Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Central Nervous System
Neural Pathways of the Central Nervous System

A neural pathway is a chain of anatomically and functionally interconnected Neurons that transmit nerve impulses of identical function in a strictly defined direction. Based on the components of a reflex arc, pathways are classified into afferent, associative, and efferent pathways (Fig. 14.16).

Afferent pathways transmit nerve impulses from receptors to the integrative centers of the Brain.

Associative pathways provide connections between the integrative centers of the brain, such as between the Cerebellum and the Cerebral Cortex.

Efferent pathways transmit nerve impulses from the integrative center to the effector (target organ).

Afferent pathways. Afferent Neural Pathways can be classified into pathways of conscious and unconscious sensitivity. Pathways for conscious sensitivity terminate in the integrative centers of the cerebral cortex, whereas pathways for unconscious sensitivity end in subcortical integrative centers: the cerebellum, Midbrain, and Diencephalon. Based on the modality of sensation, afferent pathways are divided into general and special sensory pathways.

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Fig. 14.16. Schematic diagram of the Central Nervous system pathways

The principal Afferent Pathways of conscious general sensitivity include the pain, Temperature, and tactile pathway, as well as the conscious proprioceptive pathway. The primary unconscious afferent pathways of general sensitivity are the anterior and posterior spinocerebellar tracts. Special sensory pathways include the auditory, visual, vestibular, gustatory, and olfactory pathways. All of these traverse zone II of the Brainstem.

The pathway for pain, temperature, and tactile sensitivity from the trunk and limbs (superficial or exteroceptive sensitivity) is designated as the gangliospinothalamocortical pathway, tractus gangliospinothalamocorticalis. It originates from receptors in the Skin, from which impulses travel to the sensory ganglion Cells of the spinal nerve and subsequently into the Spinal Cord, synapsing in the nuclei of the posterior horns. The information is then relayed to the thalamic nuclei and directed to the cortex of the postcentral gyrus for analysis. Thus, this pathway comprises three tracts: 1) the gangliospinal tract; 2) the lateral spinothalamic tract, which courses through the lateral funiculus of the spinal cord and the brainstem tegmentum; and 3) the thalamocortical tract.

The conscious proprioceptive pathway from the trunk and limbs is termed the gangliobulbothalamocortical pathway, tractus gangliobulbothalamocorticalis. It originates from receptors (proprioceptors) in Muscles, ligaments, tendons, periosteum, and joint capsules, transmitting information regarding the sense of weight, pressure, vibration, the degree of Muscle contraction or relaxation, and body part positioning in space. The Cell bodies of the first-order neurons of this pathway reside in the sensory ganglia of the Spinal Nerves, those of the second-order neurons in The Nucleus gracilis and nucleus cuneatus of the Medulla Oblongata, and those of the third-order neurons in the thalamic nuclei of the diencephalon. The pathway terminates in the postcentral gyrus of the cerebral cortex, where the information is analyzed. It consists of three tracts: 1) the fasciculus gracilis and fasciculus cuneatus, which course through the posterior funiculus of the spinal cord; 2) the bulbothalamic tract, running through the brainstem tegmentum; and 3) the thalamocortical pathway, ending in the projection center — the postcentral gyrus of the parietal lobe.

The pathway of conscious general sensitivity from the HEAD and Neck is the ganglionucleothalamocortical pathway, tractus ganglionucleothalamocorticalis. This pathway conducts impulses of all general sensory modalities (pain, temperature, tactile, and proprioceptive) from the head and neck. It begins with receptors in the skin and muscles, routing nerve impulses to the trigeminal ganglion and then to the Trigeminal nerve nuclei. From there, it projects to the thalamic nuclei and subsequently to the cells of the postcentral gyrus. Consequently, it comprises three tracts: 1) the ganglionuclear tract; 2) the nucleothalamic tract; and 3) the thalamocortical tract.

The anterior and posterior spinocerebellar tracts, tractus spinocerebellaris anterior and tractus spinocerebellaris posterior, are unconscious pathways formed within the lateral funiculus of the spinal cord that also convey information regarding the state of The Musculoskeletal System. The anterior spinocerebellar tract reaches the cerebellum via its superior cerebellar peduncle, thereby passing through the tegmentum of the medulla oblongata, Pons, and midbrain. The posterior tract enters through the inferior cerebellar peduncle, thus traversing only the medulla oblongata. The Role of these tracts is exceptionally critical, as they Relay sensory input from muscle, ligament, tendon, Joint Capsule, and periosteum receptors to the cerebellum, which regulates motor coordination and balance.

