily afferent pathways: the posterior spinocerebellar, vestibulocerebellar, and olivocerebellar tracts. These pathways terminate in the paleocerebellum and archicerebellum. The inferior peduncles also transmit the efferent cerebellovestibular pathway from the fastigial nucleus to the lateral vestibular nucleus, the motor cranial nerve nuclei, and the reticular formation nuclei.

The middle cerebellar peduncles consist of pontocerebellar fibers, which terminate in the cortex of the neocerebellum.

The superior cerebellar peduncles are dominated by efferent pathways extending from the dentate nucleus of the cerebellum to the red nucleus of the midbrain (dentatorubral tract) and the thalamic nuclei (dentatothalamic tract). Afferent pathways include the anterior spinocerebellar tract, which terminates in the archicerebellum. The superior peduncles also contain fibers directly connecting the cerebellar and cerebral cortices.

The main function of the cerebellum is The regulation of movement coordination. The cerebellum accounts for the effects of gravity and inertia during movement. Cerebellar damage results in impaired coordinated movements (asynergia), movement under- or overshoot (dysmetria), decreased muscle tone, an unsteady gait (ataxia), and other motor disorders.

The telencephalon (endbrain). The telencephalon consists of two cerebral hemispheres. Each hemisphere, in turn, comprises: 1 - the Pallium (the layer of gray matter covering the hemisphere surfaces), 2 - the rhinencephalon (olfactory brain), 3 - the basal nuclei, 4 - the white matter, and 5 - the Lateral ventricles.

External structure of the hemispheres. The cerebral hemispheres are separated from one another by the longitudinal cerebral fissure and are connected by the corpus callosum, the anterior and posterior commissures, and the fornix commissure. Each hemisphere has three surfaces: a convex superolateral surface; a flat medial surface facing the opposite hemisphere; and an inferior (basal) surface facing the base of the brain, which bears a relief corresponding to the internal base of the skull.

The most prominent Regions of the hemispheres are designated as the frontal (anterior), occipital (posterior), and temporal (lateral) poles. The surfaces of the hemispheres exhibit sulci (grooves) that bound roller-like elevations known as gyri.

By means of the most prominent sulci (central, lateral, and parieto-occipital), the cerebral hemispheres are divided into the frontal, parietal, temporal, occipital, and insular lobes. The latter is not visible upon external inspection of the hemispheres, as it lies at the bottom of the lateral sulcus and is covered by parts of the neighboring lobes known as the operculum.

The gray matter of the cerebral hemispheres is represented by the cortex and the basal nuclei of the telencephalon.

Structure of the cerebral cortex. The cerebral cortex is the most complexly organized structure of the nervous system. Higher forms of reflection of the external world and all types of conscious human activity are associated with the cortex.

The surface area of both hemispheres covered by the cortex ranges from 1469 to 1670 cm² in adults. Of the total cortical surface, 2/3 lies within the depths of sulci and fissures, while 1/3 occupies the gyri and the visible surface of the hemispheres. In humans, cortical thickness varies from 1.25 to 4 and even up to 6 mm.

The Study of the structure of the cerebral cortex—its architectonics—comprises several branches. Neuronal architectonics, or cytoarchitectonics, investigates the cellular COMPOSITION OF THE cortex; myeloarchitectonics examines its fibrous structure; and angioarchitectonics deals with the distribution of blood vessels within the cortex.

Phylogenetically, the cortex is divided into ancient cortex (paleocortex), old cortex (archeocortex), and new cortex (neocortex). The ancient and old cortices are located on the medial and basal surfaces of the hemisphere.

The structure of the neocortex comprises six distinct layers (laminae) that possess morphological and functional differences. The Superficial layer of the cortex is termed the molecular lamina, with a thickness of 0.15–0.2 mm. The second layer forms the external granular lamina, 0.1–0.16 mm thick, densely packed with small granular neurons. The third layer is called the external pyramidal lamina, measuring 0.8–1.0 mm in thickness. Lying deeper is the internal granular lamina, which contains small granular and stellate neurons. The fifth layer is represented by the internal pyramidal lamina, 0.4–0.5 mm thick; this layer contains the largest pyramidal neurons—Betz cells, named after the Ukrainian anatomist V.A. Betz, who first described them in the late 19th century.

