Human Anatomy Part 2 - K. A. Dyubenko A. K. Kolomiytsev Yu. B. Chaykovsky 2008

Special Part
Nervous system, systema nervosum - Special Part
Pathways of the central nervous system - Descending pathways (motor)

Descending Neural Pathways include the pyramidal and extrapyramidal pathways, as well as the descending cerebellar pathways.

Class="center">Pyramidal Pathways (Pyramidal System) (Fig. 166)

The pyramidal pathways directly connect the motor cortex of the cerebral hemispheres with the motor nuclei of the Spinal Cord and the motor nuclei of the Cranial Nerves in the Brainstem. Research shows that 30% of the fibers of the corticospinal and corticonuclear pathways originate from the motor cortex, 30% from the premotor cortex, and 40% from the parietal lobe, particularly the somatosensory area. The pyramidal system comprises the following pathways:

- the corticonuclear tract, tractus corticonuclearis;

- the lateral corticospinal (pyramidal) tract, tractus corticospinalis (pyramidalis) lateralis;

- the anterior corticospinal (pyramidal) tract, tractus corticospinalis (pyramidalis) anteńrior.

The corticonuclear tract, tractus cołticonuclearis (Fig. 166 B), connects the cortex of the precentral gyrus with the motor nuclei of the cranial nerves in the brainstem. The pathway originates from large pyramidal Cells (Betz cells) located in layer V of the Cerebral Cortex (the lower third of the precentral gyrus). The axons of these pyramidal cells pass through the genu of the internal capsule, the cerebral peduncles, the Pons, and the Medulla Oblongata, where they give off fibers to the motor nuclei of the cranial nerves on the contralateral side and, partially, on the ipsilateral side.

Within the cerebral peduncles, corticonuclear fibers course toward the motor nuclei of the oculomotor (III) and trochlear (IV) nerves; at the level of the pons, toward the nuclei of the trigeminal (V), abducens (VI), and facial (VII) nerves; and at the level of the medulla oblongata, toward the nuclei of the glossopharyngeal (IX), vagus (X), and hypoglossal (XII) nerves. In the medulla, the fibers approach the nuclei of both the ipsilateral and contralateral sides, with the exception of the lower part of the Facial Nerve motor Nucleus and the Hypoglossal nerve nucleus, which receive fibers exclusively from the contralateral side. From the nuclei of these aforementioned nerves, where The Cell bodies of the second-order Neurons reside, axons extend to innervate the striated Muscles of the HEAD and Neck.

Thus, the corticonuclear tract is a two-neuron motor pathway responsible for transmitting impulses from the cerebral cortex to the brainstem motor nuclei. Bilateral damage to the fibers of the corticonuclear tract results in central paralysis of the head muscles.

The lateral corticospinal (pyramidal) tract, tractus corticospinalis (pyramidalis) lateralis (Fig. 166 A), connects the cortex of the precentral gyrus (the motor area) to the motor nuclei of the anterior horns of the spinal cord. This pathway originates from large pyramidal cells (Betz cells), which house the cell bodies of the first-order neurons. The axons of these first-order neurons form a dense bundle of fibers that passes through the anterior two-thirds of the posterior limb of the internal capsule, the cerebral peduncle, and the pons. In the medulla oblongata, the fibers lie superficially and form the pyramids. In the pyramidal region of the medulla, 80% of the nerve fibers cross the midline to the contralateral side, forming the decussation of the pyramids, decussatio pyramidum, after which the fibers enter the lateral funiculus to form the lateral corticospinal tract, tractus corticospinalis lateralis. The axons of this tract reach the anterior horns of the spinal cord, where they terminate in synapses on cells in the lateral part of the anterior horn (second-order neurons). The axons of the motor cells in the anterior horns emerge from the spinal cord to form the anterior motor ROOT, which travels within the spinal nerve to innervate the skeletal muscles of the distal upper and lower extremities (regulating fine and precise movements).

The anterior corticospinal (pyramidal) tract, tractus corticospinalis (pyramidalis) anterior (Fig. 166 A), is formed by the axons of neurons located in the motor area of the precentral gyrus, gyrus precentralis. At the junction of the medulla oblongata and the spinal cord, these axons (20%) do not cross to the contralateral side, but instead descend caudally within the anterior funiculus of the spinal cord. The fibers of this tract undergo a segmental, incomplete decussation and terminate on the motor Cells of the medial portion of the anterior horns on both the ipsilateral and contralateral sides of the spinal cord. The axons of these anterior horn Nerve Cells emerge from the spinal cord to form the anterior motor root, which, as part of the spinal nerve, reaches the skeletal Muscles of the Trunk and proximal limb segments to provide their innervation.