The Auditory pathway carries information from the receptors of the spiral organ (organ of Corti) located in the Inner ear. Via the fibers of the Vestibulocochlear nerve (cranial nerve VIII), nerve impulses enter the pons, where the auditory nuclei are located. From there, they are relayed to the nuclei of the trapezoid body. Subsequently, nerve impulses reach subcortical auditory centers: the inferior colliculi of the midbrain, the thalamus, and the medial geniculate bodies. Reflex responses to excessively loud auditory stimuli are generated in the midbrain; an unconscious evaluation of auditory stimuli occurs in the thalamic nuclei to facilitate involuntary movements (such as running or walking); and the auditory radiation originates from the geniculate bodies as a tract that conducts impulses through the internal capsule to the superior temporal gyrus (the auditory projection center). Here, conscious evaluation of sound signals takes place. Located in the posterior part of this gyrus is the associative auditory center, where sounds are perceived as words.

The visual pathway originates from the rods and cones of the retinal layer of the Eyeball. As part of the Optic nerve (cranial nerve II), impulses travel to the optic chiasm and then proceed via the optic tract to subcortical centers: the superior colliculi of the midbrain, the thalamus, and the lateral geniculate bodies. Reflex responses to unexpected visual stimuli are formulated in the midbrain; an unconscious evaluation of impulses occurs in the thalamic nuclei to support involuntary movements (such as running and walking); and from the geniculate bodies, impulses travel via the optic radiation through the internal capsule to the calcarine sulcus of the occipital lobe — the visual projection center — where information analysis takes place. The associative visual center (the visual memory center) is localized in the cortex adjacent to the calcarine sulcus.

The gustatory pathway. Impulses originate from receptors in the anterior part of the Tongue via the Facial Nerve (cranial nerve VII) and from the ROOT of the tongue via the Glossopharyngeal nerve (cranial nerve IX), traveling to the medulla oblongata and subsequently to the sensory nuclei of these nerves. From these nuclei, a smaller portion of the information projects to the cerebellum via the nucleocerebellar tract to provide unconditioned REFLEX REGULATION OF head, tongue, and pharyngeal muscle tone, while the majority reaches the thalamus. From the latter, nerve impulses are transmitted to the uncus of the temporal lobe, where conscious analysis occurs.

The olfactory pathway begins at the receptors of the superior nasal meatus mucosa. Nerve impulses then travel along the Olfactory nerve fibers (cranial nerve I) to the neurons of the olfactory bulbs. Following this, they reach the temporal lobe cortex via the olfactory tract, where the olfactory projection center is situated in the region of the uncus and parahippocampal gyrus. From this center, a portion of the information is directed to subcortical centers located in the midbrain and diencephalon (superior colliculi, thalamus, and mamillary bodies). These subcortical centers provide unconditioned reflex Regulation of Muscle tone in response to olfactory stimuli.

Thus, a distinctive feature of the olfactory pathway is that nerve impulses initially project not to subcortical olfactory centers, but directly to the cerebral cortex. Consequently, an individual typically perceives and evaluates a smell first, and only a fraction of a second later does the unconscious emotional coloration of the stimulus develop.

The vestibular pathway originates from the receptors of the saccule, utricle, and semicircular canals of the inner ear. Via the fibers of the vestibulocochlear nerve (cranial nerve VIII), impulses travel to the vestibular nuclei of the pons and subsequently to the thalamic nuclei of the diencephalon. From there, they are directed to the middle and inferior temporal gyri, where the vestibular projection center is located. This center analyzes vestibular information (conscious awareness of body position in space).

Efferent pathways. Efferent neural pathways originating from neurons in the cerebral cortex are termed cortical pathways. Morphologically, the majority of the neurocytes forming these pathways are pyramidal in shape. Consequently, cortical pathways are also referred to as Pyramidal Tracts. Cortical pathways mediate complex, conscious motor acts. Efferent pathways originating from neurons within brainstem integrative centers are called extrapyramidal pathways. These pathways conduct nerve impulses that maintain muscle tone and govern complex, unconditioned reflex motor acts. Fibers of both pyramidal and extrapyramidal pathways terminate on motor neurons in the anterior horns of the spinal cord or on the motor cranial nerve nuclei.