The sixth layer constitutes the multiform lamina, which houses neurons of various shapes. The three outer layers of the cortex are generally grouped under the term external principal zone, while the three inner layers form the internal principal zone. The ancient and old cortices differ from the neocortex in their more primitive structure, typically lacking certain layers (such as the internal granular and internal pyramidal laminae).

The Functional Significance of the cortical laminae is determined by their cellular composition and interneuronal connections. Fibers originating from other cortical layers and the opposite hemisphere terminate in the molecular lamina. The external granular and external pyramidal laminae primarily contain associative neurons that mediate horizontal and vertical intracortical connections.

The internal granular lamina serves as the main receptive layer of the cortex, where the majority of specific projection afferent fibers from the thalamus and geniculate nuclei terminate. The internal pyramidal lamina acts as THE ORIGIN OF efferent projection pathways. The multiform lamina contains functionally heterogeneous neurons, which are believed to give rise to commissural fibers.

Concepts of functional localization in the cerebral cortex. The great Russian physiologist I.P. Pavlov viewed the cerebral hemispheres as a continuous receptive (receptor) surface—as an aggregate of cortical ends of analyzers. According to I.P. Pavlov, an analyzer is a complex system consisting of a receptor (receptive apparatus), nerve impulse conductors, and a cortical center where the higher analysis of stimuli takes place. Pavlov demonstrated that the cortex contains both localized core areas and scattered elements.

The core (nucleus) is an area of concentrated nerve cells where peripheral receptor structures are projected, and where the analysis, synthesis, and integration of functions occur. Scattered elements may be located in the periphery of the core or at varying distances from it, where simpler analysis and synthesis take place. The cortical ends of the analyzer perform the analysis and synthesis of signals.

Three groups of fields are distinguished in the cerebral cortex: primary, secondary, and tertiary. Primary fields are connected with sensory and motor organs, and it is here that the analysis of stimuli arriving at the cortex from the corresponding receptors is performed. Destruction of the core zone leads to cortical blindness, deafness, or motor paralysis.

Secondary fields (peripheral zones of analyzers) are connected to individual organs exclusively via the primary fields. They serve for the generalization and further processing of incoming information. If these fields are destroyed, a person can see and hear, but loses the ability to comprehend meaning.

Tertiary fields (analyzer overlapping zones) occupy nearly half of the cortical surface and maintain extensive connections with other cortical regions and non-specific brain systems. It is here that higher-level analysis and synthesis of information occur, resulting in the formulation of behavioral goals and tasks, as well as the programming of motor activity. Congenital underdevelopment of tertiary fields leaves an individual unable to acquire language or even simple motor skills.

Primary and secondary fields are present in both humans and animals, whereas tertiary fields are unique to humans. Furthermore, tertiary fields mature later than other cortical fields in humans.

Olfactory brain (rhinencephalon). In humans, the rhinencephalon is the phylogenetically oldest part of the telencephalon, having emerged in connection with the olfactory analyzer at a time when the telencephalon had not yet become an organ of animal behavior. Consequently, all of its components are constituent parts of the olfactory analyzer.

The human olfactory brain comprises a series of structures of diverse origins, which can be topographically divided into two sections. The peripheral section is the olfactory lobe, which encompasses the following structures: (1) olfactory bulbs, (2) olfactory tract, (3) olfactory trigone, and (4) anterior perforated substance.

Limbic System of the brain. Recently, a limbic region has been distinguished within the cerebral cortex, playing a crucial role in regulating internal organ functions. This limbic region is formed by the cingulate gyrus, parahippocampal gyrus, hippocampus, septum pellucidum, and subcallosal area. The limbic cortex, together with subcortical structures (amygdala, habenular nuclei, mamillary body nuclei), constitutes the limbic system, which serves as the neural substrate for emotions and reactions associated with primary biological drives (hunger, thirst, fear, etc.).