Thus, the pyramidal pathways are two-neuron crossings that constitute the pyramidal system, which provides direct cortical regulation and conscious control over striated muscles (the system responsible for fine and precise movements, W. F. Ganong, 2002) associated with speech, manual dexterity, and finger movements.

Damage to central neurons or the fibers of the pyramidal pathways leads to central paralysis, or hemiplegia*, impairing motor function. Below the level of the lesion, Muscle tone increases, tendon Reflexes are heightened, and cutaneous reflexes are diminished or absent. Conversely, damage to peripheral neurons of the anterior horns, nerve roots, plexuses, and peripheral nerves results in peripheral paralysis, characterized by areflexia, atonia, and muscle atrophy.

* Hemiplegia — paralysis of one side of the body caused by damage to the motor areas of the cerebral cortex.

Extrapyramidal Pathways (Extrapyramidal System)

The extrapyramidal system (Fig. 167) is a part of The Nervous system composed of the Basal Ganglia of the Brain and non-pyramidal motor pathways. It performs involuntary, automatic regulation and coordination of complex movements, modulates muscle tone, maintains posture, ensures the speed, rhythm, smoothness, and fluidity of movements, provides the postural foundation for their execution, and signals the completion of movements. Damage to the extrapyramidal system leads to involuntary movements, or dyskinesias (hyperkinesias), as well as disturbances in muscle tone. Damage to the substantia nigra causes so-called "resting tremor," which is characteristic of Parkinson's disease.

The extrapyramidal system includes a cortical center—motor area 4—and subcortical structures, namely the basal ganglia: the caudate nucleus (nucleus caudatus), the lentiform nucleus (nucleus lentiformis), the medial thalamic nuclei (nucleus medialis thalami), the subthalamic nucleus (nucleus subthalamicus, or corpus Luysi), the substantia nigra, the red nucleus (nucleus ruber), the nuclei of the reticular formation in the brainstem (nuclei formationis reticularis), and the olivary Nucleus of the medulla oblongata (oliva). In addition to these structures, the Cerebellum and vestibular nuclei are also considered part of the extrapyramidal system.

The basal ganglia serve as higher suprasegmental centers, regulating movements across various muscle groups and complex motor acts such as walking, running, climbing, and facial expressions. Via descending pathways, impulses from the basal ganglia reach the anterior horns of the spinal cord and subsequently the striated muscles. Stimuli enter the extrapyramidal system from the cells of the thalamus and Hypothalamus. Thus, the thalamus and hypothalamus act essentially as the receptive component of the extrapyramidal system.

The extrapyramidal pathways are divided into three groups: cortical extrapyramidal pathways, striopallidal pathways, and brainstem-spinal (truncospinal) pathways.

The cortical extrapyramidal pathways consist of nerve fibers extending from the cells of the cortical motor centers to the structures of the extrapyramidal system (trr. corticothalamici, trr. corticohypothalamici, trr. corticopontini, trr. corticorubrales, tr. corticonigralis, tr. corticothegmentalis, tr. corticotectalis, fasc. corticostriatici et corticopalidaris).

The striopallidal pathways are formed by the neurons of cells located within the subcortical basal ganglia (the striatum, caudate nucleus, globus pallidus, and putamen). These pathways project to the nuclei of the thalamus, hypothalamus, red nucleus, and substantia nigra. The axons of these structures form three main efferent bundles: the lenticular loop (ansa lenticularis), the lenticular fasciculus (fasciculus lenticularis), and the subthalamic fasciculus (fasciculus subthalamicus).

Fig. 167. The extrapyramidal system (extrapyramidal pathways) according to W. Kahle, H. Leonhardt, and W. Platzer

The truncospinal pathways are formed by nerve fibers originating from the nuclei of the Midbrain, Diencephalon, and medulla oblongata, and projecting to the motor nuclei of the spinal cord and cranial nerve nuclei. These include the medial longitudinal fasciculus (fasciculus longitudinalis medialis), the rubrospinal tract (tractus rubrospinalis), the tectospinalis tract (tractus tectospinalis), the vestibulospinal tract (tractus vestibulospinalis), the central tegmental tract (tractus tegmentalis centralis), and the olivospinal tract (tractus olivospinalis).

