Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

The Nervous System (general structural and developmental plan of the nervous system)
Central Nervous System
Brain - Pons

The Pons (pons varolii) resembles a transversely thickened ridge, from the lateral sides of which, both right and left, the middle cerebellar peduncles emerge. The posterior surface of the pons, covered by the Cerebellum, contributes to The formation of the Rhomboid fossa. The lower (posterior) part of the anterior surface forms a clear boundary with the Medulla Oblongata, whereas the upper (anterior) part borders the cerebral peduncles (Midbrain). The anterior (inferior) surface of the pons features transverse striations corresponding to the transverse course of fibers extending from the pontine nuclei into the middle cerebellar peduncles and further into the cerebellum. A longitudinal basilar sulcus runs along the midline of the anterior surface of the pons, housing the Basilar artery of the same name.

The anterior (basilar) part of the pons consists of nerve fibers that form conduction pathways, interspersed with cellular clusters known as the pontine nuclei. The Pathways of the anterior (basilar) part connect the Cerebral Cortex with the Spinal Cord, the motor nuclei of the Cranial Nerves, and the cerebellar cortex.

The posterior (dorsal) surface of the pons, covered by the cerebellum, together with the medulla oblongata, helps form the floor of the Fourth ventricle. Because of its shape, the floor of the fourth ventricle is referred to as the rhomboid fossa.

Cranial nerves—specifically pairs V through VIII—emerge from the pons on each side: the Trigeminal nerve (cranial nerve V), Abducens nerve (cranial nerve VI), Facial Nerve (cranial nerve VII), and Vestibulocochlear nerve (cranial nerve VIII).

A frontal section of the pons reveals two distinct parts: the base (ventral part) and the tegmentum (dorsal part). At the border between the tegmentum and the Base of the pons lies a bundle of transversely oriented fibers—the trapezoid body, which belongs to the Auditory pathway.

Structural and Functional Characteristics of the pontine nuclei and pathways. The structures of the pontine tegmentum are a direct continuation of the tegmental region of the medulla oblongata and form the floor of the fourth ventricle, or the rhomboid fossa. The pontine tegmentum contains Gray matter—specifically the nuclei of cranial nerves V through VIII, the nuclei of the reticular formation, as well as ascending and descending tracts. The motor nuclei of the pontine tegmentum include: the motor Nucleus of the trigeminal nerve (cranial nerve V), which innervates the Muscles of Mastication; The Nucleus of the abducens nerve (cranial nerve VI), which innervates the lateral rectus Muscle of the eye; and the motor nucleus of the facial nerve (cranial nerve VII), which innervates the Muscles of facial expression.

At the junction of the medulla oblongata and the pons, within the lateral Regions of the rhomboid fossa, lie the sensory nuclei of the vestibulocochlear nerve (cranial nerve VIII). Corresponding to the two parts of this nerve, paired cochlear and vestibular nuclei are distinguished. The cochlear nuclei are subdivided into ventral and dorsal divisions. The axons of sensory Neurons from the spiral ganglion of the Inner ear project to these nuclei. The processes of the spiral ganglion Cells form the cochlear nerve, which transmits impulses from the receptors of the spiral organ (organ of Corti). Efferent fibers from the ventral nucleus cross to the opposite side, forming the trapezoid body. The axons of cells from the dorsal nucleus course transversely along the floor of the fourth ventricle as medullary striae and continue into the lateral lemniscus.

The vestibular group is represented by four pairs of nuclei: the lateral vestibular nucleus (Deiters' nucleus), medial vestibular nucleus (Schwalbe's nucleus), inferior vestibular nucleus (Roller's nucleus), and superior vestibular nucleus (Bechterew's nucleus). Axons of sensory neurons from the vestibular ganglion of the inner ear project to these vestibular nuclei. Forming the vestibular nerve, these nerve fibers convey impulses from the vestibular receptors of the inner ear's Organ of Balance to the vestibular nuclei. Efferent fibers of the vestibular nuclei course within the vestibulospinal tract and terminate on motor neurons in the anterior horns of the spinal cord, thereby coordinating Skeletal Muscle activity. Some efferent nerve fibers terminate on cells within the oculomotor, trochlear, and abducens nerve nuclei, helping to stabilize images on the retina.