Basal nuclei. The basal nuclei are clusters of gray matter located in the inferior regions of the hemispheres (predominantly within the insular lobe). These are phylogenetically ancient structures, distinguished as the stem portion of the telencephalon. The basal nuclei include the corpus striatum, claustrum, and amygdala.

The corpus striatum is the largest subcortical cluster of nerve nuclei, with a volume of 11–15 cm³. It consists of several nuclei separated by strands of white matter, giving it a characteristic striped appearance in brain slices—hence its name. Phylogenetically, one distinguishes the old striatum (paleostriatum), represented by the globus pallidus, and the new striatum (neostriatum), which includes the putamen and caudate nucleus.

Functionally, the corpus striatum is a vital motor center that governs automatic movements and regulates muscle tone, operating as part of the extrapyramidal system.

The paleostriatum and neostriatum differ in their functional roles. The globus pallidus serves as the primary motor nucleus of the ancient brain, generating numerous fine, auxiliary movements necessary for a primary motor act. These movements can be produced through connections of the globus pallidus with the thalamus and midbrain (substantia nigra) independently of the cerebral cortex. Modulating this movement-generating center is another center comprised of the putamen of the lentiform nucleus and the caudate nucleus, which inhibit and restrain the activity of the former.

When the globus pallidus is damaged, patients exhibit slowness and paucity of movement alongside increased muscle tone—a condition known as hypokinetic-rigid syndrome, or parkinsonism. This state is characterized by rigidity, generalized elevation of muscle tone, and finger tremor.

Conversely, when the inhibitory function of the corpus striatum (caudate nucleus and putamen) is impaired, the opposite clinical picture emerges. Patients develop involuntary movements accompanied by decreased muscle tone, resulting in hyperkinetic-hypotonic syndrome. Manifestations of this condition include chorea and athetosis.

The claustrum is a thin sheet of grey matter located between the putamen and the insular cortex. It is connected via neural pathways to the olfactory brain, thalamus, and cerebral cortex.

The amygdala is a complex of nuclei situated in the anterior pole of the cerebral hemisphere's temporal lobe, directly adjacent to the cortex of the parahippocampal gyrus. It receives fibers from the olfactory tract, thalamus, and cortex. The Efferent Pathways of the amygdala travel within the stria terminalis. The amygdala is a component of the limbic system.

White matter of the hemispheres. The fibers of the cerebral white matter can be divided into three groups: association, commissural, and projection fibers.

Association fibers connect different cortical areas within the same hemisphere. Association fibers that do not extend beyond the cortex are called intracortical association fibers. Those association fibers that connect separate cortical regions and extend from the cortex into the white matter are called extracortical association fibers. They are divided into two groups: short and long. Long extracortical association fibers connect the cortex of distant gyri and individual lobes of the hemispheres. They form several bundles (the cingulum, superior frontoccipital fasciculus, superior longitudinal fasciculus, inferior longitudinal fasciculus, and uncinate fasciculus).

Commissural fibers connect symmetrical parts of the cerebral hemispheres. This group of fibers also forms bundles, but unlike association fibers, their course is predominantly transverse. They include: 1 - the corpus callosum, 2 - the anterior white commissure, and 3 - the fornix commissures.

Projection fibers connect the cerebral cortex with lower regions, traversing the hemispheres in a vertical direction. Projection fibers, in turn, are divided into ascending and descending (analogously to those in the spinal cord). The majority of compactly arranged projection pathways form the internal capsule.

The internal capsule is located between the thalamus (which belongs to the diencephalon) and the caudate and lentiform nuclei (which belong to the telencephalon). The capsule is divided into an genu, a posterior limb, and an anterior limb.

Lateral ventricles. The cavities of the cerebral hemispheres are the lateral ventricles. Each ventricle comprises a middle, central part located inferior to the corpus callosum in the parietal lobe of the hemisphere. Extending from the central part are extensions of the cavities—horns: the anterior (frontal) horn extending into the frontal lobe, the inferior (temporal) horn into the temporal lobe, and the posterior (occipital) horn into the occipital lobe. The central part communicates with the Third ventricle via the interventricular foramen.

In the lateral ventricles of the brain lie the choroid plexuses, which participate in The production of cerebrospinal fluid—liquor—that performs trophic and protective functions in the central nervous system.