The extrapyramidal system exerts its influence on the motor neurons of the anterior horns of the spinal cord and the cranial nerve nuclei via the following truncospinal pathways: tractus rubrospinalis, tractus vestibulospinalis, tractus olivospinalis, tractus reticulospinalis, tractus tectospinalis, tractus tegmentalis centralis, and fasciculus longitudinalis medialis.

These pathways also mediate a portion of cerebellar influences on the motor neurons of the spinal cord anterior horns. The rubrospinal tract, tractus rubrospinalis, also known as Monakow's bundle, is considered the principal pathway of the extrapyramidal system.

The rubrospinal tract, tractus rubrospinalis (Monakow's fasciculus). This pathway Functions as a projection efferent tract of the extrapyramidal system. The rubrospinal tract originates from the large cells of the red nucleus within the midbrain tegmentum, after receiving axons from the cerebellum (first neuron). The axons of the first neurons form the ventral tegmental decussation (Forel's decussation), decussatio ventralis tegmenti Foreli, in the midbrain tegmentum. After crossing the midline, the fibers pass through the tegmentum of the cerebral peduncles, pons, medulla oblongata, and lateral funiculi, ultimately reaching the nerve cells of the anterior horns of the spinal cord (second neurons) throughout its entire length. Axons originating from the nerve cells of the anterior horns travel as part of the anterior motor root of the spinal nerve and terminate in striated muscles.

This pathway transmits impulses from the cerebellar cortex and the nuclei of the corpus striatum—which are interconnected with the red nucleus—to the spinal cord. Consequently, it helps maintain posture and spatial body orientation, regulates the tone of striated muscles, and coordinates voluntary movements.

The vestibulospinal tract, tractus vestibulospinalis (Loewenthal's fasciculus). This pathway originates from the lateral vestibular nucleus (Deiters' nucleus), descends through the medulla oblongata, and then crosses to the contralateral side of the spinal cord into the anterior funiculus. The tract reaches the anterior horns of the spinal cord—predominantly in the cervical and lumbar segments—and subsequently innervates striated muscles. It connects the vestibular apparatus of the Organ of Balance with the cells of the anterior horns. Through this pathway, impulses from the vestibular apparatus reach the muscles, thereby ensuring equilibrium of the head and body in space, as well as postural adjustment reactions when balance is disrupted.

The axons of neurons from the medial vestibular nucleus (Schwalbe's nucleus) join the medial longitudinal fasciculus, fasciculus longitudinalis medialis, and project downward as the medial vestibulospinal tract to the thoracic region of the spinal cord.

* Konstantin von Monakow (1853–1930) was a prominent neurologist. Born in Russia near the city of Vologda, he pursued his career in Germany. He is renowned for his classical investigations into the visual and auditory pathways, the Localization of functions in the cerebral cortex, aphasia, and apraxia. The rubrospinal tract was named in his honor.)

The olivospinal tract, tractus olivospinalis (Helweg's fasciculus). This pathway belongs to the efferent tracts of the inferior olivary nucleus, nucleus olivarius inferior. It originates from the cells of the inferior olivary nucleus, descends within the anterior funiculus of the cervical spinal cord, and terminates on the cells of the anterior horns.

The anterior reticulospinal tract, tractus reticulospinalis. This pathway is a projection efferent tract of the extrapyramidal system. It originates from the cellular elements of the reticular formation in the pons and medulla oblongata. Scattered fibers travel through the anterior and lateral funiculi of the contralateral side of the spinal cord, terminating segmentally on the cells of the anterior horns.

The anterior reticulospinal tract regulates striated muscle tone and controls visceral motor functions, such as respiratory rhythmicity.

The medial longitudinal fasciculus, fasciculus longitudinalis medialis. This projection efferent pathway mediates labyrinthine and postural reflexes, visual innervation, and conjugate eye and Head movements. It originates in the midbrain from Darkschewitsch's nucleus (also known as The Nucleus of the posterior commissure) and establishes bilateral connections with the nuclei of the III, IV, VI, VIII, and XI pairs of cranial nerves. It traverses the midbrain tegmentum, pons, and medulla oblongata, entering the anterior funiculus of the spinal cord, where it terminates on the motor neurons of the anterior horns in the upper five cervical segments. Lesions of this pathway result in Impairment of the aforementioned functions.