The sensory pontine nucleus of the trigeminal nerve (cranial nerve V) is the rostral continuation of the spinal trigeminal nucleus and shares similar connections. Axons of sensory neurons from the trigeminal ganglion project to this pontine nucleus, conveying impulses from receptors in the scalp, facial Skin, nasal and oral mucosa, and the dura mater. The axons of the pontine nuclear cells travel within the medial lemniscus fibers to the thalamic nuclei, as well as to the motor nuclei of cranial nerves VII, IX, and X.

Autonomic nuclei and nuclei of the reticular formation are located in the pontine tegmentum among the neurons of the motor and sensory nuclei.

Dorsal to the motor nucleus of the facial nerve lies its autonomic counterpart, the superior salivatory nucleus. Axons of these neurons project to the nasal mucosa and lacrimal glands (via neurons of the pterygopalatine ganglion), as well as to the submandibular and sublingual Salivary Glands (via the submandibular and sublingual parasympathetic ganglia). The interaction between Cells of the superior salivatory nucleus and the motor nucleus of the trigeminal nerve mediates the lacrimal reflex.

The reticular Formation of the pontine tegmentum consists of scattered, isolated Nerve Cells and clusters of neurons known as reticular nuclei.

The White matter OF the pontine tegmentum is formed by conduction pathways, some of which are generated by the axons of the pontine nuclei: the medial lemniscus, trigeminal lemniscus, and lateral lemniscus.

The fourth ventricle serves as the cavity of the medulla oblongata and Hindbrain (derived from the embryonic rhombencephalon). Inferiorly, the fourth ventricle continues into the narrow central canal of the spinal cord, and superiorly into the cerebral aqueduct—the cavity of the midbrain. Because of its shape, the floor of the fourth ventricle is called the rhomboid fossa. Its upper (anterior) boundaries are formed by the superior cerebellar peduncles, whereas its lower (posterior) boundaries are flanked by the inferior cerebellar peduncles. The anterior median sulcus divides the floor of the rhomboid fossa into two symmetrical halves. On either side of the sulcus lie medial (longitudinal) eminences, marked near their centers by the right and left facial colliculi. Deep within each facial colliculus lies the nucleus of the abducens nerve (cranial nerve VI), while deeper and more laterally situated is the nucleus of the facial nerve (cranial nerve VII). Inferiorly, the medial eminence transitions into the hypoglossal trigone, lateral to which lies the vagal trigone. Within these trigones, located deep in the Brain substance, are the nuclei of the corresponding nerves. The superior angle of the rhomboid fossa communicates with the Third ventricle via the cerebral aqueduct. The lateral areas of the rhomboid fossa are known as the vestibular areas, where the Auditory and Vestibular nuclei of the vestibulocochlear nerve (cranial nerve VIII) lie deep within the brain tissue. Transverse medullary striae emerge from the dorsal cochlear nuclei and extend toward the median sulcus; these striae are located at the junction of the medulla oblongata and the pons and represent fibers of the auditory pathway.

The deep substance of the rhomboid fossa houses the nuclei of cranial nerves V, VI, VII, VIII, IX, X, XI, and XII. Sensory nuclei are located laterally, autonomic nuclei lie more medially, and motor nuclei are situated most medially. To understand this arrangement of nuclei within the rhomboid fossa, one must consider that during embryonic development, as the closed neural tube transitioned from the spinal cord to the medulla oblongata, it opened along its dorsal side and unfolded to form the rhomboid fossa. Consequently, nuclei homologous to the posterior horns of the spinal cord were displaced laterally. Thus, the sensory nuclei originating from the dorsal region of the neural tube came to lie laterally within the rhomboid fossa, whereas the motor nuclei corresponding to the anterior horns remained medially positioned. As for the autonomic nuclei, the unfolding of the neural tube placed them within the substance of the rhomboid fossa between the sensory and motor nuclei.