8.4 Functional Anatomy of the Reticular Formation

The reticular formation was first described in 1865 by the German scientist O. Deiters, who also coined the term. This term was and continues to be used to designate regions of the brain containing cells of various Sizes and Shapes surrounded by a dense network of fibers running in all directions. In the late 19th century, V.M. Bekhterev identified individual nuclei within the reticular formation.

The reticular formation is located in the spinal cord between the posterior and lateral horns, while in the rhombencephalon and mesencephalon it is localized in the tegmentum. In the human brainstem reticular formation, 22 nuclei are described, which are grouped into lateral, medial, and intermediate groups. From the midbrain, the reticular formation extends into the diencephalon, where it is represented by the intralaminar and reticular nuclei of the thalamus.

The neural connections of the reticular formation are quite extensive. Its lateral third contains receptive fields where fibers of various afferent systems ascending through the brainstem terminate. Collaterals of the medial and lateral lemnisci, as well as some sensory fibers of the V, VIII, IX, and X cranial nerves, project to the reticular formation. The medial two-thirds of the reticular formation comprise effector fields associated with the motor nuclei of the cranial nerves, the cerebellum, the diencephalon, and the nuclei of the anterior horns of the spinal cord.

The nuclei of the reticular formation located in the medulla oblongata have connections with the autonomic nuclei of the vagus and glossopharyngeal nerves, as well as the sympathetic nuclei of the spinal cord. They participate in the REGULATION OF CARDIAC activity, respiration, vascular tone, glandular secretion, and so on.

The nuclei of Cajal and Darkschewitsch, belonging to the midbrain reticular formation, have connections with the nuclei of the III, IV, VI, VIII, and XI pairs of cranial nerves. They coordinate the activity of these neural centers, which is crucial for ensuring conjugate turning of the head and eyes.

The reticular formation contains both ascending and descending pathways. Ascending pathways transmit stimuli from the lower levels of the reticular formation to the thalamic nuclei. These include the spinoreticular tract, originating in the spinal reticular formation; the spinothalamic tract, originating in the reticular formation of the medulla and pons; and the tegmentothalamic tract, originating in the midbrain reticular formation. The principal descending pathway is the reticulospinal tract, which originates in the pons and medulla and projects to the neurons of the anterior horns of the spinal cord and the intermediate zone of the grey matter. Through this pathway, the reticular formation can exert facilitating or inhibiting influences on the motor neurons of the spinal cord.

The upper Divisions of the reticular formation are connected to the cerebral cortex. Fibers project from the intralaminar and reticular nuclei of the thalamus to various areas of the cortex. The existence of direct ascending fibers from the midbrain reticular formation to the cortex is also recognized. A feature of reticulocortical projections is their diffuse character, encompassing all areas of the cortex. This fundamentally distinguishes them from specific afferent projections of various sensory modalities, which are linked to definite cortical fields. Reticulocortical fibers terminate in all layers of the cortex, whereas specific sensory pathways terminate in the internal granular layer (layer IV of the cortex).

The specific and diffuse projection systems represent two parallel pathways for afferent signals directed to the cerebral cortex. Various stimuli perceived by receptors are transmitted via specific afferent systems to the corresponding receptive areas of the cortex (general sensory, visual, auditory zones, etc.). Simultaneously, however, nonspecific afferent impulses from the reticular formation arrive at the cortex via its diffuse projection system. These nonspecific impulses provide the cortical activation necessary for the perception of specific stimuli. It is important to emphasize the crucial role of nonspecific afferent reticular fibers in the Selection (differential conduction) of information reaching the cerebral cortex. Interruption of the impulse flow from the reticular formation leads to a decrease in cortical tone, resulting in Sleep. When the passage of impulses from the reticular formation to the cortex is restored, awakening occurs.