There are two primary pyramidal tracts: the corticospinal tract (tractus corticospinalis) and the corticonuclear tract (tractus corticonuclearis). They originate from pyramidal cells in the precentral gyrus cortex, traverse the middle section of the internal capsule, and then course through the ventral portion of the brainstem (zone I).

The corticonuclear tract terminates on the motor nuclei of the Cranial Nerves and mediates conscious (voluntary) Movements of the head and Neck Muscles. In the pyramids of the medulla oblongata, the corticospinal tract divides into the lateral and anterior corticospinal tracts. The lateral corticospinal tract crosses to the opposite side, forming the pyramidal decussation (see Fig. 14.10), and enters the lateral funiculus of the spinal cord. The anterior corticospinal tract descends uncrossed into the anterior funiculus of the spinal cord, undergoing decussation further down within the spinal cord itself.

The corticospinal tracts terminate on the motor nuclei of the anterior horns of the spinal cord and are responsible for conscious (voluntary) movements of the limb and trunk muscles.

Extrapyramidal pathways originate from subcortical motor centers (such as the reticular formation, red nucleus, and olivary nuclei) and terminate, much like the pyramidal tracts, on the motor cranial nerve nuclei and the motor nuclei of the anterior horns of the spinal cord, thereby providing unconditioned reflex regulation of muscle tone and involuntary movements. All of these pathways course through the brainstem tegmentum (zone II).

The reticulospinal tract, tractus reticulospinalis, originates from reticular formation cells throughout the entire length of the brainstem and terminates on the motor nuclei of the anterior horns of the spinal cord. It maintains muscle tone.

The rubrospinal tract, tractus rubrospinalis, originates from the red Nucleus of the midbrain, courses through the lateral funiculus of the spinal cord, and terminates on the motor nuclei of the anterior horns of the spinal cord. It mediates involuntary movements (such as running and walking) and maintains muscle tone during static load (postural maintenance).

The tectospinal tract, tractustectospinalis, originates from the superior colliculi of the midbrain, terminates on the motor nuclei of the anterior horns of the spinal cord, and mediates reflex responses to unexpected, supramaximal stimuli (auditory, visual, olfactory, gustatory, and tactile).

The medial longitudinal fasciculus, fasciculuslongitudinalis medialis, arises from the nuclei of the midbrain Reticular Formation and provides associative connections among the motor nuclei of the III, IV, VI, and XI cranial nerve pairs, thereby coordinating combined movements of the head and eyes.

The vestibulospinal tract, tractus vestibulospinalis, originates in the pons from the vestibular nuclei of the vestibulocochlear nerve and terminates on the motor nuclei of the anterior horns of the spinal cord. It maintains balance and coordinates movement during vestibular stimulation.

The olivospinal tract, tractusolivospinalis, arises from the olivary nuclei of the medulla oblongata and terminates on the motor nuclei of the anterior horns of the spinal cord. Its function is similar to that of the preceding tract.

Review Questions

1. What principles form The basis of nerve cell Classification?

2. How are receptors classified?

3. Name the Main Components of a reflex arc.

4. What divisions are distinguished in The Nervous System?

5. Characterize the role of the nervous system in the body.

6. WHAT IS A spinal cord segment?

7. Which tracts run within the funiculi of the spinal cord?

8. What are the segmental and conducting apparatuses?

9. What regions are distinguished in the brain?

10. List the cranial nerves originating in the medulla oblongata, pons, and midbrain.

11. Describe the functional role of the cerebellum.

12. What structures comprise the diencephalon?

13. List the layers of the cerebral cortex.

14. Describe the dynamic Localization of Functions within the cerebral cortex.

15. List the ventricles of the brain.

16. Name the Meninges of the BRAIN AND SPINAL cord.

17. How are the PATHWAYS OF THE central nervous system classified?

18. List the general sensory pathways.

19. Name the special sensory pathways.

20. How do pyramidal pathways differ from extrapyramidal pathways?