The trigeminal nerve (cranial nerve V) contains four nuclei: one motor and three sensory nuclei (the pontine nucleus, the mesencephalic nucleus, and the spinal trigeminal nucleus). The abducens nerve (cranial nerve VI) possesses only a motor nucleus. The facial nerve (cranial nerve VII) has three nuclei: a motor nucleus, the sensory nucleus of the solitary tract, and a parasympathetic nucleus—the superior salivatory nucleus. The vestibulocochlear nerve (cranial nerve VIII) features two groups of nuclei: two auditory cochlear nuclei (anterior and posterior) and four vestibular nuclei (medial, lateral, superior, and inferior). The Glossopharyngeal nerve (cranial nerve IX) contains three nuclei: the motor ambiguus nucleus (shared by cranial nerves IX and X), the sensory nucleus of the solitary tract (shared by cranial nerves VII, IX, and X), and a parasympathetic nucleus—the inferior salivatory nucleus. The Vagus nerve (cranial nerve X) has three nuclei: the aforementioned motor ambiguus and sensory solitary tract nuclei, as well as a parasympathetic nucleus—the dorsal motor nucleus. The Accessory nerve (cranial nerve XI) has only a motor nucleus. The Hypoglossal nerve (cranial nerve XII) also possesses only a motor nucleus.

The roof of the fourth ventricle, which projects inferiorly into the cerebellum, is formed by a thin sheet of white matter—the superior medullary velum, stretched between the superior cerebellar peduncles—and the inferior medullary velum, which attaches to the floccular peduncle (a cerebellar lobule). Through three apertures in the roof of the fourth ventricle (one median posterior-inferior aperture and two lateral apertures), the ventricular cavity communicates with the subarachnoid space of the brain. The choroid plexus of the fourth ventricle lies adjacent to the inferior medullary velum on the ventricular side.

Positioned above the fourth ventricle—which essentially serves as the cavity of the pons and medulla oblongata—is the cerebellum, or little brain.

Reflexes of the medulla oblongata and pons. Functions of the reticular formation. The medulla oblongata and pons serve as integration centers for reflex arcs governing a variety of somatic and autonomic functions. These include the REFLEX REGULATION OF the salivary glands, which is mediated by the salivatory nuclei of the facial and glossopharyngeal nerves containing parasympathetic neurons.

The autonomic nucleus of the vagus nerve participates in the reflex Regulation of Respiration and Cardiac Activity. The reticular formation of the medulla oblongata and pons contains centers that play a crucial role in regulating visceral functions. The respiratory center is located in the medial portion of the medullary reticular formation. A distinctive feature of the neurons within the respiratory center is their capacity for automaticity. Even in the absence of afferent inputs, The activity of these neurons exhibits a periodic rhythm determined by the specific ionic mechanisms of their Cell membranes. The phase transitions of the respiratory cycle (inspiration and expiration) can be modulated reflexively via afferent influences. During deep inhalation and the stretching of lung tissue, impulses from pulmonary mechanoreceptors travel along afferent fibers of the vagus nerve to the respiratory center. These impulses reflexively inhibit the neurons responsible for inspiration while simultaneously exciting neurons that regulate expiration. Impulses from the expiratory neurons travel via the reticulospinal tract to motor neurons in the spinal cord, thereby triggering the onset of expiration. Structures of the Diencephalon and cerebral cortex can also participate in the Regulation of the respiratory center.

A vast region of the reticular formation in the medulla oblongata is occupied by the vasomotor center. Stimulation of the anterior (rostral) Divisions of the vasomotor center has been shown to induce vasoconstriction, elevate Blood pressure, and accelerate heart rate (tachycardia). Conversely, stimulation of the caudal division of this center leads to vasodilation, a drop in blood pressure, and bradycardia. Axons of neurons within the vasomotor center project to the thoracic spinal cord, synapsing on the neurons of the sympathetic nuclei in the lateral horns. Thus, vascular tone is regulated by the sympathetic Nervous system alone. Activation of this system causes vasoconstriction, whereas inhibition of sympathetic centers promotes vasodilation. Reflex effects on the neurons of the vasomotor center are mediated through the excitation of chemo- and mechanoreceptors located in the vascular walls.