An important role in the regulation of sleep and wakefulness has been established for the locus coeruleus and the raphe nuclei, which belong to the reticular formation. The locus coeruleus is located in the upper lateral part of the rhomboid fossa; its neurons produce norepinephrine, which is transported via axons to higher brain regions. The activity of these neurons is maximal during wakefulness, decreases in the early stages of sleep, and almost completely ceases during deep sleep. The raphe nuclei are situated along the midline of the medulla oblongata. The neurocytes of these nuclei produce serotonin, which induces processes of generalized inhibition and a state of sleep.

The action of the reticular formation on the cerebral cortex is not unidirectional. The cortex, in turn, sends signals back to the reticular formation. These signals travel along corticoreticular fibers, which originate mainly in the frontal lobes of the hemispheres and run within the Pyramidal Tracts to the reticular formation of the pons and medulla oblongata. Corticoreticular connections exert either an inhibitory or an excitatory effect on the brainstem reticular formation, modulating the passage of impulses along efferent pathways (efferent information selection). Due to this bilateral, circular feedback loop between the reticular formation and the cortex, self-regulation of cortical activity can be achieved. Muscle tone, visceral function, mood, attention span, memory, and other functions depend on the functional state of the reticular formation. Overall, the reticular formation creates and maintains the conditions necessary for complex reflex activity involving the cerebral cortex.

8.5 Main Stages of Nervous System Development

The source of nervous system development is the ectoderm. Already at the gastrula stage, the neural plate differentiates along the body midline on the DORSAL SIDE OF the embryonic shield.

Compared to the surrounding ectodermal epithelium, the neural band grows more actively, causing it to fold transversely and sink into the depth of the embryonic shield, transforming into the neural (or medullary) groove. The edges of this groove, or medullar folds, grow particularly vigorously, elevate, approach each other, meet, and fuse, turning the medullar groove into the medullar (or neural) tube. Closure of the neural tube occurs during the 4th week of development.

The spinal cord develops from the caudal (posterior) region of the neural tube. On the sides of the neural tube, the ectoderm forms the neural crests, from which spinal ganglia develop. Processes of neurons established in the ganglia grow toward the periphery, while central branches grow into the spinal cord, forming the posterior roots. The anterior roots are formed by processes of nerve cells established in the grey matter of the spinal cord. In the head region, the neural crests give rise to the primordia of the sensory ganglia of the cranial nerves.

The anterior, expanded portion of the neural tube is the primordium of the brain. In a 4-week-old embryo, primary brain vesicles are visible here, from which the 3 main parts of the brain develop: the forebrain, midbrain, and Hindbrain (three-vesicle brain stage). The forebrain (prosencephalon) is the most expanded due to the presence on its lateral walls of optic vesicles, which are the primordia of the visual organ. The midbrain (mesencephalon) is indistinctly demarcated from the hindbrain, or rhombencephalon (rhombencephalon).

During the 5th week, the forebrain and hindbrain separate, resulting in the formation of the 5 definitive divisions of the brain (the five-brain-vesicle stage). The forebrain divides into the telencephalon and the diencephalon.

The midbrain does not divide. The hindbrain divides into the metencephalon and the myelencephalon. The region between the hindbrain and the midbrain is distinguished as the rhombencephalic isthmus (Fig. 8.3).

Two cerebral hemispheres form in the telencephalon. By the 3rd month of development, a lateral fossa appears on The surface of each hemisphere, and sulcation begins from the 5th month. In the diencephalon, the thalami are laid down, and evaginations appear that give rise to the epithalamus and hypothalamus. Commissures connecting the right and left halves of the forebrain are formed. In the lower divisions of the brain, posterior (dorsal) and anterior (ventral) parts are distinguished. The posterior part of the midbrain forms the tectum, while its anterior part forms the cerebral peduncles. The cerebellum develops in the dorsal part of the hindbrain, and the pons is formed in its anterior part.

Fig. 8.3 Diagram of brain development:

1. Forebrain vesicle - prosencephalon

2. Midbrain vesicle - mesencephalon

3. Hindbrain vesicle - rhombencephalon

4. Telencephalic vesicle - telencephalon

5. Diencephalic vesicle - diencephalons

6. Midbrain vesicle - mesencephalon

7. Hindbrain vesicle - metencephalon

8. Myelencephalic vesicle - myelencephalon

9. Spinal cord - medulla spinalis