The tectospinal tract, tractus tectospinalis. This projection pathway mediates motor responses to visual and auditory stimuli and participates in protective reflex mechanisms (such as the startle reflex). It originates from the cells of the superior colliculi of the midbrain tectum. Descending through the tegmentum—where it forms the dorsal decussation (Meynert's decussation)—the brainstem, and the anterior funiculus of the spinal cord, the tract terminates on the motor neurons of the anterior horns.

The central tegmental tract, tractus tegmentalis centralis. This pathway is formed by fibers originating from the midbrain tegmentum surrounding the periaqueductal Gray matter and the red nucleus; it descends through the tegmentum and reticular formation, terminating in the inferior olivary complex.

The extrapyramidal system also includes the efferent cerebellar pathway, specifically the cerebellorubrospinal tract, tractus cerebellorubrospinalis. This pathway originates from Purkinje cells (first neurons) in the cerebellar cortex, and their axons terminate in the dentate nucleus, where the second neurons of this pathway are located. The fibers then pass through the superior cerebellar peduncles to reach the red nucleus in the midbrain, housing the third neurons. According to the PNA, this segment of the pathway is designated as tractus cerebellorubralis, s. dentatorubralis fibrae. Continuing downward through the lateral funiculi, the pathway terminates on the anterior horn cells of the spinal cord (fourth neurons), from which impulses are conveyed via Spinal Nerves to striated muscles.

Through this pathway, the cerebellum exercises control over motor coordination, balance maintenance, muscle tone regulation, and the overcoming of inertia and gravity.

Descending Pathways from the cerebral cortex to the cerebellum.

The cerebral cortex exerts its influence on cerebellar functions via the descending corticopontocerebellar pathway, tractus cortico-ponto-cerebellaris, which comprises corticopontine tracts (including the frontopontine, temporopontine, parietopontine, and occipitopontine tracts) and pontocerebellar fibers (fibrae pontocerebellaris). The first neurons of this pathway originate from cortical cells in the frontal, temporal, parietal, and occipital lobes. The projection fibers descend through the internal capsule and cerebral peduncle, terminating on the pontine nuclei, nuclei pontis (second neurons). The axons of the pontine nuclei form fiber bundles that cross the midline and project via the middle cerebellar peduncle to the contralateral cerebellar hemisphere. In this manner, functional connectivity is established between the CEREBRAL CORTEX AND the cerebellar hemispheres.

Efferent Cerebellar Pathways

Efferent pathways from the cerebellum project to other centers (nuclei) within the brainstem, thereby integrating the cerebellum into the extrapyramidal system (Fig. 168). These pathways include:

Fig. 168. Efferent cerebellar pathways

1. The cerebellorubral tract, tr. cerebellorubralis. The first neuron originates from all Regions of the cerebellar vermis and projects to the dentate nucleus. The second neuron arises from the dentate nucleus (n. dentatus), courses through the superior cerebellar peduncle, undergoes complete decussation (Wernicke's decussation), and reaches the contralateral red nucleus (n. ruber). As noted previously, the primary extrapyramidal pathway—the rubrospinal tract, tr. rubrospinalis (Monakow's tract)—originates from the red nucleus.

2. The cerebellovestibular tract, tr. cerebellovestibularis. The first neuron of this pathway originates from the cortex of the nodulus and flocculus and projects to the fastigial nucleus (n. fastigii). The second neuron is located within the fastigial nucleus, and its axons traverse the inferior cerebellar peduncle, decussate, and terminate in the lateral vestibular nucleus (Deiters' nucleus).

3. The cerebelloolivary tract, tr. cerebelloolivarius. The first neuron is situated in the cortex of the cerebellar hemispheres, with its axons projecting to the dentate nucleus (n. dentatus). The axons of the second neurons arise from the dentate nucleus, course through the inferior cerebellar peduncle, decussate, and reach the inferior olivary nucleus (n. olivaris) of the medulla oblongata.

Thus, functional communication is established between the cerebellar cortex and the brainstem.